radiowizard wrote:
The S-meter is not functioning correctly. It goes "negative" after the radio is powered up and warmed up. The meter zero control does nothing. I've used DeOxit Fader Lube on it. I just replaced the 6AS11 audio, 6HF8 and 6BZ6. Same old. Not sure what to check in that area. I do have a schematic, but it's in four parts...probably should print them off and then cut and tape it together so it makes more sense.
RW
If the S-meter seems OK after the radio is first turned ON then as it warms up (like over 30 seconds) it starts creeping negative and then eventually stays stuck down below zero you probably have a Grid Leakage problem with V6 (6BA6 1st IF) and/or V7 (6BA6 2nd IF). Grid leakage typically goes up as the tubes warm up and cause the AGC line which is connected to the grid to be drawn in a positive voltage direction which makes the S-meter pin below zero. Tube V1 can also do this but I see that you have already replaced it without improvement (6BZ6 RF Amp).
The remedy is replacement of one or both 6BA6 tubes.
I had the trouble with my SB-101 which used 6AU6 tubes and experienced the same issue. Sometimes I would have to go through several tubes to find some that would stay OK. I think the circuit stresses the tubes a little beyond their normal operating range and tends to promote this problem.
Other components that could be checked to see if they are having an effect on this are R163 (33K), R164 (100 ohms) and R167 (68 ohms). If they have shifted in value they can make it hard to zero the s-meter. Also if CR1 and CR2 become leaky in the reverse direction they will have a similar affect on the AGC line.
Curtis Eickerman
