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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Fisher - Hot Chassis? posted by Burke on March 1, 2006 at 01:45:56:
A "hot chassis" comment here often means the floating ground circuit is failing or has failed, and line voltage of significant current is reaching the chassis. This happens all the time with guitar amps of similar vintage. It happens more than appreciated with these Fishers. It's a 50/50 proposition, depending on which way the stock power plug is inserted into the mains outlet. Even when operating correctly, the floating ground often passes full line voltage into the chassis, but only in the mA range. A properly operating floating ground circuit is safe.However, if the suppression capacitor fails shorted, much higher currents can pass. The older style suppression caps used in the vintage equipment sometimes fail shorted.
Do not operate the instrument under these conditions. It can rise to a very significant electrocution risk.
Either redo the floating ground circuit, or remove it completely and replace the cordset with a grounded one. Be aware of the increased risk of ground loop hums with a grounded cord. Never simply ground an already "hot chassis".
Alternatively, you can install a polarized cord on a reworked floating ground, or "polarize" the stock cord by marking the neutral plug prong for safe insertion. As long as the neutral leg of the mains supply is running to the floating ground circuit, the risk of electrocution is greatly reduced.
There should be a lengthy exposition on floating grounds somewhere in the archives. A floating ground circuit has to be serviced in a certain way, with parts specificaly designed for this duty (Y class suppression capacitors: AC line to ground), or you run the risk of electrocution should it again malfunction.
It is far less likely that B+ voltages would reach the chassis with these Fishers, but it is possible, especially if someone was careless in a prior service.
A leaking large electrolytic will not usually cause a "hot chassis", but can burn up the power transformer if left unchecked.
Good luck!
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Follow Ups
- Re: Fisher - Hot Chassis? - sgmlaw 14:38:20 03/01/06 (0)
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