Dynaco st-70 power transformer dipped in "tar"- Don't do this at home!

davidpg

New Member
I recently completed a rebuild of a Dynaco ST-70, following the guidelines from Dave Gillespie's "base-line testing" article, and I plan to post some pictures and comments about that soon. But, there was one misadventure in the project that I thought might be worth a separate thread.

My original plan was to use only the original chassis and transformers for the rebuild, and I pulled everything off of the chassis. I then saw that the insulation on some of the leads from the power transformer were badly cracked, right where they entered the bell. I think that some of those cracks may have formed in the process of working the transformer out of the chassis. My plan then was to remove the lower bell and replace the leads with nice new wire and put everything back together.

However, when I tried to remove the bells, I discovered that they were filled with a very hard, black tar or epoxy material. I persevered, but wound up pretty much destroying the windings, as illustrated in the photo. Before trying to take the transformer apart, I did notice that there was a layer of this stuff on the outside. But, it was so badly applied, with heavy drips, that I thought that it was probably done by a previous "repairer", and I didn't imagine that it would be inside holding everything together.

In retrospect, I probably should have left the transformer in place, assuming that I could have convinced myself that the insulation on all of the leads was OK.

After making this mess, I had an e-mail exchange with Kevin Devaney of Dynakit parts (from whom I bought a new transformer). He says that he has seen Dynaco transformers treated this way and thinks that Dynaco started doing this to reduce mechanical hum. My transformer is marked 1967.

In any case, if you come across a transformer like this, don't do what I did! (unless you just want to see what is inside.)

David
 

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If you're really desperate to work with those old hardened leads, then it's often possible to soften them temporarily with a heat gun.
 
After making this mess, I had an e-mail exchange with Kevin Devaney of Dynakit parts (from whom I bought a new transformer). He says that he has seen Dynaco transformers treated this way and thinks that Dynaco started doing this to reduce mechanical hum. My transformer is marked 1967.


I would have lost a bar bet on that one. I've never had the bells off an ST70 PT and had no idea there was tar in there.
 
I usually just refrain from messing with old trafo leads unless i have no choice in the matter. I was sweating bullets when I pulled and re-installed the transformers on my Pilot SA-260. I pulled them for paint, and as soon as I started I began wondering if it was a horrible idea. Unfortunately the outputs needed to come off anyway to replace the bias pots since the mounting brackets needed new holes drilled, which of course were right under the transformers.
 
Extending leads on old transformers really isn't a lot of fun either... I had a power transformer I was going to use for my preamp and it came with leads that were about 1" long. Had to take the end bells off to extend them, and then I've got this big ball of wires covered in heat shrink coming out into this little chassis.

It was after I got it mounted, wired and cleaned up that I learned it didn't have separate 175, 50 and 25v windings. It had one giant secondary with 175, 50 and 24V taps. That transformer definitely didn't have tar, but it did have a fair bit of mechanical hum/buzz.
 
The Stromburg Carlson ASR-444 amp I built also has tar inside the power transformer, but has none on the outside.
 
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