On the winding of power chokes and transformers: Part 3 – The plate (high voltage) transformer

This is a follow-up of two previous posts in this series:

  • On the winding of power chokes and transformers: Part 1 – Chokes – link
  • On the winding of power chokes and transformers: Part 2 – A filament transformer- link

Using what we already know:

Figure 1:
Plate transformer with attached wires and end bells installed.
The windings and laminations are yet to be varnished or the end bells painted.
Click on the image for a larger version.

In the previous post of this series I described the design and construction of a filament transformer with dual 11 volt, 11 amp windings and a multi-tapped primary.  Building on the experience gained, I felt confident to take it to the next step:  The design and building of the high voltage “plate” transformer for the (yet to be described) tube amplifier.

Based on the characteristics of the tubes to be used, the plate voltage needed to be “around 1 kilovolt” with each amplifier section requiring “about 100 milliamps” of average current, or around 200 milliamps, for the pair of channels.  Because of the experience gained in the winding of the filament transformer, we could use the design of the primary winding as a starting point.  For example, we know that to achieve a target magnetic flux of 1.4 Tesla and have the transformer be capable of around 253 volt-amps and attain a rather conservative 0.4 amps/mm2, we could use:

  • 17 AWG wire
  • Taps at 220, 229 and 239 turns for 115, 120 and 125 volts at 60 Hz, respectively.

During the winding of the filament transformer it was observed that we could easily fit 41 turns of 17 AWG per layer.  This meant that the 239 turns only partially filled the final (fifth) layer, so we could afford to add a few more turns to the primary if necessary

Two secondaries needed:

While the main secondary will be a high voltage one, we will also need a 6.3 volt secondary to power the filaments of some of the driver tubes.  Because such a secondary will have relatively few turns we will need to calculate it first for reasons that will become clear.

Using the “5% rule” we calculate that our 6.3 volt secondary will actually need to produce 105% of the desired voltage (6.3 * 1.05) = 6.6 volts to account for the drop under load.  Taking our 229 turn, 120 volt primary as a starting point we determine that the turns ratio to achieve this voltage would be (120 / 6.6) = 18.182:1 turns ratio.  With our 229 turn, 120 volt tap we would need (229 / 18.182) =  12.59 turns to obtain 6.6 volts.  

What this means is that for our secondary, we should round up (reason to be explained soon) rather than down and with exactly 13 turns we end up with a primary-secondary turns ratio of (229 / 13) = 17.62:1 and from this we can calculate the actual, unloaded secondary voltage as being (120 / 17.62) = 6.81 volts – a bit higher than we’d like.

By adjusting the turns ratio of the primary a bit to get a more accurate result.    What this means is that if we change the number of turns on the primary, we should increase the number of turns rather than decrease, so what if we increase the number of primary turns to accommodate a 13 turn, 6.6 volt secondary?

Why round the number of turns up rather than down?  You may recall that when winding a primary, the magnetic flux is has an inverse relationship with the number of turns.  Because the number of turns on the primary of the filament transformer was calculated to achieve the  maximum target flux, we would not want to decrease the number of turns as that would increase that flux.  In other words, the main down side of adding a few turns is that each winding will need a proportional number of extra turns as well, taking up additional room on the bobbin:  If things are already tight, adding those turns could result in more wire than will fit.

Crunching the numbers:

  • Our voltage ratio:  120 / 6.6 = 18.182:1.  We already saw this number.
  • Since our 6.6 volt secondary should have exactly 13 turns, our 120 volt primary should have (18.182 * 13) =  236.4 turns, rounded down to 236.  This increase in turns reduces the magnetic flux from 1.4 to about 1.3 Tesla.

Clearly, a half a turn on the 120 volt winding has a fraction of the effect (18.182th, to be more precise) as a half turn on the low-voltage primary so we will round this down to 236 turns.  Let us now calculate the 115 and 125 volt taps:

  • 115 volts / 6.6 volts =  17.42:1 ratio.  13 turns * 17.42 = 226.46 turns.  I rounded this down to 226 turns.
  • 125 volts / 6.6 volts = 18.94:1 ratio.  13 turns * 18.94 = 246.22 turns.  This was rounded down to 246 turns.

Let us now do a sanity check to see if this will fit.  Since we already know from when we wound the filament transformer that we can safely put 41 turns on a layer, we can see that for 246 turns we would need (246 / 41) = 6.0 layers – so we will go with that!

Designing the high voltage secondary:

We have now finalized the design of the primary so we can proceed with the design of the high-voltage secondary.

