2. amps-preamps
  3. 133811-108496

Opinions re: debugging/repairing Cambridge Audio 540A amp stage

Hey! My old Cambridge Audio 540A has seen better days. It's been acting up recently. I'm interested in fixing it up myself if I can, but I'm looking for opinions on where to start.

Symptoms: speaker output on *any* channel (L/R, A and B) can drop instantly from the correct level to partial (including zero output). It does not happen consistently: on some audio streams, it seems more stable, but I have no way to reproduce reliably. The drop is instantaneous, not a slow shift. I can reset the audio levels if I switch input selector and back, until it happens again. If I do nothing, sometimes the audio will click back into place on its own.

This can happen on any input. Multimeter suggests that the voltage on the output drops when the audio drops; I don't suspect an issue with the speakers, but I don't have spares to test with. Headphone output is always perfect. My suspicion has been the amp stage in the 540A.

This is not a new unit, and so I popped the top off. My first suspicion was the big capacitors, because of age and the gunk they'd deposited on the board: https://i.imgur.com/u0Qnj8O.jpg  So I've cleaned those and replaced them. The problem persists, but I didn't have replacements to hand for the smaller capacitors.

I'm feeling my way around as I go; I have a copy of the service manual so I can decipher things, but I'm looking for good approaches to debugging these units to home in on the problem.

Note, obviously I can replace this unit easily. But if I can fix it, I'll feel happier, and I'm also happy to get my hands dirty. I'd appreciate pointers from folks on what to attack!
sodsto
sodsto OP
3 posts
 
One word might nail it?

From that post, I happened to land on https://www.dinkum.nl/music_and_sound/cambridge_repair/
sodsto OP
3 posts
 
Okay, the relays seem to have been the glitch. Swapped out the old ones for some replacements. The amp seems solid again.

Awesome pointer. Thank you!
teo_audio
932 posts
 
to check for relay problems, you let the unit run, usually while listening on headphones. Of course, first ascertaining that the unit has the audio amplifier proper being the source point that powers the headphone jack.

This depends upon the circuit having been run through the relays before it hits the headphone jack. Integrateds/stereo receivers are run this way, big surround amps are generally not. Big surround receivers generally run the headphones off a dedicated op amp.

then use the back end of a old toothbrush (you use old toothbrushes to clean boards and whatnot) or very small paintbrush handles, etc...to ’tap’ the tops of the relays. Generally the sound will cut in and out when you do this.

You tap gently, very gently, and slowly bring the pressure of the taps up, in order to see if the vibration will make the contacts cut in and out. Soft taps are necessary so you can be sure the vibration is not going elsewhere and thus making the problem go back to being ’anywhere’ and thus unknown in source point.

Relays these days, as in the past 15+ years, are junk, compared to the relays in +25 year old gear. Generally speaking.

you can take apart and fix 25-40-45 year old relays. They are built for it. They have removable covers, nonmagnetic conductive parts, silver contact points, etc. Which is part of the problem, silver oxide, but also their gift of quality. You can clean the contacts with 2000 grit sand paper. Just fold a small piece over double, slide it between the contact points, gently push the contacts together and then slide the sandpaper back and forth (up and down, actually) maybe 5-10 times, and you are done. Good for probably another decade.

Headphone testing is many times a good idea as an amp can sometimes be on the verge of a spectacular failure (thar she blows!.....) and you don’t want to blow up a good speaker for no reason, if you can avoid it.
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