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 Post subject: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 4:32 am 
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Joined: Sep Mon 03, 2018 2:24 am
Posts: 464
Location: Fenton, MI
I operated an old Midwest B17 for a few hours after I received the radio. I noticed the power transformer operated fairly hot. I could not keep my hand on it. After I replaced the old electrolytic capacitors the transformer ran fairly cool. I could easily keep my hand on it.

The old electrolytic caps must have leaded a lot of current and ran hot themselves. It drew enough leakage current to have a marginal power transformer temperature condition. Interesting the radio did not have hum issues with the old capacitors.


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 7:24 am 
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Location: Victoria, Australia
The hum often comes later, often accompanied by wafting smoke, when you do that sort of thing: A bad practice that can have expensive ramifications.

Old electrolytic caps anywhere, along with waxed paper, & some oil filled type caps cannot be trusted.

Marc


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 11:47 am 
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Location: Long Island
There are some cases where you can still use old electrolytic capacitors; in fact, there are certain types used in commercial, test, and military equipment which have better specs than anything you can buy new today. Some can also be reformed successfully if the correct procedure is used. But it takes a bit of experience to recognize the ones which are likely to be usable from the ones which are likely to be junk. And even with the best caps under the best conditions, you cannot assume anything. If they are to be salvaged at all, old caps have to be tested and/or reformed on the bench, never in the set that uses them!

If you don't have a variable voltage DC power supply, low range milliammeter, and some suitable resistors to limit the current and protect the meter in case a cap breaks down, and the patience to use them, it's safer to replace old electrolytics with new ones than it is to run vintage equipment on wishful thinking.

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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 1:47 pm 
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Location: Victoria, Australia
Most of the Military grade caps do not appear in domestic radio's. Therefore without doing a preliminary inspection of any "new to you set" to assess its condition, powering is out of the question.

I will concede that some high grade caps appear indestructible. I have a NP 2uF cap in the PSU of a "Tube & circuit tester from 1938: This is one of the metal jacketed type often found in military stuff & I have another high current Plessey 5S10A: These two don't leak latter is rated at 1KV.

I have serviced a couple of early Midwests from the 30's & they had no, good wax paper caps, even that they in one, had been replaced before. They had an additional issue as some caps & resistors had "Micamold" on them: All of those caps were bad.

Some types of electrolytic's once left for along period, are a write off. NOS left for more than 2 years, really need to be reformed. The reformer I use (often as a low current PSU) can step from 25V to 400VDC, as said before, often, it was mainly built to catch suspect ones, which fail in service from new. It's regulator (LR8) will lock up on overload. I do on occasion power a B+ with cold heaters, with reformer to assess leakage, or find it.

I think Chris & I are on the same page. There is more than enough evidence on this site to say cold powering with exuberance can have expensive & undesirable consequences.

Marc


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 3:27 pm 
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Bottom line is you should never have applied power to that radio before replacing the electrolytics and critical paper caps. Unless of course, you like the smell of smoke, the sound of exploding electrolytics, and the price of having a transformer custom made to replace an unobtainable original part that burned up. If the caps fail badly enough, you can burn up the transformer in seconds, before you ever realize there's an issue, and before you have the chance to jerk the plug out of the power outlet.

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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 4:14 pm 
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Joined: Sep Mon 03, 2018 2:24 am
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Location: Fenton, MI
Mr. Detrola wrote:
Bottom line is you should never have applied power to that radio before replacing the electrolytics and critical paper caps. Unless of course, you like the smell of smoke, the sound of exploding electrolytics, and the price of having a transformer custom made to replace an unobtainable original part that burned up. If the caps fail badly enough, you can burn up the transformer in seconds, before you ever realize there's an issue, and before you have the chance to jerk the plug out of the power outlet.

Perhaps so, but it was used as a daily functioning radio the day I bought it.


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 4:23 pm 
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I think part of the issue is, that old radios usually are not fused. However you would need to select a fuse that will not pop on the tenth or twentieth turn on due to surge but will pop quickly before any real damage could occur. I was just working on a piece of sixties consumer gear. I tested all the caps removed or not. The generic brand multisection lytic had issues in all the sections, but the higher quality FP type lytic tested as good as new. This device also had some solid sealed lytics and of all of them, only one had questionable leakage or other parameters.


