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75 Fender Bassman 10 blowing fuses..help

Discussion in 'Amp Tech Center' started by Faceman, Dec 30, 2019.

  1. Faceman

    Faceman Tele-Meister

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    I have a 75 Bassman 10 that came in blowing fuses. The fuse that came out of the amp was a 10 amp. Best I can tell, it had a short occur either in a tube or in the circuit and it wiped out the original PT. I installed a used replacement 606- tranny and put in a 3amp slow blow. I put it on the variac and brought it up slow. I was getting bias, high and heater voltage. Once it hit around 60 volts on the variac, the fuse blew again. I found 3 bad diodes in the SS rectifier and replaced them. Blew again at around the same ac voltage. All electrolytics are original but all tested at pretty much on the money with what value they were supposed to be. I swapped out the bias cap because it was way out of range. Same issue. The fuse will blow with no tubes and with only the mains on. It doesn’t seem to change when standby is off either. Any suggestions? Could it be the mains filter caps even tho they tested in spec? Thanks for the help.
     
  2. dogmeat

    dogmeat Friend of Leo's

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    which 3 diodes blew out? that could point the right direction. if fuse blows when STBY is open (off), there is only the x-former, a V-drop circuit(for lack of a better term), and the bias circuit. anything on the secondary is suspect, but assuming that the diodes are part of the equation, the heater circuit would be a non-issue. the heater circuit still a possibility if there is more than one problem that does not affect the diodes.

    since it is most likely, disconnect the V-drop circuit and test... break the connection to the 220k 1W and 70/350s
     
    Last edited: Dec 30, 2019
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  3. Dacious

    Dacious Poster Extraordinaire

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    Those caps are 44 years old. Testing at no load and with handheld DMM - probably the culprit.
     
  4. Faceman

    Faceman Tele-Meister

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    It was the top 3 of the 6 rectifier diodes. Thanks for the suggestion. I’ll look into it.


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  5. galaxiex

    galaxiex Tele-Afflicted

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    Light bulb limiter....

    :)
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Build a current limiter. Note: that amp schematic calls for a 2 1/2 amp slo-blo fuse.
    How do you know this old PT is good? since it is already in the amp, lift all secondaries. Does the fuse blow? If so, the PT primary is no good. If the fuse holds and with the AC plug pulled from the AC source, check continuity on the secondaries...including OT ground from each secondary. If you find good continuity and no short to the frame of the PT/ground, reattach the secondaries. Replace the filter caps. They are almost 50 years old and need to be replaced no matter what test you run shows...IF one wants the amp not only to operate but also create good sound. Make sure rectifier diodes are good. With no tubes, Fire the amp up on the current limiter....does it show a short?
     
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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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  8. Wally

    Wally Telefied Ad Free Member

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    Continuing the above...if the fuse holds, install tubes one by one...starting with the power tubes....known good tubes. Note: this is why I keep known good used tubes...just for ‘rough tests’ such as this. If the fuse blows with power tubes, the OT is suspect. If not, keep moving along the path. If you get all tubes installed and the fuse holds...or the limiter does not show a short, then I would suspect that the filter cap section was the problem. Power supply resistors should be checked while recapping....or just replace them for good measure. There is a good chance they have drifted even if they have not failed.
     
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  9. Faceman

    Faceman Tele-Meister

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    Great input guys. I do plan on building a current limiter and have for some time. Time to just do it.

    I will be doing some more testing this evening. The last thing I haven’t checked in the mains circuit as stated by dogmeat,is the 70mfd caps out of circuit to see if the main power holds without blowing fuse. The last thing I noticed before I packed it in last night was that although the caps measured at the correct value, neither cap held any voltage after powering down. The voltage measured in the millivolt. To me, that pointed to a major leak to ground. I haven’t tested that theory yet but I will go through all the diagnostics suggested here.

    Thanks a lot!
     
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  10. Wally

    Wally Telefied Ad Free Member

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    that filter section has draining resistors...2 x 220K ohm. The caps do not hold voltage for that reason.

    and...what dogmeat is suggesting..I think... is to lift the B+ at the end of the output of the rectifier circuit.
     
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  11. Faceman

    Faceman Tele-Meister

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    Right, that’s how I interpreted it. Thanks


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  12. Faceman

    Faceman Tele-Meister

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    Quick update...
    I had a very dumb moment in wiring up the replacement transformer. It is a new old stock schumacher 606- transformer with multiple secondaries other than high voltage, bias, 6v heater and 5v heater. There was a green/black wire that, in bench testing the tranny before installation, I thought I had determined to be a center tap for the heaters(unusual I know but seemed correct at the time). Well it had 6 volts on it. Lifted that wire and shrunk wrapped it. The amp is right as rain now. The diodes were part of the original issue and I created the other issue. Thanks for the help guys.
     
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