Yamaha CR-2020. Firecracker sound on one channel

[stevehertz]

A previously 'working fine' Yamaha CR-2020 receiver (1978, 105W/ch) now exhibits firecracker like sounds on the right hand channel only. The effect is quite pronounced after switch on (after relay comes in) but gradually subsides after a time. The sound is very much like a jumping jack or a firecracker; a sharp crack, crack.. crack. I've turned it on in a dark room as it does sound very much like arcing - albeit in short, sharp 'capacitory' cracks as opposed to more sustained 'fizzles' - but no visible sign that I can see unless it's under a PCB, but I'd have thought that I would have seen a faint glow even then.

To me it 'identifies' itself as one channel (obviously), and maybe the output stage concerning higher voltages and current. Alternatively the channel main smoothing cap arcing somehow. I'm going to delve in a bit further, but any ideas guys?

[Nickthedentist]

Good description, I can almost hear it

Does a bit of bashing with an insulated prod make the fault come and go?

And presumably it's independent of the volume control's setting?

Is it only audiable through the speakers or is it discernable at all with them disconnected (or before the relay pulls in)?

Is it possible to swap the L and R power supplies to determine whether it's a PSU or a Power Amp fault?

N.

[paulsherwin]

It could be a failing transistor. You will need to take some voltage readings. Be careful what speakers you connect it to until the fault is cleared, as if something goes bang a lot of expensive damage could result.

[stevehertz]

Thanks so far guys, great help. I'll do some checks!

[ms660]

Lots of small signal Jap transistors fail giving similar symptoms, often brought on by tapping or bending the board, dry joint symptoms but it isn't, it's a conductive film that can be either inside the transistors encapsulation or on the outside of the encapsulation, underneath where the lead out wires are where it shows up as an iridescent film spreading out from the centre lead to the two outer leads, wiggling the tops of the transistors in almost all cases of that particular failure mode will temp get rid of the symptoms or make the symptoms worse. I first discovered that problem back in 1976 when I was working for Sony.

If all else fails then it's always worth a try.

Lawrence.

[sp10mk11]

I am totally with the others on this fault, it is a very common fault in 1970s Pioneer receivers I have found, it is often in the magnetic pre-amp stages on these.
Good luck, very nice tuner amp BTW
Gary

[stevehertz]

Agreed in that respect, I have also had a faulty transistor on a Japanese receiver that was microphonic and very much affected by tapping and bending it.

[paulsherwin]

If you can confirm a transistor fault it may be worth swapping out all the small signal types. Most will be 2SC1815s or similar and these can be swapped for BC549s or even 2N3904s which cost next to nothing.

[stevehertz]

I have switched the preamp to power amp link to 'out' and the fault remains, proving it is not in the preamp but in the power stage of the RHC.

It crackled lovely at first switch on this morning and I thought when poking a couple of trannies that I had it, but probably a red herring Anyway, the fault disappeared soon afterwards and won't come back again while still warm it seems I have prodded trannies and frozen them but to no avail as yet. I'll get there

[sp10mk11]

Is there an easy way of removing the power supply to the pre-amp section just to totally remove it from the system?
Gary

[stevehertz]

Dunno Gary. But if the connection between the pre and power amps is O/C, I can't see how it can be the cause of the fault. But I'll stand corrected?

[sp10mk11]

The reason I do this, is because if the faulty stage in the pre-amp is still powered up it can produce large spikes on the power supply which would on a Japanese amp cause the protection to cut in...... just a thought
Gary