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Fisher 400- I let the smoke out

Discussion in 'Fisher' started by Ziradog, Mar 18, 2017.

  1. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    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:
    30,838
    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 AK Subscriber Subscriber

    Messages:
    836
    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:
    30,838
    Location:
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    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 AK Subscriber Subscriber

    Messages:
    836
    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:
    30,838
    Location:
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    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 AK Subscriber Subscriber

    Messages:
    836
    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:
    30,838
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    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 AK Subscriber Subscriber

    Messages:
    836
    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.
     
  10. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    Location:
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    I replaced the rectifier with this one https://www.radioshack.com/products/radioshack-8a-400v-full-wave-bridge-rectifiers and replaced the double 1000 mfd caps as well, and it runs with no smoke - but now I am only getting sound on one channel. So a few questions:
    1. What should I get measuring voltage on the negative side of the rectifier? I see about -26.4 vdc. Nothing marked on the schematic.
    2. As mentioned, I am now getting sound on only one channel. Actually, very faint, distorted sound on the other channel - sounds like a solid state amp with a blown power transistor. I tried swapping the tubes, including the output tubes, no change. Also swapped the speakers. I doubt this is directly from the rectifier problem, but I am open to suggestions on where to start looking. I did not have this problem when I first brought the unit up.
     
  11. rufleruf

    rufleruf AK Subscriber Subscriber

    Messages:
    866
    I got two backward polarity hayseed 1000uf caps for Fishers a while back. Wasted a lot of time trying to figure it out. Are you sure your RCA plugs are well seated? I've been stumped by something as simple as that a few times.
     
  12. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Which copy of the manual are you using. -26.4 is a little deep. It could come down a volt or 2 mainly for the heaters on the DC heated tubes. The voltage for the Rectifier should be on the right side of the rectifier image. a 100ohm 2w resistor in series between the rectifier and the caps/heater tap. If it's below -24v now, use a smaller value resistor until you get about -23.5v/ -24.0v

    Check the resistors between the phase inverter and output tubes. When the rectifier blew it could have taken a resistor or two on the affected side. Check all the voltages on the output tubes 1st. If the voltages are flakey on certain tubes, then check the resistors for that tube. But I would check all of them to be sure.
     
  13. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    Location:
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    Thanks Larry, your advice is most helpful as usual. Attached is the schematic I was using. Is the resistor you mentioned R35? I will recheck it with input voltage at 117vac (wall is 123 - 124).

    Rufleruf, I am surprised to hear that. I will dissect my dead one & see if that is the case. It looked like when I moved stuff around, one of the AC inputs to the regulator may have touched the DC + output and I did not see it.
     

    Attached Files:

    Last edited: May 1, 2017
  14. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    No the resistor is an add on. The silicon rectifier has less fwd voltage drop than a selenium rectifier hence the R. I keep forgetting the 400 manual doesn't have a voltage listed @ the Rect. However the 800c does. It lists -22 v which would put the heaters at -11v each. -22 / -23 would actually be a better target for the output of the rectifier. Check it @ 117V. If it's within the -22v / -23v target call it good if you're gonna use a variac on it all the time. Otherwise check it with the wall voltage and adjust with a resistor to get it btwn -22v / -23v on wall voltage.
     
  15. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Touching the AC to the DC side would do it. Look for welding marks on the AC and DC+ leads. Do this before tearing up what may be a perfectly good cap.
     
  16. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    Location:
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    Well replacing a fried resistor has it playing music again.

    I see no welding marks on the original rectifier. I did a quick check with the cap meter on my VOM, and the Hayseed cap reads pretty much dead on 1000 mfd on both halves. I know that isn't much of a check. Is there a way to check that the cap isn't fried at working voltage without a real cap tester? And how can I tell if it is wired backwards? For now I am leaving the replacement individual caps in it.
     
  17. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    Location:
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    I rechecked the voltage on the rectifier and still get -26 vdc at wall voltage. Had to lower the incoming voltage 108 to get -23 vdc out, so I guess I will need to add a resistor. Is 2 watts enough for this? I found a post (on Tuners & MPX) where you suggested a 5 watt-er. Also, I have not added a CL80 yet, that may help a little.
     
    Last edited: May 1, 2017
  18. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
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    5W is recommended. Your feeding the heaters, and the bias supply. Granted it ain't that much but a 5W will take any spikes a lot better than a 2w or 3 w.
     
  19. Ziradog

    Ziradog AK Subscriber Subscriber

    Messages:
    836
    Location:
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    Thanks Larry, that's what I thought. Also my post should read, "Had to lower the incoming voltage to 108..." It won't let me edit it now.
     
  20. gadget73

    gadget73 AK Subscriber Subscriber

    Messages:
    30,838
    Location:
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    Just make sure you confirm the current level at the output tubes when you change it. Raising the supply voltage (making it closer to zero volts) will increase the current draw through the tubes.
     

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