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200A Troubleshooting Fiasco

Started by chesnd, January 09, 2023, 01:06:01 PM

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Hi all,

I started working on a 200A and started with some issues that I think I've only exacerbated. Hoping that ya'll might be able to hear some of the issues that I've run into and point me in some directions.

The model that I've been working on is very clean. The only physical issue was the pedal which just needed to be tightened up. But when it came to looking at the amplifier it was clear that it was not passing signal to the speakers. There was a low constant hum that came out but didn't pass any signal from the keyboard. The AUX and Headphone Out did were working as was the tremolo.

SO getting down into the circuit showed that up on the BIAS circuit was about 15v higher than was expected. Instead of showing +.65 it was showing +15. This stretched from the base of TR-11 to the collector of TR-14. I tested the transistors 8-13 and only showed minor issues. On the basis of these issues I replaced TR 10, 12, and 14. I also looked at the ESR of the electrolytic caps inside the circuit and on that basis replaced C12. These did not seem to change anything. The filter caps looked okay, the rectifier seemed to be behaving properly, but I was surprised to see that when I went to discharge C29 tge positive filter cap, that it had already discharged. Don't know if this means anything for the cap or if there's a discharge circuit, but it seemed notable.

I was then advised, on the basis of intuition, that I might short C15. So, I did.

This caused TR-9 to bust and puff out. After this point the speakers and AUX out stopped outputting sound. I replaced TR-9 and found that the Base of TR-11 now show +22. But the rest of the bias circuit was still out of whack. Knowing that I had blown some additional components I walked through the amplification circuit again but the only real issue that I found was that R33 was low. Replaced that and found very little changed. So I decided that I might switch C15 out and see if that made any changes and low and behold, now the inside of the bias area was reading even more than it had before -- a whopping +22 instead of the previous +15. Testing the diodes showed no changes.

So at this point I thought that maybe I should check the reed bar, and, after running through all the transistors and diodes found nothing of note. I have not checked those caps yes so that's one of my next steps.


If you've gotten this far, thank you so much for reading my ramblings.

I think a lot of people are going to be thinking, "Why didn't you just replace all the transistors and caps in the first place and go from there?" answer: I didn't know better at the time.

Now I'm at a juncture of being pretty green and having gotten myself much deeper in than I was expecting. So I ask to you, the kind people of the Electric Piano Forum: got any insights?

Definitely don't expect anyone to come in and solve this for my but if there is something that rings that bell of experience I would love to hear what it told you.

Thanks so much!

TLDR; guy gets himself neck deep in Wurli amp and doesn't know what's going on anymore.

200A SchematicI've been using   <----------------


If the headphones were working, then the "earphone jack" was not working as a normally-closed switch.  The switch contacts on these jacks get dirty and corroded, and so you don't get signal through to the speaker load.  Replace that jack.

Now you have to fix the damage that you have done....