Hi all,
Can anyone point me to some good resources for troubleshooting the power amp/output section of solid-state amplifiers? I recently picked up a Fender Deluxe 90 (schematic here) and can't get any sound from it. The speaker works fine in another amp and the power light comes on, but neither regular input nor power amp input gives me any sound from the speaker. I unplugged the reverb tank and still have the same issues.
I measured +42V and -42V on the power supply, so the issue (at least one issue) must be in the power amp/output section. I don't see any burnt components or bulging caps.
Any advice for the next step? The PCB is a pain to remove, so I'd rather exhaust all other options before resorting to that. I have a multimeter and basic understanding of the signal chain, but am still a novice to amp repair.
All help is greatly appreciated. Thanks!
Can anyone point me to some good resources for troubleshooting the power amp/output section of solid-state amplifiers? I recently picked up a Fender Deluxe 90 (schematic here) and can't get any sound from it. The speaker works fine in another amp and the power light comes on, but neither regular input nor power amp input gives me any sound from the speaker. I unplugged the reverb tank and still have the same issues.
I measured +42V and -42V on the power supply, so the issue (at least one issue) must be in the power amp/output section. I don't see any burnt components or bulging caps.
Any advice for the next step? The PCB is a pain to remove, so I'd rather exhaust all other options before resorting to that. I have a multimeter and basic understanding of the signal chain, but am still a novice to amp repair.
All help is greatly appreciated. Thanks!
Is there any sound from the speaker when you switch it on or off?
If there is, chances are the power amp is OK and I would suspect J4 (power amp in). The switch on the jack may have failed interrupting the signal from the pre-amp.
Brian
If there is, chances are the power amp is OK and I would suspect J4 (power amp in). The switch on the jack may have failed interrupting the signal from the pre-amp.
Brian
Hi Brian,
Thanks for the reply. There's no sound at all from the speaker during power on or power off. I think J4 is ok, since I got continuity between the tip of a plugged-in cable (cable in J4) to the positive side of C40.
Is there any way to test the transistors while they're still in the circuit?
Thanks for the reply. There's no sound at all from the speaker during power on or power off. I think J4 is ok, since I got continuity between the tip of a plugged-in cable (cable in J4) to the positive side of C40.
Is there any way to test the transistors while they're still in the circuit?
Checking transistors in circuit is difficult although catastrophic failures such as shorted junctions will show up.
Three tests you need to do. The voltage at the output, which is the junction of those two 0.22 ohm 'emitter resistors' (R108 and R112) should be zero. Make sure to measure at the resistors and not the speaker output. The resistors should measure OK at 0.22 ohm (not open circuit) and there should be a few millivolts across the 0.22 ohms. Check this without any signal and with no speakers attached.
If those tests are OK then the problem almost certainly is elsewhere.
Edit... moved to Instruments and Amps forum.
Three tests you need to do. The voltage at the output, which is the junction of those two 0.22 ohm 'emitter resistors' (R108 and R112) should be zero. Make sure to measure at the resistors and not the speaker output. The resistors should measure OK at 0.22 ohm (not open circuit) and there should be a few millivolts across the 0.22 ohms. Check this without any signal and with no speakers attached.
If those tests are OK then the problem almost certainly is elsewhere.
Edit... moved to Instruments and Amps forum.
Thanks for the help. I was about to run through the tests the Mooly suggested, and noticed I don't have continuity between the speaker output wires and the board (I think). Are P1 and P2 (from the schematic) the output wires? I am assuming P1 is the white output wire and P2 is the black output wire.
Jumping the effects loop restores sound, so the preamp out / power amp in jacks must be dirty or broken. I'll clean them with some Deoxit and replace them if that doesn't work.
The preamp out jack does not break the signal path, so it is not faulty. Only the power amp in jack breaks the circuit. The contacts on the J4 are exposed, so you should be able to clean them.
Fixed by bending J4 tip spring contact with needlenose pliers. I'll probably replace it anyway to be on the safe side.
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