Peavey GPS 3500 - Page 1
Forum section: electronics / repair

author: JTaylor
author_rank: Newbie
timestamp: January 21, 2025, 10:41:56 am
content: Hello all,I am trying to repair an old Peavey GPS 3500 amplifier.I have the circuit diagrams. There is a 4 pin connector on the transformer, 2 wires are for the 50V 25v/25V secondary, ground and a fourth wire which connector to Prot_A and Prot_B through a pair of 1N4007 diodes. I can't find out what this 4th (yellow) wire into the transformer does, it it has no reference / resistance to the rest of the transformer taps.In the circuit diagram the connector is labelled as J8Any ideas?

author: m k
author_rank: Super Contributor
timestamp: January 24, 2025, 06:00:54 pm
content: So it's pulling down what is prot.Either it must or must not be there.What is the other end of prot?

author: JTaylor
author_rank: Newbie
timestamp: January 24, 2025, 06:18:02 pm
content: I attached the full schematic last page top left prot_an and prot_b. These are for amp protection. High DC on the output and high temperature.I can’t figure out what the purpose of them connecting toroidal transformer. The pin on the transformer is open. No reference to any winding.

author: nali
author_rank: Frequent Contributor
timestamp: January 24, 2025, 06:29:02 pm
content: Maybe a thermal switch to protect the transformer from overheating?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 24, 2025, 07:28:49 pm
content: Peavey have prior on that, they put one in the Classic 30 mains transformer.

author: JTaylor
author_rank: Newbie
timestamp: January 24, 2025, 07:29:03 pm
content: It could be. Peavey sent me a description of the protection and initialisation circuit. This wasn’t in there but it makes sense.

author: JTaylor
author_rank: Newbie
timestamp: January 24, 2025, 07:31:22 pm
content: The transformer is made by Piltron I emailed them, no reply and Peavey don’t have a wiring diagram if the transformer.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 24, 2025, 08:00:06 pm
content: Prot_A and Prot_B are triggered low in a fault condition by several parts of the circuit.They operate the relays in the bottom right corner of the schematic.The transformer winding is connected via diodes to both, so it makes sense that if the transformer overheats, and the inbuilt thermal switch connects this winding to ground, both Prot-A and Prot_B will be grounded, thus triggering both relay circuits and saving the planet.Most transformer thermal fuses I have seen are non resettable normally closed, you now need a new transformer should it open type.It is possible Peavey had a rather better idea in this case, and put a normally open one in there that would trigger the protection circuits should it overheat, then when it cools down you can do the same thing again until you get fed up with it.Dunno but it kind of makes sense, where the non resettable ones just don't.

author: JTaylor
author_rank: Newbie
timestamp: January 24, 2025, 08:04:18 pm
content: That makes perfect sense.The amp still isn’t working but I’m taking it one step at a time.I’ll report backThank all for your help.

author: CaptDon
author_rank: Super Contributor
timestamp: January 24, 2025, 08:14:40 pm
content: Indeed, it is a thermal switch and will measure open circuit at normal operating temperatures. Repairing any of the Peavey SMPS power supplies or class D power output stages is not for inexperienced technicians. Much damage can occur with faulty troubleshooting techniques. Simple deductive logic looking at the schematic should tell you pulling that point to ground shuts down the drivers and outputs and give a logical clue as to what may be hiding inside the transformer.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 24, 2025, 08:32:55 pm
content: Yes, it should also be noted that this amp is a driven ground type thing.All the emitter resistors are grounded, and the outputs instead taken from the floating power supplies.So you want to be really careful about what references you are measuring from cos the whole thing is backwards.

author: JTaylor
author_rank: Newbie
timestamp: January 24, 2025, 11:20:47 pm
content: What do you mean the whole thing is backwards?I haven’t properly looked at the output stage.I have measured the output transistors and none of themare shorted.I’ll get there one stage at a time.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 25, 2025, 01:00:13 pm
content: Usually a power amp has positive and negative power supplies, referenced to ground, and the power amps output goes through the emitter resistors, then through the speaker, back to ground.This one doesn't do that. The 2 power supplies are not referenced to ground, they are floating, and instead the emitter resistors are grounded. So the output to the speaker is actually the middle point of the floating power supply.It's the same thing really, you are just putting the ground in a different place.But this means if you try and measure voltages using a meter with the negative connected to the chassis which would normally be "ground", you will not get a meaningful reading.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 25, 2025, 01:11:01 pm
content: This amp further complicates things by actually having 4 power supplies instead of the usual two, but that is a whole new topic.The principles of the grounded emitter resistors are still valid.