[Tubesforme]

I have a model 21, which uses a model 20 chassis in it. I have a problem that is driving me nuts. I just put my third interstage transformer in this chassis. I use this radio daily and after it has been playing for several hours, withing a few weeks of replacing the interstage transformer, the volume will drop. After some time the volume comes back up to normal. This continues for several weeks until 1/2 of the secondary of the interstage transformer opens up. When I restored this chassis 1/2 of the secondary was open so I replaced it with one I had purchased from AES. This worked and the radio played for several weeks. Then the volume would suddenly drop and without doing anything, suddenly the full volume would return. What I found unusual was whenever I attempted to check the bias voltage on the grids of the 71A's, the volume would come up by itself, until the secondary would open up completely. Checking the plate voltage on the driver tube didn't affect anything. I have changed out the transformer three times as well as I finally changed the 27 driver tube and output tubes. I thought my problem was solved, but it suddenly has come back again. I know I have seen a lot of interstage transformers where the primary was open, but never the secondary side. Any ideas? Thanks guys.

[Radioroslyn]

A couple thing to check. What is the voltage from the filament -probe to the grid + probe on the 71A with a good transformer? What is it using for filter caps? Ecaps? Film? and value. Does the same half of the secondary go open? Check 71A sockets for carbon tracks or burn marks from pins 2 and 3.

[Tubesforme]

The voltage measured with a DMM is and has always been -59V on each grid, measured from filament. The caps were replaced twice, both times with new metalized polyester tubulars, same capacity as originals except 630V and mfg was Mallory. The .13 was made with a .1 and .033 in parallel. Sockest were completely cleaned, in rebuild, by hand, so no carbon tracks and yes, the same half was open all three times.

[Mondial]

Are you replacing the transformer with the identical type each time?

 Its possible there is a manufacturing defect at the termination of the secondary winding with the lead wire. The  winding is made with very fine and delicate wire and it only takes a little corrosion at the joint to have it fail. The fact that the transformer seems to fail intermittently at first seems to support this idea. If all three transformers were the same type and failed similarly, then this would be my guess. 

Really, the most likely external cause that could make the secondary fail is a grid to plate short in the corresponding 71A tube. As you have already changed the output tubes, and still had failure, then it probably is something else.

I would try another interstage from a different manufacturer.

[Tubesforme]

The tubes I put in this receiver were being used in another receiver which had no problems so I know they are good. First I put in a good used Philco  transformer and it opened. So I purchased a new one from AES and it also opened so I purchased a Hammond and it is the one that is acting up now. I had originally suspected the tube, but that has been eliminated by taking the pair out of a working receiver. The transformer from AES lasted a little over a month. The same problem is starting again with the Hammond transformer. That is why I am stumped. I originally thought that the tube was causing the problem and later thought that the winding in the transformer was the culprit.

[morzh]

If you are up to it, just measure the current in the half that keeps opening.
You will likely see the current from the screen grid through the winding to the center point which is, forgot, GND or Rect Neg. Then swap the tubes, see if it disappeared and then see if it migrated to the other half.

In any case, the output is not really loaded and has no reason to burn. If it does, follow the current.

[Tubesforme]

There are no screen grids. The tubes are triodes (71A's). I have not only swapped the tubes, I put two NOS in. So this is the second pair of 71's.  By the way, while the vol was low I hot swapped the tubes around and no change in vol.

[morzh]

Well plates then. Anyway, measure the current.

You vould start with measuring the voltage between the centertap and the end that you know will open. Compare with the other end voltage. The centertap is tied to the Rect Neg. (B).

The voltages across the half-windings should be the same and be practically zero volts. Wait till you have that volume fluctuating and see if one of them changes. Use two DMMs.
Any significant positive voltage relative to centertap should be regarded as bad.

Once confirmed, or you can go straight to it, insert milluammeter between the suspect winding end and the grid. See if the positive (from grid being positive and winding neg) current exists. If yes, remove the tube, see if it stops.


Lasy: see what resistance is of the section 1-2 of the vitreous resistor 25. As long as you are at it, check the whole resistor, all sections.

[Tubesforme]

Well plates then. Anyway, measure the current.

You vould start with measuring the voltage between the centertap and the end that you know will open. Compare with the other end voltage. The centertap is tied to the  Rect Neg. (B).

The voltages across the half-windings should be the same and be practically zero volts. Wait till you have that volume fluctuating and see if one of them changes. Use two DMMs.
Any significant positive voltage relative to centertap should be regarded as bad.

Once confirmed, or you can go straight to it, insert milluammeter between the suspect winding end and the grid. See if the positive (from grid being positive and winding neg) current exists. If yes, remove the tube, see if it stops.


Lasy: see what resistance is of the section 1-2 of the vitreous resistor 25. As long as you are at it, check the whole resistor, all sections.

I really blew this one! I made an assumption. I had replaced the interstage transformer twice and this one was the third. I thought that the same problem was happening again because the volume would drop and suddenly come back up. On the bench, the chassis would not act up so I covered it with a cardboard box after putting my VTVM on the grid of the output tube, that had been giving the problem before and the signal tracer on the input to the 1st audio tube. It finally acted up after two days. It turns out that the second 24, RF tube was the culprit, this time, not the transformer! Thanks for all the tips.