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Hi everyone:
I just got a old dirty beat up Fisher KX-200. The resistor right after the B+ voltage doubler was open, so I replace it and turn the amp on( my first time to have a vintage stereo, so I don't know much and not very smart). I was lucky, the amp work ok. However, I could not get the the B+ voltage high enough( it's about 200v-300v too low). Then, I found out that the two can caps right after the voltage doubler and next to hot hot power resistor have problem. They both are 200mf/250v, and one of the cap is in the same can with a 40mf/500v. The one is with 40mf/500 has only 20nf left( using a multimeter), and when I use a 9v battery to test it, the voltage drop very fast. Ther other cap have 120mf left, and it does not drop the voltage much from the battery.
I replaceed the caps( not original) with two 220mf/250v, now the B+ goes up to about what the schematic says.
I did not know much reforming the cap, since I'm very new with the vintage stereo. It's my old caps not reform yet, or they just not working any more? It's there a way to find out?
Follow Ups:
Yu-PO,If the value(microfarads)of a cap has dropped,reforming will not save it.REforming affects a cap's leakage current at working voltage only.In addition to the caps in your amp having dropped in value,they were clearly *leaking* excessively to have dragged the B+ voltage down so much.You did the right thing by replacing those sections.I just received a 500C that had 3 can caps completely dead. Dead, unreformable caps will become the norm pretty soon especially where high surge voltages are found in poweramps.I'm finding this more often then ever.
It's been suggested recently that caps in a voltage doubler circuit should have a higher DC working voltage than the 50 volt surge buffer used in most vintage gear.Your amp runs at 440 B+ so Fisher used 250V sections.Current wisdom dictates that those values be increased to 350-400 vdc per section.
I agree on voltage doubler replacement caps with higher than stock voltage ratings.Voltage doublers demand higher capacitor ripple current ratings & the higher voltage caps usually have higher ripple current ratings in the same UF (capacitance) rating.
Reforming is not an antidote to old cap especially those leaky ones. I observed that for cap with ESR not too far away from spec, starting up with a variac would help to decrease this value to around 1 ohm. Electrolytic caps are generally designed for 1000 hours, guess what? and some even new ones of lower mF are of 3000 hours at 105 C. You would get more hours assuming every 10 degree lower will double the life(law of chemistry)That is the problem/concern in playing vintage amp that bother me a bit. It seems that AES has got the solution in providing new production can capacitor albeit at higher cost.
Just for interest, if cap explode(it happened to one of my amp) and someone get hurt, who's liability; the seller's negligence or the buyer, I think Ebay seller should have disclaimer on this matter. Also, anyone got sued under similar circumstance? There is even some confirmation from some sellers that my amp would last for another 40 years.(may be even for another 40 hours)
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,
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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