If you are familiar with tube-type amplifiers you might have already have guessed from the voltage and current requirements that the plate impedance of the amplifier would be quite high:  10k ohms, to be precise.  The transformers themselves are designed for single-ended triode operation with 8 ohm secondaries, rated for 25 watts (maximum) output.  Going through the math one can see that the turns ratio of this transformer is approximately √(10000/8) = 35.36:1.  If 25 watts RMS were being produced into 8 ohms, this implies that the RMS output voltage is around 14.14 volts, or almost exactly 500 volts RMS on the 10k primary which translates to 707 volts peak.

According to the specifications gleaned from the Edcor support forum (a link to the message thread may be found here) the maximum “safe” voltage across the primary and secondary windings would be 1000 volts.  Clearly, assuming a 10k primary impedance, 25 watts RMS of power and any reasonable plate voltage to achieve anywhere near this output power one will have to exceed this maximum voltage rating – unless a bipolar power supply is used where the high voltage is split – that is, the standing DC voltage between the primary and secondary is reduced to half.  To do this a full wave “bridge” rectifier is used with our choke-input filter network with the centertap of the transformer being grounded.

A final (loaded) DC voltage of around 970 volts for the plate voltage was (somewhat arbitrarily) decided as the target for the tubes that will be used – a reasonable compromise between the constraints of the output audio transformer voltage rating and the efficiency of the tube.  With this target in mind, let us calculate the actual, unloaded voltage for the secondary.

We know from when we designed the choke that at 200 mA there will be a 60 volt drop, so we will need to increase the output of 970 volts by this amount, which means that we will need (970 + 60) =  1030 volts.  Because the power supply will use a choke input, we know that the loaded voltage of such a power supply is typically around 110% of the RMS voltage which means that for 1030 volts DC we will need approximately (1030 / 1.1) =  936 volts RMS.

Using the “5%” rule of thumb to take into account resistive loading of the primary itself we can calculate the actual, loaded voltage for the secondary, as in (936 * 1.05) =  982 volts, unloaded.

Using the 120 volt tap from the reference design we can now calculate our turns ratio and the number of turns, as in:

  • 982 volts / 120 volts = A 8.183:1 turns ratio.
  • 236 turns (at 120 volts) * 8.183 = 1931 turns which will be rounded down to an even 1930 turns so that the center-tap will be made at the 965th turn.

Based on the recommendations from the Turner Audio and Homo-Ludens web pages (see previous articles for the links) we can use a general rule of thumb of 0.33-0.35mm2/amp and since our current is to be 0.2 amps, we need a wire with the size of at least (0.2 amps * 0.33 mm2/amp) = 0.066 mm2.  Consulting our wire chart we see that 29 AWG has a cross-sectional area of 0.0642 mm2 resulting in a density of 0.321 mm2/amp – pretty close to our design goal.  As noted in the previous installment, Edcor seems to use a value of around 0.253 mm2/amp for their transformers and if this is applied, our primary would be capable of (0.0642 mm2 / 0.253 mm2/amp) = 0.25 amps.

As it happens I had 29 AWG wire available when the choke was wound (it, too, was designed for 200mA) so this is the wire that I used.

Will it fit?

At this point the question must be asked:  Will all of these windings fit on the bobbin?

We know from when we wound the choke that approximately 161 turns of 29 AWG wire will fit per layer, and with 1930 turns total, we’ll need 12 layers.  With 30 AWG wire having an outside diameter (with insulation) of 0.33mm and the tape from each layer adding 0.05mm of thickness, each layer will occupy 0.38mm or, with 12 layers, 4.56mm of of bobbin “height”. 

We also know from our winding of the filament transformer that one layer of 17 AWG wire plus 0.05mm of insulating tape has a total height of 1.274mm and with 6 layers, that comes to 7.644mm.  Put together, the combined height of both windings is 12.204mm – approximately 73% of the 16.5mm available height.

Figure 2:
Center tap of high voltage plate winding located in the middle of the winding
before Nomex insulation was added.
Click on the image for a larger version.

This figure does not include the low voltage secondary winding (one layer of 17 AWG, adding another 1.274mm) or the extra insulation that must be added between windings (approximately 0.5mm for each of the three) all of which adds another 2.774mm, taking us up to 13.704mm – about 83% of the available space.

While this will be kind of a tight fit, we ended up with the same sort of numbers when we built designed and successfully built the filament transformer so we can have good confidence that this, too, will work.

The winding:

While it may seem customary to wind the primary first, that may be because most transformers that are seen these days are step-down, with the secondary winding handling more current than the primary and thus using larger wire.  It usual to place the smallest wire on the inner-most winding since it is more flexible and  easier to handle on the smaller-diameter first layers of a bobbin, going around the square-ish corners and leaving the larger wire for later when the diameter is larger and the corners more rounded.

