Weird output transformer

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Weird output transformer

Postby Leif B on Sun Jan 29, 2017 1:08 pm

I started a thread elsewhere about some funny DMM readings, and the epic story led me to discovering (among many faults in an amp) a strange output transformer. For a start, after a complete overhaul of the power supply, the power amp sounds terrible. Distorting at low volume, it sounds as if there's a short in the OT – I'm guessing secondary.

It was in a Sound City L120. Never seen an OT like it.

The primary has four wires – as if it's two single-ended primaries. The two black wires are joined together to make a CT, and from there we get a familiar three legged OT primary.

The secondaries (plural) are strange too. It LOOKS as if there might be two separate windings for two speakers – like perhaps this OT could be used in a stereo hi-fi amp with two single-ended outputs.

It definitely **sounds** as if there's a short in the OT somewhere, but I can't put my finger on it completely.

Primary – DCR in the primary is around 15 ohms CT to each leg. (around 30 ohms leg-leg)

Secondary – it's as if there are two 'sides' to the secondary – they are electrically separate from each other, BUT, when wired for play, each colour is joined like-to-like – as if perhaps it's two 60-watt windings combined to handle 120 watts.

The colours are (2 x) black (common), orange, brown, yellow, red
Measuring DCR from the common, both 'sides' give familiar readings for each respective tap – until you get to red.
So it looks like this:
Side 1:
Bk (comm)
Or – 0.6ohm
Br - 0.7ohm
Y - 0.9ohm
R - 1.6ohm

Side 2:
Bk (comm)
Or - 0.5ohm
Br - 0.7ohm
Y - 0.8ohm
R - 3.8ohm

There is no short from side 1 to side 2. They remain isolated.
Curious, no?

I'd love to hear some similar experiences if any of you have one
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Re: Weird output transformer

Postby Pitchfork on Mon May 01, 2017 5:48 pm

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Re: Weird output transformer

Postby Leif B on Tue May 02, 2017 6:33 am

Thanks, but what am I looking for? I haven't seen any OT that resembles what I've seen.
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Re: Weird output transformer

Postby pdf64 on Tue May 02, 2017 7:22 pm

Your description of the OT seems like a regular UK Electrovoice / Partridge SC spec OT to me, nothing unusual.
Note that a single OT can't be used to output independent stereo signals.
From the site previously linked, see http://soundcitysite.com/sc_120_ot.jpg
As you have a concern about the OT, I suggest to separate and isolate all windings.
Then feed a signal into one of them and see what appears at the other windings.
If it all looks ok but you still have doubts, try http://www.geofex.com/FX_images/xform_test.gif
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Re: Weird output transformer

Postby Leif B on Tue May 02, 2017 8:46 pm

Yes, the secondary side of that drawing represents what I've seen – as far as the external wiring goes.
It doesn't show how they connect internally to the windings. My DCR measurements showed they were isolated from each other.

As for the primary in that drawing, it shows it as a regular push-pull with 3 wires.
But mine has four wires, and DCR test shows the pairs are isolated from each other. It's like a power transformer primary, with a 120/240 option.

I remember testing it with signal, and it was hard to discern anything wrong by looking at waveforms.
When the OT was in the circuit – that's when you could really tell things weren't right.

That neon test is a good tip, thanks for that.
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Re: Weird output transformer

Postby pdf64 on Wed May 03, 2017 2:09 pm

Have a read up on OT interleaving and it may become clear how better quality OTs have their windings in sections.
These sections are then connected together as required.
I think it may be looking strange to you because (some of) the section leads have been brought out, rather than being connected together under the tape wrapping around the windings.
Note that if there was a hard short across even one turn of a winding, your test results would have looked bad.
The winding being driven would have drawn much more current than expected, the inductance of all windings would have collapsed, as the shorted turn/s sucked most of the energy.

It may be that there's a (intermittent) short across some turns that only manifests under certain conditions, eg high signal levels.
Other than an autopsy, swapping in a good OT (doesn't need to be an exact spec equivalent) can be the only way to id such faults.
Good luck!
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Re: Weird output transformer

Postby Vortexion on Wed May 03, 2017 3:00 pm

pdf64 wrote:Other than an autopsy, swapping in a good OT (doesn't need to be an exact spec equivalent) can be the only way to id such faults.

Wot he said. As your tests (involving injecting signals straight into the OPT) revealed nothing odd, I do wonder whether you're looking for a fault that's not there - not in the OPT, I mean. Unless you can try the amplifier with another OPT, you've no proof that the distortion problem isn't being generated by a fault earlier in the amp's circuit. It's just a shame that OPTs aren't exactly the cheapest or easiest things to lay hands on if you haven't already got a known good one lying around your workshop... :cry:
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Re: Weird output transformer

Postby Leif B on Thu May 04, 2017 6:58 am

Thanks for the replies. I replaced the OT a while back and – oh yes – the original OT was indeed at fault.

This was a Sound City 6 x EL34 power amp, and I thought the stated 120 watts is far too much amp for today's sensibilities. Plus, an exact replacement for an amp regarded as inferior cost a small fortune.

So I stripped the innards, and hand-wired a 100W Marshall-esque amp, using a Hammond OT and... yeah it moves air.

Intermittent fault might well have been the problem, but now the amp has been reborn, I'm more interested in seeing if this OT's wiring looks familiar to anyone. I've just never seen any OT with this may wires hanging out of it.

PDF64 – I shall look up 'interleaving', thanks for that. Your description makes sense.
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