DC voltage @ speaker output

[Jazz P Bass]

If the relay contacts are truly always closed, then the relay is bad.

It needs to be replaced.

[Enzo]

I may need to be replaced, but I'd imagine that it is asking a lot from even a good relay to drop out faster than a pop can form when the power switch opens.


I consider it a pop if it is from the switch sparking. if it is some artifact from the power down sequence, I consider that a thump. Point being, a pop will result instantly at switch off, the relay power supply will take a brief moment to discharge and open the relay contacts, and by then the pop has occurred. A thump you might be able to catch, depending upon whether it occurs before or after the point the relay drops out.

[methodofcontrol]

by enzo's definition, i'd say we're dealing with more of a thump than a pop.
sounds like i need a new relay.

can someone verify that my theory about the relationship of IC7 to the relay is correct? i just want to make sure my understanding is accurate for future reference.

[Enzo]

Yes, the IC is a relay control IC. Pin 6 drives the relay by completing the circuit to ground. When it is off, the relay supply voltage comes through undiminished, so there is 52v at pin 6. No current means no voltage drop. Then pin 6 drops to ground and the full 52v is placed across the relay coil AND 1.2k R44. They are in series. The relay IS being supplied by the 52v rail, but the series resistor drops roughly half that voltage. Relays are not NASA space probes, 30v is fine on the 24v relay coil.

With power OFF, measure resistance from those 0.22 ohm resistors at the power amp R37,38 or if more convenient L901/R36 . Measure from one of those to the hot post of the speaker output. If there is continuity with power off, then your relay contacts are welded shut.

[The Dude]

HERE is a link to the datasheet for that chip. There are all manners of detection input to the chip- DC offset, overload, AC, etc. If the relay is stuck on as you describe, this is all defeated. Likely the contacts of the relay have "arc welded" themselves together. Replacing the relay will likely solve your "thump" problem, but more importantly give you the speaker protection that IC and relay are designed for if the amp should decide to put out DC voltage. If you look at the schematic, there is a feed taken from the output and fed through an op amp to pin 1 of the chip to detect if something "bad" happens. And then, pin 2 detects offset. If the relay is stuck, I'd be more concerned about this loss of protection than a "thump".

[methodofcontrol]

new relay on the way! hopefully IC7 is just fine and the new relay will solve the "thump". thankfully the speaker doesn't seem to have suffered any obvious damage from the DC or the thumping yet. will post an update once the relay is in

[methodofcontrol]

new relay arrived yesterday. took about 10 minutes to swap out the old one and now the amp is acting as new. thanks for all of the advice, gents!