fisher L phono out???????

[hinodit]

My fisher 800c has decided to stop playing in stereo???? It plays both channels in mono mode but only the left channel in stereo mode. Any ideas as to start my search for the problem.:scratch2:

 

[Pio1980]

Easiest first, flip ALL of the function switches back and forth several times and return them to their normal positions. If no joy next twist/re-insert the phono signal plugs with the loudness down.
Also suspect the phono player if swapping the RCAs swaps the defective channel.

 

[hinodit]

tried all that still nothing.

 

[Pio1980]

Does the defect swap channels with the phono lead swap? if not you may have a 12AX7 phono tube (V19) with an open heater (doesn't light up).

 

[Dadbar]

When kludgy crap like this happens to me, I always look first at the tape monitor switch. Try working it in vigorously. Also the input selector...and the external preamp loop connectors.

 

[hinodit]

i qill check that. Would it still play in mono with yhat tube bad?

 

[Pio1980]

i qill check that. Would it still play in mono with that tube bad?

Yes.
(Edit) Or not, see following posting.

 

[dcgillespie]

Well, no -- if I may. If the heater of one phono preamp tube is open, the heaters in both tubes will go dark, because the heaters of these tubes are wired in series and powered from a DC source. If the heater in either tube goes bad then, both channels will go silent.

Dave

 

[Pio1980]

Well, no -- if I may. If the heater of one phono preamp tube is open, the heaters in both tubes will go dark, because the heaters of these tubes are wired in series and powered from a DC source. If the heater in either tube goes bad then, both channels will go silent.

Dave

According the the pertinent skiz details Dave is absolutely right, a bad tube heater will take out both channels. V18 (L) and V19 (R) heaters are shown wired in series.
So if the symp's don't swap sides with the phono leads it gets "interesting".

 

[hinodit]

The problem does not swap sides with the phono lead swap. it is persistent with the left side.

 

[Pio1980]

This is just on phono, the tuner does both channels, right?

 

[Trio]

Unsolder the RCA sockets for the phono section and see via "buzz test (just touch the leads with amp on). If you have separation with those leads disconnected from the original porcelain centered sockets, replace sockets. I have seen those get that screwy. If you do not get separation, reattach and buzz test back through the audio stages until you get both channels responding when one itme is contacted with the screwdriver blade.

While it may be stereo/mono switch is at issue, or the wafer switch is developing a problem, it may also be that a terminal strip mounting rivit on a strip that is common to both channels in the phono stage has corroded enough to lose the chassis contact so both channels blend, which may have revealed itself because of a grid short in one of the later phono stages, or a grounding rivit in a later stage on a tube socket may have corroded as well.

This assumes the issue arises only when the switch is set to "Phono" and other input sources work just fine.

 

[s-petersen]

I would try moving the 12AX7's around to see if the problem moves with them

 

[s-petersen]

I would try moving the 12AX7's( 18, 19) around to see if the problem moves with them

 

[kvflyer]

You might want to check the voltage on the plates of the 12AX7 tubes. Often, the plate resistors of our vintage tube amplifiers goes high or even open. If open, your plate voltage will be very low or not at all. That is often the problem with Knight-Kit tube amplifiers which use a 220K ohm resistor. (Must be from a bad batch )

Just a thought.

 

[Pio1980]

This is just on phono, the tuner does both channels, right?

If so replace V19 with a known-good tube as per s-peterson's suggestion.