Fisher 800-C Help

[Karga USA]

Hey Friends, I'm working on Fisher 800-C and have few questions. If you can help me to answer them that'd be great! Thank you.

Question #1:
There is black/yellow shielded wire attached to volume pod (see the picture), but it looks like wire (not the white shielded one) is detached from the volume pod. Where should I solder that wire? When I trace the cable I see it's connected to ground on the other end.

Question #2:
I'm running amp at 117V 60HZ through Variac, I read 16.20V bias from output tubes. The issue is when I pass signal through phono/aux and the volume is low (10-15%), it sounds good. When I increase the volume its start clipping.

Voltage readings with 117V.
Output Tube:
PIN 3 = 358V
PIN 4 - 8 = 330V
GRID BIAD = 16.3V

Phase Inverter/Driver Tube:
PIN 1 = 251V
PIN 6 = 97V
PIN 3 = 100V
PIN 8 = 1.05V

Thank you!

 

[Karga USA]

Attached the photo of the volume pod.

 

[jvstratman]

That's a double pot (left & right channels) with 2 wires coming to each. Emulate the connections of the front pot and you'll be good to go.

 

[dcgillespie]

Karge -- The ground lead at the volume control end of those cables that extend from the volume control back to their respective driver tubes, should not connect to anything -- they are intentionally cut short so as not to touch anything. The ground lead is grounded in the driver tube area.

Your B+ voltages are quite low -- either the power supply caps are bad, or the output stage (or something else) is overloading it. The un-connected ground leads at the volume control are not your problem.

Good luck with it!

Dave

 

[Karga USA]

Hey @dchgillespie, thanks for your reply.

I've recently renovated bias supply, I get steady 21V (this is after replacing R119 (5.6K) with 2.7K).
You can see the recent measurements while the DUT plug into the wall outlet.

Output Tubes:
Heater Pin 2-7 : 3.2 VAC
Plate Pin 3 : 454 VDC (it's fluctuating 1-2 Volts, not sure if it's normal but the voltage reading seemed a bit higher to me based on the schematics which is stating 430V)
Grid-2 Pin 4-8 : 411 VDC (same it's higher than schematics which is 375VDC)
Cathode / Grid-3 Pin 5 : (.274) VDC to ground through 10ohm Resistor.
Grid-1 Pin 6 : 20.9 VDC

Unit sounds good, but output tubes seems little hot to me. Any suggestions to get the voltage readings within specs or acceptable ranges.
Thank you!

 

[larryderouin]

Plate and Screen voltages are proportionatly high but within the 10-15% Fisher specified variance due to component tolerances. What is/was wall voltage? 117vac is the test standard and schematic values are based on this. Cathode voltage could be better (about 34ma) vs 27.4ma you have now. Take your new R119 and replace it with a 10KΩ 1/4w pot (bournes 3386 is fine) and see if you can adjust the bias voltage higher (Cathode voltage) for now. Your voltages in Post 5 could be a result of a higher than spec'ed Wall voltage, worn out tubes, and or a combination of the 2.

Measure all 4 tubes. Take the highest one and use that as your bogey. As you increase Output tube Cathode load the plate and screen voltages should come down some. See if you can get the bogey tube to 34ma (340mv) with a pin 6 voltage of NLT -15vdc (toward ZERO).

Tubes get hot, Usually enough to relieve you of your fingerprints. No way around it. Ever consider why the British call them Thermionic valves???? As long as they don't glow red on the plates, you're generally in pretty good shape unless the tube is dead. Or gone to air (getter turns white from Oxygen contamination of the barium mix in the getter) in which case it can be used as a plinker .22cal target. Gotta give them that chance.

 

[dcgillespie]

There ya go.......

Dave

 

[larryderouin]

My thumb ain't sore this time......

 

[Karga USA]

Thank you very much Dave, I'll work on it tomorrow and update here. I've just got some additional measurements and would like to share here and yes Tubes are original ones, and I think they abused (see little plastic cracks on the base of the tubes) for a while. Wall outlet was 118VAC when I took the first measurements that I posted above.

 

[dcgillespie]

Tubes are not running hot, and likely not your problem. Need more detail as to what inputs the unit operates properly on (full volume range), when it doesn't work properly, and what input is selected when it doesn't operate properly.

Dave