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 4:47 pm 
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allied333 wrote:
Perhaps so, but it was used as a daily functioning radio the day I bought it.

It may have been functioning, but the capacitor short had aleady developed as evidenced by the hot transformer. Hope you caught it in time.

I had an Eico audio amp where the transformer was previously stressed. Probably internal insulation was compromised due to heat or something else. Even with new capacitors, the transformer started smoking after about 20 hours of playing normally. Fuse did not blow.

So keep an eye on your radio. Hope it'll stay ok.

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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Thu 27, 2018 5:02 pm 
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Joined: Sep Mon 03, 2018 2:24 am
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Location: Fenton, MI
AJJ wrote:
allied333 wrote:
Perhaps so, but it was used as a daily functioning radio the day I bought it.

It may have been functioning, but the capacitor short had aleady developed as evidenced by the hot transformer. Hope you caught it in time.

I had an Eico audio amp where the transformer was previously stressed. Probably internal insulation was compromised due to heat or something else. Even with new capacitors, the transformer started smoking after about 20 hours of playing normally. Fuse did not blow.

So keep an eye on your radio. Hope it'll stay ok.

Operated for an hour then replaced all capacitors in the radio. That is why the transformer operates cool now. This Midwest has a fairly large transformer or quite a bit larger than my same power output Wards 62-402


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Fri 28, 2018 12:40 am 
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"Caveat Emptor" still applies with any radio new to you, that you have bought in. Over a period of decades I have seen incredible things done to radios, one involving a "stick welder", & loud speaker.

Just because it works does not mean that it is safe (for it, or you) and I have had radios alleged to work at a sale end up on my bench with dangerous faults and a cap fell out of one when the chassis was removed. The greatest achievement in recent times was a 'wedding present' radio from the 50's. It developed hum but was working, that in itself saved a lot of work. However, it had a figure eight cable of what appeared to be rubber. It was dumb luck, it never caught fire or electrocuted him unplugging it. It was no different than a lot of the degraded stuff we find inside them.

Some of those Midwests with lots of tubes needed a near welding transformer; just looking at filaments & heaters. One with two 5Y3's is 4A; One had four 6F6 that's 2.8A
Lets say 300mA for dial lights & the rest of the tubes (18tubes total) that would be close to another 4A+. So that's something around 50+ watts in room heating alone.

Marc


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Fri 28, 2018 12:43 am 
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Joined: Sep Mon 03, 2018 2:24 am
Posts: 464
Location: Fenton, MI
Marcc wrote:
"Caveat Emptor" still applies with any radio new to you, that you have bought in. Over a period of decades I have seen incredible things done to radios, one involving a "stick welder", & loud speaker.

Just because it works does not mean that it is safe (for it, or you) and I have had radios alleged to work at a sale end up on my bench with dangerous faults and a cap fell out of one when the chassis was removed. The greatest achievement in recent times was a 'wedding present' radio from the 50's. It developed hum but was working, that in itself saved a lot of work. However, it had a figure eight cable of what appeared to be rubber. It was dumb luck, it never caught fire or electrocuted him unplugging it. It was no different than a lot of the degraded stuff we find inside them.

Some of those Midwests with lots of tubes needed a near welding transformer; just looking at filaments & heaters. One with two 5Y3's is 4A; One had four 6F6 that's 2.8A
Lets say 300mA for dial lights & the rest of the tubes (18tubes total) that would be another to another 4A+. So that's something around 50+ watts in room heating alone.

Marc

My Midwest B17 is kind of a 'bad' boy. But so are those Zenith 15 tubers & Scott 20 tubers too.


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 Post subject: Re: Replace those old Power Supply Capacitors
PostPosted: Sep Fri 28, 2018 3:34 am 
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Location: Berkeley, CA 94709
Mr. Detrola wrote:
Bottom line is you should never have applied power to that radio before replacing the electrolytics and critical paper caps. Unless of course, you like the smell of smoke, the sound of exploding electrolytics, and the price of having a transformer custom made to replace an unobtainable original part that burned up. If the caps fail badly enough, you can burn up the transformer in seconds, before you ever realize there's an issue, and before you have the chance to jerk the plug out of the power outlet.


Yup. Caps are cheap. Power transformers are expensive.

Bob

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