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I have an old Grunding V2000 audio amp. It has an output protection circuit that consists of an anti-pop circuit (delayed output relay turn on/instant turn off), and what I think is a DC detection circuit.

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The way I understand this circuit is:

  • Anti-pop: during normal operation, T607 is held at -2V due to D605 rectifying (negative) half-wave via R617, which is much smaller than R616. This keeps T607 out of the circuit for now.
    • On power on, R613 slowly charges C602. When the voltage is high enough, T605 conducts, which enables T606. I understand T605 is a 24VDC regulator and BD138 is its output stage
    • When the unit is powered off, the 24VAC disappear and R616 quickly charges C603. T607 conducts, and the voltage via R613 drops, which shuts down T605. D600 makes sure the delay works on power on, but not on power off.
  • Output protection: output protection detects DC at the output of either channel. If an output transistor is shorted, there will be DC at L or R. this will charge C601 via R605 or R606. In normal operation, there should be 0V here. Whenever there is some voltage present, T601 or T602 will conduct. This will make the base of T603 go low, which will allow the base of T604 to go up, which will discharge C02, and make T605 turn off.

The problem I have is that T604 is normally at 0V but after some time it starts going up to 400mV which is enough to pull T605 down. This makes the output relays either shut down completely, or start clicking.

I don't understand why T604 is going down, since T601 base/T602 emitter is only at 32mV. The accepted DC offset at the output according to the manual is 100mV so it's well within spec.

I'm thinking maybe T603 or T604 are bad. Unfortunately I don't have BC546 to replace them and BC548 won't do as the power supply is 60V.

As a workaround, I've disabled the shorted-output protection by removing T604 and replacing it with a resistor between C and E to allow C602 to discharge.

Is there something obvious I'm missing here?

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  • \$\begingroup\$ Did you check C601? If it is dry, there may be quick responses to audio signals. \$\endgroup\$ Commented Apr 7 at 21:43
  • \$\begingroup\$ @Jens all capacitors were, oddly, overrated. that 100uF one measured 200uF on my multimeter. most of them measured 2x as rated. I confirmed my multimeter is good by measuring a "new" capacitor and the reading was within spec. I replaced all 3 caps before even investigating the circuit but they weren't really the problem \$\endgroup\$ Commented Apr 7 at 23:14

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T604 is switched on when Vb is about 400 mV, so it likely working properly. Probably T603 is faulty, or R607 or R608 opens (increase resistance) and T603 cannot be activated.

Assuming T604 is Ok, when T603 is not switched on, T603 collector will rise up to the voltage of a resistor divider (R611/R612) plus 0.6 V (Vbe of T604). Calculate, Vc will be maximun of about 4 V, so you can test T603 with a BC548 without any problem.

BTW, when T605 is activated, T606 Vbe will get D601 drop voltage, and emitter is connected to +60 V via R615 . This configuration is a current source of 4.1 V/ R615, about 23 mA. This limited current is used to activate Rel. I , Rel II or both in serial, depends of switches (a, b, a+b).

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