Following this convention a hole was “drilled” in the side of the nylon bobbin with a hot soldering iron and a piece of Teflon™ insulated wire was pulled through, attached to the start of the winding and then insulated with several layers of polyimide tape and a layer of Nomex™ insulation.  With that task completed, the winding proceeded carefully with care being taken on the first layer to assure both neatness and tight packing – the latter being done by pausing every few turns to slide the wire over to minimize the gap between adjacent conductors.

Figure 3:
End of the high voltage secondary winding, insulated with both
polyimide tape and Nomex ™ paper.
Click on the image for a larger version.

The first layer done, a single layer of 0.05mm polyimide tape was placed over the top.  When I wound the choke I had only a single width of this tape available, but this time I had a selection of widths so as I proceeded with the layers, the location and widths of this tape was changed with each layer to minimize “piling” of the turns which would later make it difficult to keep the layers even.

After a few hours of intermittent winding over several days – with each layer individually insulated with 0.05mm polyimide tape – the center tap was reached and for this a loop of wire was made in the conductor at right angles to the lay to which another piece of Teflon wire was soldered which was brought through the side via a hole made in the side of the bobbin with a hot soldering iron.  This joint was carefully placed in the middle of the flat side of the bobbin that would face outward from the core and insulated it with a few layers of polyimide insulation and Nomex paper to prevent it from damaging or being damaged by the pressure of turns in the layers above and below.

Figure 4:
Overlay of Nomex ™ insulating paper atop the finished high

voltage secondary winding before the top layer of polyimide
tape and its “creepage” insulation along
the sides of the bobin was added.
Click on the image for a larger version.

After a few more days of occasional winding the last turn was laid down, nearly filling the 13th and final layer.  I soldered to this a piece of Teflon wire and insulated it and the wire was brought out through the side of the bobbin and the entire secondary was covered with several layers of polyimide tape and 0.05mm Nomex paper.  As a final covering over the Nomex, another layer of polyimide tape was laid down – this time with the tape slightly going up the sides to increase the “creepage” distance between the primary and secondary – a sensible safety precaution, particularly with a high-voltage transformer!

Now, the primary…

The conductors of the primary were now laid down atop the insulated secondary.  As with the filament transformer the 17 AWG wire was brought directly out through the side of the bobbin and tucked out of the way:  The connection to flexible wire would be done later.

Figure 5:
The three “end” taps of the primary winding:  Top-left is the 115 volt tap,
below it is the 120 volt tap with the 125 volt finish on the left.  After
this picture was taken small pieces of Nomex paper and additional
tape were placed below and above the taps.
Click on the image for a larger version.

As with the start of any new winding, the first layer of the 17 AWG primary was done with special care to make it neat and tight and each layer was individually insulated with 0.05mm polyimide tape.  When the 220th and 229th turns (for the 115 and 120 volt taps, respctively) were reached, loops of wire were put in the conductor, which was brought out through marked holes in the bobbin at right angles to the conductor alongside the final, 239th turn for the 125 volt end.

With each tap being insulated with polyimide tape and Nomex paper where they crossed over other windings, the entire primary was then covered with several layers of polyimide tape and Nomex paper.  Again, a bit of insulation was brought up along the sides of the bobbin to provide extra “creepage” distance to provide good insulation for the 6.3 volt secondary to maximize both safety and reliability.

More about the 6.3 volt secondary winding:

Because it was on-hand, 17 AWG wire was used for the “6.3 volt” additional secondary.  With a cross-sectional area of 1.04mm2, we can calculate its current-handling ability:

  • Using the 0.33 amps/mm2 recommendation from the Turner Audio site, a safe current is:  (1.04mm2 / 0.33 amps/mm2) = 3.15 amps
  • Using the 0.253 amps/mm2 design Edcor guidelines a safe current is:  (1.04mm2 / 0.253 amps/mm2) =  4.11 amps.
Figure 6:
The completed winding – including the 13 turn, low-voltage secondary –
with the just-started core stacking.
Click on the image for a larger version.

Even in the worst-case scenario the addition of a 4.11 amp secondary would add only another 28 volt-amps of load to the transformer – well within its capacity.  Because this winding is on the outside of the bobbin and “exposed”, it has good opportunity for cooling by convection and thus the Edcor rating would seem to be applicable – and 4 amps is plenty of current for several 6.3 volt tubes.

Comment:  If more current is needed it will be easy to add another parallel 17 AWG conductor to double its capacity.

