In Reply to: RE: Need Futterman H3aa schematic and biasing instructions posted by airheadair on September 12, 2013 at 19:01:45:
Hi everyone,
I am servicing a pair of H3aa and I see that indeed a test point is not shown on the schematics. But the most important is that test procedure is wrong if someone follows it exactly! All bias test points are wrong. I believe that the text refers to another schematic.
The idea of balancing is to connect a DVM between upper bank's g1 and lower bank's g1. Then to adjust upper bank's pot to make 2 bias voltages equal so DVM will show ZERO volts.
Step 2 is to measure each bias and see if indeed are equal plus minus 2 Volts. It is performed by connecting DVM between one upper bank's cathode and g1, and then similarly at the lower bank.
Note that lower bank has a not adjustable bias (-42V) because it originates from rectifying 35Vac while the upper bias comes from a voltage divider pot fed by a stabilized ps. That ps voltage depends on it's Zener diode selection. It is fed by lower bank's plate voltage. Am I correct?
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Follow Ups
- RE: Need Futterman H3aa schematic and biasing instructions - adam2a3 00:46:20 09/15/13 (3)
- RE: Need Futterman H3aa schematic and biasing instructions - rhinohifi714 08:18:49 09/15/13 (2)
- RE: Need Futterman H3aa schematic and biasing instructions - adam2a3 16:35:31 09/15/13 (1)
- RE: Need Futterman H3aa schematic and biasing instructions - rhinohifi714 18:55:47 09/15/13 (0)