Heathkit transformer replacement...

[kmcarpenter]

Hello all, I have an old pair of heath amps, usually I'd be on here blathering on about issues with the Power amp, an AA-121 (EL-34 PP stereo)- but now I am going to blather on about an issue with the matched preamp.

The Preamp is an AA-141A unit and what has happened is that I lost plate voltage the other night running the tuner quietly.

After poking around with the meter, I have determined that the B+ tap on the power transformer has gone open. Must've popped the winding. It still powers up and I have filament heat and a pilot light on the front, so I know that at least the 6 volt taps are still working, but that doesn't amount to a **** hole in a snow bank without voltage on the plates.

Now, the heathkit part number on the top of the xfmr is 54-71, and after googling around I have come up with zilch, so I may have to order an aftermarket unit.

The question is, what the hell am I looking for. I've never had to buy a transformer before.

With a designed input voltage of 117VAC I have 2 6.3V taps for the filaments and a 238V tap after a couple of Selenium diodes for the B+ supply. I never had a meter on the winding while it was working, so I dont know what the actual voltage on the winding was. These numbers are "what I should have" according to the schematics.

What does having a transformer re-wound usually cost?

[thaumaturge]

Best bet is find another piece of old tube equipment, like a really old tek scope. They had oodles of transformer taps to choose from. Physical size would of course be a consideration. Doc

[kmcarpenter]

I am a bit of a nostalgia freak -I would never dismantle a piece of equipment like that for some iron unless it was total and utter junk.

I've got a couple of old Tek 564B's and I love them both.

But yeah, physical size is kind of a big deal and the iron on my old tek is huge.

[HollowState]

Finding a suitable replacement power transformer should not be very difficult. But I can't find a schematic on line. Only pictures of the inside, so I know about what physical size tranny you need. Can you post the diagram here? Even if you can scan just the power supply area and post that. There may well be something from Edcor, Hammond or even eBay that will work.

Also, I have had luck in the past repairing transformers much like yours. You have to remove the shells and perhaps even the outer layer of insulation to find a break. Sometimes the break occurs near the top where the lead wires are attached. If it's really open, you have nothing to loose by trying.

[VictoriaGuy]

If you post a picture of your transformer, it would help.
Is it a 'laydown' - type?

[VictoriaGuy]

Quiet evening here, so I hit Google for a bit.

Looks like the 54-71 was used in several Heathkits, including the AA-20 and SP-2. Looking at those schematics (available online) the PT has a 117/120 v secondary (ie 1:1 winding ratio with the primary), feeding a voltage doubler to give a B+ in the 290volt range.
Finding a 120volt 'isolation' transformer that can handle a few (50?) mA should be pretty easy, but.... My guess is that there's not a lot of extra room on the chassis? So you'll need something with the 6.3 vAC for the filaments as well if there is no room to add a 2nd transformer. You can figure out how many amps of heater current you need from the tube specs.
That should get you started......

PS- Any ideas on what caused the HV secondary to fail?

[Tom Bavis]

If there's room for another transformer, a Triad 120:120V transformer is about $15 new. I have a couple of transformers with 120V and 6.3V windings - and 230V could be used as well, with a full-wave bridge, or 230-0-230 with a center-tap full-wave. The seleniums should go anyway. What are mounting dimensions? MAXIMUM transformer size?

[jackinnj]

Rewinding a transformer like this isn't difficult. I saved this article from Radio Electronics -- perhaps a bit difficult to read:

Rewinding Transformers

Good transformers for preamplifiers can be had from old Eico/Heath/Knight test equipment -- get an idea of the output voltage you need. I've repurposed a bunch of Eico audio signal generators for this end.

Lastly, Antek has some low VA torroids for tube stuff on their website.

[v4lve lover]

Practical Transformer Winding

have a good one

[kevinkr]

Edcor could probably make one if you provide the specifications, there might even be a replacement market for this part if as widely used as it seems.

Here: EDCOR Electronics Corporation

You would have to provide them with a very good idea of what you need, (core dimensions, etc.) and note that their idea of customer service may not be yours.. I recently had them make me some custom power transformers which turned out very nicely and were very reasonably priced - yet to finish the project they will be used in.