As with the primary winding – which also used the same 17 AWG conductor – the ends of this 13 turn secondary were brought straight out the sides of the Nylon bobbin for later connection to flexible conductors and this additional secondary was overcoated with polyimide and polyester tape.

Finishing and initial testing:

With the addition of the low voltage secondary, all layers were over-wrapped with another layer of polyester tape to both secure and insulate the windings.  The transformer was almost ready to be tested

Figure 7:
The stacked transformer undergoing initial testing with a
a variable transformer.
Click on the image for a larger version.

Although there are approximately 111 pieces of iron to be inserted into the core, the process is pretty easy:  Simply lay the bobbin on the table, one of the “outer” faces (where the taps are made and wires are attached) and alternately place the “E” sections atop each other.  With the “E” sections done, the transformer is then set end up to give access to the vacant slots between every other “E” section into which the “I” sections were dropped.  Once these sections were added to one side, the bolts were slid through the laminations with the “I” sections to prevent them from falling out as I turned the transformer over and the final pieces were added to the other side.

With all E and I sections installed, a block of wood and a small hammer were used to abut the pieces of laminations against each other, a process that required several passes on all four sides.  With this done some nylon shoulder washers were installed (visible under the screw heads in Figure 7) to prevent the effect of currents that might be caused by the “shorted turn” effect of the screw and the bolts tightened.

Using a variable transformer the transformer was then tested, first noting that the unloaded (magnetizing) current of the transformer was comparable to that of the previously-tested filament transformer indicating that there seemed to be nothing amiss.  Very carefully, the high voltage secondary’s voltage was then tested on each side of center tap and I noted that they were within a fraction of a volt of each other, and exactly at the calculated value with 120.0 volts applied:  491 volts.  I could not directly measure the 982 (unloaded) volts across the entire secondary since I have no voltmeter that is “officially” rated above 750 VAC.

After a test of the low voltage secondary, which was also measured to be at its designed voltage, I attached permanent wires and the end bells as seen in Figure 1 at the top of this page.  At this point the transformer  only awaits being dipped in insulating varnish – something that will happen after inital testing of the (yet to be described) amplifier prototype.

A future post in this series will describe the final steps in finishing these transformers:  Impregnation in “insulating varnish” and the final painting of the end bells.


Surecom KT-8900D: Mobile

Via Nate… thanks for the tip!

Surecom KT-8900D 136-174/220-260/350-390/400-480 Mini Color Screen Mobile Radio for $97.00.

  • 1 x SURECOM KT-8900D 136-174/220-260/350-390/400-480 MINI COLOR SCREEN MOBILE RADIO
  • 1 x Microphone
  • 1 x Car Power Cable
  • 1 x Fuse
  • 1 x Mounting bracket
  • 1 x Screw
  • 1 x User”s Manual

General specification

Fequency Range
VHF:  136-174MHz  /  220-260MHz 
UHF:  350-390MHz /   400-480MHz
Number of channels 200
Channel spacing 25KHz、  20K、12.5
Phase lock step step 2.5KHz、5KHz、6.25 KHz、10 KHz、12.5 KHz、15 KHz、25 KHz
Working voltage 13.8V DC±15%
Squelch  way CTCSS、DCS、5Tone、2Tone、DTMF
Frequency stability ±2.5ppm
Operat temperature -20+60℃
Dimension 98(W)*43(H)*126(D)mm
Weight 448g
Receive(ETSI EN 300 086 Standarddized.Test)
 Wide Band  Narrow  Band
Sensitivity ≤0.25μν ≤0.35μν
Channel Cholce ≥70db  ≥60db
Intermodulation ≥65db  ≥60db
Spurious Rejection ≥70db ≥70db
Audio Response +1~~3db(0.3~3KHz)   +1~~3db(0.3~2.55KHz)
Signal to noise radio ≥45db  ≥40db
Audio Distortion ≤5%
Audio output power   ≥2W@10%
Transmit(ETSI EN 300 086 Standarddized.Test)
 Wide Band Narrow  Band
Output Power 25W/20W(VHF/UHF)
Modulation Mode 16KΦF3E  11KΦF3E
Channel Power ≥70db ≥60db
Signal to noise radio ≥40db ≥36db
Parasitic Harmonic ≥60db  ≥60db
Audio Response  +1~~3db(0.3~3KHz)  +1~~3db(0.3~2.55KHz)
Audio Distortion  ≤5%

Review – SharkRF openSPOT

by John ‘Miklor’sharkopen-200

OK, so what is a hotspot?

