My neighbor dropped an old PA amp this week end he uses for Guitar. I
am a novice HIGH FI guy and offered to help. The amp had a blown fuse
and the Rectifier tube was burnt up.
I removed the Transformer to test. The power trans has three outs, one
510, CT, 510. One 5 & One 6.3 CT 6.3. They all worked when I removed
them. I put it back together and tested, with the Rectifier tube out
and the 510 Center tap to ground it blows fuse. With the ct pulled off
ground it runs no bown fuse. If I put the Rectifier tube in and leave
the CT off ground it burnt up the tube then the fuse in about 3
seconds.

Any thoughts? Every thing else tests good to my knowledge.

Oh an ohm meter across the Windings shows continuity from both 510 to
CT, but not across both 510's...

Ed Houston
Cincinnati

On 8 Jan 2006 16:48:01 -0800, wrote:

My neighbor dropped an old PA amp this week end he uses for Guitar. I
am a novice HIGH FI guy and offered to help. The amp had a blown fuse
and the Rectifier tube was burnt up.
I removed the Transformer to test. The power trans has three outs, one
510, CT, 510. One 5 & One 6.3 CT 6.3. They all worked when I removed
them. I put it back together and tested, with the Rectifier tube out
and the 510 Center tap to ground it blows fuse. With the ct pulled off
ground it runs no bown fuse. If I put the Rectifier tube in and leave
the CT off ground it burnt up the tube then the fuse in about 3
seconds.

Any thoughts? Every thing else tests good to my knowledge.

Oh an ohm meter across the Windings shows continuity from both 510 to
CT, but not across both 510's...
Was there any OBVIOUS mechanical damage to the Xfmer?

Huh! What was the readings in DC ohms? Did you reverse the meter leads
(I know, it SHOULD not matter on a AC winding)

MEGGER (High voltage Meg Ohm Meter) time.

A normal DMM/VOM will not show winding insulation breakdown. You can
try readings (highest ohms scale) BETWEEN windings, Windings to Xfmr
frame. IIRC The breakdown voltage on the 5V should typically be over
5000V, The 6.3Vac heater leads would nominally be 600V. Never exceed
the windings LABELED breakdown when selecting the Megger test voltage.



Ed Houston
Cincinnati