Re-building backelite capacitors - Printable Version
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Re-building backelite capacitors - morzh - 11-18-2011
I just tried the recommended procedure by the "Philcorepairbench.com", and so far rebult two caps, 3615-J and 3615-L.
http://www.philcorepairbench.com/capbuild.htm
As promised, it was a snap, no fumes, not even slightest smell, very clean procedure.
The tar does not even start to melt, when it can be pushed out of the shell.
I use Milwaukee gun, and first I gently warm it on Low around for 2 minutes or so, and then I put the gun on Hi, and for 30 or so seconds from a very respectable distance (5-6 inches) warm it around, and then go back to Low, and in about 20 sec I push it out and it just flies out, leaving a very clean shell. The tar when picked up, is barely warm and even cool to the touch.
Instead of stiff wire to push the block out (wire may bend) I used the thinnest Allen wrench I could find, that actually fit through the central rivet, and as it is steel, it is as stiff as they come, and also dull at the end, so no puncturing occurs.
PS. I almost regretted this the caps inside look like new. All of them measured 0.1uF i/o 0.05uF, and I am not even sure it is aging - may be it was intentionally made higher.
With one cap the wire readily fell off the solder on the cap's side, the other was OK.
I am not sure what can go wrong with this caps actually, and rebuilding them only because everyone says they go bad after a while. They are actually working caps.
Re: Re-building backelite capacitors - Arran - 11-19-2011
morzh Wrote:I just tried the recommended procedure by the "Philcorepairbench.com", and so far rebult two caps, 3615-J and 3615-L.
http://www.philcorepairbench.com/capbuild.htm
As promised, it was a snap, no fumes, not even slightest smell, very clean procedure.
The tar does not even start to melt, when it can be pushed out of the shell.
I use Milwaukee gun, and first I gently warm it on Low around for 2 minutes or so, and then I put the gun on Hi, and for 30 or so seconds from a very respectable distance (5-6 inches) warm it around, and then go back to Low, and in about 20 sec I push it out and it just flies out, leaving a very clean shell. The tar when picked up, is barely warm and even cool to the touch.
Instead of stiff wire to push the block out (wire may bend) I used the thinnest Allen wrench I could find, that actually fit through the central rivet, and as it is steel, it is as stiff as they come, and also dull at the end, so no puncturing occurs.
PS. I almost regretted this the caps inside look like new. All of them measured 0.1uF i/o 0.05uF, and I am not even sure it is aging - may be it was intentionally made higher.
With one cap the wire readily fell off the solder on the cap's side, the other was OK.
I am not sure what can go wrong with this caps actually, and rebuilding them only because everyone says they go bad after a while. They are actually working caps.
They use a paper dielectric just like any other bypass or coupling capacitor from the 1920s through the 1950s and it's the paper itself that goes bad. Quite often paper caps will read the proper value on a straight capacitance test but once you start putting a voltage across them they will fail.
Regards
Arran
Re: Re-building backelite capacitors - morzh - 11-19-2011
Actually the third cap I replaced was showing (at 0.05uF nominal value) 0.27uF in one direction, and 0.37uF in another direction.
Sometimes it would show 0.1uF as the other two, but only in one direction.
Still, it worked
Re: Re-building backelite capacitors - morzh - 11-19-2011
Today finished re-stuffing all the bakelites.
When I replaced the last one, number 8, the one with 250 Ohm resistor and 0.05uF cap, all started sounding quite a bit better.
The cap was giving me a weird reading - it would start with 0.06uF on on polarity, then, if re-polarized, would read 0.370uF and when put back to the first polarity, read 0.13uF. If disconnected from the meter and given time to discharge, it would fall back to 0.06uF.
I had two caps acting in this way out of five of them. I am not sure of the electrical mechanism behind it, and it is not important. Just interesting.
Re: Re-building backelite capacitors - etech - 11-20-2011
morzh Wrote:Today finished re-stuffing all the bakelites.
When I replaced the last one, number 8, the one with 250 Ohm resistor and 0.05uF cap, all started sounding quite a bit better.
The cap was giving me a weird reading - it would start with 0.06uF on on polarity, then, if re-polarized, would read 0.370uF and when put back to the first polarity, read 0.13uF. If disconnected from the meter and given time to discharge, it would fall back to 0.06uF.
I had two caps acting in this way out of five of them. I am not sure of the electrical mechanism behind it, and it is not important. Just interesting.
This is called leakage. Probably all of the caps you replaced were at least somewhat leaky. When the caps are as bad as the ones you have described above, you can actually measure the leakage on an analogue vtvm or vom on the highest ohms scale. On the paper caps that are not as bad, you will still be able to measure the leakage on a good cap checker, like a Sprague Tel-Ohmike and others.
Ed
Re: Re-building backelite capacitors - morzh - 11-20-2011
I have Stanford Research SR720 LCR meter.
Though never measured those on it - I just used my Fluke 189 DMM.
It'd be interesting to see what leakage it shows.