Peavey M-7000 Mark Power Amp - Slow Power Up

[TomCarlos]

Gents...

I have read the notes on the recent M-7000 thread titled Peavey M7000 won't energize output relays. I am not sure if the amp I have has the same problem or not.

The M-7000 amp I have may have two issues, both related to Channel B. The first issue is that when I apply the power, it takes approx 30 seconds to hear a "Click" (presumably it is the output relay) that engages. I then see the Power LED is light on Channel B. Channel A turns on immediately.

Once Channel B is on, there could be a thermal issue. I noticed the output drops considerably when the amp warms up.

But, first things first... the delay in the Power LED. Is that related to the output stage?

Thanks!

[Enzo]

Could be, who knows? get on the inboard side of the relay and watch the output bus of the amp. WHile waiting for the relay to click, is there any DC offset on that output?

[TomCarlos]

Thanks Enzo...

At first, I measured -8.5 volts on R43, the 22 ohm resistor that sits across L1. When the "click" kicked in (around 45 seconds), I was then measuring 1.5mv.

I powered down the unit and cleaned the Input and Output jacks with some DeoxIt. When powering up now, the Channel B Power LED comes on immediately. Coincidence or is the jack completing some type of circuit? I will keep testing this tonight.

[Enzo]

Probably less about cleaning those jacks, and more about the taking it apart and putting it back together.

So the relay circuit was doing its job. The circuit had 8v offset to start, then faded down to a millivolt and released the relay.

[TomCarlos]

Agree... now that I am studying the schematic, I do not see the connection between the output jacks and the relay.

I am going to run a signal through Channel B for a while... 15 minute increments... slowly increasing the power to see if it operates properly. That is the problem I first noticed with this amp.

Will get back to this by the weekend.

Thanks Enzo.

[Enzo]

The output jacks directly connect the speaker jacks with the output of the amp circuit. The relay COIL is elsewhere driven by the protection circuit. The coil is drawn next to the speaker jacks, but look at the wire from it tracking down and around to the group of transistors lower left right above the mains wiring. Q28 drives the coil. See the SBS14 Q25 there? The output of the channel is sampled and runs through Q25 to get to the transistors. Note that just below that part another line sneaks off back to the right to the thermal breaker on the heat sink. There are a lot of things going on down in that corner, a number of which have something to say about the relay coming on.

Once you have it running, and clicking on promptly, the first thing to do is feed it a signal with NO load, and see if it makes a clean waveform all the way up to the rails. If it does that OK, THEN put a load on it and see how it responds.

[TomCarlos]

Thanks Enzo...

Today's test thus far - the amp fired up right away and both channels have a clean signal up to the rail. So I will run some music through B and turn up the volume!

The one thing this amp could use? Some additional open space on the front to let the hot air flow out. This amp does generate some heat.

I'll report back over the weekend.

[TomCarlos]

The amp has been working over the last few days. The Channel A and B power LEDs light up immediately. Other than some cleaning, I didn't do any repairs. Must have been divine intervention that fixed the problem. For now, case closed.