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Upgrading a blown power transformer?

Discussion in 'Amps and Cabs [BG]' started by RumbleBot, Mar 11, 2005.

  1. So here's a silly question. I've got an older Yorkville BassMaster 200W head laying around with a blown transformer in it. I was thinking of putting a new transformer in it, mainly because I hate just having it sitting around. Now, I can get the identical replacement transformer, but would it be possible to swap in a power transformer from the 400W model?
  2. Dumb qustion: what blew the old power transformer? If that ain't fixed, the new PT will also go up in smoke, or if it doesn't something else will. Second dumb question: are you sure it's the PT? Usually PT's are fairly robust.

    Exact replacement PT's may not be available, but there's a wide selection of PT's available and one of them could probably be made to fit physically. You need to know what output voltages your amp requires and go from there...Angela Instruments www.angela.com has a selection of Hammonds.

    The 400W PT probably won't work, the output voltage will probably be too high. You juist need to get a schematic or somehow find out what the voltage needs to be.
  3. I'm thinking that it's the PT mainly because I'm getting a normal signal out of the Effect Send. Actually, I'm looking at the service diagrams right now, and the Effect Loop is right before the limiter circuit, which feeds into the power amp. Hmmm... Think I'm gonna dig in and do some more checking on the board. Getting this amp running again is more or less a pet project.
  4. Since the PT also powers the preamp, actually that kinda rules out the PT.

    Do you have a volt-ohmeter? (VOM)

    Look at the output of the PT, there is probably 3 wires, but maybe more. One wire goes to the chassis ground. Then two more will probably go to a diamond-shaped gizmo, with two more wires going out of it...these will go to a couple of large capacitors. The diamond shaped thing is the bridge rectifier. Measure the voltage from each of the 4 corners of the bridge rectifier to ground. (The two wires coming from the PT to the bridge rectifier are AC. The two wires exiting the bridge rectifier are DC.) Note: some amps have individual diodes. Still, there should be two AC wires from the PT plus the center-tap which is ground...If you have AC voltage present going into the bridge rectifier, the PT is good.
  5. Sorry about that. I was getting my power transformer and power amp mixed up. Anyways, after closer inspection I found two loose 470uF caps, which may have been the instigators, and a burned out trace on the ground of the headphone jack. According to the scematics, the power amp runs the ground from the speakers in series with the ground for the headphone out. I'm gonna try and skip the headphone jack altogether and wire a bypass to the next ground. Here goes...
  6. YEA!!! It worked! Bypassing the headphone jack resurrected my old amp! Thanks a bunch for getting me to reevaluate the problem. I just fixed it with a little bit of solder and a 2" 12 guage piece of shielded wire. Just saved a bundle. Now I'm gonna reinforce everything and try it out with my Aguilar cabs! Wow!
  7. Good work, congrats.
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