630v vs 400v & 200v caps in AA5 radios

3/19/2008 1:28:04 AMPeter G. Balazsy

(I've thought a bit about this question and and can't find any clear answer in my head.)

I have always so far only replaced old paper caps in any radio that I've re-capped with the 630v variety with the exception of a big bag of new-style film caps .05uf 400v that I had around here that I always use to replace the AVC cap because I know for sure the voltage is low.

Now the reason I bring up today's question is that I accidentally ( my stupidity) received a quantity of new mylar caps that are:
.01uf @ 200v
.002uf @ 200v
.005uf@ 200v
.005uf@ 400v

So I was trying to think of where in a typical AA5 radio that these caps should NOT be used.

It seems to be that the real critical place for only 630v caps would be:
1.) from the plate of the output tube to ground
2.) the AC power line coming in

Otherwise, to my reasoning, it seems safe to use these 200 & 400volt caps for the coupling cap from the volume pot to the grid of the detector and other grid input points.... perhaps even the grid intput to the audio amp stage.

However it has also been my experience that the caps that are typically found to be failing in old AA5 radios are always the coupling cap to the audio tube and the volume control cap.

So why is that? Where, if so, is the higher voltage that may cause failures for those caps coming from?

And can I safely easily use a 400v or 200v cap there?

3/20/2008 12:28:40 AMThomas Dermody

Mylar isn't as likely to fail as paper. The caps in old radios usually become leaky, which is only because paper is porous, and can retain water and minerals.

3/20/2008 1:38:05 AMPeter G. Balazsy

Thanks Thomas... yeah that seems to be the right answer ... from anyone else over at arf too... and the more I think about it ... that's certainly all it is with those old paper caps...mostly. "leakage"... not the voltages applied per se.
Esp to the vol control cap and audio coupling cap for instance...
thanks..

3/20/2008 7:43:14 AMRadiodoc

Peter,

And... I am pretty sure the manufacturer of the caps did not envision the caps needing to last 70 years or so.

Radiodoc
************

:Thanks Thomas... yeah that seems to be the right answer ... from anyone else over at arf too... and the more I think about it ... that's certainly all it is with those old paper caps...mostly. "leakage"... not the voltages applied per se.
:Esp to the vol control cap and audio coupling cap for instance...
:thanks..

3/20/2008 10:53:56 AMEdd

I also heartily concur on the sleazy characteristics of paper dielectric materiel…..along with its potential inner contained contaminants. In a most recent evaluation of an AA-5 in restoration for a pulled out of the attic restoration for a little ole “granny goodknockers” old time acquaintance; I initially used my time proven, simple capacitor testing technique of floating all paper capacitors by one lead with the lead having B+ on it being left connected in circuit. Then the hooking of a high impedance DC metering mode between the floating lead and B- and then you power up the set and take a test lead and short across the metering leads such that the cap gets charged up immediately and then you look at metering to see if ANY DC voltage shows up….with a premium capacitor only leaking thru, well down in the ~20-50 millivolt levels. On stages where the DC voltage supply level connection is minimal, the set is then powered down and those caps get fed their high side DC voltage from an Eico 950’s DC supply…. ( I still like to see analog metering versus an eye tube.)

Of the 6 paper capacitors being used in that unit……leakages were:

.1 ufd / 400WV-------------110 millivolts leakage (AC Line Capacitor)

.05 ufd /200WV-------------6.6 Volts leakage

.001 ufd / 400WV----------16 Volts leakage

.0022 ufd /400WV---------1.2 Volts leakage

.05 ufd / 200WV------------66 Volts leakage

.0047 ufd /200WV---------9.9 Volts leakage

Need I say that those conditions could cause some SERIOUS performance degradation along with bias transformations?

73's de Edd

3/20/2008 12:06:27 PMRadiodoc

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:I also heartily concur on the sleazy characteristics of paper dielectric materiel…..along with its potential inner contained contaminants. In a most recent evaluation of an AA-5 in restoration for a pulled out of the attic restoration for a little ole “granny goodknockers” old time acquaintance; I initially used my time proven, simple capacitor testing technique of floating all paper capacitors by one lead with the lead having B+ on it being left connected in circuit. Then the hooking of a high impedance DC metering mode between the floating lead and B- and then you power up the set and take a test lead and short across the metering leads such that the cap gets charged up immediately and then you look at metering to see if ANY DC voltage shows up….with a premium capacitor only leaking thru, well down in the ~20-50 millivolt levels. On stages where the DC voltage supply level connection is minimal, the set is then powered down and those caps get fed their high side DC voltage from an Eico 950’s DC supply…. ( I still like to see analog metering versus an eye tube.)
:
:Of the 6 paper capacitors being used in that unit……leakages were:
:
:
:
: .1 ufd / 400WV-------------110 millivolts leakage (AC Line Capacitor)
:
:.05 ufd /200WV-------------6.6 Volts leakage
:
:.001 ufd / 400WV----------16 Volts leakage
:
:.0022 ufd /400WV---------1.2 Volts leakage
:
:.05 ufd / 200WV------------66 Volts leakage
:
:.0047 ufd /200WV---------9.9 Volts leakage
:
:
:Need I say that those conditions could cause some SERIOUS performance degradation along with bias transformations?
:
:
:73's de Edd
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Edd,

I was beginning to think I was the only one left that used that simple condenser test. Right on...

Radiodoc
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3/20/2008 6:30:46 PMPeter G. Balazsy

But why bother testing them at all?
The cost to replace them is sooooo little .
....and it's far less than the time than it takes to test them...right?

I have only one EASY capacitor test.

Q. Is it an old paper cap?
A. Yes = Replace them!

anything else is futile... so why bother?

3/20/2008 9:16:08 PMRadiodoc

Peter,

After several decades of repairing (not necessarily restoring) electronic equipment, old habits die hard I guess. It was a quick check (the method Edd described) to see if a suspected cap was leaky causing the malfunction. If the cap was leaky, then one lead had already been disconnected.

Radiodoc
************

:But why bother testing them at all?
:The cost to replace them is sooooo little .
:....and it's far less than the time than it takes to test them...right?
:
:I have only one EASY capacitor test.
:
:Q. Is it an old paper cap?
:A. Yes = Replace them!
:
:anything else is futile... so why bother?

3/20/2008 11:55:17 PMThomas Dermody

Usually I don't test them anymore, except to confirm problems, or if I don't have a lot of capacitors at hand, and don't want to replace all of them in a set. Back when I was learning a lot, though, it taught me quite a bit....why certain problems occur (for those who choose not to replace all capacitors beforehand). I think that it's somewhat bad practice to just fire up a set with the old parts still in place, so now I either just replace the parts or test the ones under the most stress. My 1940 Silvertone radio recorder still has almost all of its original caps, and is working very well, though most of my other radios have at least the critical caps replaced. Some, like the ones 'bypassing' low value cathode bias resistors, can have quite a bit of leakage (typically in the millions or thousands of ohms) without causing any real trouble, so I don't usually replace them. I have replaced some fairly leaky ones to see if there was any difference, and there almost never was. For absolutely reliable performance, however, all new metalized film caps is the way to go.

3/21/2008 7:52:52 AMBill G.

I have found it interesting just to see how bad they are. I ran into an odd one about a year ago, a 0. MFD 200 volt paper wax capacitor in a radio from the 1930's and it was good! I can't understand it. It is the only good one of that vintage I have found. It tested identically to the new one I was going to put in.
It was like finding a 70 year old puppy. How can that be?

Best Regards,

Bill Grimm

3/21/2008 6:23:32 PMPeter G. Balazsy

a 0. MFD ??