A year ago I bought an old Alpha PA-76, a two holler, serial nbr 5032.
It was a smoker but I have that mostly cleaned up. It is working
however there is almost no time delay on start up. This could be a 18
meg resistor, a 15 mfd 20 volt tantalum cap, or Q1 which is a n
channel dual gate mosfet. I think tantalum caps lose capacity when
they age and the diagram has a date on it of 9/1/76. This cap could
be replaced with an electrolytic as any leakage would not matter. When
the Time Delay relay is closed the cap is out of the circuit. I will
replace the 18 meg resistor, the 20 mfd cap and the mosfet.
Here is the problem, the mosfet, a MFE122 or MPF121 is no longer
available, any direct substitute if you can find one is very high
priced, scarce you know, since the gates are directly connected
together in the original why not replace it with a single gate mosfet
like a IRF520.
Anyone been down this road, ideas? suggestions? any url's info, ect
appreciated.
This is a fine old Alpha it is solid, the tubes are good, I run it
easy never over 1k pep out.
I have the utmost respect not only for the amp but also for the
engineers who designed it.
73,
Bob
w5pvr@goquest.com
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