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Post by DavidR on Sept 3, 2019 18:12:13 GMT -5
I bought an old Sencore TC136 to check vacuum tubes. It's a basic tube checker and basically gives you how much life is left in the tube, or not left.
I found a website that has info on tube checkers and it had a nice write-up about my model and how to restore/repair. It has 5 capacitors. Three are the type that don't have to be changed due to age.
Of the two that should be changed are an AJAX 20MFD (uF) DRY ELECTROLYTIC 100VDC 85'C and has a (+) on one end telling me its a polarized cap.
The other is labeled: STANDARD .1MFD 200VDC. There is no indication of it being polarized. No other info.
I'm having a hard time find a 20MFD that has a tolerance better than -20% / +70%
My question is can I use polypropylene non-polar film caps (crossover caps) ?
Dayton has NON-POLAR 20MFD in 5% and 1% in 250VDC
and
NON-POLAR 0.1MFD 250VDC in 5% and 1% in 250VDC or 400VDC Film and Foil
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Post by donh50 on Sept 4, 2019 14:12:58 GMT -5
Yes you can use non-polarized caps. They may be larger and cost significantly more but I doubt the circuit will care.
The tolerance for electrolytic caps is usually spec'd that way to meet their lifetime specs; they lose capacity over time. You can buy electrolytics with tighter tolerance and higher temperature rating (105 degC is the typical next step up) from places like Digikey. I did not do a search for your specific cap, however.
Those old testers tended to be shock hazards so be careful working on and using it.
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KeithL
Administrator 
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Post by KeithL on Sept 4, 2019 15:11:07 GMT -5
In general you can always substitute a film cap for an electrolytic.
It's also worth pointing out, however, that a spec like -20%/+70% is pretty typical for an electrolytic capacitor... especially one not in a crossover. So, the original cap probably had a similar tolerance, and replacing it with a new one of similar tolerance would be just fine.... In that application, as long as the value is over the minimum, a little extra usually really doesn't matter.
Odds are a good quality film capacitor will have a longer service life - but you'll be spending a lot more for a higher quality part that won't actually work any better.
In something like a crossover, where the capacitor value helps determine the crossover point, tolerance is somewhat critical.
In a power supply, or as a cathode bypass capacitor, as long as the value is above the minimum, going significantly above that value (within reason) usually doesn't make much difference. Also bear in mind that, if that specific value is difficult to find, a 30 uF capacitor at the same voltage rating will also fall within that range just fine. (It looks like that one is a cathode bypass capacitor.)
I bought an old Sencore TC136 to check vacuum tubes. It's a basic tube checker and basically gives you how much life is left in the tube, or not left. I found a website that has info on tube checkers and it had a nice write-up about my model and how to restore/repair. It has 5 capacitors. Three are the type that don't have to be changed due to age.
Of the two that should be changed are an AJAX 20MFD (uF) DRY ELECTROLYTIC 100VDC 85'C and has a (+) on one end telling me its a polarized cap.
The other is labeled: STANDARD .1MFD 200VDC. There is no indication of it being polarized. No other info.
I'm having a hard time find a 20MFD that has a tolerance better than -20% / +70% My question is can I use polypropylene non-polar film caps (crossover caps) ?
Dayton has NON-POLAR 20MFD in 5% and 1% in 250VDC
and
NON-POLAR 0.1MFD 250VDC in 5% and 1% in 250VDC or 400VDC Film and Foil
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Post by DavidR on Sept 4, 2019 16:46:28 GMT -5
I was pretty sure I could go with film caps. Cost isn't a big issue.
So a non-polar cap would work; even for the 20uF with the + on one end? That is what I'm unsure of as voltage could go both ways with a non-polar.
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Post by donh50 on Sept 5, 2019 13:34:18 GMT -5
I was pretty sure I could go with film caps. Cost isn't a big issue. So a non-polar cap would work; even for the 20uF with the + on one end? That is what I'm unsure of as voltage could go both ways with a non-polar. Yes it will work fine. Polarized capacitors will have problems if the voltage across them is reversed; non-polar capacitors will not. So you can substitute a non-polar for a polarized without issue but not the other way around (in general you cannot replaced a non-polar cap with an electrolytic or other polarized cap when the signal is bidirectional). |
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Post by DavidR on Sept 5, 2019 18:32:16 GMT -5
I was pretty sure I could go with film caps. Cost isn't a big issue. So a non-polar cap would work; even for the 20uF with the + on one end? That is what I'm unsure of as voltage could go both ways with a non-polar. Yes it will work fine. Polarized capacitors will have problems if the voltage across them is reversed; non-polar capacitors will not. So you can substitute a non-polar for a polarized without issue but not the other way around (in general you cannot replaced a non-polar cap with an electrolytic or other polarized cap when the signal is bidirectional). Thank you.
The voltage is way higher than what I need but it will fit. Using a film cap will eliminate my having to change it in future.
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Post by donh50 on Sept 5, 2019 19:25:29 GMT -5
Film caps fail too but are usually more reliable than electrolytics. That said, the cap you linked is rated to 105 degC and meets the other specs so I'd use it.
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Post by DavidR on Sept 21, 2019 18:14:27 GMT -5
I pulled the old caps this afternoon and installed the new ones.
The old ones were still in spec and fairly close. The new caps are a little closer in spec.
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Post by AudioHTIT on Sept 21, 2019 20:11:39 GMT -5
You’ve inspired me to try and fix a bouncy meter on my tube tester, which I think is actually the test switch.
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Post by DavidR on Sept 21, 2019 21:21:51 GMT -5
Be careful using deoxit. Some can damage plastic and many switches in old tube testers have several layers and have many sandwiched plastic spacers. I was lucky and ran across a nice write-up on repair of Sencore units.
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Post by vcautokid on Sept 22, 2019 5:54:36 GMT -5
Yeah I concur with Keith, you should be good. Plus old caps after while start having ESR go through the roof. And caps are cheap. Also changing the caps could prevent damage later.
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