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Combining the priomaries and running varaic at 27 volt, again high current drawn, there is about 1 volt from 1 secondary and 2 volt from the other. One secondary is for main power rails and the other for HT supply for a pair of valves. Its a job to say which has some shorted turns. DC resistance of the secondaries is 0.5R and 4.1R I will try feeding low voltage AC in the secondaries and see what happens.

Shorted turns in either primary will show up as a high current when driving either one since the non-driven one acts as a secondary.

Or shorted turns in a secondary.

-- sam | Sci.Electronics.Repair FAQ:

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There is a light gauge secondary, presumably about 18V ac , full wave rectified to give 25.2 V dc for the seriesed heaters of 2 ECC83, CTs N/C. The main secondary winding supplies the TO3 o/p devices and a diode pump for the valve HT. Not determined where the opamp supplies are from.

Putting 20V ac on the heater supply gave 50 V on main and 96V on each primary Putting 20V on the main gave 7.6V on the heater one and 38 V on each primary.

I assume from this the primaries are ok and something wrong with the main secondary, perhaps 2/3 of the run is shorted somewhere, 1/3 only functional. Luckily they always dilute the epoxy infill with chalk or something so easy to chain drill out. Nothing obviously wriong with the outer layer of secondaries but I assume the problem is due to only one layer of polyester, half overlapped on the outside and only that separates squashed secondaries right up against the metal top of the 1U case. Rubber disk on the bottom face of the toroidal but nothing at the top other than the standard toroidal polyester wrap strip. At least they did not decide to pass the coach bolt through top and bottom of the case. If the secondaries are re-windable (if no primary damage) then some extra padding will have to go in over the Tx and some extra space found in the cab.

My wild guess would be the HT secondary since that has the most turns of fine wire with the msot stress on it, but who knows? I would think that it will be rather difficult to sort it out in a well made transformer without knowing what the original specs were exactly, including resistances, or unwinding it.

What are the spec'd voltages on the outputs?

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Don't know the specs. What is Murphy's Law - If something can go wrong it will go wrong at the most inopportune time or the most awkward place. I always thought the primary was buried on the inside of transformers - conventional and toroidal. Not this one, the primaries are on the outside and the main secondary is on the inside, presumably to save on the expensive large gauge copper wire. Basler Electric, Highland, Illinois

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Can a toroidal former become permanently magnetised and cause excessive current draw. ? This one does not attract a small piece of iron. I don't know what the main secondaries are supposed to provide, but can give 300 watts into 4 ohms. I've wound off the primaries to try and understand what is going on with this transfornmer. Applying 70 volt ac , from a variac, across series secondaries is ok but increase to 75 volt and the current draw goes up to 1/4 amp. Should be able to take twice that voltage. Absolutely no visible problem , to the wiring or the coating of the toroid and both secondaries measure DMM the same DC. Tomorrow will apply something a bit over 70 volt with temperature sensitive labels fitted around. I will even try a TV degaussing coil.

No, it isn't magnetized.

That fact that you can now apply 70 VAC to it without significant current drawn suggests that whatever short was present is now gone.

How many turns were on the original primaries? Call that "n".

Wind a few turns on the core, call that "m".

Apply the rated input voltage x m/n to your winding and report back. :)

-- sam | Sci.Electronics.Repair FAQ:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

"N_Cook" wrote in news:gqe45e$16e$ snipped-for-privacy@news.motzarella.org:

The magnetic field on a toroid is contained within the toroid.

You will have a hard time degaussing it unless you wrap wire through it.

You will need at least a single turn of wire THROUGH the toroid to check to see if it 'remembers'.

See how core memory works: