Help please oddball DAC90 fault - UK Vintage Radio Repair and Restoration Discussion Forum

Boom · 28th Mar 2008, 5:22 pm

I've picked up an extremely clean DAC90 which I would like to get going.
First I removed the eternal speaker socket that someone had fitted
(yep really) and while doing so noted the single pole on/off switch
is wired through the neutral to chassis! This means the chassis becomes
live when the set is switched off even if wired the 'correct' way around!

The problem:

The output is very quiet almost silent but the AF stages are fine with lots
of 'finger hum'. Very occasionally the set will work normally but then drop
volume again.

Voltages are pretty much what they should be except for the AGC which
seems to be all over the place but I suspect this could be a symptom rather
than the problem. The CCH35 and EF39 have been replaced with no joy.

The problem seems to be in the IF stages and I have a sinking feeling that
the IF transformer feeding the EF39 itself might be faulty. Continuity is
present on primary and secondary windings so if faulty I would suspect
shorting turns.

Can anyone confirm or deny that the IF transformers go faulty in this way
or if you all say 'Not in a Million years, it doesn't happen on a DAC90' I'll
look again but I really am flummoxed.

One small oddball I noticed is that I get finger hum from the EF39 top cap.
Is this correct? It's an IF stage after all.

The problem is made hard to investigate as I have mislaid my isolation
transformer so can't get a 'scope connected.

Thanks for any help or suggestions - Dave

Ian - G4JQT · 28th Mar 2008, 5:37 pm

The mains-switch wiring is normal. This thread explains: https://www.vintage-radio.net/forum/...6336&highlight

If the IF transformers have not lost continuity, check that one of the ferrite cores has not come lose from their brass thread and knocked it right off resonance. But if so, carefull "supergluing" can fix it, but don't take the IF can appart to do this until you have checked everything else. To be honest, you should get something even with a poorly alighed IF stage, so a broken core sounds like a last resort!

Ian

Steve_P · 28th Mar 2008, 5:40 pm

Change everything on the AGC Rail if you're not sure!

Have you subbed V3?

Have you done a capacitor change, including the ones across the IF Cans?

Have you got a Signal Generator. If you have, it's 465 kHz and go via suitable caps to the anode of V2. Then the anode of V1 and tune the IF up if you can. It could have been tweaked onto the wrong frequency.

Cheers,

Steve P.

The manual is available up top if needed.

Boom · 28th Mar 2008, 5:48 pm

Quote:

Originally Posted by Steve_P

Change everything on the AGC Rail if you're not sure!

Have you subbed V3?

Have you done a capacitor change, including the ones across the IF Cans?

Thanks Steve. Yes I have changed V3 and can't change the IF caps' as they are inside the can which is going to be a pig to get at so I want to leave that to a last resort. I'm really asking if anyone else has had these pack up or not.

The high values resistors on the AGC line all measure ok as do the caps'.

No way of genneying a 455 at present. When the fault clears the IF's do indeed peak. This is what is saying to me it might not be them. I'd have thought the caps' would be either good or/s but then again shorted turns might just come and go. It really is doing my head in.

Boom · 28th Mar 2008, 5:58 pm

Many thanks all. Think it is sussed. The set came to life again and the IF peak is indeed changing during the fault so it looks almost certain that either a cap across the transformer is going o/c or is dry jointed. I'll have to take the can out and sit down with it. Looks a horrible long job.

Many thanks again for all your help.

af024 · 28th Mar 2008, 8:01 pm

Hello Dave,

I had one with a dodgy mica cap inside the IF transformer and another where an IF transformer winding was intermittent.

I hope you find it. These intermittent faults can be a pain.

Best of luck,

Andy

Boom · 28th Mar 2008, 8:03 pm

Quote:

Originally Posted by Ian - G4JQT

The mains-switch wiring is normal. This thread explains: https://www.vintage-radio.net/forum/...6336&highlight

If the IF transformers have not lost continuity, check that one of the ferrite cores has not come lose from their brass thread and knocked it right off resonance. But if so, carefull "supergluing" can fix it, but don't take the IF can appart to do this until you have checked everything else. To be honest, you should get something even with a poorly alighed IF stage, so a broken core sounds like a last resort!

