Yamaha SY-22 synth issue

[frisc0]

Thanks, I just ordered a replacement transistor and a 5.5V 0.47F super capacitor, hope this will fix things; keep you posted!

[frisc0]

Good news: TR9 was the issue! Today I've received the required parts, the BD241 and the 5.5V supercap, soldered them in and everything works fine again! Voltages around TR9 are what they should and the patch memory is retained after power down.

So happy with this result, thank you very much for helping out, learned a lot from this quest and your suggestions on how to trace down the issue. Great to be able to use this synth again, its vector synthesis produces such a unique sound..!

However, I do wonder if I could have spotted the faulty TR9 sooner in the process? I mean, without taking TR9 out of circuit, using a multimeter with diode setting? @Elecdonia you mentioned the 'diode drop voltages of only 300mV' are an indication; could you please explain?

[elecdonia]

Bipolar transistors should measure as 2 back-to-back diodes with a multimeter in “diode-check” mode. This test is usually (but not always) effective with the transistor “in-circuit.”

For NPN transistors:         E  —|<|— B —|>|— C
With - probe on E and + probe on B, meter should read between .5V and .7V
Next, - probe on C and + probe on B should also read .5 to .7V
With - probe on E and + probe on C the reading should be “open circuit” (over-range). E to C should never measure as a dead short.

For PNP transistors:         E  —|>|— B —|<|— C
With + probe on E and - probe on B, meter should read between .5V and .7V
Next, + probe on C and - probe on B should also read .5 to .7V
With + probe on E and - probe on C the reading should be “open circuit” (over-range). E to C should never measure as a dead short.

A faulty transistor is often either a dead short or open circuit when measured this way. However there can be times when it is useful to remove the transistor and test it “out-of-circuit.” If you get E-B diode drop of only .3V with the transistor out-of-circuit it is likely the transistor is faulty. Also, when E-B is checked with probe polarity reversed, meter should read “open-circuit.” I have seen faulty transistors where E-B measured the same in both forward and reverse.

[frisc0]

Great explanation, very clear, thanks! This will help with future trouble shooting..!