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In Reply to: Is my old can cap O.K.? posted by Po on December 26, 2003 at 03:20:59:
Hello and welcome to the Forum. Working on these old amps is a learning proccess. I never went to school for this but have poked around them since age 11 (1961) Your post is a little unclear. You only reform a cap if its old and the unit has been sitting around.
Its not neccessary with new caps. Think of it as if you were get the amp new. Plug in and a way you go.As far reforming goes I was new to this. Picked up a Fisher 500c receiver this fall and was not sure how long it sat. Borrowed a Variac from a retired tech. Variacs allow you to bring up the voltage slowly. His instructions were to bring the unit to 50 volts and let it sit for 5 minutes. Then shut it off for a few hours. Then, bring it to 75 volts for about five minutes. Again, let sit for a few hours. Then fire it up at line voltage. You only need to do this with old caps--not new ones.
There are two schools of thought. Replace the caps or reform them. My Fisher 500c was OK. Turns out the previous owner was using it. There are a lot of posts on this subject. And, a lot of mods. I plan to put 10 ohm resistors on each output tube in place of the ground. They will act like a fuse if the tube shorts out protecting the transformer. Beware of the floating ground issue on vintgage amps. See the post done by "sgmlaw(A)" here on 10/18/03. I tested my 500 and it was OK. Still I replaced the line cord with a polarized three wire cord just in case this problem developes in the future. I plan to replace the coupling caps while I am in there. I trust you have a schematic for your amp. If not its a good idea.
Good luck with your amp. Sounds like you have some back ground knowledge of this stuff. They are really nice amps. My Fisher cleaned up well. Refinished the cabinet as well. Its a strong one. Just got to figure what to do with it. Too much stuff.
Regards,
Follow Ups:
My opinion is replace the B+ filter capacitors (cans). I replace all caps at 30 years of age just as a precaution even if the capacitors withstand the B+ voltage.Antique Electronic Supply (http://tubesandmore.com/) in Tempe, AZ has many parts to service older tube based equipment. My 2 cents.
Thank you, guys! This place is so cool, I've learn a lot just from reading the post.
When I was growing up, most of people have already listening
to C.D. players, but when I went to Hi-fi stores in Taiwan when I was in high school, I've seen new made tube stereo before( way too much for high school kids to even think about buying it).
Last year, it took 5 months for the local amp tech to change new tubes and a new input jack for my Marshll JTM45, so I thought I should learn how to change new tubes by myself, but I ended up re-build the whole thing, since it's so much fun!I got the Fisher KX-200 form a local shop( a high school dream came true). It's so dirty, I don't think it's been used for a long time. The can caps looks original, since they all have Fisher's part number on it. I don't have a Variac, so after I unplug the tubes and checked a few things, I just power it up. Maybe it's me who killed the can caps. Sorry, old can caps, I killed you.
Since I couldn't find 200mf/250v caps, so I replace with two 220mf/250v caps. Will the bigger caps give the power transformer more stress and kill the transformer in the future? Also, I'm still using the 40mf/500v cap which is in the can with one of the broken cap. Will the 40mf/500v cap be effected by the broken cap, since they are in the same can?
Thank you for helping!
Yu-Po
Replacing a cap with a larger one like you did is OK. Higher voltage caps are OK too. But never lower! Also, if a section or part of a can is bad, you can replace just the bad part. My good friend who was like a Father to me taught me that trick. Another trick is to make a diagram of where the wires go--even if you have a schematic. It makes replacement easier.
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