Opinions on state of AU-20000 sought

[KeithD]

Sorry, I ended up editing my post #178 while you were typing. Will get the voltage at TR03 base...

 

[KeithD]

OK, I think I've got a bad solder connection around TR03. As I moved the transistor a bit the relay started clicking. Will repair and give things another try.

 

[richardpp]

The one change I made that I am less certain about are the transistors. I followed one recommendation to put in KSC1815, but I see another used KSC2383. Others seem to have not changed the transistors at all. So, I may try putting the original transistors back in, or trying KSC2383. I don't know if this makes sense in terms of how the circuit works, but the transistors are the one thing I'm less certain about.

The transistors are simply acting as switches in this circuit. I don't think there's any reason to be concerned over the KSC1815. Maybe their gain is lower compared to the originals but they should still work. The KSC2383 is far more robust but I'm not sure that kind of beefiness is really needed. The original 2SC1364's seem to have lasted quite well over the years.

Another thing that is not the same is using 20VDC instead of 24V (though the schematic indicates the PS is providing 22VAC). I can see If it can find something that goes up to 24VDC at work, or alternatively I've got a variac and could supply 24VAC at pin 12. Don't know if this would make any difference, but it is one thing that different.

The amp supply is ~25VDC, the extra voltage should give a small increase in delay time.

Obviously this is frustrating, and I feel bad that you are taking so much of your time assisting to find the issue. I'm sure if you had the board in your hands, you'd likely find the problem quite quickly. Troubleshooting form a distance can be very challenging. Thanks again for your assistance!

Sorry too, for the frustration. I've taken you the long way around this problem and made a few poor calls along the way. Thanks for bearing with me!!

 

[KeithD]

Success! Turns out TR03 was shorted. Figured this out after I pulled it out and tested with a Peak meter. Replaced that, and relay was clicking in at a few seconds.

To increased relay time, I tried 22 µF at C10, and that increased delay time to about 4 seconds. Put in a 33 µF cap, and that gets me to about 6 seconds, so I think all is working at this point. Anything else I should look at before testing the board in the amp?

 

[richardpp]

That's great news, Keith! I don't think there's anything else to check, other than you've reconnected D18 and D19.

 

[KeithD]

That's great news, Keith! I don't think there's anything else to check, other than you've reconnected D18 and D19.

Yes, D18 and 19 were reconnected before putting in the replacement transistor.

Put the board back in the amp. Passed DBT and on full mains. With a 33 µF capacitor at C10, I'm getting about 4-5 seconds before the second relay click, which is enough that now I'm not getting a "pop" when the speaker relay clicks in. So, I think all is good here. When I order parts again, I'll get two 3.9 Ω/7W resistors for R01 and 02 and revisit the board at that time.

Unfortunately, we were not able to find the cause of the shorter relay timing, but as long as using a larger cap for C10 is OK, I think we can call it good.

I may put the rail fuses in this weekend before I take a break from working on the amp.

Thanks again for all your help and sharing your insights and knowledge about this board and amp! You are a wealth of knowledge on the AU-20000, Richard!