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Troubleshooting a 2275 130.


Scott Farrell
Posted on Monday, September 20, 2004 - 11:26 pm:   

Hi all,

I have a bit of a problem with an old musicman amp. I have no sound out of the little bugger. It turns on, the tubes are lit up but no sound. I did some troubleshooting to try and isolate the issue. Power supply seems fine. The +- 16v line is fine. The 700v line is fine and so is the 350v line. I tried to set the bias by measuring voltage across the 3.9 ohm resistors, but there was no voltage. I checked the transistors with a buddies transistor checker and they both seem fine. I checked the resistors that make up the bias circuit for drifting values but there was nothing out of the ordinary. I replaced all the output tubes. I checked the 16 volt line to the tubes (pin 4)and it was fine. I checked all the grounds in the output section and they were fine. Does anyone have any idea as far as what voltages I should be seeing around the transistor driver section? I am really curious to see how high a voltage is needed on the base of the driver transistors to turn them on. Any help would be appreciated.
Steve Kennedy
Posted on Tuesday, September 21, 2004 - 10:15 am:   

If the 700Vdc isn't making it to plates of the output tubes the amp will be silent even if everything else is working perfectly. This is what the standby switch does. Check to see if the output tubes have 700Vdc on the plate and if not, see if a defective standby switch is the culprit.

An open primary winding on the output transformer will also prevent 700VDC from reaching the plates of the output tubes.

Check your speaker output jacks as well. If this chassis is from a head or a 2-speaker combo, the two output jacks are wired in series and one of the output jacks has a switch on it that shorts the jack to ground when it is not used. If this jack is NOT the one being used AND the switch connection to ground is not being made there will be no continuity on the ground leg of the speaker connection and no audio output. Try moving the speaker output to the other jack and see if this makes a difference.

Of course, a bad op-amp in the preamp could cause no audio signal being fed to the output stage too.

If all the main DC voltages are present, an audio generator, an oscilloscope, an audio load and the schematic will be your new best friends.