Dead 800-C left channel DEAD - Please help

Vinylcafe

Linvin' the Dream
Found an old barn stored 800C and decided to rebuild and give it to my daughter this Christmas.
Was an ugly duckling with a little more corrosion than I'd like, but went ahead.
One metalbone kit later and 3 replaced can caps and I can't get anything but a faint whisper out of the left channel.
Tubes are all good, checked and double checked my wiring. Voltages for the 7591s good (drawing about 28 milliamps per tube)

Channel is dead in aux, phono and FM (AM I think is dead).

I am loking for a strategy to find the problem.

Reading the schematic it looks like the best way to go is work backwards from the left channel outputs and compare readings to the working right channel.
So V12, half of V16 and half of V17 seem dedicated to the Aux level preamp.
I am assuming that the high voltage side is good (but maybe I shouldn't).
I'm confused around the heater pins and voltages around V12 (left side) and V15 (right side)
Looking at the schematic for these tubes there is no heater voltage given (I believe 12AX7s can run 6 or 12v depending on series or parralel).

V15 and V12 show pin 9 to ground and pin 4/5. I assume this is because they are notsplit to separate channels.

Readings are as follows- both tubes have Pins 4&5 jumped together.:

V15 pin 4/5 0 volts
pin 9 0 volts


V12 pin 4/5 0 volts
pin 9 0 volts


Does this mean for these heater voltages I cannot just gound my meter to the chasis, but must run it across pins 9 and 4/5?

Clearly both tubes are getting warm, though I am not sure the voltages are correct.


They pretty well match readings on each pin of the two tubes, and I get a nice loud crackel on each channel as I move my probes from pin-to-pin (this make me believe its a problem before th 7591s).

On V16 and V17 here are the readings, again the schematic does not give any voltages for pins 4 and 5.

V-17

Pins
1 182v (should be 167 - out of spec resistor)
2
3 1.05v
4 -24.6v (?)
5 -11.5v (?)
6 180v (bad resistor)
7
8 1.07


V16

Pins
1 190v (should be 167 - out of spec resistor)
2
3 1.0v
4
5 -11.56v (?)
6 192.7v (another bad resistor)
7
8 .972v



If pins 4 and 5 are heater pins, how do you get negative voltages there?

Cleaned and measured voltages out of both phase pots, they look similar.

Any thoughts would help.

Hoping to get at least the amp portion done so I can deliver this Christmas!
 
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Channel is dead in aux, phono and FM (AM I think is dead).

I am loking for a strategy to find the problem.

I'm working on an 800-c right now as well and I came across a weak channel. Turned out to be attributed, in part, to the selector switch. Hit it HARD with deoxit and that cleared up a lot of the noise. Before going electrical, try hammering the pots with deoxit, especially the balance and selector pots and see where that lands you.
 
If you have it connected to speakers and wiggle tubes and hear noise out of the weak channel, it's amplifying but signal isn't getting to it. Back to the switches.
 
One time I worked on an 800C with a similar problem.

It was fixed after cleaning each tube socket 3 times!

Give it a shot if you haven't already done so.
 
Thanks guys will do all of the above and report back!

I have done Deoxit thing on all pots, including phase pots. Did not look at jumpers or clean sockets.
 
Went through all of the above again and you were all right!

Was mechanical rather than electronic.

Unit sounds very good, better base than my 500C.

FM section is very weak, but there is one known bad tube in there.

AM is dead, not sure I gave the skills to fix.

This Fisher will be under the tree instead of in the graveyard.

I will post pics after it's all back together.

Still have some out-of-spec resistors to get in there.

If anyone could expalin some of the voltage readings I am getting, I'd still love an explanation. One learns a little bit more with each project.

Thanks againn for all the suggestions.
 
The 12AX7's are wired in series for heaters. Look at the schematics. measure from pin 4 on V17 to ground and then pin 4 on V16 to ground. You should have -22V and -11V or very close to that. They are fed off the NEGATIVE side of the Rectifier.

Look at the B+ voltages for V16 and V17. On the schematic (Pins 1 and 6) you see A4 or another "Ax" alpha-numeric at the end of an arrow. This corresponds to an "A" number on the power supply. FISHER also noted that voltage variance of 10-15% was acceptable due to part variation, Line voltage fluctuations, etc. 10% of 167 is 16.7 so round it up to 17. 184 is 10% and 192 is 15%.

Considering that BOTH PIN's 1&6 take their voltage from the same spot, and they are consistent, between sides on each tube, your "problem" lies upstream at the resistors between the filter caps. Check the voltage drops between each section of C87 and then from C98 to C87D. Also bring your line voltage down to 117V for testing purposes. ALL of these voltages are DC.

The AM Section (If I'm reading this right) is V8, V7, for the Osc and mixer then goes to V5 and V6 for RF. The FM Section is V1 thru V6. So check for Weak / Dead tubes on V8 and V7 for The Dead AM as you have a weak FM. The two tubes that are problematic here for the FM are V1 and V2. Especially V2 as it's run hard. I've not seen a good 6AQ8 in that position yet.



Larry
 
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Thanks for the thorough explanation Larry.

I will go at the FM and then AM sections next.

I just find those sections so complex and intimidating.

Also all those variable capacitors and other weird components I'm not familiar with.

Pix tomorrow.
 
Pix as Promised

Deck is in really bad shape though underneath, not so much.

Had a couple can caps around that only met 3 of the 4 values, so put in some outboard caps underneath.

Faceplate in good shape.

Have ordered some brass caps for the knobs.

Interesting that treble and bass are different nobs all togther. Brown arrows in the center, the back piece (of the split knob) has no plastic ridge at the zero mark, but a black dot on the brass ring.

Finally, 4 x 500K trim pots connected to vary the grid voltage so they all match up, 5 dollars in parts, and voila, matched tubes.
 

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The bass and treble knobs are not original to that piece. Are they split knobs so you can do each channel or are they solid?
 
AM is probably one of the easier things to fix ( at least to me) FM is another thing (again to me). When I bought my 800b, the AM was dead. After taking a few readings with the meter vs what was on the schematic, I found the IF oscillator wasn't running. It turned out to be a broken wire to one of the IF coils. I soldered it back and the AM was fine. My point is that you might be able to fix the tuner sections by just doing a little troubleshooting with the meter.
Don't give up.

Glenn
 
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