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Old 24th February 2004, 05:15 AM   #1
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Unhappy Fisher 800c neg bias problem

Hi There

I am having a problem with the negative supply in my Fisher 800C
receiver.
After changing all the filter caps I notice that 4 of the tubes (pre-amp + 2 others) were barely glowing and one channel is distorted.
I checked the voltages on them and they are low. about 4.5 volts when they should be 12 Volts each (12AX7)
I have read that this model receiver uses a silicon rectifier (which it does)
and a couple of 1000 MFD caps(replaced) as the source
of neg bias on the output tubes (this is low too at -14 Vdc) book says it should be -21 Vdc and the filament voltages for the pre-amp tubes.
So can anyone tell me:
1) Is this correct? should there be negative 12 Vdc on filaments of each of these tubes?
I checked the voltage output from the rectifier and it is -24Vdc which seems
about right.
There is a possibility that I hooked something up wrong but I was very careful and made diagrams.
P.S. I understand the need for neg voltage on the output grids but on filaments?
Any advice greatly appreciated.
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Old 25th February 2004, 05:52 AM   #2
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They used the negative DC voltage for preamp tubes as it's there anyway, and quieter than AC. 11V per tube - they should last forever! Schematic is on my web page if you don't have it: http://www.audiophool.cjb.net


17V is listed for bias, but most prefer to see 19-20V with today's high line voltage (and 7591 prices...). Check the resistors, increase R122 or decrease R119 as necessary.
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Old 25th February 2004, 04:21 PM   #3
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Thank you soooo Much.
That is just what I was looking for.(schematic)
I think the old selenium rectifier was dropping too much voltage.
I am going to replace it .
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Old 26th February 2004, 12:55 AM   #4
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Selenium? In your first post you said silicon, which eliminated the most obvious possibility. I'll wager that the selenium rectifier is your problem. Get it out of there and use a silicon rectifier and all will probably be well.
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Old 26th February 2004, 01:29 AM   #5
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I've lost count of how many 400/500/800 bias supply failures I've repaired, but they are always the same: replace the Siemens bridge with a 4 amp Si bridge, replace the 2X1000 uF can with a couple of radial lead 1000 uf@35 or 50 volt caps, use the existing corpses to mount the new stuff on, and adjust R219 and 221 as Tom suggested until you get 21~22 VDC across the heaters of the preamp tubes and a minimum of -17 on the grids of the 7591s. There's a third resistor that sometimes has to be tweaked, also, but I can't remember which one. It's pretty obvious on the schematic, though. I can't give hard values either, because each fix was unique.

This problem, and the leaky coupling cap failure, were the driving force behind the fried output transformer syndrome. If any of you has one of these classics sitting in the bone pile and would like to replace the OT, I have drop-in replacements available for both the B and C series. Email me if you're interested.
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Old 29th February 2004, 03:34 PM   #6
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Default Phase inverter values

Thanks to all who replied to my post on the Fisher 800C.
Well with a new rectifier in place that fixed the problem and now have healthy voltages all round.
I was able to adjust one phase inverter to exactly what it said on the schematic 310 Vdc. But the other one would only go down to about 317 or so.
The amp sounds fantastic to me and I can't hear any distortion.
I am however concerned about it becoming un-linear in the future.
I replaced the finals with Audio Harmonix 7591A Matched Quads.
I suppose the specs are just different enough to exceed the adj pots range.
Should I be concerned about this?
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Old 29th February 2004, 06:29 PM   #7
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The phase inverter adjustment should be adjusted for equal levels out of the 7591s - do you have a 'scope? Best way is to add a 10 ohm resistor in series with both cathodes to ground. Adjust the phase inverter balance for minimum signal across it. Better yet, add three resistors (1/2W is fine) - One to each cathode, tie other ends together. Third from the common point to ground. Now you have a place to check idle current, and a point to check balance too.

Click the image to open in full size.
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Old 29th February 2004, 06:31 PM   #8
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Let's try that picture again...

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Old 12th September 2008, 07:03 AM   #9
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Quote:
Originally posted by erichayes
I've lost count of how many 400/500/800 bias supply failures I've repaired, but they are always the same: replace the Siemens bridge with a 4 amp Si bridge, replace the 2X1000 uF can with a couple of radial lead 1000 uf@35 or 50 volt caps, use the existing corpses to mount the new stuff on, and adjust R219 and 221 as Tom suggested until you get 21~22 VDC across the heaters of the preamp tubes and a minimum of -17 on the grids of the 7591s. There's a third resistor that sometimes has to be tweaked, also, but I can't remember which one. It's pretty obvious on the schematic, though. I can't give hard values either, because each fix was unique.

This problem, and the leaky coupling cap failure, were the driving force behind the fried output transformer syndrome. If any of you has one of these classics sitting in the bone pile and would like to replace the OT, I have drop-in replacements available for both the B and C series. Email me if you're interested.
does it matter on the voltage rating of said rectifier? I have a 400 volt one I bought, is that too small? thanks!
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Old 12th September 2008, 12:23 PM   #10
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For a rectifier, you want it rated for at least twice the peak voltage. 100V would be enough, certainly 400V is OK.
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