Ian

Many thanks Ian. Dead on. The slugs had fallen off the threads and were floating about as I turned the set upside down to work on it hence the sometimes working bit! I Araldited what was left of them back in place which has got it working but it has taken me 2 hours. I'll stick to DAC90A's in future. Far less hassle.

Thanks again - Dave

telstar · 28th Mar 2008, 8:31 pm

Hi. The problem on both my DAC90's was with the I.F.T slugs breaking away from their brass formers. It really was a pain to superglue the fragments of the slugs together again (luckily the breaks were fairly straightforward, and not any worse).
Another strange fault that I had was the cathode resistor of the CL33 output valve changing value when the set was switched on (its value was ok when "cold"). I have never experienced this problem in any other radio - I replaced the resistor and it seemed to work normally again.
Regards
Mike

28th Mar 2008, 5:22 pm	#1
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Help please oddball DAC90 fault I've picked up an extremely clean DAC90 which I would like to get going. First I removed the eternal speaker socket that someone had fitted (yep really) and while doing so noted the single pole on/off switch is wired through the neutral to chassis! This means the chassis becomes live when the set is switched off even if wired the 'correct' way around! The problem: The output is very quiet almost silent but the AF stages are fine with lots of 'finger hum'. Very occasionally the set will work normally but then drop volume again. Voltages are pretty much what they should be except for the AGC which seems to be all over the place but I suspect this could be a symptom rather than the problem. The CCH35 and EF39 have been replaced with no joy. The problem seems to be in the IF stages and I have a sinking feeling that the IF transformer feeding the EF39 itself might be faulty. Continuity is present on primary and secondary windings so if faulty I would suspect shorting turns. Can anyone confirm or deny that the IF transformers go faulty in this way or if you all say 'Not in a Million years, it doesn't happen on a DAC90' I'll look again but I really am flummoxed. One small oddball I noticed is that I get finger hum from the EF39 top cap. Is this correct? It's an IF stage after all. The problem is made hard to investigate as I have mislaid my isolation transformer so can't get a 'scope connected. Thanks for any help or suggestions - Dave

28th Mar 2008, 5:37 pm	#2
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,326	Re: Help please oddball DAC90 fault The mains-switch wiring is normal. This thread explains: https://www.vintage-radio.net/forum/...6336&highlight If the IF transformers have not lost continuity, check that one of the ferrite cores has not come lose from their brass thread and knocked it right off resonance. But if so, carefull "supergluing" can fix it, but don't take the IF can appart to do this until you have checked everything else. To be honest, you should get something even with a poorly alighed IF stage, so a broken core sounds like a last resort! Ian Last edited by Ian - G4JQT; 28th Mar 2008 at 5:44 pm.

28th Mar 2008, 5:40 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Help please oddball DAC90 fault Change everything on the AGC Rail if you're not sure! Have you subbed V3? Have you done a capacitor change, including the ones across the IF Cans? Have you got a Signal Generator. If you have, it's 465 kHz and go via suitable caps to the anode of V2. Then the anode of V1 and tune the IF up if you can. It could have been tweaked onto the wrong frequency. Cheers, Steve P. The manual is available up top if needed. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Mar 2008, 5:58 pm	#5
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Help please oddball DAC90 fault Many thanks all. Think it is sussed. The set came to life again and the IF peak is indeed changing during the fault so it looks almost certain that either a cap across the transformer is going o/c or is dry jointed. I'll have to take the can out and sit down with it. Looks a horrible long job. Many thanks again for all your help.

28th Mar 2008, 8:01 pm	#6
af024 Octode Join Date: Jan 2003 Location: Ashby-de-la-Zouch (it's not by the sea) Posts: 1,255	Re: Help please oddball DAC90 fault Hello Dave, I had one with a dodgy mica cap inside the IF transformer and another where an IF transformer winding was intermittent. I hope you find it. These intermittent faults can be a pain. Best of luck, Andy __________________ Now where on earth did I remove that from?

28th Mar 2008, 8:31 pm	#8
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Help please oddball DAC90 fault Hi. The problem on both my DAC90's was with the I.F.T slugs breaking away from their brass formers. It really was a pain to superglue the fragments of the slugs together again (luckily the breaks were fairly straightforward, and not any worse). Another strange fault that I had was the cathode resistor of the CL33 output valve changing value when the set was switched on (its value was ok when "cold"). I have never experienced this problem in any other radio - I replaced the resistor and it seemed to work normally again. Regards Mike