Power transformer replacement??

[Tubejunke]

Heres the latest on the TC-127. I finally got around to unbolting the original transformer. I removed the endbells. Now I know where the burning smell came from. No bare wires shorted. Just burnt. Really that is good since I have invested in the Merit P-3070. The Merit unit is a bit smaller which means less weight. That always helps.

There are some differences in specs from the original to the Merit. John said that the original damper winding was 6.3v 1.2a. for instance. The Merit is 6.3v 2.7a. There are other differences but my Merit specs out at more than the original amperage in all cases. I would think that would be OK. Somebody make me feel good on this one.

Since I have real nice cloth wire I am thinking of simply connecting to everything up near the mounting point. I will not have to thread and tuck away new wire if I do this, plus the aspect of a more original apearance. I have to locate a big 2 watt 100 ohm resistor before I fire up the old thing. I noticed one that was cracked in two. Not really burnt, but cracked.

Now that I work a regular 9 to 5 its hard to get to one of the FEW electronics suppliers around here. Too bad Radio (joke) Shack aint what it used to be...

They probably wouldnt know what a resistor is.....Remember buying and testing tubes at the Shack? Those were the days!!

[bgadow]

Originally Posted by Tubejunke

The Merit is 6.3v 2.7a. There are other differences but my Merit specs out at more than the original amperage in all cases. I would think that would be OK. Somebody make me feel good on this one.

Yep, more amps is more better!

[polaraman]

Tubejunkie,

The SAMS went out today. I have the same set in storage.


polaraman

[Tubejunke]

Thanks friends. I'm going to start the install tonight.

[Tubejunke]

Just for the sake of conversation here are some resistance measurements from the old (fried) transformer. Pretty weird!!

First I notice that none of the windings are color coded like my replacement. Merit had sense enough to group the windings in code. Like wire colors are seperate windings in this crazy thing. You get infinity across all but yellow to yellow.

Now between the Blue/yellow stripe to a Black I measure .7 ohms, 1.4 on the other set.
Between a Brown and one of the Blue stripes I measure .4 ohms, same on other.
Between one Yellow and a Green stripe I get 9.2, to the other yellow I get 14.2. This is weird because my measurement between the two yellows was 15.2.

I thought I had found the winding with center tap. Actually I probably have but I must remember that I am taking measurements from a faulty unit.

No wonder in the beginning I was having trouble taking resistance measurements. I was trying to match colors as far as windings go. Live and learn....

[Tubejunke]

Well, the replacement bolted in fine. My game plan was to cut the old leads up near the old transformer and using a minimal amount of the replacements wire as it is rubber instead of the better looking cloth. On top of that I think this would be the best way to go in terms of common sense. The replacement did not have enough lead to go to the various connection points, so either way you would have to add some wire. Why not use what is there AND already routed real nice?

So, everything is now wired in. (Hope its all right) I went ahead and purchased enough caps to replace the whole power supply filter section and MOST other electrolytics. I also got a bunch of orange drops to replace most of the wax coated paper caps. Now I see why a lot of pros say change all of the caps. Tonight I noticed a 100 ohn 2 watt resistor burned in two in one of the horizontal circuits. There was a .0022 near by that measured like .012 and also there was a .22 that measured .001. I'm not sure about the mathmatics or exactly how much tollerance is allowed without noticable performance issues, but a measurement is a measurement and that is way off from the new stuff I put in. At least I will have peace of mind. I did not fool with the crazy stripped black caps which I have heard are troublesome as any, if not more. I may regret this move...

I'm trying to do this right so I have not yet applied line voltage to the set. I may break out the 100watt bulb again. I believe you are supposed to pull the horizontal output tube when doing the reduced voltage run. Anybody know?

I should have some results this week. I have a goal of seeing a picture by Christmas. Unless I really screwed up some wiring I should be a winner. Hopefully the power supply was the major problem area back in the sets use. That way all the RF coils and other odd ball things will be ok. No mysteries....

Hopefully!!

[Chad Hauris]

Yes you will regret it if you leave those striped caps in, please replace them or you will be asking for trouble...you can end up with hard to diagnose troubles or even if the cap holds together now it will decide to crap out and cause damage when you have it all back together.

