Dead Transformer??

[Jaicen_solo]

I've got a dead Evans tape echo unit that I need to get fixed ASAP (as in, before noon tomorrow!).
At some point it appears the transformer and it's brackets have parted company with the chassis, and have been thumping around inside.
No PCB damage is visible, and all the wiring seems ok, but i'm not getting any mains voltage after the fuse. The transformer primaries are reading very low ohms, as in almost a short, but i'm not sure what they're specc'd to be since I can't find a schem.
So, does anybody have any advice on what I need to do to get this thing going?? I'm leaning toward it being a short in the transformer primaries, so it'll probably need a new one, I just thought i'd see what everybody else thinks.

EDIT: Not sure if it was clear, but the power supply fuse blows immediately on powering up, hence my assumption that it's a short in the primary windings.

[R.G.]

Go to GEO. Read the section of the tube amp debugging page on "fuses blow" as regards testing transformers and the "shorted transformer tester" article.

[distortedskies]

the primary winding will read low when measured with an ohmmeter because your only measuring the DC resistance of the wire that is wound on the primary. Did you disconnect the all the secondarys and then check to see if it still pops fuses? If it does then you can be pretty sure the transformer is shorted somewhere and will need to be replaced. If it stops blowing the fuse with the secondarys disconnected then something downline from the transformer is causing a short circuit, could be a shorted filter cap or something.

[Jaicen_solo]

Ok,

I finally got around to looking at this again. I disconnected all the secondaries from the transistor, but it still blew the fuse. When I hit the power there was an instant green flash and the fuse popped. So, I started looking at the way it's wired to see if I could see anything untoward. From what I can tell, the power indicator lamp is wired across the primary windings, in series with a 10K resistor. I guess that's ok, but the transformer is only measuring about 1.5 Ohms, which sounds absurdly low to me!
Does this seem to indicate a short on the primaries??

[dxm1]

Did you read the articles on GEO? They present a number of things to look for, and ways to test them:

http://www.geofex.com/ampdbug/ampdebug.htm

and

http://geofex.com/FX_images/xform_test.gif

[Jaicen_solo]

Yeah, I read everything over a GEO, but it was mostly related to tube stuff.
I did all the tests I could without actually being able to apply power! As I say, I suspect that there's an internal short in the trannie, I doubt that running the power lamp  thru a 10k resistor drew enough current to blow the (slow blow) fuse. I'm sure that the resistance I measured across the primaries was way too low.

[R.G.]

I disconnected all the secondaries from the transistor, but it still blew the fuse. When I hit the power there was an instant green flash and the fuse popped. So, I started looking at the way it's wired to see if I could see anything untoward. From what I can tell, the power indicator lamp is wired across the primary windings, in series with a 10K resistor. I guess that's ok, but the transformer is only measuring about 1.5 Ohms, which sounds absurdly low to me!
Does this seem to indicate a short on the primaries??

Only maybe. Quoting myself:

Go to GEO. Read the section of the tube amp debugging page on "fuses blow" as regards testing transformers and the "shorted transformer tester" article.

I mentioned the shorted transformer tester because is impossible to tell from the outside of a transformer with a DMM whether there is a short inside the transformer. I've had grizzled veteran amp techs offer to buy the beer when I showed them how to find shorts this way. Here's the link: http://geofex.com/FX_images/xform_test.gif
You need a neon indicator bulb and a battery. It's quick, it's simple, and it's definitive, because it measures the energy that comes out of the magnetic core when something changes.

To test a transformer, you use a DMM to verify that the windings are not burned open, and not shorted one to the other, then you use the shorts tester to find out if there's an internal short. If it passes all these, it's probably good.

As an alternative, you can hack together a test lamp. This is a setup to run a 120Vac light bulb in series with the unit under test. If the unit is shorted, the lamp lights like normal, limiting the current to its normal current drain and preventing your fuse from blowing and everything stopping. If the unit is noy shorted, the voltage is shared with the lamp, and the lamp glows dimly.

Did I mention to be careful with AC power and not get yourself electrocuted?

[Jaicen_solo]

Thanks for chipping in again R.G

I'll give that a try tonight and see what's going on..