[kbm37-630]

Hello,

Here come the first round of questions from the newbie. I have schematics, parts locator and parts list my new 1937 Philco. Verified all tubes are the correct tubes and functioning. Radio plays on all 3 bands, but read that the very first thing to do is replace all caps.

1} Should I replace all caps including the mica caps ?

2} Capacitance values are listed on the parts list, but not voltage ratings. How do I know what voltage to get ? The electrolytic Radial caps currently installed show a voltage of 450V is that correct ?

3} How can you determine the voltage of all other caps without removing them ( I can't see the V rating on them }, I would like to order all of them prior to removing any of them and then replace them one at a time ( carefully ).

Would appreciate any input on a logical way to go about it......

Who makes a good quality Cap ?, etc.

Thanks,

Kurt

[codefox1]

A lot of us support this hobby friendly source Just make sure you make a complete list with a couple of spares, and mind you the smaller carbon resistors in the set should probably be replaced as well. Makes a huge difference when you get it together and perform an alignment. If you're really doing a restoration, go for it, it's only a few dollars and a couple of more hours at the bench.

http://www.justradios.com/capacitors.html

Jameco, Mouser, Digikey also fine sources, and our colleagues will provide other sources.

[37silverstreak]

Hi Kurt.
It's Best to replace all the Electrolytics and cardboard tubular caps. The Mica caps are generally okay. I've gotten most of my Capacitors from "Just Radio's" they are great to deal with and has a good selection. You can buy capacitor kit's from them as well. that will give you a good selection of the sizes most often used in pre-war radios. I use the metalized polypropylene caps with a 630V rating. you should always use a voltage rating either equal too or greater than the original, never smaller. Also when replacing the electrolytics remember to observe the correct polarity when installing. Installing an electrolytic cap backwards can turn it into a firecracker !!!!!!!! Regular caps can be installed either way.
Kevin

[Arran]

If it's working on all three bands then the mica capacitors are more then likely fine. Actually if it seems to be working properly someone may have already overhauled the set and replaced the paper caps as well as the electrolytic filter ones, it may be worth checking to see if someone restuffed the bakelite block condensers. If the filter caps have a 450 VDC rating that is more then enough, if they look reasonably new, like a sealed aluminum can with a shrink wrap label, or have a date code that indicates they were installed in the last 20 years they will probably be fine as long as the set has been powered up on occasion since they were installed. You could replace them if it worries you but that's up to you.
I've never had an electrolytic short in an operating set, all of the ones that shorted out were from cold starting a long disused set without using a variac or a dim bulb tester to slowly power them up. Tha being said I wouldn't trust a 50+ year old filter cap, definately wouldn't trust a 70+ year old cap of any kind.
Regards
Arran

[kbm37-630]

Hello,

It's the newbie again.

If you can not find the exact value capacitor or resistor is it OK to use a different value that is within less say 5% to 10% of the specified value. I understand that voltage rating must equal or exceed the specified values..

For example if you need a 50K ohm 1/2 watt can you use a 47.5K to 52.5K 1/2 watt resistor, within +/- 5% ?

I believe I read that caps can vary by 20% of specified value, is this correct? ? If so, does in hold true for all caps electrolytic or non-electrolytic ?

What do you do in a case such as " .015 - .015 mfd - double bakelite ?

Kurt

[codefox1]

Tolerence question yes. Make up dual sections and odd caps out of discrete ones and mind dress lead carefully.

[exray]

For example if you need a 50K ohm 1/2 watt can you use a 47.5K to 52.5K 1/2 watt resistor, within +/- 5% ?

47k is the normal replacement value for an older 50k.

I believe I read that caps can vary by 20% of specified value, is this correct? ? If so, does in hold true for all caps electrolytic or non-electrolytic ?

More or less. Old electrolytics sometimes had a wide range, as much as -50 to +100%. That doesn't imply that you should use a replacement that far off but the standard values, just like the resistors, are different these days. Use the closest available value.

What do you do in a case such as " .015 - .015 mfd - double bakelite ?

.015 can be found. This may be a loaded question in this example if that is the AC line bypass cap. They were often a dual .015. There's a special type of cap that is preferred in AC-line bypassing nowadays.

No need to get nutty trying to achieve the exact values of the old components. If you're within 20% you'll almost always be within spec.

-Bill

[kbm37-630]

Bill (exray),

I was fairly confidant that the closest value would be acceptable. Thanks for the hand holding, I am new at this and didn't want to make a incorrect assumption.

Looking at the schematic it appears that the .015 - .015 Double Bakelite caps are in the AC circuit. Shows that it leads to the on/off switch...
The on/off switch is intermittent.. always will turn on , but sometimes will not turn off, my first thought is that the tone control/Ac switch is defective, could the issue be with one or both of the .015 caps ?

Kurt

[rghines1]

Not the caps. Probably the grease in the switch has petrified. Try shooting some WD40 inside the assembly.

So many questions but keep asking away.

Richard

[exray]

Richard is correct. Those switches do get gummy and WD40 is good for dissolving the grease.

More on the .015 caps. The preferred 'modern' replacements are AC-rated caps since its an AC circuit! Justradios sells them under the name of "safety caps" . Its not a must to use them but they do have some worthwhile advantages. He has a page on his site explaining all that. You can get them at Mouser as well. In the old days they used over-rated DC voltage caps but when a little leakage sets in they tend to heat up and pop with spectacular results Their purpose is simply to bleed off some of the interference that may exist on the incoming power as well as bleeding off any rf from the radio that may bleed backwards. In some cases they aid the RF circuitry in finding a ground path back thru the power neutral. Value is not critical. You can use plain jane 630 vdc yellow caps but theres nothing to lose by spending a few cents more on the 'better mousetrap'.

[kbm37-630]

Hello,

Last question on Caps, I promise. The electrolytic caps currently installed are Aerovox 20uF 450 vdc ( They say Dry , labels are paper black and yellow ) don't know how old they are. Would like to replace but can only find one the same diameter as my clamps 1 3/8", but value is 250uf @ 500vdc. The only other ones that will fit are multi section at 32uf/32uf @ 500 vdc. They are pretty close in price.... can you use the multi section only using one leg ?

Kurt

[exray]

Those sound expen$ive. What many people do is leave the existing cans in place and wire in the new caps underneath mounted on a terminal strip. The radio would work fine with 10 or 22uf @ 450v. 33 is pushing the limit for the rectifier's surge current rating. It work probably *work* but the rectifier won't be real happy about it. And yes, you could use one section of a dual electrolytic.

Note carefully how the cans are isolated from the chassis via a cardboard tube and note which one is connected with positive to chassis.