Hey folks,

i took this radio to a tech and he recapped it. it was still crackling, and he said it would need new valves, I bought some nos valves but it was still crackling. so was wondering if faulty mica caps would make this symptom? ive heard it can make a radio crackle. its the last thing to fix that I can think of,

thanks guys!!

Steven

There is C11, 150pf mica cap off the anode of 6BM8 (pin9).
That would be highly suspect, just take off one end and see if it fixes the problem.
Can also be caused by a failing speaker transformer.
JJ

I would never let a set doing that, out of the door. That person was probably all at sea with that radio and had not the part to fix it or the gumption to find the source.

Ok! you told him to change the caps, granted he did that.

If it not that cap have a look if there is grot in the tuning gang, or a joint not soldered. Chopsticks are good for prodding. Bad & corroded valve sockets cannot be ruled out.

Look up "silver mica disease"

Usually a failed Mica does produce a "Snap, Crackle, &sometimes pop. So do a few other things. However, its principally the ones going to ground with high voltage on them that fail and in Australian sets, that's rare.

The Americans however, have a lot of trouble with theirs. They indeed call it Silver Mica Disease (SMD) and is caused by Silver like a few other metals (Tin another) growing metal "whiskers" which short.

No tech is going to tell you it needs new valves if it is working. The fault is obviously not a valve.

I would, as said, not have let that out until I fixed it: Did this person have spares? In retrospect: Really should not have taken on the job.

You are wrong with the valves; Obviously without equipment, or the skills to apply them, or spares: There is an issue. However, Pentagrids, 6BA6/12BA6, 6N8, are tubes that are notorious for going short and often produce those effects along with socket & pin corrosion.

Never had that problem with those valves, but I have had the later 6N8 (the ones with the silver mesh) go crackly and/or low gain. The original Philips ones (with the grey shield) seem to work forever, as long as the silver-plated pins are kept corrosion-free.

But in this case, as the valves have already been changed, it would very much likely be a mica cap, on the anode of the audio triode.

If not already found

https://www.kevinchant.com/uploads/7/1/0/8/7108231/11-80.pdf

Some of those Silver coated pin valves had a bad habit of tracking and depositing the silver on the socket. My rather brutal method of dealing with the valve was to dip the bottom of it in Hydrochloric Acid: Rinse well.

As for the Audio it has 100pF and 0.0047 on the pentode plate the 100pF also to the triode plate making it less vulnerable due to the lower voltage, however with a bit of grot & being stacked Mica, might just flash over? The 0.0047μF is the most vulnerable and 600V may not be enough? I tend to use kV rated there.