I have completely rebuilt a Zenith 5-S-127 from the chassis up according to the Rider's Schematic. The volume control doesn't work at all (pot tests good). I only get a sound out of it when I touch a metal object to the plate (pin 3) of the 6Q7. I have checked my wiring and component layout, and it looks fine.

When I checked the pin voltages, I found that several were severely out of line. The following is a list of the voltages I read; The voltages from the Riders chart are in parentheses:

6A8 Pin 4 96.3V (85V)
Pin 5 6.49V (-1V)
Pin 6 254V (166V)
6Q7 Pin 3 272V (75V)
Pins 4&5 214V (0.1V)
Pin 8 232V (1.5V)
6F6 Pin 5 -29.8V (-5V)

My power transformer tests good, but I believe it has a higher output than planned for in the schematic because I'm reading 45V higher on the rectifier filament than the Rider's chart shows. Does anyone know what's going on here? I would deeply appreciate any help at all on this subject.

Thanks in Advance,
Larry Rickard

Hi Larry

There are a couple problems..

Pin #8 of the 6Q7 must be near zero. R7, 590 ohms to ground must be open. Pins #4 and #5 will come down when you correct pin #8.

Pin #5 of the 6A8 needs to be negative several volts for the oscillator to be operating. With pin #6 so high you may have a bad 6A8 tube or poor connection.

Correcting the 6Q7 circuit may take care of the 6A8 voltage. By having the 6Q7 circuit problem the AVC line is positive. This circuit goes to the 6A8.

The 6F6, pin #5, is a little high but should be ok. Voltage should be around -20 volts. The schematic may call out -5 as meters in those days would load down the circuit.

Norm

: I have completely rebuilt a Zenith 5-S-127 from the chassis up according to the Rider's Schematic. The volume control doesn't work at all (pot tests good). I only get a sound out of it when I touch a metal object to the plate (pin 3) of the 6Q7. I have checked my wiring and component layout, and it looks fine.

: When I checked the pin voltages, I found that several were severely out of line. The following is a list of the voltages I read; The voltages from the Riders chart are in parentheses:

: 6A8 Pin 4 96.3V (85V)
: Pin 5 6.49V (-1V)
: Pin 6 254V (166V)
: 6Q7 Pin 3 272V (75V)
: Pins 4&5 214V (0.1V)
: Pin 8 232V (1.5V)
: 6F6 Pin 5 -29.8V (-5V)

: My power transformer tests good, but I believe it has a higher output than planned for in the schematic because I'm reading 45V higher on the rectifier filament than the Rider's chart shows. Does anyone know what's going on here? I would deeply appreciate any help at all on this subject.

: Thanks in Advance,
: Larry Rickard
:

Norm,

Thanks for the advice! Sure enough, there was a cold solder joint on R7... a little heat got me back in business.

Thanks again,
Larry