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Author Topic: "Reforming" FL-2100B capacitors  (Read 4982 times)

Posts: 293

« on: January 09, 2003, 07:12:03 PM »

Well, I finally powered up the FL-2100B that belonged to my Dad, that hasn't been powered on in 20 years, in series with a 100W light bulb. The plate voltage came up to 1200V, the filiments lit, there are no lethal voltages presenton the caase, and nothing blew up, so that's cool.

What's be best way to baby the capacitors? Should I run the caps up to part voltage for short periods then turn off, full voltage for short periods then turn off, leave on at part voltage for a few days, or leave it on at full voltage for a few days?

One more problem is that, during shipping, one of the 572Bs was damaged in such a way that the glass envelope became loose in the socket. (I know you should always remove the finals when shipping, but in this case Dad' widow hired a moving gang to toss the stuff in boxes and send it via UPS.) I cemented the glass back into the socket with high-temp ceramic adhesive. The tube did not lose its vacuum (else it would have blackened) and the filament lights OK. My question is, if the grid is open, what happens to the plate current? I'm thinking off all the bad things that could happen with an open grid in one of the tubes - like all of the plate current being dissipated in the bad tube's plate? Well if the grid is open the tube is bad anyway, noharn doe, except I don't want an explosion on my hands.

Unfortunately the shack is not wired for 220. It will be interesting to see if the current draw at 117 is too much, even when running the amp at very low drive power.

If you've restored an old 2100B post your experiences and thanks . . .

Posts: 293

« Reply #1 on: January 17, 2003, 03:58:04 PM »

Well, I've applied full voltage in standby mode now a couple of times, each for about 30 sec. The first the amp made a couple of clicks, not too loud, and the HV dipped momentarily. The second time nothing happened, maybe whatever caused it to arc got blasted out of the way. I'll have to inspect it to see if I can see signs of where it arced.

I don't like loud noises so I'm a little afraid to just leave it on. I've been around when a big cap blows. OK well it was our college radio mad scientist putting 117VAC into at 50000uf, low voltage, beer-can sized electrolytic. I'm sure the student center is still a toxic waste area from the PCBs that flew eveywhere :-) The cans in the FL2100B are more modest. Would these things explode or just quietly leak I wonder.

Posts: 97

« Reply #2 on: January 27, 2003, 05:53:19 AM »

  IMHO, trying to "reform" caps is not only taking a chance but why would you want a cap that old thas been sitting for 20 years?
  I know some try to "reform" caps, some are sucessful..some are'nt..but not worth the time and stress when you can just put new ones in.
  Old caps can short without appears they may have already given you a warning..: )
  Or there may be other issues.
  Bad/resistors caps can also take out transformers or other components.
  I don't know what the specs for the caps are, but try to get as close to spec as you can.
  You might try Harbach Electronics.
  I know they deal in Heathkit SB series amps. They may have something or can refer you to where you can get the parts you need.
  As far as the tube, depending on the tester, it will tell you what sections are good.
  Word of caution. I don't know your experience. You are dealing with lethal voltages. If you are not experienced in working high voltage and equiptment of this type, I STRONGLY recommend you seek help from a competent and qualified technician. Nothing wrong with knowing ones limitations.
  There is enough voltage and amperage on those plates to knock a couple of horses off their feet. Nothing to play around with if you don't know what your doing.

Posts: 2198

« Reply #3 on: January 29, 2003, 09:08:06 PM »

For one thing, pull the tubes first, and then after re-forming the caps (even relatively new caps can fail for unknown reasons...)  proceed cautiously.

    I don't know if you meant thea the HV is dead completely, or if it's back to normal.  If the VV is NOT at normal, but not completely dead, turn odff the amp, pull the plug, move the plug AWAY from the A.C. socket (these things sometimes have a mind of their own)  and wait about a half hour.  (I'm being conservative, but safe.)
  Then, dig into the amplifier, and carefully short out all of the electrolytic capacitors (not just those in the HV supply; even the bias supply could have quite a kick, and if the bleeder resistors have gone bad, there could still be a hefty charge!)
    Then check across each HV cap for a high resistance; anywhere from about 47K and about 100 K, (I'm not familiar with the actual circuit) these readings should be the same in either direction.  If not, the cap is probably bad.  If you get infinite resistance, then that bleeder resistor is gone, and needs to be replaced.  Also, if any of the resistances across any one capacitor is much different than the other resistor values, that resistor also needs to be replaced; it might be prudent to replace the whole bleeder string.

    Finally, check the rectifiers; there should be conduction in one direction only; reversing the ohm meter leads should have a very high resistance.  Since this design probably had equalizing resistors across the diodes, these should have a high resistance corresponding to  their marked value in the reverse direction of the diode conductance.
    Also, do not try to replace a string of resistors with a single unit.  Resistors have a voltage rating, as well as power and resistance ratings. Only replace single resistors with single resistors.  (I.e., if a capacitor rated say, 450 volts is bridged by two 100K resistors, don't try to replace the two resistors with a single 200K resistor!  That's because the original's had a voltage rating of only 250 volts, and you'd like have failure if you replaced it with a single resistor.
    Good luck, and keep us posted as to your progress!
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