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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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30th Apr 2019, 10:18 am | #1 |
Triode
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 28
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IF cans
Hi all, not really into repairing radios usually repair amplifiers, but here I have a Ristaucrat radio/record player. I tested the valves and replaced all the caps, it worked and pulled in some stations for about 1/2 hour then it would not. All I get now is a thunder/rushing noise and nothing else. The audio stage is OK fine on phono, it has a PC150 couplet, but looking looking at the couplet circuit appears to be mostly after the volume pot and this noise is volume dependent, so I am wondering now about the caps IN the IF cans, these are small, I am nervous of taking them apart, so thought I would post here, in case anyone has suggestions for my next step.
Thanks. |
30th Apr 2019, 10:23 am | #2 |
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Re: IF cans
Lost me, thought I knew valve circuitry too. PC150?
#All faults on valve sets are not due to capacitors especially silver mica ones that are normally extremely reliable.# |
30th Apr 2019, 10:39 am | #3 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: IF cans
I used to have a Hammarlund receiver which used similar.
Specs for the PC150 couplet (couplate) here: http://pacifictv.ca/schematics/centralabcouplates.pdf Lawrence. |
30th Apr 2019, 10:43 am | #4 |
Dekatron
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
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Re: IF cans
Caps in the IF cans or should say across the IF's when they fail and I only ever came across once in a Radiomobile car radio, in my case intermittent caused very poor performance.
As they would because the IF in fact is acting as though off alignment. Don't risk causing another fault that is not there.
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30th Apr 2019, 10:48 am | #5 |
Dekatron
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Re: IF cans
Some of the "built in" capacitors in certain types of IF transformers are well known for giving problems.
Lawrence. |
30th Apr 2019, 11:17 am | #6 |
Dekatron
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Re: IF cans
Ah ,interesting and I wonder which IF's those are?
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30th Apr 2019, 11:25 am | #7 |
Dekatron
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Location: Cornwall, UK.
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Re: IF cans
I can't remember a specific manufacture but Google silver mica disease.
Lawrence. |
30th Apr 2019, 12:24 pm | #8 |
Dekatron
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Re: IF cans
Spot on, it does indeed sound like those caps. Fault fits the description on YouTube .
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30th Apr 2019, 12:28 pm | #9 |
Triode
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 28
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Re: IF cans
Thanks for replies. Here is the schematic, as I said, everything's been changed apart from couplet.
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1st May 2019, 10:38 pm | #10 |
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Re: IF cans
Could be IF instability/oscillation. Recheck decoupling capacitors and possible IF alignment (as a last resort). The noises you describe sound like IF oscillation to me. Maybe the replacement capacitors are a different type and if stability was marginal anyway, changing them may have been enough to push it over the top and cause it to take off. Could you scope it to get a better idea of what was happening? It looks like a live chassis set to me so the usual precautions apply.
Alan. Last edited by Biggles; 1st May 2019 at 10:39 pm. Reason: typo |
1st May 2019, 10:59 pm | #11 |
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Re: IF cans
There is a feedback pathway, that can result in instability and IF oscillation that often gets overlooked, the heater chain. Initially it can be ok, but as the valves age the heater-cathode leakage can increase. Just as a check, try adding small bypass caps on the heater pins to ground. Also, after the screen bypass cap is checked on the IF stage, try a new IF valve, I have had instability from gassy ones. Assuming everything is perfectly bypassed and the IF valve good, and you have also tried the bypass cap from the screen to the cathode, if it still unstable and oscillating, try either a small amount of damping on the IF transformer with something like a 470k resistor. Or put a 100R resistor in series with the IF amplifier's control grid.
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3rd May 2019, 11:02 am | #12 |
Triode
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 28
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Re: IF cans
Well it turned out to be the mica caps in the IF cans, I might add, I did not repair it as did not fancy going in there I took it to a guy in Bristol [Gerry] who is on this forum, who repaired it overnight for me! So all working now.
Thanks to all who responded, Bob |
3rd May 2019, 11:04 am | #13 |
Dekatron
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Re: IF cans
Good result,that is all that matters.
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3rd May 2019, 11:00 pm | #14 |
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Re: IF cans
Good result. Sounded like a tricky one.
Alan. |
4th May 2019, 8:04 am | #15 |
Dekatron
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Location: Chesterfield, Derbyshire, UK.
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Re: IF cans
I found this problem with certain American radio's as has been said it is caused by the silver mica migrating across , it usually causes low gain and flat response when aligning the If's ,sometimes crackling ,the cure is to drill small holes ,as cut of the connection to the pin out , the capacitor is part of the base you cannot see it. I do this with both can's faulty or not , then fit the appropriate capacitor in or outside the can .Mick.
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15th May 2019, 7:14 pm | #16 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,883
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Re: IF cans
Just repaired a Bush AC34, it turned out to be the 2nd IF. I decided to take a look inside and to my surprise the lead from the mica capacitor was fractured and open circuit. I don’t know how this could have occurred, there was no obvious stress on the lead, all I had to do was to bridge the gap and it now works fine.
Cheers John |