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

TS-950SD repair

Long time - no post !

Well, I've been buzy, changed jobs (twice) and so any time I've had I've concentrated on home and fixing radios.

But now things have settled down a little, I will return to occasionally blogging about fixing kit that I've acquired - in order to help others and record how they were repaired.

TS-950SD  

I found this lovely radio on the UK website hamradiodeals.co.uk which was out for a swap and clearly labelled as non working with an interesting fault.

After a little thought I contacted the poster (a lovely guy named Mike) and a deal was done for the transceiver.

I picked it up (never trust couriers even for a faulty radio) as whilst the fault might not get worse the physical condition of the radio may well do so !!

The fault as described was the receiver had a 60db carrier (FSD on the S meter) after warm up and no signals could be received regardless, the TX was perfect - full output on FM and SSB. On the bench sure enough on switch on there was no receive (also no FSD on the S meter), TX was indeed perfect in all modes. So the description was (unusually in many cases in my experience) spot on :)

So left the transceiver on for a few hours and sure enough after a while the s meter went to FSD. During the warm up I checked controls etc - all were fine with the exception of the RF gain control (which apparently didn't function) and a total lack of audio / signal receipt. I also checked the sub receiver - which did have audio and did receiver albeit at a poor (insensitive) level.

So I'd already down loaded the service manual and familiarised myself with it and so I started to look at the receive chain / receiver gain control parts of the transceiver due to the symptoms noted above.

I checked the audio amp by injection a suitable signal into it on the main receiver  input to the AF amp and sure enough it worked fine. So onwards.

I doubted that it was a PLL problem as the sub receiver (which shares elements of the PLL) was working fine with low receive, but thought I would check, by carrying out an alignment as per the manual all was fine.

This looked more and more like an AGC type fault (as there was no RF gain control operative) effectively the RX looked liked it was turned hard off when the fault was present. I checked the manual to see where I could check the voltage and noted that there was a couple of nice test points used on the alignment that would give me a good start (see below).

So Fluke to fore, measured on main AGC and to find that it was way off (can't remember the actual voltage and didn't make a note - D'oh!!!) , the sub rx was sligtly off but was adjustable to the required level (2.8V).

Making progress :) 

So started to measure voltage in the AGC area and established that they were not correct in the fault state - also they weren't stable (to be expected in an agc circuit - but these varied by volts not mv and were not signal related!). (circuit below)

the voltage (13.5, 14.5 circled in red) were all over the place and are derived from the 15v line (red on the right).

So why is the 15V line variable and where does it come from ?

Answer it comes from a sub board on the control unit called the trx board (circuit below)

And sure enough its a right pain to get to :( - its location is below

So removed the Control unit and decided to remove the trx board from the unit (thank you decent desolder station - makes this a breeze!) to take a look at the board - sure enough R156 and R157 solder flow was poor (in fact when I touched R157 it fell off the board !!

The components check out OK but I decided to replace them both just in case, I also re-flowed the rest if the components on the board as well - just to be sure.

Replaced the trx unit onto the control unit, and ensured that both that unit and the one close to it were bent away from each other to help with any heat issues - although I'm sure sooner or later (an certainly in hot climates) this fault will re-appear, the control unit is located behind the front panel as so there is not much (if any) air movement and so this almost certainly is the root cause. - I considered a small fan but in the decided that this wouldn't really work effectively.

I re-assembled the control unit with all its connectors, rechecked all and then tested again.

First thing I noted was there was noise in the receiver (I forgot to turn down the AF gain!!) and it blasted me :), secondly (when I turned the audio gain down!) - was the the RF gain control now worked correctly - ie as I reduced it the s meter slowly lit up until it was fully lit (ie fully reduced gain).

I checked the test point on the signal unit and it was still wrong (to high at 6.3V) but I could now reset it to the correct level (2.8V).

The receiver was now functioning correctly - I then did a full re-alignment of the transceiver as per the service manual - and it is now totally within spec and working exceptionally well.

Hope this might help others in the future.

Job Done :)

73s

Chris