800B Restore Dumb Mistake

Discussion in 'Fisher' started by ncwalz, Mar 20, 2017.

  1. ncwalz

    ncwalz AK Subscriber Subscriber

    Messages:
    65
    Location:
    Alberta, Canada
    I'm in the process of restoring a Fisher 800B. I replaced most of the caps and resistors. Still waiting for new electrolytic cans from Hayseed Hamfest and some new tubes including output tubes from Jim McShane. Everything was going well until....

    I decided to remove the noose and adjust the phase investor pots using the quick and easy method outlined in the sticky. I've done it a couple of times before and was in a hurry and kind of careless. After completing the adjustments, I forgot to remove the short from the positive terminal of the electrolytic can to ground that supplies the phase inverter B+. Of course I simply powered up the receiver quickly on the variac without the use of a dim bulb tester and was greeted with a loud hum and magic smoke coming from a resistor on the same electrolytic can.

    After realizing my mistake I shut everything down quickly. Upon restarting the receiver I have noted that although it is operational, the sound on both channels is very distorted. I'm hoping the damage is mostly to the electrolytic can and resistor which are going to be replaced, but would appreciate any suggestions as to other checks I should make.

    Thanks
     
  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,369
    Location:
    Ball Ground, GA
    Likely, the B+ dropping resistor was burned into such a high value that all the circuits it provides power to are now operating at much too low a voltage. If true, then most particularly, the phase inverter stage is now way, way over biased, causing the distortion you're hearing.

    Good luck with it!

    Dave
     
  3. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    30,328
    Location:
    Southern NJ
    Been there, done that. Usually the issue was what Dave describes, a resistor cooked and went off value. Not much harm done other than to the ego.
     
  4. ncwalz

    ncwalz AK Subscriber Subscriber

    Messages:
    65
    Location:
    Alberta, Canada
    Many thanks gentlemen. I will replace resistor and try to be a little more careful.
     

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