Bruce Esquibel wrote:

Harry Lavo wrote:
snip

snip

As far as repairing them, am 99% sure it's a factory job. I'm somewhat of a
technician, did tv repair from the vacuum tube days and into vcr's up
through the year 2000. Also had minor forte's into power amps, both home and
pro gear (musicians, rock stars, hate the lot).

Relatively trivial to repair the interfaces.

snip
It looks like each time they were repaired, the only thing changed was the
one transformer, not power supply but the one that interfaces to the audio
side of things. Difficult to say 100% because they are doped (potted),
probably more for mechanical reasons than trade secrets.

Interfaces consist of basically two parts:
- audio step up (with or without caps and resistors)
- high voltage supply


If the panel was totally dead, I'd agree with you on a possible cap repair.
Like I mentioned in the original post, the panel works but it doesn't take a
trained ear to figure out one doesn't sound like the other. The "bad one" is
slightly lower in output and you can hear a raspyness to certain vocals.

This could be either a fried xfmr or bad HV supply or both. The HV
supply is usually a moderate voltage xfmr followed by a diode/capacitor
multiplier "stack". If the caps get leaky, the string drops in voltage.
Going over the peak voltage of the supply with the audio causes clipping
on the ESLs... "raspyness".

Almost all ESL repairs I have done end up being either the diodes or the
HV caps.

With a HV probe you can check the HV supply directly...
Or you can jury rig the "good" interface box's HV supply to the "bad"
panel and use the audio part (assuming it isn't all potted) to run the
panel, diagnosing the HV supply vs. the audio step up xfmr.




I think it's some kind of insulation breakdown in the transformer, else just
an old fashioned short in it. From memory (this going back 10-15 years), I'm
pretty sure the banana jacks your amp connects to is simply across the input
side of the transformer. All the voodoo with that pot/adjustment to match
the panel's crossover is on the secondary side, the power supply and step up
stuff is isolated from both.

You can check all the transformers with an ohmeter. Start with just
measuring the primaries and secondaries between the two channels. If you
find a reasonably large diff, then remove the inputs or loads from them
and see what you measure - this eliminates the load as the cause or
points to it. (smell is also a tip off to a toasty xfmr...)



As you can imagine, that transformer isn't something you'll run across at
Radio Shack, again from rusty memory, there isn't even a part number or code
on it, assumed it was custom wound, maybe by hand.

Kind of irrelevant - these are not exotic transformers at all.
If you know the turns ratio, the primary inductance, and the freq
response of a good one, you know everything you need to know to replace it.

The HV xfmrs are generic anyhow.


Thanks to everyone for the responses so far, something to chew on.

-bruce


_-_-bear