Hi All
Last year, I restored an old valve radio from 1950s. It was not working when I got it, but when I recapped all the original paper capacitors to new ones, it started working.
It is AC Mains powered, and sounds quite good - and it is still working well. But when switched on, and then after about 4 hours constant receiving AM stations, it starts sounding popping and crackling from the reception.
What could this be? Where should I look to sort this problem? Thanks for your info and advice.
Last year, I restored an old valve radio from 1950s. It was not working when I got it, but when I recapped all the original paper capacitors to new ones, it started working.
It is AC Mains powered, and sounds quite good - and it is still working well. But when switched on, and then after about 4 hours constant receiving AM stations, it starts sounding popping and crackling from the reception.
What could this be? Where should I look to sort this problem? Thanks for your info and advice.
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It could be a lot of things..
The first easy one is, is it still their with the volume turned all the way down..If it is then chance is either PSU caps or leakage in the output stage possible coupling cap. If its gone when the volume is down then its in the reciever stage..again possible coupling cap..
Might be easier with a schematic..
Sorry its a bit vague...
Regards
M. Gregg
I once had an old Zenith radio which developed similar crackling noises.
I traced the problem to the IF transformers. Inside the transformer cans, Zenith had designed fixed capacitors. The capacitors were made up of a pair of flat metal plates with a thin sheet of mica between the plates. The mica had deteriorated and the two metal plates had started to arc through the mica.
I was able to replace the mica with a thin sheet of teflon.
I traced the problem to the IF transformers. Inside the transformer cans, Zenith had designed fixed capacitors. The capacitors were made up of a pair of flat metal plates with a thin sheet of mica between the plates. The mica had deteriorated and the two metal plates had started to arc through the mica.
I was able to replace the mica with a thin sheet of teflon.
Arcing? There are few RF volts across a IF coil winding. Surely, they make false contact and then introduced noise, but arcing?? Mmmmm....
Yes but lots of DC!
I would tap the 35W4 (35Z5 in older versions) tube as that is most likely to be the problem. Next try the 50C5 (50L6)
Also look to see if any of the tubes are glowing red on the plates.
FB we cross posted!
No, no and no. The circuits gives no DC voltage across primary winding, as the logic and common sense tell me. So, no voltage = no arc.
The original arc may have been from the primary to the grounded can.
The mica showed a few holes.
Whatever ... it did arc through the mica. As you can see on the schematic, I marked "REP" to indicate that I had replaced the mica insulator.
Thanks for your posts - great
I will have look at them through time one by one, and will come back with any findings.
But one question which makes me baffled is, - why does it go popping and crackling after 4 hours of operation? It sounds perfect for first hour or two, and even three or four, so one day I measured time as soon as it is switched on, and around 4 hours of continuous playing, it pops and crackles.
It is playing music very well now in front of me as I am typing this message - but in 3 hours from now, I know it will start crackle and go popping.
Thanks & regards to you all
I will have look at them through time one by one, and will come back with any findings.
But one question which makes me baffled is, - why does it go popping and crackling after 4 hours of operation? It sounds perfect for first hour or two, and even three or four, so one day I measured time as soon as it is switched on, and around 4 hours of continuous playing, it pops and crackles.
It is playing music very well now in front of me as I am typing this message - but in 3 hours from now, I know it will start crackle and go popping.
Thanks & regards to you all
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This is a common problem in certain brands/models of radios. Usually the restoration guys remove them,and P2P solder some new silver mica caps to the underside of the cans.
I guess it's caused from silver migration? I dunno. I've never seen it myself,but I've read about it dozens of times.
It could be what is generally termed as "silver mica disease". A web search on the term should yield many references on the subject. Here’s one of them: IF Transformers. Here’s a youtube clip of what it sounds like: Silver migration "disease" sound. - YouTube.
Since the repair involves tedious rework of the IF cans, I suggest eliminating other causes first such as loose connections or possibly dirt between the plates of the tuning capacitors as previously noted.
Since the repair involves tedious rework of the IF cans, I suggest eliminating other causes first such as loose connections or possibly dirt between the plates of the tuning capacitors as previously noted.
Many IF transformers use one mica sheet with the primary cap and the secondary cap side by side. Silver sometimes form trace leakage paths between the two, which will make a lot of noise. see my web page wa2ise's Home Page of Radios
about halfway down.
about halfway down.
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