testing Seeburg Output Transformers

on an early stereo Q160 ( chassis SHFA2 ) I am going through some
spare amps and trying to figure out whether the transformers are any
good. I ohm'd out 3 different Output Transformers and they all read
the same to within an ohm or so BUT they all were unsymetric in the
same way which causes me to believe the winding is partially shorted,
but all 3? and the same to within an ohm?

Center Tap to each grid reads 30 ohms but grid to plate reads 50 ohms
for one tube and 75 for the other in the pair --- in every case. These
are with the 6973 tubes, the sockets were empty of course and there
are no other components attached to those leads so I am measuring just
the winding resistance.

Anybody ever see this before?

Thanks in advance for your comments, Travis

I think I answered my own question. Will post it for future
reference.

It is normal for the DC resistance to be non-symmetric because what is
critical is to have the TURNS be symmetrical and they are, but because
one part of the winding is at the core and another part is at the
winding surface, the ohms will be more for the same number of turns on
the larger diameter winding. Also, typically an Audio Output Xfmr will
fail open at some point.

In article
,
Travis wrote:

on an early stereo Q160 ( chassis SHFA2 ) I am going through some
spare amps and trying to figure out whether the transformers are any
good. I ohm'd out 3 different Output Transformers and they all read
the same to within an ohm or so BUT they all were unsymetric in the
same way which causes me to believe the winding is partially shorted,
but all 3? and the same to within an ohm?

Center Tap to each grid reads 30 ohms but grid to plate reads 50 ohms
for one tube and 75 for the other in the pair --- in every case. These
are with the 6973 tubes, the sockets were empty of course and there
are no other components attached to those leads so I am measuring just
the winding resistance.

Anybody ever see this before?

Thanks in advance for your comments, Travis

I once figured out I had a bad transformer on an AMI D by adding a very
low voltage (like 2 or 3) AC to the speaker, with no tubes or speaker
hooked up (!), and looking at the waveforms on a 'o scope at each tube
plate. With the unit off. They should be opposite, symetrical, and
more or less cancel if added. Bonus points if you've got an audio
generator - you can sweep the band.

In a pinch, a 6.3V transformer, variac, and voltmeter can do a good 'go'
'no go' test. Suicidal folks will just use a variac

Philip Nasadowski wrote:
In article
,
Travis wrote:

on an early stereo Q160 ( chassis SHFA2 ) I am going through some
spare amps and trying to figure out whether the transformers are any
good. I ohm'd out 3 different Output Transformers and they all read
the same to within an ohm or so BUT they all were unsymetric in the
same way which causes me to believe the winding is partially shorted,
but all 3? and the same to within an ohm?

Center Tap to each grid reads 30 ohms but grid to plate reads 50 ohms
for one tube and 75 for the other in the pair --- in every case. These
are with the 6973 tubes, the sockets were empty of course and there
are no other components attached to those leads so I am measuring just
the winding resistance.

Anybody ever see this before?

Thanks in advance for your comments, Travis

I once figured out I had a bad transformer on an AMI D by adding a very
low voltage (like 2 or 3) AC to the speaker, with no tubes or speaker
hooked up (!), and looking at the waveforms on a 'o scope at each tube
plate. With the unit off. They should be opposite, symetrical, and
more or less cancel if added. Bonus points if you've got an audio
generator - you can sweep the band.

In a pinch, a 6.3V transformer, variac, and voltmeter can do a good 'go'
'no go' test. Suicidal folks will just use a variac

Yes, that's a good way to test the output transformers without having to
first replace all the B+ power caps (if not all the caps) before
powering it up.