A hotspot an interface that allows you to connect to a digital network directly without the need of an actual repeater. There are several variations of hotspots available. Some may require your PC or Raspberry Pi interface, but the SharkRF openSPOT is a small stand alone IP gateway that connects directly to your internet router via the Ethernet connection. No other equipment required.

The openSPOT was developed by two hams that not only understand what hams want, need, and enjoy, but know how to make it work with minimal setup. With features not found in other hotspots, the bottom line is, they did it right.

openspot01So, here’s my take on openSPOT…

What’s In the Box

–   The SharkRF openSPOT
–   120-240V / 5V 1A  wall supply
–   USB / micro USB power cable
–   Small UHF antenna (SMA-Male)
–   39″ (1m)  Ethernet cable

Chassis Size: 2.75 x 2.5 x 1.0″
(71 x 67 x 25mm)
Weight:  5oz  (150g)

To start, the openSPOT is a very solidly built unit.  Don’t let the size fool you.  The chassis size is only 2.5 x 2.75 x 1.0″, but the weight is 5oz.  This is due to the weighted base plate inside
the enclosure to stabilize it when the cables are connected.  There is no heat build up inside, which allows me to run mine 24/7. The antenna jack is a standard SMA-F, with the antenna terminating with an SMA-Male. Four small rubber feet assist in keeping the chassis stable.

The Manual

The openSPOT manual never goes out of date because it’s web based. The instructions and tutorials include both graphics and videos to guide you through the entire setup. As new features are added, the manual is updated online, so it’s never out of date.

Setup and Interface

To set up the openSPOT, I connected it to the USB power source and my WiFi router using the supplied Ethernet cable. The openSPOT has two internal micro-controllers, designed to use a web based interface. Once connected to my WiFi router, the openSPOT was accessed by simply logging into //openspot.local.  I didn’t need to load any additional software or drivers. Everything is self contained.

At that point, all that was necessary was to enter:
–  The operating mode (DMR, C4FM, D-Star)
–  My callsign
–  My DMR ID (available from DMR-MARC)
–  The desired server
–  The input and output frequency of my handheld
Note: The openSPOT allows you to select two different frequencies if desired.
After that, a 30 second calibration, and I was on the air.
The LEDs give a clear status indication during operation.

Once configured, time slot, truck group, etc. information is controlled by your handheld. You set it, and forget it.
Cross Mode Operation

The openSPOT has cross mode capability, allowing a DMR transceiver to access C4FM, as well as  C4FM access to DMR.  This allows you to operate both modes with one handheld. I personally haven’t ventured into the cross mode operation, but understand it works perfectly.
OTA Audio Quality

This is where the openSPOT excels. In the past few months, I have learned to identify some hotspots by their robotic audio, much like R2D2 and BB8.  I can honestly say that I have never heard  an openSPOT with less than perfect audio. My reports have been nothing short of excellent.  I use mine on a daily basis, and have had absolutely no issues.

Firmware Updates

The openSPOT firmware is totally upgradable.  Periodically, SharkRF will post FW Beta versions on their site, however my personal preference is to wait until a final version is posted.  I do, however, like to review what is included in the beta versions to get a glimpse into the future. The developers are constantly keeping up with network changes so you always have the latest version.

The power level of the openSPOT variable up to a max of 20 mW.  That may not sound like a lot of power, but 1 mW is enough for me to hear the signal solidly throughout my entire house.  20 mW on an outside antenna will allow you to use a digital radio throughout your neighborhood.

But how about Mobile Installation

Here’s where I became a bit creative. Rather than connecting directly to my router (downstairs), I wanted to see the lights flashing, so I purchased a TP-Link TL-WR802N and set it up as a client to access my in home WiFi. The openSPOT never missed a beat.

Why the TP-Link? Well, it is just slightly smaller than the openSPOT, connects via an Ethernet connection, and fits nicely on top of my RAVPower 22000mAh battery. I linked the TP-Link to my Cellular WiFi hotspot and now, instant digital mobile. I also successfully tried a Vonets VAR11N-300 mini.
Now when traveling or on vacation, I no longer need to program a codeplug for every repeater along the way.  I set the radio to mate with the preselected openSPOT frequency, and that’s it. I now have access to the worldwide Brandmeister network

The entire mobile hardware configuration is approximately 2 x 3 x 6.5″ (5.5 x 8 x 16.5cm), not counting a few cables sticking out. The entire configuration fits inside a pencil box.

I also noted that the cellular data required is relatively small. Approximately 6MB per hour.
So, why an openSPOT

Hotspots were not developed to replace repeaters, but rather to supplement them. In areas where there is No repeater, a hotspot allows the user to connect directly to a digital network via the internet. In areas of Heavy repeater use, a hotspot allows the user to access the network without competing for an available time slot.

