I was fully prepared to install bleed down resistors on the Fisher 90-T tuner I recently acquired. What I discovered was that the electrolytic capacitors do not stay up very long. In fact, within about 20 seconds, the voltages are below 1VDC. Here is a link to the service manual on fisherconsoles.com website:
THE FISHER 90-T SERVICE MANUAL
I am having trouble figuring out where this energy is being burned. The electrolytics are at the bottom right C118 in this schematic.
THE FISHER 90-T SERVICE MANUAL
I am having trouble figuring out where this energy is being burned. The electrolytics are at the bottom right C118 in this schematic.
When you turn the unit off the cathodes of the unit's tubes are still hot so they continue to draw plate current for some time after the power is removed - that is what is draining down the capacitors. They are relatively modest in value as well so do not store a lot of energy - in addition if these are the original caps they are probably more than a bit leaky at this point in time, and should be replaced as a matter of course IMO.
I have been reading a thread on radiomuseum about reforming capacitors rather than chucking them. see thread here I will be investing in an insulation tester in the near future.
I went through a phase of attempting to reform old electrolytics several decades ago, it worked reliably about 25% of the time, and best on units that were obviously low hour to no hour pieces. I've also had reformed electrolytics explode in equipment a few months later. You'd really probably be better off replacing that filter capacitor even if it now seems OK. It's over 50yrs old at this point and far beyond its design life time.
Does this look good for C118 which requires 40/40/40/40uf - 250/250/300/300VDC
http://www.tubesandmore.com/products/C-EC40X4-350
I don't know anything about CE manufacturing, and the Fisher specs don't include temperature or tolerance ratings. So is 55C and -10 to +50 reasonable?
http://www.tubesandmore.com/products/C-EC40X4-350
I don't know anything about CE manufacturing, and the Fisher specs don't include temperature or tolerance ratings. So is 55C and -10 to +50 reasonable?
I'd correct Fisher's over sight and include bleed-off resistors. You can't count on hot cathodes bleeding off the voltage after power down. What happens if you power cycle? You could retain nasty voltages if you do that. If it's just for safety, the bleed down doesn't need to pull more than a milliamp or two -- hardly enough to over burden the power supply.
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