Is it possible that an electrolytic capacitor can fail to withstand an operating voltage of approx. 350VDC and still check high resistance when discharged? Just curious.

I would say "yes".

The cap could be flashing over during the high voltage stress, but check "good" with an ohmmeter. Don't forget, when you check leakage, "bad" means that the needle on your ohmmeter moves AT ALL on its' highest range.

Lou

:Is it possible that an electrolytic capacitor can fail to withstand an operating voltage of approx. 350VDC and still check high resistance when discharged? Just curious.

What are you calling "high resistance" ?

The only acceptable reading when measuring cap leakage is "OPEN" or infinite.
You ought to never have any leakage in any caps at all.

That doesn't mean the radio won't work... just that "leakage" means it is leading to failure or mis-operation.

I think that he his referring to my "at all" test with paper and mica caps. To test them, if the meter moves up at all, it must fall back to exactly where it started. Electrolytics are permitted quite a bit more leakage than paper or mica caps, and since they are usually of a comparatively high value, they will deflect the meter noticably. While it is best that the meter fall back to exactly where it started, it can also fall back to the megohm region. Also, a VOM test isn't all that accurate, not only because it doesn't test a cap at its working voltage, but also because electrolytics can sometimes retain a semi-permanent voltage in the order of 1 volt or so, which will cause the meter to linger up a bit (indicating false leakage), or even deflect negatively.

An electrolytic can break down or have leakage under high voltage. Other types of capacitors can, too, which is why the best cap test is one at its working voltage (such as what a Solar cap analyzer does...but be careful not to grab the leads while making the test. YOUCH!). However, with electrolytics, leakage at operating voltage can also simply mean that the cap needs reforming. You can reform the cap by feeding it its operating voltage through a 5K to 10K resistor. Allow at least 15 minutes for current to drop. If the current doesn't drop evenutally, then the cap is most likely shot (or hasn't been used in half a century, and therefore needs one hefty reform......probably shot anyway).

Always be sure to observe correct polarity when testing electrolytics. Testing an electrolytic with the meter leads backwards yields a meaningless answer.

T.

:I think that he his referring to my "at all" test with paper and mica caps. To test them, if the meter moves up at all, it must fall back to exactly where it started. Electrolytics are permitted quite a bit more leakage than paper or mica caps, and since they are usually of a comparatively high value, they will deflect the meter noticably. While it is best that the meter fall back to exactly where it started, it can also fall back to the megohm region. Also, a VOM test isn't all that accurate, not only because it doesn't test a cap at its working voltage, but also because electrolytics can sometimes retain a semi-permanent voltage in the order of 1 volt or so, which will cause the meter to linger up a bit (indicating false leakage), or even deflect negatively.
:
:An electrolytic can break down or have leakage under high voltage. Other types of capacitors can, too, which is why the best cap test is one at its working voltage (such as what a Solar cap analyzer does...but be careful not to grab the leads while making the test. YOUCH!). However, with electrolytics, leakage at operating voltage can also simply mean that the cap needs reforming. You can reform the cap by feeding it its operating voltage through a 5K to 10K resistor. Allow at least 15 minutes for current to drop. If the current doesn't drop evenutally, then the cap is most likely shot (or hasn't been used in half a century, and therefore needs one hefty reform......probably shot anyway).
:
:Always be sure to observe correct polarity when testing electrolytics. Testing an electrolytic with the meter leads backwards yields a meaningless answer.
:
:T.

If an electrolytic cap is leaking, it will get at least warm, usually hot, and sometimes all over the inside of the box, whatever kind of box it is. You can measure the leakage current in circuit, and deduce the amount of the leakage from that.
Lewis

::I think that he his referring to my "at all" test with paper and mica caps. To test them, if the meter moves up at all, it must fall back to exactly where it started. Electrolytics are permitted quite a bit more leakage than paper or mica caps, and since they are usually of a comparatively high value, they will deflect the meter noticably. While it is best that the meter fall back to exactly where it started, it can also fall back to the megohm region. Also, a VOM test isn't all that accurate, not only because it doesn't test a cap at its working voltage, but also because electrolytics can sometimes retain a semi-permanent voltage in the order of 1 volt or so, which will cause the meter to linger up a bit (indicating false leakage), or even deflect negatively.
::
::An electrolytic can break down or have leakage under high voltage. Other types of capacitors can, too, which is why the best cap test is one at its working voltage (such as what a Solar cap analyzer does...but be careful not to grab the leads while making the test. YOUCH!). However, with electrolytics, leakage at operating voltage can also simply mean that the cap needs reforming. You can reform the cap by feeding it its operating voltage through a 5K to 10K resistor. Allow at least 15 minutes for current to drop. If the current doesn't drop evenutally, then the cap is most likely shot (or hasn't been used in half a century, and therefore needs one hefty reform......probably shot anyway).
::
::Always be sure to observe correct polarity when testing electrolytics. Testing an electrolytic with the meter leads backwards yields a meaningless answer.
::
::T.
:
:
:If an electrolytic cap is leaking, it will get at least warm, usually hot, and sometimes all over the inside of the box, whatever kind of box it is. You can measure the leakage current in circuit, and deduce the amount of the leakage from that.
:Lewis
:Ray Thanks lewis and Dermody, I'll follow up on your advice. I have also discovered that with nothing in circuit besides a known good electrolytic on the B+, the rectifier flashes over. Once this happens to a rectifier tube I assume that it is shot; What physically occurs inside the rect tube when this happens and does a momentary overcurrent pose a risk to a new $68 transformer? I have also checked for wiring errors and hidden shorts.

