Hello all!
I am very new to the world of vintage radios, and have recently come in to possession of a beautiful old "multisonic" stereo and have enjoyed learning about it so far. It produces a beautiful tone, but in the last little while it has developed a fault which I was wondering if you might be able to help me diagnose.
I accidentally left the radio on for a couple of days while I wasn't home and returned to hear a horrible crackling sounding something akin to pulling a stylus across a record coming from the left channel only. I immediately switched off the unit and waited an hour or so, and switched it back on gently - the sound was gone and the radio sounded fine, but as it warmed up, the sound gradually came back, getting louder and louder, until it reached a peak and then just stopped, and everything sounded fine... Then, gradually again it would come back, then peak and stop, over and over. It seems to be getting worse... any ideas what it could be? I'm hoping the fact that it only happens on the left channel means it's fixable...

That's quite an impressive looking unit. It looks like there's three speakers on each channel a 12", a 6"x4" and an electrostatic tweeter. Try to figure out which of the three the noise is coming from or if it's coming from all three.

If it's only coming from the tweeter, C22b a .01 cap could be the culprit.

There's a press button switch that cuts out some of the speakers, does it make any difference when it's not in the 6 speaker position?

WARNING Don't go poking around if you're not familiar with these old things. There's lethal voltages where you least expect them. C22b is on HT3 and could have 220v on it!

Cheers,

Warren

Thanks Scraps!

It really is wonderful, I do hope I can fix it!

When I drop down to just two speakers, I still hear the crackle - and I noticed when I go back to multisonic the crackle is the same in the tweeter as it is in the woofers (with a different tone obviously) so I figure the fault must be somewhere between the balance knob and the speakers.

Interestingly, yesterday I doused the tone, volume and balance dials in WD40 for a moment I thought I had fixed it - but the crackly came back eventually, and today is back to its old broken self. But for four tracks of a record it sounded beautiful... could it be one of the 1M pots perhaps?

I used to work in electronics and have had a fiddle (with some slightly shocking results) already... I have a diagram and parts list, so I might just replace all the caps on the left channel and see if that improves things - from what I am gathering so far, the sound I am experiencing seems to usually be faulty capacitors.

Here is something you can try:

Swap the pairs of 6GW8's around and see if the problem presents itself on the right channel.

Cheers.

Thanks for the tip - tried that, still isolated to the left channel.

In addition, I plugged some external speakers in to the extension sockets on the radio, and the problem is replicated there too... Gradually narrowing it down!

Are the C21's, a, b, c & d (.0047uf), wax paper Ducons or the mustard Philips types?

I agree it'd be a good idea to replace any wax paper capacitors that are in there. The circuit I've got shows the only wax paper caps are in the RF section but it would be wise to actually check all the others to make sure. The C20 electros look interesting but I suspect they'd cause more of a howl.

Strange that spraying the controls changed the problem, it might be worth spraying some contact cleaner in them whilst working them back and forth. I'd have thought a dirty contact would only be a problem when you were moving it.

A dry joint on C18 on the tone control? A bit of WD40 might have made it come good for a while.

Marcc would advise that this needs to be diagnosed from the OP back and there's a lot of other things to check before getting back to the tone control though.

I'm not too keen on using WD40 around radio switches and pots. I prefer CRC contact cleaner for general use and Deoxit for the tough jobs.

Cleaning switch contacts is a good move. I've been suckered by dirty contacts before.

No petroleum-based lubricants should be used either for lubrication or dry-cleaning of parts. Any residue that remains will attract new dust to the affected surface and unreliability will return.

Ball bearings in larger tuning condensers should be given a light grease but other moving parts such as pots should be given a dose of contact cleaner followed by silicon spray. Silicon spray also doesn't have an odour.

Whilst you can get crackle from arcing in valves (bad cathode resistor sometimes) dirty valve sockets, switches and other mechanical causes.

Constant crackle is normally the realm of Silver Mica capacitors and some of those little "Styroseals" had their moments.

Silver Mica's with High DC volts on them are the more likely to fail.

Marc

Is the noise still there with the volume turned down ,the problem could be the speaker transformer.
John

Hi all! Thanks for the many replies, I'm sorry I've been caught up of late.
After further testing, I have chalked the previous WD40 solution to chance as I haven't been able to replicate it. Thanks for the advice about not using petroleum-based lubricants, that does make sense and I will keep it in mind (got that tip from here ( http://thebakeliteradio.com/page102/page105/page105.html , will take info from here with a grain of salt now!) To answer some questions:
Monochrome625: C21s are Philips type.
Zamiac: Noise is still there with volume turned down…
This might help in the diagnosis - I have recorded the sound via the extension speaker sockets on the unit, and have uploaded it here:
http://www.discreetservices.com.au/1197cracklesample.mp3
You can hear at about 00:00:45 when the crackle just stops, then gradually builds up again before stopping at 00:02:06, and then slowly starting again…

(By the by, I seem to have swapped channels during recording, so the crackle comes through on the right channel in the MP3.)

Thanks so much for your help so far everybody!