Transformer question

[E biddy]

Before I throw away a perfectly good transformer.

If all of the primaries (5 of them) short to each other, it means that the transformer is bad, correct?

[Tom Phillips]

Before I throw away a perfectly good transformer.
If all of the primaries (5 of them) short to each other, it means that the transformer is bad, correct?

What you call "...all of the primaries (5 of them)" is equivalent to one long wire with three taps placed between the two ends. What do you consider to be a "short?"
Tell us the actual resistance readings that you are measuring.

[frus]

So all 10 combinations are dead short?

[Justin Thomas]

If you use the continuity function instead of the resistance function, depending on your meter, this may not be out of line... as Tom said, what are the actual resistance readings?

Justin

[E biddy]

Thank you, I wasn't processing the different wires at taps. For some reason, I was thinking separate wires.

I have resistances from 50 down to 3. So this might be ok.

From a power supply for a dbx586 mic pre that is blowing fuses.

[nickb]

So all 10 combinations are dead short?

You cannot tell in general by just measuring resistance. What kind of transformer is it? What are the voltages / impedances of the windings?

Almost nothing is ever a 'dead short'. It could be that the winding you are measuring has so low a DC resistance that it cannot be measured by your meter. As Tom asked, "tell us the actual readings'. Tell us about the transformer.

Edit: Uh oh. Posts crossed in the aether...

[Chuck H]

What they said^^^^^^^^^^^^^^^^^^^^^^^

Can you show us a schematic that we might try to determine what else, other than the PT, might be causing your fuse to blow?

Nevermind. Found it. But it doesn't spec or even show the transformer or the fuse. It just shows the power supply from the rectifiers forward following the ribbon cable plugs.

ftp://ftp.dbxpro.com/pub/pdfs/discon...0Schematic.pdf

[The Dude]

....or just hook up the primary, disconnect the secondary, and measure the output.

[E biddy]

I've checked the resistance between the pins on HD13 and HD14. The only place that is low is between pins 1 and 5 on HD13. The resistance is 9.5.

[Tom Phillips]

I've checked the resistance between the pins on HD13 and HD14. The only place that is low is between pins 1 and 5 on HD13. The resistance is 9.5.

Q1) Is that with the transformer secondary plugged into HD13 or with the transformer secondary disconnected?

Q2) If you unplug the power transformer secondaries, does the mains fuse still blow when you turn on the power?
We are trying to isolate the problem with these tests.

[E biddy]

The secondary is unplugged.

I don't have any more fuses to blow until I buy some more tomorrow

[Justin Thomas]

I don't have any more fuses to blow until I buy some more tomorrow

Might be time for a light bulb current limiter... a.k.a. fuse-saver.

Justin

[The Dude]

So, you checked the primary windings, but have you checked to see if there is a primary to secondary short anywhere?

[Tom Phillips]

The secondary is unplugged...

Understood.
Since you are reading 9.5 Ohms between pins 1 and 5 on HD13 with the transformer disconnected, the fault possibilities are:
1) Bad components in that 5V power supply circuit. Start with the four diodes in the bridge rectifier and the two capacitors shown on the schematic.
2) Something loading down the +5V Unreg line. What is that line powering? Should it be drawing high current?
3) Something loading down the A/C+ or the A/C- line. Same what is it powering question as above.
Look for other connections that can easily be opened to disconnect circuitry hooked to those lines to further isolate the problem area.

[E biddy]

The 5v circuit powers an optional digital output that this unit doesn't have, and I think that the A/C+ and -A/C are for the lights on the meters.