Please help with PT diagnosis

[tubeswell]

I have a PT that I accidentally shorted-to-ground a while back (bad circuit wiring on my part - the Mains fuse blew instantaneously at the time IIRC) and today I thought I'd test it properly, just for a what-the-heck? moment. It used to work fine for years.

So I have wired up the PT into a chassis and have all the secondaries wired to unconnected terminal strips. If I disconnect the CTs for the HT and heater windings from the chassis, there is no DC resistance between any of the secondary windings, or between any of the secondaries and the chassis. There is no DC resistance between the primary and any of the secondaries. When I connect the CTs for the HT winding and the 6.3V winding, they are bolted securely to the chassis and I get DC resistance/continuity accordingly.

The 3-chord main is grounded with a dedicated chassis ground. I have a 1R 5W power resistor in series with the active/phase mains wire, and without any meter across any of the secondaries, there is virtually no VAC voltage drop across this resistor (0.11VAC/1R = 0.11A, i.e. 0.0121W), and the fuse is fine. I have the active/phase wire fused with 250mA Slo Blo.

What's more, I can meter across either the HT winding or the rectifier winding okay (and get good VAC readings - 695VAC and 5.3VAc respectively).

But when I try to VACmeter across either the 50V winding or the 6.3V winding, the mains fuse blows (instantaneously in either case). How come this happens, even though there is no current to speak of across the primary (as measured through the 1R power resistor?) and otherwise no load on the PT secondaries? Go figure (please?).

Schematic of wiring diagram FWIW

[Tom Phillips]

Do you, by chance, have the test lead plugged into the current measurement input on the meter?

[tubeswell]

Do you, by chance, have the test lead plugged into the current measurement input on the meter?

No I had the lead in the voltage measurement input for all readings.

Could it be a trick of a small fuse rating? (But why would the 5V 3A winding and the 660V 150mA winding work okay with a 250mA mains fuse, and not the 6V 5A winding nor the 50V winding?)

[Enzo]

If your meter is a typical digital meter, it should have about a 10meg impedance. That makes it difficult to blow fuses with. It may have a fuse in it, but that usually protects the thing from current, not voltage.

In a working transformer, you should be able to put your AC volt meter between ANY two points on the transformer and take a reading. The reading may even be meaningless, but you shoulkd be able to take the reading.


I don't know why the meter should do this, but at this point I would try the same setup with a known-good transformer just to test our testing procedure. What if for some reason the meter has some internal failure. Though why it would show up on 6 and 50 but not 5 si beyond me.


And another test might be to put a resistive load on the winding instead of the meter. For example a 12 ohm resistor on the 6v winding should draw a half an amp or a 24 ohm load a quarter amp. A 1200 ohm resistor draws 5ma. There you go, clip a 1200 ohm resistor across the 6v winding and power on. Does it still blow that fuse?

So my message is: verify your testing protocol, and use alternative means to test the transformer. Once we know just what part of this story is blowing the fuses, then we can figure out why.

The unmentioned part of this story is the terminal strips. Any chance putting probes on the terminal strip is causing some close things to go ahead and touch? Try clip leads to your meter instead of probes. Try pulling those wires OFF the terminal strip and se if you can measure them hanging in air.

[tubeswell]

TFT Enzo

I loaded all the secondaries a bit and re-tested with a fresh battery in my meter. It blew less-often*, and didn't blow at all with a 1A Slo Blo fuse with successive testing. Maybe the smaller fuses couldn't handle the switch bounce or something?

* i.e.: I could actually get a reading on most of the secondaries without blowing the 500mA fuse. However, when I tried to meter the HT after I loaded it with 2M2, the 500mA fuse always blew (altho' it ran fine without the meter leads attached).

The total VA on the secondaries with the loads depicted in the following (revised) schematic is 0.85VA - which is puzzling because this doesn't seem to equate to the calculated .287VA on the primary side (Wall supply is a steady 230VAC here, and I measured 0.125mV across '1R' in series with the active/phase mains wire).

Surely the secondary total VA should be less than the primary VA if the secondary was losing power somewhere? Where is the extra power on the secondary coming from?

Or is the rated 1R (5W power resistor) on the primary side too inaccurate to rely on for proper power calculation?

Or are the secondary loads too light to accurately assess the efficiency of this PT?

Or is this something to do with the operating characteristics of transformers?

(Or is it something funny about this PT?) Or am I missing something? (which has been known to frequently happen)

[Steve Conner]

Some transformers have a big inrush current that will blow fuses at random. Maybe the fuse is blowing at random and you're imagining a pattern.

New transformers have worse inrush than the old Tweed-era ones, due to the introduction of grain-oriented steel, so a fuse value from an old Fender schematic may not be appropriate.

[tubeswell]

Some transformers have a big inrush current that will blow fuses at random. Maybe the fuse is blowing at random and you're imagining a pattern.

Yeah - it's possible I've got an overactive imagination :-)

New transformers have worse inrush than the old Tweed-era ones, due to the introduction of grain-oriented steel, so a fuse value from an old Fender schematic may not be appropriate.

TFT. Handy to know.