Harvey Wells mod tranny

[stevef]

My TBS50-C has a bad mod iron.  The primary/modulator side (a balanced center-tap winding I presume) shows 50 ohms across one side and 3 megs across the other.

There are no specs in the manual to go by, so I'm not sure what values to be looking for in a replacement.    Push-pull 6L6's to an 807.  I went to the Radiotron Designers Handbook for clues on how to calculate it but no luck.

[W2XR]

If you can provide the following information, we can determine with reasonable accuracy the impedances of the primary and secondary windings and the audio power rating required for a replacement modulation transformer:

1) Plate voltage on the 6L6 modulators.

2) Are the 6L6s triode or pentode connected? My guess is pentode connected, but it's worth verifying just to be safe.

3) Plate voltage on the 807 final amplifier. Probably the same voltage as on the 6L6s.

4) Plate current that you normally load the 807 up to.

73,

Bruce

[w3jn]

You might try pulling it out and seeing if one of the windings broke loose at the end (not sure if this is a tranny with terminals, or one with wires).  I've successfully and easily repaired several trannies where the wire burned or broke where it was connected.

If broken inside the tranny somewhere, your only option is replacement or rewinding. 

[stevef]

1) Plate voltage on the 6L6 modulators:  420

2) Are the 6L6s triode or pentode connected? My guess is pentode connected, but it's worth verifying just to be safe:  Yes, pentode

3) Plate voltage on the 807 final amplifier. Probably the same voltage as on the 6L6s:  425

4) Plate current that you normally load the 807 up to:  100 mA

Some more info:
The 807 screen is linked to the modulated B+ line via 10k + 15k series resistors.
The secondary (807 side) of the mod transf. measures 76 ohms.

I'm going to open the transf. up today for a look - there's nothing to lose.

[W2XR]

Hi Steve,

For the operating conditions you have described, the replacement modulation transformer should  meet the following requirements:

1) Primary plate-to-plate impedance: 6000 ohms. I am assuming the 6L6s are biased for class AB1 operation, so as to obtain sufficient audio power to fully modulate the 807.

2) Secondary impedance: 4250 ohms

3) Secondary current rating: 125 ma. min., unbalanced DC

4) Audio power rating: 25 watts, min.

5) Hi-pot rating of 1000 VDC, min., on the secondary side

If you can source a transformer that meets these requirements, you should be in good shape. This website or Fair Radio would be a good starting point.

Such a transformer should not be too difficult to find.

Good luck!

73,

Bruce

[w1vtp]

Hi

My aunt (W1UET) had one of those beauties along with a SX-28.  There was some serious wiring that had to be done in the back of the xmtr. I'm assuming that has been done as per the manual.

BTW. What's the second secondary winding for on the mod xfmr -- to excite the screen?  I haven't printed out the manual and compared the hookup in the back with the schematic.  Kind of odd that it was the only xmtr I know of that included 2M on it.  I have rescued some xfmrs by taking off the winding core and carefully removing the outside insulation and tracing the connections in a ways.  Some times I lucked out and found the break near the end of the winding. 

Good luck.  Al VTP

[stevef]

The wiring across the back terminals is for configuring it for various power supplies or for exciter-only operation, which is what the other secondary winding is for.  It provides 20-25 watts of audio at 500 ohms.

I dug into the transformer and found one break in the first outer layer and repaired it.  But there is obviously another, or more, as indicated by resistance readings.  I can't imagine trying to rewind this thing.  All of the wire ends appear to come out of the outer windings.  How in the world do you have a winding start and end in the out layer?  Maybe I should learn something about transformer construction first.

Meanwhile, still hunting for a replacement.... 

[w1vtp]

.....

Meanwhile, still hunting for a replacement.... 

I just got a nice 60 watt Thordason mod xfmr off Ebay at a reasonable price, ~$30.  I'd keep a watch there and, of course, local festers.  You just have to be careful with Ebay as you know. I think you need about a 30 watt unit if memory serves me right.  You have some good suggestions in this thread

Happy hunting es 73 Al