[Charlie]

Chad is right... change those black caps! All kinds of quirky little problems go away when you get rid of them. Also, I've had those blow out... and it will certainly cause you to soil your britches... they sometimes sound like a 22 pistol going off.

[andy]

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[Tubejunke]

Good info.. Thanks folks!! No progress today. I feel like I can try the set with the black caps still in place. Sure there will probably be less than perfect performance but I can kind of check my progress so far with the whole power supply being replaced. Actually I went for a long time leaving those things alone in other sets because of that stupid color code. I have a book now that helps decipher what value thet are. Before that I just changed electrolytics, tubes, and bad resistors.

[Chad Hauris]

The sams with the pictorials also makes it easy to verify you have the right value on those black color band caps.

[Tubejunke]

Well, my "by Christmas" goal will not come to pass. I finally plugged it in. First with a 100watt bulb in the line for about one hour. Everything seemed OK. The light slowly dimmed, so I knew my SUPER SHORT days were over. The bad thing about the bulb method is that the voltage is so reduced that the DC never gets pumping enough (or at all?) to slow charge and reform the paper caps. I think it works this way.

Needless to say after a couple minutes of operation with no sound or raster I heard a sizzleand quickly disconnected the line. Low and behold one of those stupid black stripped paper caps had gotten hot. A .018mfd 1000v damper filter. I unsoldered one end of the thing and it measured like .15mfd. So until I get my hands on the 7 or 8 1000v caps that I will purchase to cover everything needed I am at a standstill. I have enough orange drops to keep me busy with the soldering iron. Just no 1000v jobs.

I feel good even though I still have a dead set. I started off with something that could not safely even be plugged in. I may not be as far away from success as it seems. This cap is probably why I never got a raster. Then I can look into getting sound. I didnt expect a much better result. I just wanted to make sure that there were no screw ups with the power supply re-vamp.

[Charlie]

IF you get those caps in time, you could set a new goal of "by New Year's" and possibly have a good enough picture to watch the ball drop for 2007.

[andy]

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[kbmuri]

Originally Posted by Tubejunke

The bad thing about the bulb method is that the voltage is so reduced that the DC never gets pumping enough (or at all?) to slow charge and reform the paper caps.

Correct. When you "warm start" a piece of tube equipment by variac or series light bulbs, the transformer is operated at reduced voltages. This means the 5V supply to the 5U4 filament is reduced as well. At reduced voltages (especially at the low end of the variac sequence), the 5U4 filament isn't hot enough to "boil" any electrons, the electron cloud is not present, the plates do not conduct at all (no emission). You'll measure little or no B+ until a significant percentage of "full throttle" is reached. I believe it's just a threshhold where, when reached, the 5U4 suddenly fires and a sudden, near-full jolt goes to your vintage caps. This is why most in-circuit "reform" attempts fail.

Build a couple of these (see photo). It is a solid-state 5U4 that uses a pair of 1N5408 diodes as the rectification method. They work at all AC input voltages from 0 to 1000, incrementally and/or smoothly, to avoid this problem.

You'll want to have two of them in your toolbox, one I call left-handed and one I call right-handed. The unbanded side of the diodes go to pins 4 and 6 of the tube base (salvaged from bad metal radio tubes such as 12SA7). The banded side of the diodes, connect together, then connect the joint to pin 2 for a left-handed unit, or pin 8 for a right-handed unit.

It will be up to your specific application as to which unit to use. Either pin 8 or pin 2 will be connected to your B+ supply and power filters. The other will go thru the 5V windings of the power transformer. I don't know that it matters if the high(er) voltage runs thru the 5V windings or not, but I prefer not to do so. So use the unit that corresponds to the pin going to your power filters (i.e. a choke or the 1st electrolytic). 5U4 pins 2 and 8 are symmetrically interchangeable in practice, so don't necessarily trust the Sams schematic. Look at the wiring. I have at least one example where the sams is wrong (my 1949 Magnavox).

This all having been said, I agree that reforming 50-year-old can caps is a risky business. I just use the solid-state 5U4's for slow-starting to avoid smoke-and-flames until I'm sure everything's wired up correctly. After putting in new capacitors.