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-   -   Fisher 800C phase inverter adjustment (http://www.diyaudio.com/forums/tubes-valves/3833-fisher-800c-phase-inverter-adjustment.html)

Albert 30th May 2002 06:09 PM

Fisher 800C phase inverter adjustment
 
Hello All,

I just re-tubed my Fisher 800C and would like to adjust it to optimum. I noticed two adjustment lugs on the top for "phase inverter adjusment", pardon my ignorance but what do these adjustments do and can I do the adjusments by ear?..can I harm anything if I turn them? also any other suggestions for getting this receiver tuned up would be greatly appreciated.

Thanks,

Albert

Thatch_Ear 14th June 2002 03:46 PM

Fisher recievers can be very touchy. I would check and try to find someone who knows how to service Fisher and take the thing in for a complete calibration since you retubed it. Changing the tubes in the tuner section usually throws it off, and you won't have to do it again unless you retube it. If it sounds good to you as is don't touch anything unless you can mark it to get it back to exactly where it was incase it doesn't work out.

Brett 14th June 2002 06:24 PM

Hi Albert,

When you rolled the tubes, you probably threw it all out of adjustment. I know I did the same thing in my 800B last week, trying some JJ's.

David is right about the RF section. LEAVE IT ALONE. Sorry to shout, but it'll save you a lot of grief and money. The audio sections of Fisher receivers are pretty standard design for the era.

You asked about the phase inverter. I've included a <a href="http://members.optusnet.com.au/~xx308/800B%20amp.jpg">snip</a> from my 800B schematic, which is more or less identical to yours. Signal path is from left to right. The first tube section, is the gain for the power amp. The second is the phase inverter, and the two next to the transformer are the output tubes. On the diagram are some voltages. On the phase inverter are two voltages, one at the plate (top 288V) and the other on the cathode (lower 139.5V). Above the plate is the variable resistor (R111 25k) that you see on the chassis of the receiver. Adjusting it whilst measuring with a multimeter until you get as close as possible to the two voltages listed will bring it as close to alignment as possible with simple test gear. This assumes that the B+ is at the correct voltage. Do this for both channels, with the volume control at minimum.

The phase inverter needs to be set right or else you'll get a lot of distortion. It can't be done properly by ear.

Unless you are familiar with high voltage electronics and electrical safety, don't go playing inside the chassis of any amp, especially a tube one. There are LETHAL voltages inside.

Take it to someone who knows what they're doing if you don't.

The full 800B schematics can be found at http://www.one-electron.com/FC_Consumer.html, and full service manuals can be obtained from the Fisher Doctor www.fisherdoctor.com . You don't <i>need</i> a service manual for the audio stages.

If you don't mind me asking, why did you re-tube? Were they bad, or did you just want to try a different flavour?

Albert 14th June 2002 08:15 PM

Hi Brett,

I replaced the OT's because they were a total mishmash of everything and none of them very good. I figured a matched quad of new EH's for 65 bucks wasn't much of a gamble. I noticed a marked increase in (good tight) bass since changing the OT's but I still have a volume discrepancy between the two channels which I had hoped would go away with the new OT's. I have not changed any of the other tubes besides the outputs; do you think by changing any of the other tubes I could fix the volume issue? By volume I mean amplitude but also it seems like the lower amplitude channel is also lacking presence even when I adjust the amplitude to the same on both channels. The low channel however still sounds better than any of my SS gear!

Thanks,

Albert

Thatch_Ear 18th June 2002 09:42 PM

If you are having a volume control problem that you can fix by adjusting the balance that would have me looking 1st to the driver tubes. I have never owned an 800 but it is basically a 500 with AM tossed in. The 2 12AX7s in the front close to the trio on the little plate that is your multiplexer could be the problem if it is tube related. I think there is another farther off to the right but not sure and if so exactly what it does. Do not even pull the 12AX7s from the multiplex unless you mark them to go back in the same sockets. It should be OK to replace the 2 driver tubes and be sure and get a matched pair. Cheap ones like Sovtec are fine to see if this fixes your problem. Buy better ones later and keep the Sovtecs as backups. Even with closely matched tubes it will not be perfectly balanced. There is always some difference in tubes. You have a very nice reciever and if Fisher hadn't made as many pieces of HiFi as they did they would cost a lot more because the quality is high. I would suggest that as a start you get a overhaul kit from Al the Fisher Doc. He sells a packet of the most likely parts that would need to be replaced with instructions, a schematic and a copy of the manual for around $70 (a couple of years ago) depending on model. He has knob end pieces too.


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