If your local repeater gives you access to a network such as DMR-MARC, an openSPOT can give access to networks such as Brandmeister. You will now have access to the best of both worlds.


DMR repeaters are being placed in service daily, but currently there are only around 800 repeaters in the US.  As for me, I am 25 miles away from my two nearest repeaters and require an outside antenna to reliably use them (and I’m one of the lucky ones). In some areas of the country, digital repeaters don’t exist.

The solution quickly became obvious. To enjoy the freedom of a handheld, I needed the help of a hotspot.  After listening to several configurations over the past several months, I am convinced daily that I definitely made the right decision with an openSPOT, with everything I need condensed into one small package.

SharkRF –  Home Website
SharkRF –  Purchase


GMRS: The Other UHF Band

I’ve always had a liking for the General Mobile Radio Service (GMRS). It’s a licensed radio service but does not require a technical exam so it works great for basic personal communications. When our kids were young we made good use of GMRS communications. This was back in the Pre-Cellphone Era, shortly after the dinosaurs left the earth. I still have my GMRS license: KAF1068

Midland MicroMobile GMRS Transceiver

GMRS uses frequencies in the general vicinity of 462 and 467 MHz. When the FCC created the Family Radio Service, they intermingled the FRS and GMRS channels, creating a real mess. See this page for a good explanation of how FRS and GMRS frequencies are arranged. Many of the low cost walkie-talkie radios sold in stores are combination FRS/GMRS radios.

I recently came across this really sweet little GMRS rig, the Midland MXT-100 Micro Mobile GMRS Radio. This thing is nice and small with an external mag-mount antenna for the roof of the car. It only has 5W of output power, which is not much more than a typical FRS/GMRS handheld radio but the external antenna should help a lot. (I’ve heard there are newer models on the way so stay tuned for that.)

I’ve encountered 4WD / Jeep clubs that use FRS radios for on the trail communications. This Midland radio would be a good upgrade for that kind of use, providing additional radio range. Some of these 4WD enthusiasts have gotten their ham ticket via our Technician license class. Ham radio provides a lot more capability but not everyone in their club is likely to get their ham license. GMRS is a great alternative…the other UHF band. It will work for other outdoor, community and club activities that involve “non radio” people.

FCC recently reduced the cost of the GMRS license to $65 for 5 years. I suspect that most people don’t bother with getting a license…but they should. For more detail on GMRS, see the FCC GMRS Page or for some good bedtime reading see the FCC Part 95 Rules.

73, Bob KØNR

The post GMRS: The Other UHF Band appeared first on The KØNR Radio Site.

Repairing the power switch on the Kenwood KA-8011 (a.k.a. KA-801) amplifier

Back around 1990 my brother mentioned to me that there was an amplifier, in a box, in pieces, in the back room of the home TV/electronics store where he worked at the time and that if I made an offer I could probably get it for cheap.  Dropping by one day I saw that it was a Kenwood KA-8011 Integrated DC amplifier (apparently the same as the KA-801, except with a dark, gray front panel) laying in a box from which the covers were removed with a bunch of screws and knobs laying in the bottom.  I also noticed with some surprise that it had a world-wide voltage selector switch on the back and that the power cord had a Japanese 2-prong wall plug and U.S. adapter – and still does!  All of the parts seemed to be there so I offered some cash ($50, I seem to recall) and walked out with it and a receipt.

Figure 1:
 Spoiler alert:  This is the KA-8011 with the repaired power switch.
As noted in the text, the original, blue-painted panel meter lights were
replaced long ago with blue LEDs.

When I got home with the amplifier I knew that I had my work cut out for me – particularly since, in those days before the widespread internet – I had no schematic for it and no-one that I contacted seemed to be able to find one.  Powering it up I noted that the speaker protection relay would never engage indicating that there was a fault somewhere in the amplifier.

A visual inspection of the awkward-to-reach back panel’s circuit board revealed several burned-looking leads sticking up from the circuit board where transistors had exploded and several burned resistors.  After a few hours of reverse-engineering a portion of the circuit I realized that the majority of the circuit at fault was one of four identical phono preamp input circuits (there are two separate stereo phono inputs) and associated low-level power supplies.  Between the intact amplifier sections and being able to divine the color bands on the smoked resistors – along with some educated guesses – I was able to determine the various components’ values and effect a repair.

Figure 2:
The power switch, with a broken bat.
Click on the image for a larger version.

