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800C power

Discussion in 'Fisher' started by GHutch58, Jul 26, 2018.

  1. GHutch58

    GHutch58 New Member

    Messages:
    3
    I have a power question about my 800C. It is probably just a generic tube amp question. I have an 800C that has been sitting in an attic for 25 years. I pulled it out and hooked it up a few weeks ago. It was working, somewhat for a few days. The tuner was a little weak but my antenna was not too good either. But it was driving the speakers pretty well with close by statoins.

    Then one day I turned it on and there was no more noise. The lights all still light up and the tube heaters all still heat but there is not even a hum. Is this indicative of capacitor problems or something else?

    GHutch
     

     

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  2. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,256
    Location:
    NC
    You had it stored for 25 years and just plugged it in?
     
  3. audmod01

    audmod01 Super Member

    Messages:
    1,249
    Location:
    Tioga, TX
    Al asked a very important question. Electronic equipment that has sat un-used for years should be brought up very carefully on a Variac, gradually increasing the AC voltage going to the unit's power transformer or power supply. This is especially true of vacuum tube equipment, but also applies to solid state electronics. Most experts recommend replacement of the electrolytics in the power supply of a unit and examination of parts underneath looking for any that show signs of overheating. Discoloration may be an indication of a part that has been overstressed for some reason and would indicate a need for some investigation of the circuitry it is installed in to determine if there is a problem in that area.

    Most of us replace electrolytic capacitors in the power supply and some key areas elsewhere that are known problem areas. We often install a thermistor in the AC input circuitry of the unit to protect against turn-on transients and current rush. This extends the life of many components.

    The fact that you still have dial lamps and tube filaments lighting up show that at least the low voltage AC to those items is still intact and operating. However the HV B+ in the set may have failed somewhere. The failure(s) will only be found by some good troubleshooting procedures. You will at least need a VOM (Volt-Ohm-Milliamp Meter) to do some measurements following guidance from people here who are willing to help.

    Let us know what test equipment you have and we can begin to help. Switches both the volume-on-off power switch and other slide switches used in these units develop corrosion over time and often need cleaning to correct intermittent problems. Since you said that the problem is now continuous, It may make troubleshooting somewhat easier.

    Joe
     
  4. GHutch58

    GHutch58 New Member

    Messages:
    3
    Thanks Joe. I was cautious with the power on but don't have a Variac. I do have volt/ohm meters as well as the schematics and some rusty electronics knowledge. Unfortunately, many of my tools are packed for an impending move. I will probably not be able to spend much time troubleshooting for several weeks. I will be working on it as I can.
     
  5. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,774
    Location:
    Southern NJ
    No point in the variac at this point. Age has commanded some repairs and maintenance though. No sound at all usually means some sort of power supply problem
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,837
    Location:
    Glen Burnie Md.
    I would pull the S.M. for your Serial Number group. Then check power supply voltages to the plate, screen, and output transformers. Also the bias supply for the output tubes (the cardboard covered cap underneath). One more thing! pull clean and heatshrink the "U" portion of the jumpers for the space expander. Pushing them until they stop, creates a grounding of the signal at thtat point and pretty much 99% of sound doesn't get thru.
     

     

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  7. GHutch58

    GHutch58 New Member

    Messages:
    3
    Thank everybody for the replies. I will not be working on this amp any further. I have sold the system in the condition is was in. I still have a pair of XP7 speakers and will be making those available for sale.
     

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