Gibson GA-20 Bad Tranny?

[BigE]

Got a Gibson GA-20 that has no output.  I suspect the OT.  I ohmed from the Center Tap to each leg and get around 220OHM on each leg.  But when I ohm across the secondary i get 0 ohms and it tests positive for continuity.  Is that not a bad OT?

If so, what would be a good OT to use for a replacement?

thanks in advance.

[sluckey]

I suspect the OT.  I ohmed from the Center Tap to each leg and get around 220OHM on each leg.  But when I ohm across the secondary i get 0 ohms and it tests positive for continuity.  Is that not a bad OT?

Sounds like a good one to me.

[PRR]

> across the secondary i get 0 ohms

It should be in the area of 0.1 ohms to maybe 1.6 ohms.

Most ohm-meters will NOT read such small Ohms correctly. "Zero" is a probably-right answer.

Anyway, "no output" can be almost ANY part in the amplifier, from cathode resistor to speaker jack. Work through all the cheap/easy parts before you even think about the expensive/tough ones.

[eleventeen]

What PRR said. Trannies *do* go bad, mostly by arcing internally...and whether or not they show carbonization/charring on the outside, such a transformer usually stinks of burnt electrical. Pretty rare.

When testing transformers (both power and output) that I salvage from equipment, I am often amazed at the incredibly low readings I often get on high current ( = fat wire) windings. I start with simple continuity tests to match up the ends of corresponding windings, if necessary, and tie them together. A good sized fat power transformer you'd throw across the AC line can often read .4 ohms on the 120 VAC winding. And my test leads are probably .2 ohms of that! That's not a short??? Nope. (Well...judgement withheld until I plug it in for the smoke test.)

[PRR]

> That's not a short??? Nope. (Well...judgement withheld until I plug it in for the smoke test.)

An Ideal transformer would BE a "short" to DC.

The question is: does that short go-around enough iron to induct power/audio AC voltages?

You think 0.4 ohms is low. I think it is annoyingly high.... when it is the power-line to my house. It looks like 240V 100 Amps. However the wire is l-o-o-o-o-n-g, 0.4 ohms worth. If I actually tried to pull 100 Amps through it, I'd get 100A*0.4r = 40 Volts drop! Which is bad enough on 240V loads, but absurd on 120V loads. (Interesting that the wire won't smoke. It has 100A*40V = 4,000 Watts of heat in it, but spread-out over 500 feet that isn't so much.)

The starter-motor in your car is 12V at 200 Amps or 0.06 ohms. Perfectly good load... for a fat heavy battery and short fat wires.