The amplifier now worked… sort of.  I then had to sort out a problem with the rear-panel input selector switch, operated by a flat, thin ribbon of stainless steel in a plastic jacket that was engaged from a front-panel selector.  I managed to cut off the portion at the front that had been damaged where it was pulled-on from the front panel having been loose in the box, punch some new holes in the ribbon, align the two (front and rear) portions of the switch mechanism and restore its operation.


Having done the above, the amplifier was again operational and I have used it almost every day in the 25 or so years since, needing only to replace the blue-colored incandescent meter lights with LEDs, powered from a simple DC filtered supply.  In the intervening years I also had to replace some of the smaller electrolytics on the main board that had gone bad, causing the speaker protection circuit to randomly trip on bassy audio content and with slight AC mains voltage fluctuations.

Figure 3:
Comparing the old (top) and new (bottom) switch components.  In order
to prevent it from interfering with the body of the switch some of the
metal on the new bat would have to be removed.
Click on the image for a larger version.

I was annoyed when one day, a few months ago, the power switch handle – which had been bent before I got the amplifier – and then “un-bent” during the repair – broke off in my hand when I turned it on.

Using the “bloody stump” of the power switch for a few months  I finally did a search on EvilBay to look for a new switch.  While I didn’t find a power switch I did see a “tone control” switch for the same series of amplifier – so I got that, instead.  When it arrived I noted, as expected, that most of it did not mechanically resemble the power switch or look as though it would easily mount in the same location, but it did have essentially the same metal bat on the end as the original that I figured I could fit onto missing portion that had broken off the power switch.


Even though the “new” switch was much too small and of insufficient current rating to have been used to switch the mains (AC input) power, it would have sufficed to operate a relay.  To have done this would have required that new holes be drilled in the front sub-panel to match those of this new, smaller switch. While this would not be “original” circuitry, it would have looked the same from the front panel and is a possible option should the power switch itself become unreliable some time in the future.

Removing the original power switch I laid the two side by side and made notes of the differences between and the metal bat of the original, which was narrower in some places to clear parts of the switch body, and taking a file to the new one I took off some metal to clear the possible obstructions.  I then noted on a crude drawing the length and orientation of the new bat based on the axis of the switch’s pivot point.  Because the bat of the original switch was embedded in a block of molded Bakelite I knew that I would have to somehow attach a portion of the new switches’ bat to the old, so I carefully disassembled with old power switch, cutting off and saving the original rivet on which the switch pivoted, and carefully noting where everything had gone, saving the small springs, contacts and some small Bakelite pins.

Figure 4:
The new bat, butt-soldered on the old switch.  Note that the bat from the
“new” switch has been filed to better-resemble the shape of the
original bat to clear the switch body.
Had I not been able to find a “similar” switch on EvilBay I could have
probably measured the original switch, found some scrap
steel of similar thickness and made a suitable replacement entirely
by hand with careful filing using another switch as a template.
Click on the image for a larger version.

Clamping the old part in a vise I cut off most of the original bat, leaving about 5mm of metal remaining.   Carefully comparing the old and new piece I then marked where, on the new bat, that I would have to cut to allow the repaired piece – consisting of the new and old butted and laid end-to-end – have the same length as the intact original.  Doing so – purposely cutting the “new” bat slightly long – I did some fine tuning with a file until the two pieces laid down precisely lined up as they should.

Attaching the new piece

Using some silver solder intended for stainless steel I applied some of its liquid flux – apparently a mixture of chloric and hydrochloric acid – and using a very hot soldering iron, “butt-soldered” the two pieces together in careful alignment and then filed the surfaces flat to remove excess.  While the bakelite switch body can handle a brief application of a soldering iron, I knew that it would not tolerate the heat from a proper, brazed joint.

This (weak!) solder joint was intended to be temporary, need only to be good enough to allow a sleeve to be made by wrapping an appropriately cut piece of thin, tin-plated steel (from my junkbox) around the joint.  Once this sleeve was checked for proper fit and folded tightly, additional flux was applied and the entire joint – sleeve and all – was soldered, the result being a very strong repair with the restored bat being of the same length and at the same angle as the original.


The trick was now to get every thing back together.

Figure 5:
The steel sleeve being installed over the butt solder joint,
before soldering.
Click on the image for a larger version.

Reinstalling the pivot and making a few clearance adjustments to the original switch’s frame with a small needle file, the original rivet was then soldered into place and the entire assembly washed in an ultrasonic cleaner to remove the remnants of the corrosive flux from the bat and switch body.

In the base of the switch, the contacts, which were the same as those had it been an SPDT switch, were reinstalled – this time, rotated 180 degrees so that the previously unused contact portions would now be subject to electrical wear.  These contact were then “stuck” into place with a dab of dielectric grease so that they would not fall out when the switch body was inverted.

