help with warm up noise?

[awangotango]

mid 70's pioneer sa800 int amp........ Using headphones, the sound will cut out a bit every now and then UNTIL THE AMP HAS BEEN WARMED UP FOR AN HOUR OR SO. And then all seems fine. I don't have any speakers to try and see if it is just the headphone input or the speakers outs as well..............any ideas what cap/resistor and where may be the culprit here? Also, when the problem occurs, it can be activated/aggrevated by using the volume knob........thanks for any ideas.

[Mooly]

Tarnished speaker output relay contacts (assuming it uses relays). If the relay comes apart just give the contacts a wipe with paper soaked in WD40.

Other suspects are any mechanical contacts the audio passes through.

Are all inputs affected ?
One or both channels ?

Also check for poor soldered joints which could be anywhere but usually are around components that get hot or suffer any mechanical movement such as around sockets/switches/controls.

[awangotango]

I found that it was this relay that was not fully engaging so I hotwired it in the on position.....any problems with doing this? The amp works fine now without cutting out, but I also don't want to blow anything. I'm sure the relay serves a function, but........?

[lanchile]

Originally Posted by awangotango

I found that it was this relay that was not fully engaging so I hotwired it in the on position.....any problems with doing this? The amp works fine now without cutting out, but I also don't want to blow anything. I'm sure the relay serves a function, but........?

Bad practice ! you can damage your speakers or your amp. relays work as a "switch", some are there to protect speakers if there is a short or DC to speakers. jumping it is like putting a big piece of wire in the line of fuse...it will work but it will not protect nothing!!!! I would check some caps that dry out and affect relay to engage!.

[awangotango]

so you think a bad cap was not allowing it to fully engage? I will swap out the relay for a new one just in case, and also snoop around for associated caps...thanks for the tip.

[Mooly]

You need to determine if it is really a relay problem or a fault. Monitoring the voltage across the relay coil may give a clue. If the correct voltage to operate the relay is present but it fails to pull in then its looking like a faulty relay.

Also when the relay doesn't pull in (when the amp is in the faulty state), measure and confirm that there is NO dc voltage at the amplifier outputs. This must be measured before the relay, not on the speaker output side.

Common general faults are poor solder joints, typically on components that get hot and on power transistors. Also certain power transistors have a habit of failing open circuit.

Any DC faults like the above will cause loud pops and bangs through the speakers so if that isn't happening it may well be a "safe" fault such as the relay or its drive.

[awangotango]

the relay 'works' in that it clicks and engages within a sec or two of turn on, but it isn't making a strong enough contact because the sound will cut out if the relay is tapped - and will respond to tapping of the chassis as well. A relay seems to be a delicate mechanical contraption and the engage/disengage tolerances seem quite small?

[Mooly]

If the relay engages but there is no sound then it might be just a faulty relay but you need to be sure.

If the relay is common to both channels then it is unlikely both channels would be affected identically if the contacts alone were the problem. If both channels drop out together then either the relay holding voltage is insufficient or the relay really is dropping out due to a fault.

Hard to see in the picture but if you have soldered onto the relay contacts rather than the terminals then the relay will need replacing anyway I suspect.

Relay contacts are easily cleaned by pulling a bit of paper or card soaked in WD40 through the contacts.