[Joeztech]

Dynaco A470 Xfmr Autopsy

I decided to dig into the Dynaco A470 output transformer that tested almost twice the DC resistance measured plate to plate as the good transformer. I had nothing to loose.

I began by removing the end bells from the transformer. This exposes the interior assembly.

I removed the E and I leaves one by one until I had them all out. There were 130 E leaves and 128 I leaves.

I carefully removed some outer layers of wax impregnated paper and a Mylar tape wrap (1 turn). An extra layer of paper tape and some slender 3/8in. wide black tape was removed to reveal the primary winding terminations. Here is what I noted at this point. The magnet wire used appears to be one of the following kind: 34AWG if single insulating film; 35AWG if heavy film insulated; 36AWG if triple film insulated.

The two plate leads have two wires going to them. The two screen grid leads have four wires going to them. The center tap has four leads attached to it.

I found one wire to a blue/white lead that was broken in two about 3/8in. from were it came out of the wire bobbin. I decided it was worth trying to reattach the wire. I carefully un-soldered the rest of that wire from the end of the blue cloth covered wire. Then I tinned it and the free end of wire coming out of the transformer winding and bobbin with solder near where it had broken. Once I had a good solder take to the inner copper (the solder heat burns away the insulation), I tack soldered it to the free end coming out of the transformer winding and bobbin. I slid a very small OD Teflon sleeve over that repaired area, then used some clear flexible plastic hobby glue and some 3M electrical tape to hold it in place so that it would not move. I re-wrapped the free end of the wire previously un-soldered from the blue wire and soldered it there.

I took time out to use my ohm meter and compared the DC resistances of the repaired transformer to the good one. Both transformers now read the same DC resistance plate to plate. Checks from center tap to screens and plates of both transformers now match within a few ohms of each other – well within expected tolerances.

Next time was spent securing each primary wire to the end of the winding assembly/bobbin and making sure that they were all held tightly. Then each E leave and I leave was carefully placed back around the bobbin and winding. The last item to go back in place there was two wooden shims that help hold the winding and bobbin tightly against the E & I leave stack-up. The end bells, screws and nuts were put back on and tightened.

This took about 6 hours all together.

I now have two good transformers that could be used. I have a new one on order and intend to install it. This repaired transformer will be held in reserve as a potential replacement if the other transformer happens to fail.

Joe