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In Reply to: RE: GRAAF GM 200 posted by Frederichabert@hotmail.com on August 12, 2015 at 08:28:24:
I am guessing that what you are talking about is what is called an MOV (Metal Oxide Varistor). It is a component that acts like a high-value resistor until a certain voltage across it is reached, at which point it becomes a low-value resistor. Until the voltage across it rises to the cutoff point, it will dissipate quite a bit of power and it is normal for it to get very hot. Once the cutoff point has been reached (when it becomes a low value resistor) the dissipation will be significantly reduced and it should cool down significantly.
It is usually used to prevent current surges in an amplifier when the power supply is ramping up to its working voltage. Once the PS working voltage is reached it essentially switches itself out of the circuit as its resistance is reduced. I would not be too concerned about this component getting hot unless there is some other fault present or it is looking like it is burnt out.
Here is an article on them:
http://www.cliftonlaboratories.com/metal_oxide_varistor_%28mov%29.htm
Of more concern in the GM200 are some pretty large wire-wound resistors in the power supply that are about 2-3" long. There are two of them, almost immediately after the rectifier bridges, that get EXTREMELY hot if there is a fault in the amplifier (normally they are not used but plugging the amp into an incorrect line voltage e.g. 220V when the amp is set for 110V, would be such a case or if there is a short in a power tube). As they were mounted directly onto the PCB in early amps, there have been rare occasions where they have caused the PC board on some GM200 amps to catch fire! The solution to this (that can be seen on the later constructed GM200s) is that they are installed with long leads that lift them off the board by about 1.5-2" so that they can dissipate the heat properly and not heat up the PCB. If you find your amp has these two resistors mounted directly on the PCB you should probably install two exact replacement resistors that are mounted above the board on their leads.
Hope this helps...
Kind regards,
Mark.
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Follow Ups
- RE: GRAAF GM 200 - Mark S Allen 22:05:29 08/14/15 (3)
- RE: GRAAF GM 200 "My Precious!" - Frederichabert@hotmail.com 04:27:19 08/16/15 (2)
- RE: GRAAF GM 200 "My Precious!" - rhinohifi714 09:09:44 08/20/15 (0)
- RE: GRAAF GM 200 "My Precious!" - Banat 05:18:05 08/16/15 (0)
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