[spfradios2]

Hi all you experts...

I've been to this "phorum" before regarding my audio interstage transformer and the fact it had a burned out primary. Well, I have secured a replacement.

I'm back again because I want to learn more about how these things work. I'm asking the question... Why did the primary of the transformer burn out?

Here is my theory, tell me if I'm thinking right or wrong. The primary of the transformer is in the plate circuit of a 6F6 driver. That 6F6 has a primary grid that requires 10 volts according to the schematic and associated materials. That grid voltage is controlled by basically three resistors connected in series directly to the power supply. Referring to the schematic those resistances are R114 (330k), R108 (240k) and an IF expander pot (500k). I have found that R114 was totally open, and the pot had intermittent contact. The bottom line... the grid probably had zero volts. Would this situation have caused an uncontrolled current to flow through the plate of the 6F6 and probably burned out the transformer?

Opinions welcome.
Thanks
John Shaffer

[Alan Douglas]

Is the transformer physically charred? Then the answer is yes, the extra current overheated the windings. It wouldn't have melted the copper wire, but could have caused a break at a weakened (corroded) spot. The current couldn't have been too excessive, though; it was limited by the power transformer, rectifier, DC resistance of the primary, and the emission capability of the 6F6.

[spfradios2]

Actually, it was not charred. I took it apart just in case the break was observable. Not sure about corrosion. I've seen it before, but this transformer looked clean... no copper sulfate stains, etc.

Continuing on with the issue... I attended the MARC Extravaganza, Lansing, MI, yesterday. I met a very interesting man from Melvindale, MI who is 92. He said he was a radio repair man before the radio I'm restoring even hit the market. (By-the-by, his chemistry teacher in high school was Charles Lindbergh's mother.) He said my theory is not exactly what really cooked things. First of all, there is no current to speak of in the grid circuit, so it's not like a grid resistor will burn out. He said if it was a high current issue, the root cause was the coupling capacitor connected to the 6F6 grid. It was probably leaking severely, thus wiping out the grid bias and the tube went to saturation. He said that if you really want to protect the primary of the audio interstage tranformer, just make sure the upwind coupling capacitor is very healthy. He suggested a 1000 volt to make it last beyond my life.

When I got home, sure enough, that capacitor was nearly fully shorted out, probably delivering a lot of B+ to the grid.

Whether it was corrosion or high current, at least I learned something from a really neat guy with a wealth of knowledge.

Thanks all,
John

[Alan Douglas]

He was correct; a leaky grid-coupling capacitor often causes high plate current. Whether the high current caused the break or only contributed to it, is another question.

Breaks are usually caused by corrosion where there is a defect in the wire enamel. The corrosion is either caused by acidic impurities in the paper, or electrolysis between the positive winding and the grounded core. Or if it's at the wire ends, it can be from acidic soldering flux.

Heating could expand the windings and pull a weakened spot apart.

[spfradios2]

Thanks for the insight.

I was wondering if I could fix it by rewinding. Only problem is the primary is probably not on top.

Has this ever been done before?

John

[Alan Douglas]

It's tedious but can certainly be done. For starters, remove the laminations and then remove the cardboard layer right next to the core. This should expose the starting end of the primary. Unwind a layer or two and watch for a break, If you find it, bring out the wire and make a new cardboard tube to fill the space (manila folders glued with Elmers work well. Use the old core as a template to form the cardboard).

If the break is elsewhere, or the entire winding disintegrates, you could wind an entirely new primary but the secondary is probably in no better shape.

Re-using the old wire is usually more work than it's worth, and a bad idea if the wire is prone to breakage.

If this were a 1920s transformer, the primary is typically 4000 turns of #40.

[spfradios2]

Sorry for the late response. I've been busy rebuilding the chassis. It's Done! And it WORKS! No mistakes in over 300 connections. I replaced all wire, caps, resistors, etc. This thing is like new.

Anyway, thanks for the rewinding info. I've never done a re-wind before and I think this would be a great test case. If I fail, no big deal as I have a used, original replacement transformer.

Now on to alignment... and that's a whole new question I have. See new posting.

John