IF cans - UK Vintage Radio Repair and Restoration Discussion Forum

sparkey58 · 30th Apr 2019, 10:18 am

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.

Boater Sam · 30th Apr 2019, 10:23 am

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.#

ms660 · 30th Apr 2019, 10:39 am

I used to have a Hammarlund receiver which used similar.

Specs for the PC150 couplet (couplate) here:

http://pacifictv.ca/schematics/centralabcouplates.pdf

Lawrence.

HamishBoxer · 30th Apr 2019, 10:43 am

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.

ms660 · 30th Apr 2019, 10:48 am

Some of the "built in" capacitors in certain types of IF transformers are well known for giving problems.

Lawrence.

HamishBoxer · 30th Apr 2019, 11:17 am

Ah ,interesting and I wonder which IF's those are?

ms660 · 30th Apr 2019, 11:25 am

I can't remember a specific manufacture but Google silver mica disease.

Lawrence.

HamishBoxer · 30th Apr 2019, 12:24 pm

Spot on, it does indeed sound like those caps. Fault fits the description on YouTube .

sparkey58 · 30th Apr 2019, 12:28 pm

Thanks for replies. Here is the schematic, as I said, everything's been changed apart from couplet.

Biggles · 1st May 2019, 10:38 pm

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.

Argus25 · 1st May 2019, 10:59 pm

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.

sparkey58 · 3rd May 2019, 11:02 am

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

HamishBoxer · 3rd May 2019, 11:04 am

Good result,that is all that matters.

Biggles · 3rd May 2019, 11:00 pm

Good result. Sounded like a tricky one.
Alan.

vinrads · 4th May 2019, 8:04 am

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.

John10b · 15th May 2019, 7:14 pm

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

30th Apr 2019, 10:18 am	#1
sparkey58 Triode Join Date: Jun 2010 Location: Weston Super Mare, Somerset, UK. Posts: 28	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
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	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
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	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. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

30th Apr 2019, 10:48 am	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	Re: IF cans Ah ,interesting and I wonder which IF's those are? __________________ G8JET BVWS Archivist and Member V.M.A.R.S

30th Apr 2019, 11:25 am	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: IF cans I can't remember a specific manufacture but Google silver mica disease. Lawrence.

30th Apr 2019, 12:24 pm	#8
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	Re: IF cans Spot on, it does indeed sound like those caps. Fault fits the description on YouTube . __________________ G8JET BVWS Archivist and Member V.M.A.R.S

30th Apr 2019, 12:28 pm	#9
sparkey58 Triode Join Date: Jun 2010 Location: Weston Super Mare, Somerset, UK. Posts: 28	Re: IF cans Thanks for replies. Here is the schematic, as I said, everything's been changed apart from couplet. Attached Thumbnails

1st May 2019, 10:38 pm	#10
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	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
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	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.

3rd May 2019, 11:02 am	#12
sparkey58 Triode Join Date: Jun 2010 Location: Weston Super Mare, Somerset, UK. Posts: 28	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
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	Re: IF cans Good result,that is all that matters. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

3rd May 2019, 11:00 pm	#14
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: IF cans Good result. Sounded like a tricky one. Alan.

4th May 2019, 8:04 am	#15
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,754	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.

15th May 2019, 7:14 pm	#16
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,882	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