HW-101 mic amp stage questions.

[Kenneth G. Gordon]

Recently, my restored HW-101 suddenly started exhibiting severely 
reduced mic gain.

In checking the components around V-1, 6EA8, I discovered a 
really ancient 100 K ohm carbon-comp resistor soldered across the 
bottom of the board from the +275 VDC line to the trace on the 
board which connects to pin 6, the plate of the pentode section.

Checking the voltages around that tube following the values listed 
in the manual, I found 8 VDC on pin 6, whereas, according to the 
manual, it should be "around" 40 VDC.

Yanking the resistor, I found that instead of 100 K, it had drifted 
high to about 225 K. Also, the small metal-film 100 K resistor that 
had originally been mounted on the board, and was still there at R-
1, was open.

After I replaced R-1 with a known good resistor, the voltage at pin 6 
is now "around" 40 VDC, sometimes more, sometimes less by a 
volt or two.

However, what surprised me is that the voltage on pin 3, the 6EA8 
screen, is around 130 VDC !  This seems strange to me. In 
addition, there is no listing for the voltage on pin 3 in the manual, 
which I took as a hint that Heathkit didn't consider it important.

Lastly, the rig still shows considerably reduced mic gain when 
compared to how well it worked just after I put it on the air.

I still can get the ALC meter to kick up to around S-6, but it requires 
the mic gain to be set quite high in order to get it. Furthermore, if I 
attempt to adjust the mic gain above a certain point, the output 
power is reduced.

So, at this point, I believe I had, or have, two problems: 1) the high-
value resistor caused V-1 to not provide enough gain, but 2) there 
appears to be a problem further on, and I suspect the balanced 
modulator stage, or perhaps the isolation stage, V-2 6AU6.

Has anyone here experienced low mic gain being caused by 
problems in the balanced modulator area? If so, what was found?

Ken Gordon W7EKB

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