Amp Troubleshooting

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Esmo, Apr 13, 2003.


  1. Esmo

    Esmo Guest

    Hi there. I was playing my 80's fender concert amp the other day and it was working just fine. I turned it off and came back and turned it back on. The tubes and the jewel light light up but there is no sound coming through it. The speaker is connected and the fuse is fine. any suggestions on fixing it?

    thanks

    dylan
     
  2. VaughnC

    VaughnC Supporting Member

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    While it could be a lot of things, the tubes are probably the best place to start by either testing them in a tube tester or replacing them one at a time. If the amp has 2 channels, and both channels are dead, it's probably the phase inverter (small tube closest to the 2 large power tubes) or rectifier tube (if the amp has one). The chances of both power tubes going bad at the same time is very slim. If the tubes are ok, it's time for a tech if you don't feel comfortable inside the amp yourself.
     
  3. John Phillips

    John Phillips Member

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    Do you get any visible blue glow (or other sign of them working) in the power tubes when you turn the standby to 'on'?

    If not... those amps have extra internal fuses too (some of them have the extra ones on the back panel) - at least the Export models certainly do, I assume the US versions will as well. It sounds like the HT (B+) fuse has gone - but the question is why...

    Most likely is a blown power tube, but a shorted filter cap or rectifier diode is a possibility too. These amps are getting just old enough that re-capping may be a good idea anyway.

    If you do still get signs of the power tubes working... these amps also have an FX loop which is switched in and out of the circuit by the FX return jack - a bad contact here will stop the amp working too. You can check it by putting a lead from the send straight to the return (set both the levels to around halfway to start with).

    After that, Vaughn's suggestion of the phase inverter is probably the most likely.


    As usual with this amp series, if you or a tech is in the amp for any reason, replace the 47-ohm resistor in the bias supply chain (on the tag strip beside the transformer) with a 1W wirewound version - assuming it hasn't already been done before. This resistor is the 'known fault' in Rivera-series amps, and could even be the cause here, although I would have expected it to sound very loud for a minute or two before it stopped working if it was - when the resistor fails the power tubes run wide-open.
     
  4. Esmo

    Esmo Guest

    thanks guys. I looked at it and saw that only one power tube was glowing blue while the other had the orange glow but no blue glow. I turned it on and plugged a cord into it. When i touched the end of the cord to something metal, I could hear the nomal buzzing sound but it was very quiet. Do you guys have any ideas for me to test of the possible problems with that are enexpesive. How much would a shop charge to look at/fix something like this

    thanks

    dylan
     
  5. John Phillips

    John Phillips Member

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    OK, so there is still B+ supply getting through (the blue glow in one tube shows that - the other tube isn't necessarily dead, some just don't glow).

    If you connect your guitar directly into the FX Return socket and turn the Return level up, do you get any output?

    If you do, the problem is earlier than the FX loop (ie in the preamp), or could be the switch in the FX Return socket itself. If you get no output, it's probably either the Phase Inverter tube (the preamp tube closest to the power tubes), or (something I forgot before) could it be the speaker? It's not very likely - they don't usually just die suddenly - but check that the connections are good and tight, and the plug is fully into the correct socket.

    It's very hard to give a cost estimate at this stage, but keep posting here and we'll most likely be able to identify the potential cause before sending it to a tech.
     
  6. Esmo

    Esmo Guest

    i tried the fx loop thing and got nothing. I checked out the speaker connections and everything looks fine. Any other ideas?

    thanks to all

    dylan
     
  7. John Phillips

    John Phillips Member

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    Try it with a different speaker cab just to check the speaker hasn't blown (not likely, but needs eliminating).

    Another possibility is the internal impedance switch (in the speaker jack). This could have developed a bad contact too. Unplugging and replugging the speaker might 'cure' this, but if it is the cause it needs sorting out properly on the inside, in case it happens again while playing.

    Try swapping the Phase Inverter (12AT7) for the reverb driver (the other 12AT7, 3rd from the far end of the amp and next to the little transformer). This will check whether it's the PI.

    If it's not tubes or the speaker, it's likely to be a job for a tech if you're not familiar with this stuff. Probably not too expensive since the transformer appears to be OK. At least you now know the problem is not in the preamp.
     

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