40-201 IF Transformer Failure & Repair - Printable Version

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+--- Thread: 40-201 IF Transformer Failure & Repair (/showthread.php?tid=7968)

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40-201 IF Transformer Failure & Repair - Joeztech - 11-22-2013

Today I began some continuity checks of the 40-201 also discussed under Philco 40-201 Speaker. I checked the 1st. IF transformer primary and it was OK. I checked the secondary and it showed open. I was going to remove the transformer anyway at some point to scrub the chassis as part of preparation for painting the top of the chassis to eliminate continuing corrosion. I made a sketch of the wires to/from the IF transformer and what tube pins or other circuit nodes they connected to. I also have photographs of the underside of the chassis to aid in reconnecting things.

I cut the wires not far from the IF transformer and un-screwed the nuts and star washers that held down the IF transformer can to the chassis, then removed it from the chassis. There are two spring loaded washer-nuts on the side of this transformer that allow turning of the ferrite slugs in and out of the transformer to align the IF circuit. The ends of the brass screws that are attached to the ferrite pieces have a screwdriver slot in them for adjustment. I had to carefully remove these two spring washer-nuts in order to remove the internal parts of the transformer. Unfortunately even with the washer-nuts removed and the internal parts loosened up, they would not come out of the IF transformer can or shield due to the deep depression in the IF can where the adjustment shafts and the washer-nuts were fastened. I had to use a right angle scribe to bend the lower edge of the opening where the adjustment shafts pass through the IF can or shield to provide more space to remove the internal parts.

Once I had the assembly out, I checked each winding with the ohm meter again. Each circuit showed continuity. I finally spotted the cause - the secondary winding has the signal ground end of the winding attached to a solder lug that is also part of the captive nut that the ferrite adjustment shaft screws in and out of. The only thing that provided circuit continuity of the secondary is this spring washer-nut on the outside of the IF can or shield. So the continuity depends on this connection plus the quality of the connection between the IF can shield and the chassis.

In this set mice had peed all over the top of the chassis and tubes. Some of it got on the IF transformers. This causes extensive corrosion on the chassis top surface and anything metal that it comes in contact with.

I managed a repair by soldering a No. 24AWG tinned copper wire to the internal lug for this ground and brought it up and out around the secondary ferrite adjustment shaft. I then reassembled the transformer while carefully making sure I could access the added ground wire for the secondary winding. I wrapped the wire around the adjustment shaft while screwing the washer-nut back onto the adjustment shaft. A quick check with the ohm meter now showed continuity to the aluminum shield. I will have to also make sure that I have good nuts and star washers when I assemble the IF transformer to the chassis.

I noticed something about this IF transformer that I have not seen before. It has one winding that is connected to the suppressor grid of the 7B7 IF amplifier tube. The other end goes to ground. There is no tuning capacitor associated with this winding. It is a rather large winding on the secondary coil form in the IF transformer. It is scatter wound which would indicate they were making an attempt to minimize the capacitance within the winding. Right now, I have no way to check the phase of the signal that might be sent to the suppressor grid of the IF tube.

If anyone knows about this added suppressor grid circuitry and what it is supposed to contribute to performance, please explain.

I tried to take pictures of the internal parts of the transformer while it was disassembled, but they were significantly out of focus due the camera I have. I could make a sketch and scan that in if anyone is interested.

Joe

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