My Collins 75A3 ceased playing. I measure -115 VDC on the grid of the
audio stages (6AQ5)and it is supposed to be -10 VDC. Plate and screen
grid voltages are fine. Would appreciate suggestions of what the most
likely problem might be.

apsent posted:
My Collins 75A3 ceased playing. I measure -115 VDC on the grid of the
audio stages (6AQ5)and it is supposed to be -10 VDC. Plate and screen
grid voltages are fine. Would appreciate suggestions of what the most
likely problem might be.


I don't have a schematic for the receiver, but my initial thought is the grid
bias is created by a cathode resistor to chassis ground. Did you measure the
-115V from the 6AQ5 cathode to the Control Grid (G1)? If yes, the cathode
resistor is open, or has fried to a very high resistance value.

Don

I posted:
apsent posted:
My Collins 75A3 ceased playing. I measure -115 VDC on the grid of the
audio stages (6AQ5)and it is supposed to be -10 VDC. Plate and screen
grid voltages are fine. Would appreciate suggestions of what the most
likely problem might be.


I don't have a schematic for the receiver, but my initial thought is the grid
bias is created by a cathode resistor to chassis ground. Did you measure the
-115V from the 6AQ5 cathode to the Control Grid (G1)? If yes, the cathode
resistor is open, or has fried to a very high resistance value.


--------
I found that the 75A3 audio output tube is NOT self biased, so what I posted
before is not valid.

The 75A3 has a negative bias supply. I still haven't found a schematic for the
A3. The negative bias supply is used for both the "working" bias, and the mute
bias.

Double check that any switches used for xmt to rcv change-over are in the
receive position, as this might affect the 6AQ5 grid bias.

Can anyone direct me to a schematic for the 75A3? The BAMA site doesn't have
one.

Don