MR67 Updating

I have an MX110 where the internal caps in the equivalent transformer (associated with the 6U8) developed a case of silver migration causing a pri to sec short. I disassembled the transformer, removed the faulty caps, removed the migrated silver from the caps and measured them at about 900-980pf each. I replaced them externally with two 1000pf dipped silver mica caps. Obviously, the whole thing required realignment.

The mfgr part numbers on the MX110 transformer are the same as on the MR67 transformer and the basic circuit is the same so I assume the transformers are also the same including the value of internal caps.

At the risk of stating the obvious, repairing a transformer such as this is an exceedingly delicate and tedious operation with a fairly high probability of failure because the internal caps are an integral part of its physical construction and there’s no good way to simply disable them: they have to be physically removed. You'll also probably be required to unsolder and resolder some super fine wire...something like #46. Very easy to break the leads as they enter the winding proper. Break a lead and you’ll be looking for a replacement transformer.
 
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FYI: a few months ago I spoke with Audio Classics, and they told me that they do repair these transformers for$100. Fortunately the replacement I got for mine is still good, but it is great to know I can get it rebuilt if mine goes south.
 
Thanks Steve. I purchased a second donor or more correct a cadaver MR67. I removed the 19khz transformer and disassembled it. It was seized up so I had nothing to lose by using it as a test mule. I freed up the ferrites and re-lubed them. I did not drill out the rivet to get at the caps. I know what you are saying about the coil wiring. It is definitely a concern. Care to share any lessons you learned while doing it? Did you use any kind of a fixture to hold the plastic housing together? I am seriously thinking of building one to keep things together and keep things away from the coil wiring.

If things go south on my original transformer, I still have the second one. It is known good because the donor tuner was and still is fully functional.
 
I’m surprised your ferrite cores were frozen. Both of my transformers are the “Ktran” version with modified slotted adjustment. Have never encountered a frozen Ktran “slug”. Compare to threaded hex core slugs where about 25% are split and useless. OTOH, have encountered lots of Ktrans with shorted internal silvered mica caps. While optimized for automated assembly, these things only last a few decades before silver migration/whisker problems occur.

Used no special fixture. Just one of those dual-screw woodworking clamps.

Lessons/tips:

Do:
1. Use vacuum type temp controlled desolder station.
2. Use top quality 63/37 eutectic solder.
3. Use temp controlled solder iron set to as low a temp as is consistent with good soldering practice.
4. Use extra flux.
5. Be patient and perform work in a non-distracted environment where you won’t be interrupted.
6. Make a detailed pictorial diagram of everything so you get it back together exactly as it came apart.
7. Use good quality magnifying optical visor.
8. Keep solder iron tip clean and properly tinned.

Don’t:
1. Use desolder braid or mechanical or bulb type solder sucker.
2. Use lead free or silver bearing solder of any type.
3. Don’t flex winding lead wires.
 
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