Fisher 400- I let the smoke out

Discussion in 'Fisher' started by Ziradog, Mar 18, 2017 at 6:21 PM.

  1. Ziradog

    Ziradog Well-Known Member

    Messages:
    760
    Location:
    Apex, North Carolina
    This is a late model 400, in really rough shape. When I originally brought it up on a variac & DBT, it generally worked - FM played, and my phone through the AUX worked ok. The tuning tube did not light up, but that is a problem for another day.

    So today I replaced all 4 can capacitors, carefully checked my work, & fired it up again. I had sound from one channel only. I switched the speakers, same channel from the amp had no sound on either FM or AUX, just fairly loud hum. While I was poking around, the twin 1000 mfd can cap started to smoke. Long story short, the speaker wires on one channel were shorted together. I am 99% sure that was not the original issue, that the connector pulled partway out while I was tilting the receiver up to look underneath. Does it make sense that shorted speaker wires would cause this (I hope)?

    So while I am waiting for my replacement caps to come (5 - 7 business days), what else can I check for collateral damage, and what else can I check for my original problem of no sound in one channel (other than my wiring work)?
     
  2. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    29,513
    Location:
    Southern NJ
    That cap needs the positive leads going to chassis, so make sure the polarity is right.

    Speaker terminals shorting wouldn't bother it, but reverse polarity caps definitely would. If that was going on, voltage to the preamp tubes would be wrong, possibly low enough for one to not do anything.
     
  3. Ziradog

    Ziradog Well-Known Member

    Messages:
    760
    Location:
    Apex, North Carolina
    No, those are correct (just checked again) but I may have been wrong about where the smoke was coming from. I'm not sure what this little black component was but it looks like it has seen a better day. The black wire going to one end of it is the - side of the two 1000 mfd caps.
     

    Attached Files:

  4. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    29,513
    Location:
    Southern NJ
    That'd be the bridge rectifier unless I miss my guess. If that shorted, it would send AC to the caps, and they would not appreciate that either.
     
  5. Ziradog

    Ziradog Well-Known Member

    Messages:
    760
    Location:
    Apex, North Carolina
    So replace the rectifier and the caps, or start with just the rectifier? Looks like there is a Rat Shack rectifier that is commonly used as a replacement.
     
  6. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    29,513
    Location:
    Southern NJ
    At least the rectifier, but the caps don't like AC either. They may be bad, you'll know fairly quickly. If they were individual caps, I'd go straight to replacing them but if you bought a can that tends to be a little more pricey. If one decides to die later though, you'll lose the bias supply and that would do badly for your output tubes.

    Bridge rectifiers are fairly common items, if you don't have an RS around anymore there are probably a few hundred at Mouser that will work fine. Off the top of my head, probably anything rated larger than 1 amp will get it done. This is low voltage so most of the other ratings aren't real critical. There is probably an in-line package that would more closely duplicate the original, a lot of them are a square package.
     
  7. Ziradog

    Ziradog Well-Known Member

    Messages:
    760
    Location:
    Apex, North Carolina
    The original appears to be the FWLA 200 mentioned in other posts, rated at 4A & 400VDC. RS has the identical one, plus one rated at 400 vdc 8A that other folks used as a replacement.

    Any thoughts on why it may have died now? I soldered to one leg, so over heat or force moving stuff around is possible, but worried about other errors I might have missed.

    I hate that I probably killed a Hayseed replacement can. I'm putting individual caps from Mouser in there for now, if it all ends up good I may put another Hayseed in as a present for the next guy.
     
  8. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    29,513
    Location:
    Southern NJ
    If Hayseed perhaps built the can wrong, that would do it. About all that kills bridge rectifiers in a way that they blow a hole in them is extreme current load. If the output was dead shorted or the caps were backwards, that would explain it. They otherwise last just about forever.
     
  9. Ziradog

    Ziradog Well-Known Member

    Messages:
    760
    Location:
    Apex, North Carolina
    Well I have been back through the schematic, the pictures I took before I started, and the sketches I made and it all looks correct. I can only guess that I damaged the rectifier somehow by overheating or moving stuff around.
     

Share This Page