Figure 6:
The repaired switch, reassembled,  with the new bat spliced on.
Click on the image for a larger version.

After reinstalling the springs and pins, the rear part of the switch with the contacts was placed over the top of the moveable portion, held in the mechanical center, and the base was carefully pushed into place, compressing the internal springs and pins.  Holding everything together with one hand the proper operation of the switch was mechanically and electrically verified before bending the tabs to hold everything into place.

In reality the reassembly didn’t go quite as smoothly as the above.  During one of the multiple attempts to get everything back together the smaller-diameter rear portion of the small, spring-loaded Bakelite pins used to push on the contacts snapped off.  To repair these pins the front, larger-diameter portions – that which pushed against the metal contacts – were placed in the collet of a rotary tool and a shallow hole was drilled into the rear portion where the broken pieces had attached to fit short pieces of 18 AWG wire:  By rotating the piece into which the hole was to be drilled, the exact center is automatically located.  The pieces of wire were then secured using a small amount of epoxy – a process accelerated by placing the pins in a 180F (80C) oven for an hour.  After the epoxy had set the wires were then trimmed to the length of the original sections that had broken off and the ends smoothed over with a small needle file to prevent their snagging on the spring.  The result was a repair that was stronger than the original pins and these easily survived the reassembly.

The results:

Figure 7:
After reassembly it was noted that the gray “skirt”
was hitting the front sub-panel frame, preventing it from
being set to the “off” position.  A bit of heat was applied to
set a permanent bend so that it would clear this panel.
Click on the image for a larger version.

The amplifier was then put back together, very carefully.  The only real issue that I noted was that the gray plastic skirt/escutcheon on the bat ended up about half a millimeter farther away from the switch body and closer to the sub panel than before, causing it to snag on the front sub-panel’s cut-out when I attempted to move it to the “off” position.  Careful softening of the plastic with the rising heat of a soldering iron and bending it very slightly allowed it to clear.

Putting all of the knobs back on, tightening the bushing nuts and screws as necessary before doing so, I then tested the amplifier on the bench and was pleased to find that I’d not managed to break anything.

Finding that everything was working fine I put it back on the shelf where it belongs where I continue to use it often.


Connect Systems: Product Line Overview

Jerry of Connect Systems posted the following to the CS7000 Yahoo Group:

“CS580: This radio was designed to be in direct competition to the MD380 radio but have the advantage of being much easier to use because of a few new features we added. We sold over 100 radios before it was officially released and we have about 200 radios left before we have to order more. It takes at least 30 days from placing an order to receiving more radios. We have already received follow on orders from some of the people who have ordered the first batch of radios. It cost $130 each.

CS750: We reduced the price from $239 to $180. At this price, it is a competitor to the Tytera products but you get a better radio with more features. After the CS760 is stabilized, we will be adding more features to this radio as well as fixing any remaining problems.

CS760: This is the follow on to the CS750 series of radios. Besides having the features of the CS750, it has optional Bluetooth, GPS, Man Down, and vibrator. Its IP67 rated (waterproof) and has a color display. It will have the ability to be compliant with ETSI Standard Tier III. Being a new radio with new features now and more new features in the future, you can expect this radio to have frequent firmware updates both to fix bugs and to add new features. It was originally scheduled to ship December 1, then December 10, and now December 19. The basic radio cost $299 and with all the options it cost $399.

CS800: This is our single band UHF mobile (Available in VHF). After the CS760 is stabilized, we will be adding more features to this radio as well as fixing any remaining problems.

CS800D: This is a dual band CS800. I expect the VHF band to be the same as the CS801 and the UHF band to be extended to 512 MHz. It is initially going to be released as an amateur product but later we will get it part 90 certified. We do not have our cost yet but I am hoping the selling price will be $100 more than the single band CS800. We have preliminary schematics and we have a target of getting a working sample shipped by January 23, 2017 to show at the various amateur shows and production starting about two months later.

CS108G+: This is a Xiegu X108G with custom firmware to include a spectrum analyzer and an ID Timer. It covers all the HAM bands from 160 meters to 10 meters. We expect to get our first shipment next week.

CS7000/CS8000: This is a single band multi-protocol (DMR, DSTAR, Analog, and others) radio. We are still working on it but I will not give a definite date for initial shipments.

E-Commerce Web Site: Connect systems now has an online store where you can order most of our Amateur Products 24 hours a day. Please look carefully at the product description to see if in stock. The site is at csi-radios.com. It can also be reached from our main site of connectsystems.com”