SWR 400 Strange Humming/cutting out

[Pryde]

Reluctantly I am looking at a SWR 400 Workingpro bass amp with an unfamiliar issue (to me)

Schematic attached

The amp powers on with a constant hum for 3-4 seconds then 2 or 3 small thuds, then dead silence for another 3 seconds. Then this sequence repeats over and over like clockwork.

During the humming portion of the cycle you can hear faint instrument signal when strumming, during the silent part no signal at all is present. Very strange that this whole ordeal cycles over and over ???

I ran the preamp out to another power amp and it is fine. I then ran the power amp out to another speaker cab with a separate preamp and the problem occurs again.
So the power amp and/or supply is the problem area.
I chopsticked everything for loose connections and found a loose molex connection but that did not solve the issue.

Unsure what to look at next. Any advice is appreciated. Thank you.

[Pryde]

OK I am getting -8.2vDC on the +HV rail (TP-1 should be +71.7 per schematic)

The -HV rail I am getting -70vDC so that looks fine (TP-2 on schematic)

So my positive line is getting a negative DC value. Is the big 10kUF filter cap bad? Anything else suspect to cause this?

[g1]

The bridge rectifier could also cause this. Do you measure AC volts on the +HV rail?

[The Dude]

Check that +HV rail for AC content. If it is high, you could have bad filters or a bridge rectifier as suggested by g-one. Another cause could be shorted output transistors. The likely reason the amp cycles is that it is detecting DC on the output or an overcurrent problem and protecting itself. The protection circuit will engage, the amp will try to turn on, the protection circuit will engage, the amp will try to turn on again, etc. I wouldn't hook up a speaker until you get it sorted.

[Pryde]

Bridge rectifier was it. Close inspection revealed a cracked solder joint. Sucked it out, re-flowed and we are in business

Interestingly it appears the rectifier gets very hot as the 4 solder points looked pretty stressed. I may clean and reflow the other 3 for good measure.

Is that normal for a 12 year old amp or is something else happening here?

[The Dude]

Probably normal and you would be right to solder them all.

[Pryde]

Thanks for all the help, you folks are the best.

[audiopete]

How hot does it get? Can you touch it? Keep a finger on it? Burning hot? What model is on the rectifier? Schem says it is a 400V 40A part, so that should not be getting hot enough to desolder. How is it mounted for heat dissipation? Perhaps you are drawing too much current still from another issue and that caused the rectifier problem and the original problem still exists.

[Pryde]

How hot does it get? Can you touch it? Keep a finger on it? Burning hot? What model is on the rectifier? Schem says it is a 400V 40A part, so that should not be getting hot enough to desolder. How is it mounted for heat dissipation? Perhaps you are drawing too much current still from another issue and that caused the rectifier problem and the original problem still exists.

It is tightly mounted to the chassis for heat dissipation. I ran it for 30 minutes and it wasn't hot so I am not sure what happened. I re-soldered everything so we will see how it does with some time.

[audiopete]

Sounds good. Probably a crap solder from the factory. The wave soldering machines sometimes go too fast and thicker leads don't get the heat they need to properly bond, so you get a cold solder, and/or not putting enough solder on the joints, so they crack later down the road with heat/cool and regular abuse and vibration. Ampegs are bad for that but it can happen to any brand. I always go over the board and heap the solder on things like pots, jacks, board connectors, tube socket pins and any other dodgy looking joint. I use a jewelers visor to magnify everything.