TS-790 Repair

Sometime ago (in fact over nearly a year ago!) I purchased a Kenwood TS-790 from eBay. It was described as for parts only or repair -which if you read my occasional post's you will realise its how I like  them !

I've repaired three of these for others and I own one which I love. In the repairs I've carried out previously no PA blocks had failed but one had ALC problems (faulty diodes in the 144mhz section), one had poor soldering the PIN diodes of the PA and one had been transmitted into (!!!) and so needed the front end of the 2m section rebuilding.

This one was especially interesting as it had the rare UT-10 (23cm) module, the pictures of it showed that it was in a dirty state and un-loved, the sub receiver LCD display was not working. The advert did say that it received on 144Mhz (the only band they tested) but there was no output on transmit at all, the inference being that the PA blocks in TX section were blown.

So when the auction ended I found I had won it for a very, very good price, especially remembering the UT-10 module faulty or otherwise !!!

Upon receipt the 790 was as described - dirty, unloved and definitely in need of work but did indeed have the UT-10 module !   

Initial testing proved the advert accurate, no transmit at all, it did receive (actually really well to spec on the Stabilock 4031) and the sub display was indeed dead.

Easy bits first - the Sub LCD display was working just not illuminated - the two 12v bulbs had were blown. I ordered (a long time ago!) 100's of the right bulbs with wire ends and current and so, 20 mins later the sub receiver was illuminated evenly and working nicely.

I also noticed that on SSB there was no ALC action at all, and regardless of the mike gain, the ALC was end stop (!).

At this point work intervened and I put it away for another day !

Fast forward nearly 12 months, my conscience pricked me and I had some time so I thought I'd take a closer look.

I started on the 144mhz, sure enough no output. I'm lucky enough to have a genuine service manual and so set about checking it against that, basic's first, voltages, tx switch voltages, all were OK. I went through the setup process, all were there or there, with a little adjustment all were put as per the service manual, but no output at all, checking the PA section everything looked fine. I disconnected the input to the PA stage just in case it should spring into life and to aid fault finding.

I checked again and the ALC on SSB was end stop and so started to check the ALC area's of the circuit (below)

(Copyright Kenwood)

It has a fairly standard approach to ALC with feedback from the PA sections being fed into the ALC board within the IF unit - its not unusual for this to fail, but the voltages were within reasonable levels. However on pin 7 of the unit the ALV should have been around .62v but there was none. Checking where this is derived from the 'SW' unit which turns out to be the RF power and Mike control board. 

So I have a close look at the RF Power / Mike control, it seems very slightly loose (i'd assumed it needed the collet ring tightening up) but upon closer inspection its the pot itself. Now all have a little movement, but this seems slightly more that usual.

I remove the board from the front panel and have a closer look. The front of the pot is definitely not quite right (see below)

So I de-solder the pot from the board and test it - sure enough the dual gang power pot (front one) is open circuit.

Luckily these are fairly often used in Kenwood kit and I know that I have one in a damaged front panel that I replaced on a TS-850 for someone else. - result !

I remove it from the panel, check its working (it was) and then put it in the SW board and re-assembled the front panel. 

I check the voltages now as sure enough they are correct now, the ALC isn't end stop on SSB and so I reconnect the PA stage, connect to the Stablilock and sure enough there the right output that can be controlled by the RF power control. 

Result - Fixed :)

So 144Mhz fixed and onto 433Mhz

Start again checking all the usual voltages and suspects - they are again all correct, I work back from the PA and check the input to it from the 433mhz RF section - there is an output (ie input to the PA section) but it seems very low (according to the service manual). I check and reset / tune the whole 433Mhz sections . PLL, 430Mhz section etc and get a massive output of 0.2w (200mW) output (according to the Stabilock !)

There clearly isn't enough output from the 433Mhz section to drive the PA blocks, so I disassemble the 430Mhz RF board so I can get to the bottom (all surface mount of course!) where the active components are and start checking the voltages vs the service manual (below is the section from the manual).

(Copyright Kenwood)

Prior to this element of the circuit all the inputs and voltages on each active device were correct, I expected the final pre driver (Q244) to be faulty but the voltage were correct, working back I then checked  Q223 - again all voltages correct, then Q222 (2SC3357) and found that the emitter voltage (.75v) was not present - I then check the bias components - all were OK, these are all SMT so I checked all the soldering - all were OK, so a possible faulty transistor.

I didn't have stock - so ordered up a replacement (in fact 5) NEC equivalents.

They arrived a couple of days later, genuine NEC ones (watch eBay for fakes there are lots of them!), I removed the 'offending' transistor with the hot air rework station, and replaced it then cleaned the flux off.

I re-assembled the board and tested the bias on TX and - nothing :(

Oh dear - I Then realised that in turning on and off the 790 I had selected 144 not 430 (!) 

I then set the right band, and voila the right bias was present and the output of the driver board was about the right level.

I reconnected the 433 PA unit and there was the correct output.

I noticed on this 790 as with ALL the others I've seen there was no heat sink compound on any of the PA units and so I will ensure that there will be some before I finish,

I spent some time doing a complete setup from the service manual, and all was well

RESULT - fixed.

Now onto the 1.2Ghz section..

TBC