Two Kenwood TS-50 repairs - capacitor problems and a catastrophic transistor failure !!!!

 Hi,

I love the TS-50.

They are really great radios - work really well and are very rugged, but as per many radios of this age they do suffer from capacitor break down / leaks which causes odd problems both by damaging the PCB traces but also the PCB through connections as well !

I recently picked up a couple of these both non-working in one way or another ! - The first one looked to be working quite well but had no receive audio and the other was working quite well but with no backlight and was making a funny buzzing sound - not from the speaker but from the display !

I decided to have a look at the TS-50 with the display problem first, 

After carefully dismantling the front panel, as perhaps expected it was a capacitor that had/was causing the problem (below)

I've circled the offending capacitor - the connectors are corroded and its leaked badly onto the board and is also affecting the transistor next to it as well as the PCB itself.

The capacitor was so badly damaged that as I touched it - it fell off entirely leaving just to pcb connection on the board ! - see picture right.

Anyone that works on these (or other radios with similar problems) will know how difficult it is to clean up this type of corrosion - and will recognise that smell (I think it's a little fishy) when you solder and clean up any capacitors that have leaked !

After some careful cleaning work around the capacitor and removing the transistor to try and ensure removal of all the mess and also to take a very close look at pcb tracks to see if they have been damaged at all - not that unusual really the capacitor leaking can cause massive problems!

So having done that it was quite clear that some damage to the capacitor mounting lands had been damaged and there was some erosion of the small tracks near there as well. After some careful cleaning and examination, the tracks were actually not too bad, and so I cleaned and tinned them in order to protect them and give them a little longer life hopefully !

As you can see - I also re-worked and remounted the transistor as well - I checked it was OK with the transistor tester, it was and so after cleaning and tining it and its pcb mounting location I resoldered it back into place.

You can also see the cleaned and tinned tracks as well in this picture - thankfully they didn't need re-working as once clean they still had continuity - good news!

This just left the mounting of a new 47uf capacitor to replace the one that had caused all the problems! Sometimes the leaking can cause real problems in soldering capacitors into place, as was the case here, it needed to be cleaned several times as well as some burnishing with a fiberglass pen to enable a clean surface to solder too. 

The picture right shows everything after mounting and tidy up but before final cleaning, as you can see far better than the original!

All that was left was to now replace the backlight bulbs - so I removed and checked that they were blown- they were. 

I don't like to replace backlights with LED's which seem to be quite popular - for me, they seem too bright and lead to a 'patchy' illumination style and are too harsh to my eyes, and so I replace them with similar original bulbs they are quite straightforward to do and provide consistent illumination as per originally designed.

This TS-50 also had some quite bad screen marking/scratches and so needed some extra work using specialist plastic polish I have to remove the scratches and put it back to pretty brand new condition - it takes time - about an hour of careful work but it gets all but the very deepest of scratches out - with this unit it now looks like new.

That just left cleaning the rest of the front panel whilst it was disassembled and of course the case as well - something that I always enjoy doing as it brings everything together again and for me concludes the repair. Final testing took place - including checking the brightness controls works correctly as well as full receive and transmit. - All worked well and so chalk up a successful repair! - I have ordered a complete set of replacement surface mount and through-hole capacitors, as they will need replacing based on the condition of the display capacitor - I'll write that up in another blog write-up when I get the chance!

The second TS-50 I acquired is in good condition but with no receive output. I tested it upon receipt and sure enough transmit is fine a signal is received on the S meter display but no output on the speaker.

Usually, this tends to indicate a problem with the capacitors leaking badly around the audio amplifier in my experience which means replacing all the capacitors (as usual!), I checked visually and sure enough, there was leakage evident - not too bad but enough to warrant a complete replacement of all the capacitors but rather than jumping in and starting to do that straight away I thought I'd trace the audio chain through to check and see if there was anything else that I'd need to repair.

Upon tracing out the audio circuit, I work from the speaker backwards when the receiver is working - don't forget to check the speaker (yes some do open circuit) but not in this case also the rear speaker connector as sometimes they get damaged and this prevents audio to the speaker - but not in this case -  No output from the audio amplifier so check at the input (using a scope) on pin 1 and yes there is input to the amplifier which goes up and down in level with the AF front panel control which is good news, so I then check that the amplifier has the correct voltages - and .... no voltage on pin 8 (circuit below copyright Kenwood)

So check the voltage on the input to the switching transistor (Q13) and sure enough, there is the correct voltage but no output and so it's likely faulty.

Now Q13 is located right at the back left-hand side of the TS50 and is tucked away so I had a closer look (after my cursory usual check around) and what do I find when I look more closely?

Wow a lovely circular hole right in the middle of the switching transistor !

No wonder it is not switching! - this is a 5A 50V switching transistor which is generally not available anymore and so I have to order a suitable equivalent replacement which I am waiting to arrive. It does pose the question of what caused it to fail in such a catastrophic manner and what might it have damaged during its failure ?

I've seen such failures before but never in a TS-50 and so this is a little unusual in my view.

I will go ahead and replace all the surface mount and through-hole capacitors, but I am a little concerned that the AF amplifier might have caused the failure although I have carried out a few static tests including feeding the right voltage into the amplifier chip from my desktop power supply and it didn't draw too much current so its at least it's not a short circuit !!

 I'll update here when the transistor arrives and see how the repair progresses 😊