TS-50 repair continued

 Hi,

I promised an update on the TS-50 with the catastrophic audio switching transistor failure, and here it is ;

The original transistor is virtually unobtainable and so I found an equivalent transistor ZTX851. Interesting to note it has the same specification (actually slightly better) but is a lot smaller - I guess the onward march of technological improvement !

Replaced the transistor (if you do this watch out as it's not a direct pin-compatible replacement you'll need to carefully swap the base and the collector) carefully insulating it to ensure no short circuits.

I could have possibly shortened the leads a little but decided to leave them, they are perhaps a little long, to be honest, but no great matter.

So put it back together again and......

Still no audio 😔

But.... now I have the correct switched +ve on pin 8 of the amplifier chip so some progress at least!

Essentially only one thing left which is that audio chip itself - all other static components and interconnections/voltages were correct and so I ordered a new upc1241H audio chip from Cricklewood electronics, I looked elsewhere and they were by far the cheapest for an original chip even with the postage - but I also needed some HV electrolytic for the collins PM-2 rebuild and so I even got free postage !

Chip arrived the next day (great service guys!) and was quickly replaced, with the resultant bags of audio on the test, don't forget to put the clip back on the audio chip so it presses against the chassis heatsink (I always put a little smear of heatsink compound on it - not sure it actually helps but makes me feel a little better!)

So a little bit of an odd one, often replace the electrolytics due to leakage and high resistance but this is the first time I've ever had an audio chip failure.

My working assumption is that the chip failed and drew some much current it took out the switching transistor - however looking carefully at the audio chip I can see no cracks or damage at all - I would have perhaps expected that looking at what it did to the transistor !

Now all back together and on the bench for a soak test...another one of these great little radios back on the air