Marantz 2275 with a popping power amp

[Matt Mincer]

Hey Folks,

I have a 2275 with a loud pop from one speaker. It's loud enough to trip the speaker protection relay. The cause is definitely on one of the power amp boards. I know this because the boards are connected via Molex connectors. So I can completely remove each one and switch them, left for right, and the pop always follows the board. Also, I can remove the pre/power jumper so I know it's nothing before the amp board. I replaced 3 transistors from the Audiokarma bad transistor list, but it didn't solve the problem. The caps test okay in circuit with my ESR70 meter. Any suggestions on where I should go next?

Thx
Matt

 

[eiraved]

Are you getting the pop with nothing connected to the 2275, except for the speakers of course. Pop happens just after turning on the 2275?
Regardless, I would consider replacing all the transistors on the suspect board, then see if the issue continues.

 

[mbz]

Pop happens just after turning on the 2275?

Good point.
Is the pop immediately at power ON or after 4-5seconds (relay mute timing)

If pop is immediate then a really long shot would be a welded relay contact, if immediate then try...
with amp powered off/unplugged measure the following resistance on the P800 board
- between J808 and J810
- between J807 and J809

expect OL in both cases

It's loud enough to trip the speaker protection relay.

Suggests that it happens somewhat randomly well after power on.
Differential pair H701,2 would be candidates, replace with a hfe matched pair


Otherwise as per eiraved, probably a transistor, may be helpful if you can identify the sign of the voltage spike.
Monitor voltage a J810, note +/-sign when pop occurs

 

[Matt Mincer]

The pop is intermittent, and I replaced the dif pair. They were on the AK known-bad list.

 

[Osibisa]

Consider this .... a 2275 that I recapped a while back operated fine for a few weeks then exhibited the same behavior that yours does. What I did not do during the course of the recap was to apply deoxit to the molex plug and pins on each amp board, after treatment there was no further occurrence. This may be something you should do unless it has already been done.