Old Radio Trips Breaker After Recap

bodonnell

Member
2015-08-18 3:54 am
Hello Everyone,

I have been doing a recap on an old Pilotuner T601 FM Tube Tuner from 1945-ish. There was a multi-electrolytic cap that I replaced with separates. There were 4 caps that I replaced it with, original 40uF, 40, 20 and 2, I put in 39uF, 39, 22 and 2 all with higher voltages. Once I changed them, and the bad tubes, the radio powered on nicely and tuned in stations quite well, it basically seemed fixed. Unfortunately after about 5+ minutes it tripped the breaker in my room. Even switched off, plugged in, it trips the breaker after about 5 minutes. It had a selenium rectifier that I changed out with a 1N4007 and resistor, hoping to solve the problem, but it's still tripping the breaker after a few minutes. There are a lot of paper caps that I am also planning to change out, but just looking for a solution to this problem first. Thanks for the input!

-Brad
 
Check the switch. Check the components (and wiring) which are still connected to the mains when the switch is off - there won't be many, in some cases none at all!

A mains filter cap could be the culprit. Old sets just used a paper capacitor (which often explodes); these days you must use an X-rated cap which is guaranteed to fail safely.
 

bodonnell

Member
2015-08-18 3:54 am
I am not sure if there is too much current or not. I think I have an in line power meter I can hook up.

It does have a power transformer. The radio is set up with 2 wires, no ground, I have not converted it.

The switch is something that I have been wondering about, the radio powers off when turned off, but I have a feeling that its still letting some power through. I changed what I though were all the filter caps, but I might as well change out all the paper caps. There is one .05UF paper cap on the switch, maybe that has some issues.
 
There is one .05UF paper cap on the switch, maybe that has some issues.
Agree it's a big suspect.

Hope you replaced main filter caps witrh modern, fresh ones .... this is the place where avoiding NOS makes sense.

In any case, go step by step: first connect *only* PT primary, with all secondary wires unhooked and taped for safety ... then add filaments .... then rectifier but not filter caps ... and so on.

Measure current at every step and let it "burn in" a at least 1/2 hour each time.

EDIT:I guess DF96 means changes around the RF area might detune it, and recalibrating, besides being a PITA, requires specialized equipment which is hard to find today at a Tech's bench.
Barely remember how to "align" an AM radio, haven't done so for 45 or 50 years, might do it "by ear" , but on FM? ... forget it.
 
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bodonnell

Member
2015-08-18 3:54 am
That is the one I mean.

I replaced the 0.05 and its been running for 15 minutes no problems! I was planning on changing that, as it was a paper cap, but I am very happy to see a problem and fix it. If I had just replaced them all at the same time I would never have known...

What was going on to make it trip the breaker, even off, after 5 minutes, if you don't mind giving an explanation?

Thanks DF96!!!
 
bodonnell said:
Could you expand on that idea just a little bit more?
In the RF and IF section the exact value of a capacitor matters, so you can't always replace a '500pF' cap with another '500pF' cap, as the first one might really be 485pF and the second one might be 512pF. Fortunately, this doesn't matter as these caps rarely give trouble so there is no reason to replace them unless faulty. Actually, there is rarely a reason to replace the audio section caps unles faulty. Some of the higher value caps in the RF section (used for local decoupling) may need to replaced if faulty - but only if you can read a circuit diagram and so know which are decoupling caps.

In the FM front end even the exact placement of a cap may matter so don't touch anything there unless you have the knowledge and test equipment to realign it after repair.

What was going on to make it trip the breaker, even off, after 5 minutes, if you don't mind giving an explanation?
As it tripped even when switched off it must have been on the live side of the switch. It should have been on the switched side of the switch. Why it waited a while before tripping is less clear. Could be that the leakage current built up as the cap heats up. Is the cap across live-neutral, or live-ground? If live-ground then best to remove it.
 

bodonnell

Member
2015-08-18 3:54 am
Actually, there is rarely a reason to replace the audio section caps unles faulty.

The radio works great now, but there are still about 5 paper caps, which I assume are in the rf sections. Do you think its ok to leave those, I thought they were a fire risk, but it sounds like they will probably mess up the tuning if I change them out. Thoughts?

Service Manual - http://www.nostalgiaair.org/PagesByModel/587/M0014587.pdf
 
Only caps across the mains are a likely fire risk.

Caps which you could replace without too much risk of problems are C6, C18, C19 and C20. Replace with the same value as the original and don't disturb anything else when you do it. Personally, I would leave them unless they are faulty: either lost value or becoming leaky.

Other decoupling caps (e.g. C4, C17) only have low voltages across them so it doesn't matter if they become leaky. Don't touch anything around the first 6BA6 and 6BE6.
 
Well I still have problems.... After running the radio for a few hours straight, the next day I put it back in its case, thinking I was 100% done, and the breaker tripped while the unit was off. So I started prodding around and noticed that there is 40VAC coming off of the antenna line #2 terminal on the back of the unit. This connects to 120VAC from the power cord through C25. I would have to assume that means C25 has gone bad. Because it can't pass VAC right? Its a very interesting looking cap, looks like a skinny yellow lego piece. C25 is rated on the service manual as 10uuF.... what in the world is that, a mistake? There are a ton of uff's in the service manual though....

If I do need to replace this cap what should I replace it with?

Apparently you connect terminal 2 to terminal 1 in order to use the internal antenna and just leave terminal 2 alone if you have an external antenna.... so in theory I don't even need to use the circuit, but it would be good to fix it, especially if it's causing the breaker to trip.

Any Thoughts?

-Brad

The cap in question looks like M4 in this picture (except yellow): http://www.radiomuseum.org/forumdata/users/6435/capacitors/pic3.jpg

The cap is yellow with 6 circles on top three are black and the other three appear to not have anything (its very very hard to see the cap its sandwiched in between to many parts)

B - Black Dot
N - No Color and or too hard to see

B N N
B B N
Power from outlet ------ Cap ------- wire to terminal 2 (antenna)
 
'uuF' is an old-fashioned American way of referring to 'pF'.

C25 is a 10pF cap which seems intended to allow the incoming mains lead to be used as a VHF antenna in the absence of a proper antenna. Daft idea. Remove the cap.

I have just noticed C28, 10nF from mains to chassis. Remove this cap. It serves no useful purpose and is potentially dangerous. If present, it must be a Y-rated cap (like an X-rated cap, but extra safe because your life could depend on it).
 

bodonnell

Member
2015-08-18 3:54 am
I have just noticed C28, 10nF from mains to chassis. Remove this cap. It serves no useful purpose and is potentially dangerous.

Thank you. Looks like that cap was never even installed. I removed the other cap, no telling if it fixed the problem, only time will reveal the truth. Thanks again for the help. Neat radio, but sounds like a questionably designed death trap.... I guess they never intended people to use them in 70 years....
 

bodonnell

Member
2015-08-18 3:54 am
Well it was working great outside the wood case, I had it running for about 2+ hrs, I put it back in the case tuned to a different channel and 3 minutes went by and breaker tripped.... I think I'm going to just give up on this one.... but its hard because I'm so close.... I doubt the wood case is having an effect and I'm all out of ideas.