Rumble and popping in amplifiers

[NOON]

Hi everyone. 
  I need another small amplifier around the place, so I'm sorting through the 'junk' pile to see if there's something worth repairing rather than buy another one. I have a couple of classic 70's hi-fi amps that are intermittently making loud rumbling and popping noises. Confined to 1 channel (so not from power supply) and not affected by volume control (so post preamp).
  I'm thinking it's a leaky cap somewhere in the power amp section, but wanted some second opinions before I go diving in. Any thoughts?
  Thanks

 

[CJ]

tube amps?

wiggle the tubes in the sockets, maybe some crusty contacts,

or maybe the tubes themselves, do a tube swap,

transistors?

don't know much about transistors,

 

[jwhmca]

As a repair tech, I hate hazarding a guess, but I would guess bad opamp.

 

[NOON]

All discrete transistors, classic mid to late 70's gear. The Marantz is the main one I'm hoping to get going easily.

 

[SSLtech]

...I'm thinking it's a leaky cap...

Every time I hear this line  -and I hear it a lot-  it's always seems to be from someone who doesn't really understand what caps actually DO.

-And every time I hear this line, it's pretty much never a cap.

Forty years ago, I used to hear "I think it's just a loose wire"

 

[JohnRoberts]

Loose wire..  ;D ;D  Perhaps a bad solder connection, dirty connector, bad cable?

JR

 

[NOON]

It's these kind of pointless, arrogant, self-aggrandising comments that make me just not bother with these boards most of the time.
  I understand perfectly well what capacitors are and what they do, and in this case they are generally being used to block DC voltages between discrete transistor stages. If one is not doing it's job and allowing the DC bias voltages of the transistors to interfere with each other then the result could easily be rumbling and popping. Seeing as I've already done some resoldering of suspect joins, tapped and poked at the boards and wires looking for loose connections and compared signal paths between channels looking for obvious differences with a signal generator and oscilloscope, my next step is to suspect some of the electrolytics, which are the main components that degrade with age in this kind of circuitry.
  These kind of intermittent faults can be a right bastard to chase down, and just when you think you've solved it they can come back again. I merely asked for advice from anyone who may have come across a similar issue in the past and solved it if they had any further insight before I dived into changing components. It's a waste of everyone's time to say it may not be that, try and disparage my level of knowledge to make yourself look good and then not offer any further insight. If you would care to explain why it's not likely to be caused by old electrolytics, educate us all about capacitors and what they do that you suggest I am ignorant of, or you can suggest a more likely candidate to try first, please go ahead, I'm all ears, that's why I asked. If you have nothing of value to contribute, please don't bother in the future.      /end rant

 

[JohnRoberts]

I always find it amusing when people ask for free advice, then complain about it.  ;D (Ask for your money back).

A "leaky cap in the PS section" is unlikely to cause the noise you describe, more likely a fault in the direct audio path.

I repeat my suggestion that a bad electrical connection, either solder or mechanical is suspect.

Alternating spray cold, and heat from an iron can help in troubleshooting. Tapping components with a pencil eraser can sometimes reveal intermittent connections.  I found a microphonic film cap (one time in several decades).

You mentioned a couple amps with the same noise, which makes me suggest checking that your hook up cables are all good. It seems unlikely that two old amps would share an identical failure (while anything is possible).


JR

 

[jwhmca]

John, you forgot to ask the client to "unplug" it for a full 15 minutes.

Also, it  "could be a fuse."

Seriously, what's the model number? Do you have a Scope? ESR meter? Schematic? Is it "Thermal?" i.e. Does it happen after it warms up? Stop after it cools down?

The tip of your soldering iron is a great proximity heater and canned air turned upside down is great "freeze spray."

 

[RuudNL]

I'm afraid NOON has left the building...

 

[NOON]

Sorry, had some other things come up. The first patient is alive and well, a Pioneer SA-6500II. It has an interesting 'protection' circuit where if it detects DC on the output it shorts the rails to ground with a triac, blowing the fuses. Crude but effective.
  The rumbling and popping would eventually set off the protection circuit and blow the fuses, so troubleshooting was difficult.
  I ended up just replacing the majority of the electrolytics, a few had been changed in the past and were no longer at factory values, I assume with whatever was on hand at the time. With a new set of electrolytics it's now quiet and clean, no longer rumbling or blowing fuses.
  Quite a few of the original caps I removed were well out of spec. To be expected in an amp that is pushing 40 years old.

 

[Whoops]

Every time I had that sort of problem that you described it was either a transistor or an Op-Amp,
but anything can happen I guess.

You time I had 10K ringing in an amp, it was one colder joint.
Another time I had a 10K ringing and it was a ground loop.

Maybe next time I have a 10K ringing  it's just a "Loose Wire"