Ampeg SVT Classic fault issue

[Apexelectric]

I have an Ampeg SVT Classic on the bench from around 2013 that won’t come out of fault mode. The relay on the power input board does not allow the HV to pass to the circuit. The relay tests fine from an external source and the connections seem to be engaging when the relay clicks closed. There is 6VDC present on the board for the coil and there is about 3V coming from the preamp board to the power input board to the transistor that switches the coil voltage. The transistor was suspect but I pulled it and it tests ok on my tester. Will replace shortly anyway. The resistors in the transistor switching circuit test good too.

Is the 3V to the drain on the transistor adequate enough to switch on the transistor or should there be more? The schematic shows a range from 0-15V on that wire to ground so I would assume that it’s getting enough. The voltage on the wire from the preamp board to the drain goes up when I disconnect it from the power input board. From about 3 to about double.

Amp HV does work and passes a signal when I bypass the relay. Tubes
test good and output tubes test close to being matched, or at least three matched for each side. Should be able to balance a little from there.

I was going to bypass it and test the output section for uneven cathode current next but it seems like the HV should be present for at least a little while when taking it out of standby.  This is only the second one I’ve had on the bench and the first was a simple bias adjustment.

Anyone with experience with this amp have any hints on where to go next?

Thanks as always!

[DummyLoad]

not without a schematic.


--pete

[Apexelectric]

not without a schematic.


--pete

Sorry ‘bout that

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/

[Ritchie200]

Man what a mess of a schematic!   If I am seeing this correct, the heater circuit (which I assume is also on a relay?) is being triggered by the same transistor? Do you have power to this section?  What about the connectors and their corresponding soldered pads?

Jim

[Apexelectric]

Man what a mess of a schematic!   If I am seeing this correct, the heater circuit (which I assume is also on a relay?) is being triggered by the same transistor? Do you have power to this section?  What about the connectors and their corresponding soldered pads?

Jim

Heater circuit works and does not get triggered by the relay. The relay is a single pole single throw essentially. The contacts are paralleled to get a higher current rating. All the solder connections look good around the relay.

[Ritchie200]

If you look here:

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/SVT-CL_AC-Term_Brd.pdf

Q1 appears to be triggering both the heater circuit through J36 (to a different relay?) and "Relay control from power amp" through J35.  I was thinking if you had power to the heaters, Q1 was able to engage that relay.  If it was able to engage the heater circuit relay, it should be able to engage the power amp relay? 

Sorry, in my first post I meant to check the ribbon connectors to the power amp for voltage and their associated pads.

Jim

[sluckey]

Jim, could you fix that broken photobucket link in your signature?

[Apexelectric]

If you look here:

http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/SVT-CL_AC-Term_Brd.pdf

Q1 appears to be triggering both the heater circuit through J36 (to a different relay?) and "Relay control from power amp" through J35.  I was thinking if you had power to the heaters, Q1 was able to engage that relay.  If it was able to engage the heater circuit relay, it should be able to engage the power amp relay? 

Sorry, in my first post I meant to check the ribbon connectors to the power amp for voltage and their associated pads.

Jim

Jim,

I think what your seeing is the coil voltage is being fed from the filament supply. It comes from the preamp board at the control panel to the power input board.

I know the tubes on the amp heat when in standby and with the standby switch on so that part seems fine. The amp does work when the relay is bypassed so all the hv supply voltages are working. I just need to figure out the operation of the fault circuit so I can properly diagnose what’s going on. If I can find a scenario that explains why the relay won’t click on, even briefly, based on the conditions I’ve explained, it would hopefully lead me in the right direction.

The relay is good, the coil supply is present, the transistor tests good, the diode and resistors around the relay test good, the output tubes test good and close to all matching(within about 10% of one extreme to the other), preamp tubes are good and power supplies test good.

[Apexelectric]

I pulled both the output tube board as well as the main board in the chassis and found no issues associated with any of the 10R cathode resistors that I’ve seen mention of online. Nor did I see or measure any other issues with components. I might just put it back together with the relay bypassed to see if it biases properly or if there are any mismatch issues under operating conditions.

[shooter]

 

could you fix

I'm thinking it's on purpose 

[mresistor]

When the amp is turned on for at least 5 minutes - what is the voltage at IC2B pin 7 of the Power Amp board?

[Apexelectric]

Will check that and report back. Might not be till tomorrow.

Thanks!

[Apexelectric]

After much time and effort I finally got to the bottom of this SVT.

I put it back together after the replacement of the Darlington transistor on the AC board that controlled the coil voltage to the relay and the HV came back online. The transistor tested ok on my tester but in circuit must have been a different story. The fault indicator on the front panel turned green again and I got the bias lights lit again on the back panel.

After that there was output to the speaker but just a hum and no signal passing. I divided the preamp from the output by the jacks on the back and found that the preamp half of the circuit was the issue. I finally was able to trace it back to a shorted half of an op amp on the output board, IC2A, that is the buffer for a non-existent effects loop in this amp. The output board is a shared board for another SVT version. The other half of the op amp was used for the fault trigger to the drain on the Darlington transistor.

Not sure what might have caused this issue in this amp as it was messed with by another tech before I got to it and the output tubes were changed and the output board was removed but no components seem to have been changed.

Hopefully this can be of help to someone in the future.

[Willabe]

Thanks for posting the fix.