It is usually sufficient to match bias point. This permits matching to the *bias point* of devices. With a SET DC on the gate, you look for devices that are close in terms of ammeter reading for a given gate voltage. You turn the volts up, the ammeter shows the current through the device. nothing fancy, or high current needed, even for power Mosfetters. The main supply does NOT have to be equal to the usual operating voltage, it can be somewhat less, but 35volts or so is a good target. The 10T pot is a voltage divider off the main supply, to put 0- "X" volts on the gate. The voltmeter is set to read DC to the gate. so maybe a shunt on a lower current meter is a good idea. you'll need to be able to see about 100-200 ma fairly clearly. Ammeter should be probably like 0-2 amps or so. Harbor Freight and others sell perfectly serviceable DVMs for $3.00. You can use a DVM for one or both meters if you like. Power supply, 10turn pot, voltmeter and ammeter. It will need to be about >75% of AC mains for the amp to work. Check the AC voltage at the input and see where it lands.
Put a lightbulb of about 40 watts in series with the primary, and that will drop the B+ by a fair amount and provide series current limiting as well. Quiescent current in the amp is likely about 250 ma or less. Just do not exceed the power rating of the fuse! For testing this may give you some peace of mind.
You can use a fast blow 500ma fuse in all probability. I will use a grounding strap and great care to keep static electricity at bay.įirst off, if you have no load on the Mosfets, you do not need a large amperage fuse. With no power to the MOSFETS I am assuming that pulling them out of the circuit will not hurt anything. Then I will try removing them one at a time to see if I can identify the bad component(s). With the fuses out of each board, I am going to compare voltages and see if I can figure out which bank of MOSFETS has a problem. The MC boards have the fuses on the board and the only thing fused is the power connection to the MOSFETS. And the only other thing connected are the P and N channel MOSFETS. My thinking is that since two boards from Musical Concepts have the same apparent problem, it is not the Musical Concepts boards. They are the same as the board that works.
I have left off the input and output connections so i only have B+, B- and ground from the supply and the three connections to each bank of MOSFETS. I have been over the connections multiple times and since there aren't that many connections, I am pretty sure they are right.
I am pretty methodical when it comes to doing this type of work.
Yes I did install the 47k resistor and moved it to the replacement board when I got that from Musical Concepts.