Fun with Wurlitzer 200A

[Rhodesplyr]

So, I'm just finishing up some repairs on a 1977 Wurlitzer 200A Electric Piano--I'd replaced all the electrolytics and corrected some errors a previous tech made in the Aux and Headphone outputs. It sounds great and is much quieter than it was. Until....

While it's just sitting there turned on, a loud hum suddenly comes through the speakers because it had just suffered a cascade failure of two of the small signal transistors on the amplifier board (TR9 and TR10).

You fix a Hammond Organ, and it runs for years. You fix a Wurlie, and it runs for half an hour before it decides to have another nervous breakdown...

Were these early solid state amps simply under-designed? For a fairly low-power amplifier, they seem to self-destruct pretty often.

[J M Fahey]

Well, they were *early*
Somebody (was it RG Keen?) wrote a lot about early SS VOX amps: germanium power (this alone is a death sentence), no termal tracking (ditto), undersized heatsinks, poor air flow, no Zobels, no series inductors, no anti-kickback diodes, no short protection, many were marginally stable and could not survive open circuits either, you name it.
Much of SS poor image comes straight from that era.
40 years of continuous progress have not yet compensated that first image.

[Rhodesplyr]

In the end, I replaced all six smaller transistors in the power amp section of this 200A, matching critical ones as differential and complementary pairs. Four had shorted; two I replaced in the hope that they'll be less likely to fail in the near future. It would be interesting to know what happened first; the first failure seems to have taken out the other three transistors like dominoes.

I used this cross-reference to standard transistor types, and it seems to be a good resource:

Wurlitzer 200A | Transistors

I'm told that TR-7 and TR-8 need to be matched and that it can help to match TR-10 and TR-12 as a PNP/NPN complementary driver pair.

Another source gives 2N3859A as the equivalent for TR-9.