Ampeg SVT4 Pro with output relay issues

[Version_6]

Well, I’ve got one of these monsters on the bench and it DOESN’T have shorted mosfets! In fact there’s signal all the way to the output relays, however they don’t close when the amp is engaged. I don’t have any DC on the output and I’ve replaced the relays and the transistors feeding them. The old relays were dead and I can now at least get them to close briefly when powering up on the variac.

Anyone else had a similar issue? I’m thinking of shotgunning the TL074 next, any other recommendations would be great. I’m going to open it back up and do the obligatory dry/cracked solder check and touchup while I’m there.

Schematic is at the link below. Can’t post it as an attachment as I’m posting via phone.

http://schems.com/bmampscom/ampeg/Am...t_4pro_sch.pdf

[SoulFetish]

Well, I’ve got one of these monsters on the bench and it DOESN’T have shorted mosfets! In fact there’s signal all the way to the output relays, however they don’t close when the amp is engaged. I don’t have any DC on the output and I’ve replaced the relays and the transistors feeding them. The old relays were dead and I can now at least get them to close briefly when powering up on the variac.

Anyone else had a similar issue? I’m thinking of shotgunning the TL074 next, any other recommendations would be great. I’m going to open it back up and do the obligatory dry/cracked solder check and touchup while I’m there.

Schematic is at the link below. Can’t post it as an attachment as I’m posting via phone.

http://schems.com/bmampscom/ampeg/Am...t_4pro_sch.pdf

Well, What are your voltages on the TL074? Are you getting your +/-15 voltages?
What are the voltages at the base, collector, and emitter of Q220 (which you replaceD)?

[nevetslab]

There is an LM-35 temperature sensor IC2 (TO-92 package) that's clamped to the heat sink between Q11 & Q13 on the busy side of the power amp PCB assembly. Not very accessible until you get the rear panel PCB's out of the way, then remove the clamp that holds Q11 & Q13 up against the heat sink. It's output is fed thru IC3-D, whose outputs feed the inverting inputs of the Relay voltage comparators IC3-A & IC3-B. If there is a problem with the sensor IC or it's companion IC3-D, it can prevent the relays from closing.

[Version_6]

Ok, problem solved. I made the stupid mistake of not noticing the fan whirring off its head at startup. Replaced the LM35 and the TL074 for good measure and success, she's alive.