:::I think that he his referring to my "at all" test with paper and mica caps. To test them, if the meter moves up at all, it must fall back to exactly where it started. Electrolytics are permitted quite a bit more leakage than paper or mica caps, and since they are usually of a comparatively high value, they will deflect the meter noticably. While it is best that the meter fall back to exactly where it started, it can also fall back to the megohm region. Also, a VOM test isn't all that accurate, not only because it doesn't test a cap at its working voltage, but also because electrolytics can sometimes retain a semi-permanent voltage in the order of 1 volt or so, which will cause the meter to linger up a bit (indicating false leakage), or even deflect negatively.
:::
:::An electrolytic can break down or have leakage under high voltage. Other types of capacitors can, too, which is why the best cap test is one at its working voltage (such as what a Solar cap analyzer does...but be careful not to grab the leads while making the test. YOUCH!). However, with electrolytics, leakage at operating voltage can also simply mean that the cap needs reforming. You can reform the cap by feeding it its operating voltage through a 5K to 10K resistor. Allow at least 15 minutes for current to drop. If the current doesn't drop evenutally, then the cap is most likely shot (or hasn't been used in half a century, and therefore needs one hefty reform......probably shot anyway).
:::
:::Always be sure to observe correct polarity when testing electrolytics. Testing an electrolytic with the meter leads backwards yields a meaningless answer.
:::
:::T.
::
::
::If an electrolytic cap is leaking, it will get at least warm, usually hot, and sometimes all over the inside of the box, whatever kind of box it is. You can measure the leakage current in circuit, and deduce the amount of the leakage from that.
::Lewis
::Ray Thanks lewis and Dermody, I'll follow up on your advice. I have also discovered that with nothing in circuit besides a known good electrolytic on the B+, the rectifier flashes over. Once this happens to a rectifier tube I assume that it is shot; What physically occurs inside the rect tube when this happens and does a momentary overcurrent pose a risk to a new $68 transformer? I have also checked for wiring errors and hidden shorts.

Almost always, heat is what kills a transformer, and they heat up slowly. The tube may or may not be shot, but I am thinking the tube is bad before you put power to it. You might try putting a couple of cheap Radio Shack 1N4004, 5, or 6 diodes in the rectifier socket temporaraly, and see what happens then. A few cents worth of diodes will be better than a few dollars worth of tubes.
Lewis

If the tube is flashing over it seems there is a load greater than the tube was designed to handle.

Sometimes if the 1st electrolytic filter cap is much larger than the the original circuit design it may exceed that specified in the tube manual for that rectifier.

If the cap is the correct size and it is good.
And the tube is known good too... and you still get flash over ... then it seems there is something in the filter or b+ circuit drawing too much current.

Definitely be careful with excessive electrolytic capacity when dealing with the 6X5.

T.

:Definitely be careful with excessive electrolytic capacity when dealing with the 6X5.
:
:T.
Thank you all for the advice. I had a 22uf cap ats the first filter instead of a 10uf as called for. Also, I had a 150uf cap after the 1k ohm field coil.I have ordered a new transformer for closer to the B+ voltage called for it is a 650 VAC CT (-325VDC B+).A 630 VAC CT would have been exact, but Antique Electronics did not have a 630... If 10VDC is too much difference,I'll select a dropping resistor. Any opinion about RCA 6T2 'Tombstone' AM 7 SW?

Definitely is good looking. Hopefully some others on here actually own one so they can give you actual user opinions.

Regarding your 1st electrolytic, 20 MFD isn't very excessive, though if you can get away with 10 MFD, then go for it. My Philco 60 uses 8 MFD caps (I believe...can't remember...I restuffed the caps back in 1995), and is entirely hum-free. If the radio you are working on uses the 6X5 rectifier, these are highly prone to flashing over (even with small capacity electrolytics). That they flash over doesn't mean that they have failed. They can flash over many times without failing. It isn't good for the tube, though, and if a cathode to plate short was to occur, it could damage the power transformer, and possibly even the field coil. Also, if damage is done to the cathode, and it shorts to the heater, that can damage your power transformer. A #47 pilot lamp or .25 to .5 ampere fuse on the power supply center tap will save the transformer. For some reason the 6X5 and 6X4 tubes really like to flash over. Other similar tubes, like the 35Z5 and 117Z6, rarely flash over, though I have come across a few 35Z5s that do flash on a somewhat regular basis. These tubes usually operate at lower voltages than the 6X5.

The best thing to do if you cannot avoid flash-over by reducing electrolytic size without sacrificing lack of hum is to avoid turing the radio off and then back on with the tubes still glowing. If the tubes aren't glowing, then the rectifier won't be able to conduct, and so it won't be surged. As the rectifier warms up slowly, it will slowly charge up the electrolytics and will slowly apply the radio's load to itself. This is why tubes like the 5Y3 and 80 are so much nicer, because their cathodes cool very quickly, and they are also not close spaced rectifiers, though they can flash over under extreme conditions. It isn't too common. One tube that is structurally similar to the 6X5 that is electrically pretty much an equivilant to the 5Y3 is the 5V4. Amazingly I haven't seen a 5V4 that will flash over even under extreme conditions (like 350 volts and a 50 MFD cap at the cathode).

T.