Speaker was not attached when the photo was taken.. However, when the speaker is attached it is very loud.. If I lower the input slightly, the clean wave returns, and the volume drops drastically. Audio is clean, but very low.
For the rest of the tests, keep the audio clean (not clipping).
Find the point where R3 and R4 connect. That's the input to the differential amplifier. Gradually increase the input level comparing the signal level at that point with the output. The gain should remain constant between those points. Is there a point where the gain changes significantly?
Find the point where R3 and R4 connect. That's the input to the differential amplifier. Gradually increase the input level comparing the signal level at that point with the output. The gain should remain constant between those points. Is there a point where the gain changes significantly?
The R3/R4 point shows no signal until the input is loud enough that the distortion starts (and the gain "kicks in").
Interesting.. if I tune R1 (input level control), it is set at the "loudest" setting. Turning is (as expected) will lower the output until it cannot be heard. However, if I bypass the variable resistor, the amplifier appears to operate "normally".
The variable resistors both seem fine..
The variable resistors both seem fine..
With the pot in the circuit, do you get a smooth transition of output level from the wiper as you adjust it from minimum to maximum gain?
I was about to remove it to test, but decided to wiggle and tap it while the amp was on, and I think that is the problem.. with enough movement the amplifier started working.. UGH, something so simple.. 🙁
Now, I need to find a source for them?? Or, is it safe to just bypass them? I would typically leave them at the maximum level anyway..
In the meantime, I broke something else. The relay is not kicking on. I think one of my test leads fell and hit something. May have blown Q201, which is a MPSA14. I bought some spare MPSA42's, will that work? If not I have to pay $10 to ship a 50 cent transistor...
Now, I need to find a source for them?? Or, is it safe to just bypass them? I would typically leave them at the maximum level anyway..
In the meantime, I broke something else. The relay is not kicking on. I think one of my test leads fell and hit something. May have blown Q201, which is a MPSA14. I bought some spare MPSA42's, will that work? If not I have to pay $10 to ship a 50 cent transistor...
I doubt that the high value resistors driving Q201 will allow the A42 to switch reliably. You can jump collector to emitter to allow further testing.
You can bypass the gain controls but I think the gain would be too high. Two series resistors could be used in place of the pot.
You can bypass the gain controls but I think the gain would be too high. Two series resistors could be used in place of the pot.
Is there a suitable replacement available now for those variable resistors? (same foot print)
Thanks
Thanks
Mouser, Digikey, Arrow have those available. It should have numbers on the side...such as 5k,10k,20k or 50k.
Says: Piher Spain on top.. 942 and 20K on the side..
Plenty of parts listed, but I'd like to find one mechanically compatible (same size, and able to get to it with the case closed).
Plenty of parts listed, but I'd like to find one mechanically compatible (same size, and able to get to it with the case closed).
Parts came in today. I installed the 2 POTs and Q201 (for the relay). Amp works GREAT.. Let it play on the bench for a while to make sure all was good, and after about 10 min the relay kicked out.
Replaced Q201 again (good thing I got extra), and it is playing yet again. Turned it back off for now.
Any idea what would cause Q201 to blow out?
Thanks
Replaced Q201 again (good thing I got extra), and it is playing yet again. Turned it back off for now.
Any idea what would cause Q201 to blow out?
Thanks
I tested it, and it was bad.. But, I may have damaged further while testing it "live" because at one point the relay kicked back on... It was behaving like a short across E-C, no delay when powering up.
Just tested the voltages on the new transistor..
E-B is 1.3V
E-C is .7V
I did replace D202 previously with a BAW76, I can do the same with D203.
Just tested the voltages on the new transistor..
E-B is 1.3V
E-C is .7V
I did replace D202 previously with a BAW76, I can do the same with D203.
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