Output Transformer Resitance

[marv]

I have finally finished rewinding the OT. Quite a task. The CT primary consists of 3900 turns of #37 in 26 layers. I took a gear drive out of my pellet stove that rotates at 2rpm. I mounted the core on a 3/4 x 3/4 wood shaft attached to the drive of the motor. Shaped two cams on the end of the wood shaft that actuated two micro switches. Took an old calculator and wired to the 1 and + terminals so with each rev it counted the turns for me. Took about 1 hr and 20 minutes per layer.

I have the OT back in the original metal and ready to mount it back on the speaker frame. The resistances of the primary and to the CT are within a few ohms of what the schematic says so I know I used the right size wire and number of turns. Am hopeful it works once I get it done. I need new glasses now.

[morzh]

Wow! Respect, Marv! I would probably just do it by hand, though last time I did it was when I was a teenager, and even though back then I was much crazier about doing this kind of stuff (I am paid for it now, that's why) I still did not enjoy this particular one.
I still have this fear of inductors in me...even if it is just 10 turns.
But if I ever have to choose between making a tool and winding by hand.....by hand it will be.

Did you also put the wax paper between layers and soak the whole thing in the hot wax, or you just did layers without separation?

[marv]

I used "magic" tape between layers, don't know if that is good or bad or how long it will hold up. I was going to soak the completed winding in varnish (did this back in 1976 when I rewound a power xformer for a AK 89) but decided against it. The #37 is so fine I was afraid if I broke a wire during final assembley (which I did) it would be dificult to fix. I have the OT mounted on the speaker now but have yet to try it with the radio.

[Arran]

(04-20-2012 12:13 AM)marv Wrote:  I used "magic" tape between layers, don't know if that is good or bad or how long it will hold up. I was going to soak the completed winding in varnish (did this back in 1976 when I rewound a power xformer for a AK 89) but decided against it. The #37 is so fine I was afraid if I broke a wire during final assembley (which I did) it would be dificult to fix. I have the OT mounted on the speaker now but have yet to try it with the radio.

I've never tried rewinding a transformer before but have considered it and read some articles on the procedure. As far as I know "Magic" tape is fine, that is if it's the tape I'm thinking of, the frosted type that is made out of polyester or some other plastic. The stuff that is NOT alright to use is ordinary Scotch sticky tape, the tape itself breaks down and the glue goes with it, I believe that it may be corrosive. That is one thing that I have learned with splicing eight track tapes, Scotch tape is bad (glue goes gummy then the tape dries up and falls off) but Magic tape is fine if you can't get real splicing tape. Once you know that the rewinding job is successful you could take it to a motor winding shop and have it dipped, if the price is reasonable, they vacuum the air out so the varnish gets into all of the layers.
Regards
Arran

[codefox1]

(04-07-2012 03:56 PM)exray Wrote:  

(04-07-2012 12:39 AM)marv Wrote:  OK, I am getting ready to attempt a rewind of this thing and have one more question. I counted the turns of the bad OT and measured 3984 turns on the primary and 39 on the secondary. I think the turns ratio agrees with what Ron and Morzh stated above. By my measurent the primary used #37 wire and the secondary #20. I have some new 37 on the way but do not have any 20.

What will happen if I use #22 for the secondary maintaining the correct number of turns? Will the impeadance still be correct but result in a lower watt rating?

Hi,
I'm a bit late to this thread. You might be able to sneak by with #22 but it would probably be worth the effort to get some 20 so as to not have to worry about it later. I guess re-doing 39 turns wouldn't be a terrible job if you had to rewind it.

On that high 6K5 voltage. If its still high after you get the xfmr redone then you should probably question the type of meter you're using. That's a very high impedance point and the meter they used originally to create the voltage chart probably loaded down the circuit somewhat. A DMM will read high at points like this.

So long as you have enough "real estate", secondary guage not that important.

Make sure you have corrected voltage divider circuits before firing up again. I like to insert an old fashioned analog DC ammeter in circuit while bringing set up slowly on variac to see if there is a lingering problem

[marv]

Forgot to mention I used #19 for the secondary which was the correct size. A local motor rewinding shop gave me 20' of it free. They have been kind enough to help me out over the past 30 yrs.