I have been restoring this old amp for awhile now, I was so close to finish it but then I saw smoke coming from R30 in zobel network, it happened twice, at first I thought some short caused this because it was momentary and amp worked fine after reseting the power, but now R30 is constantly heating, what could cause this? That resistor used to be always cold.
Transistors are new, I measured all of them no sign of short, caps are also new. Resistors are old but they are all in range of +/- %5.
My multimeter just stopped working properly—it can't measure voltage correctly, although the other functions still works fine. How can I identify the issue? ChatGPT suggested that if the resistor is still heating up even with no speaker connected, it could mean either C13 is faulty or there's a 100kHz oscillation present.
Any suggestions? Meanwhile I'll look for new cap and replace the old resistor with 10r 5W one, too bad can't measure speaker's output DC voltage right now.
Transistors are new, I measured all of them no sign of short, caps are also new. Resistors are old but they are all in range of +/- %5.
My multimeter just stopped working properly—it can't measure voltage correctly, although the other functions still works fine. How can I identify the issue? ChatGPT suggested that if the resistor is still heating up even with no speaker connected, it could mean either C13 is faulty or there's a 100kHz oscillation present.
Any suggestions? Meanwhile I'll look for new cap and replace the old resistor with 10r 5W one, too bad can't measure speaker's output DC voltage right now.
All the capacitors, transistors, and output transistors, as well as the op-amp, were replaced. After the changes, it was working fine for several days.
I swapped all the TIP29/30C transistors for TIP41/42C ones because one of the old TO-220s was shorted. I also replaced the BC204Bs since they were shorted too; the new TO-92s are BC547s and two BC327s. The electrolytic capacitors were replaced as well—this circuit is from 1976, and the original caps were still intact. The op-amp is still a 741, but a new one, since the old one wasn't working. The output transistors were fine, but I replaced them with MJ15003Gs.
I swapped all the TIP29/30C transistors for TIP41/42C ones because one of the old TO-220s was shorted. I also replaced the BC204Bs since they were shorted too; the new TO-92s are BC547s and two BC327s. The electrolytic capacitors were replaced as well—this circuit is from 1976, and the original caps were still intact. The op-amp is still a 741, but a new one, since the old one wasn't working. The output transistors were fine, but I replaced them with MJ15003Gs.
Stick a scope on the output, it's almost certainly oscillating beyond the audible range (as per davidsrsb) - it could be something as simple as you've fitted the wrong output transistors, can you try the old ones back?. Anything else you've changed could be causing the issue as well.
You've replaced several components and this may have changed (or I'd say certainly) the openloop frequency and phase response.
This may cause oscillations and you'll need to readjust the compensation capacitors.
Not a big deal, but you need a scope and some experience with readjusting the compensation.
Look for the pF and nF capacitors - these are the ones that provide the compensation. You'll need to try decreasing or increasing them while monitoring the output with a scope.
Always perform this troubleshooting with a current limiter at the AC (bulb or any equivalent resistor) to prevent frying something if strong oscillation takes place.
This may cause oscillations and you'll need to readjust the compensation capacitors.
Not a big deal, but you need a scope and some experience with readjusting the compensation.
Look for the pF and nF capacitors - these are the ones that provide the compensation. You'll need to try decreasing or increasing them while monitoring the output with a scope.
Always perform this troubleshooting with a current limiter at the AC (bulb or any equivalent resistor) to prevent frying something if strong oscillation takes place.