SVT-CL: No "balanced" line out, but otherwise works fine

[mac1amps]

I've got an SVT-CL in my shop that works fine, except there's virtually no signal at all at the XLR "balanced" out. This model (the later version, I believe) has the [line] transformer (as opposed to the [earlier] version that is driven by op-amps). Even before the transformer, there is barely any signal coming from the preamp, yet the speaker out is still putting out full power! Damn, this bugger is driving me batty! Has anyone out there run into this scenario?

[Enzo]

Run into an amp that has one part not working but the rest does? Yes, most every day.

You have the simple one or the one with the pre/post switch and a level control? In any case the post comes from pin 9 of the conector, and back on the preamp side it appears pins 9, 14, and 12 are wired together. That means the signal at the FX return jack on 14 is routed to the power amp in jack (and preamp out jack) at 12. And it also comes through pin 9 to the transformer.

The pin 9 connection could be bad, because to come out the speaker all we need is 14 to 12 working. From pin 9 to the transformer, the signal goes through 220 ohm R91, so that could be open or damaged. Is it? Got signal at the pin 9 end of R91? Got it on the other end? The transformer could be bad. The signal at the top of the transformer should be the same signal as is on the power amp in and FX return jacks. Disconnect the transformer and see if the pin 9 signal is th same as the other two without it, and then does reconnecting it kill the signal there? The secondary side could be shorted or the output shunted somehow.

[mac1amps]

Even with the [line] transformer primary disconnected, there's still a barely detectable signal at either end of the preamp/ power amp ribbon cable (which includes the aforementioned common points of pins 12 & 14). What's really confusing is that this barely detectable signal resides (via scope probe readings) as far "back" in the circuit as the master volume (??.....in spite of the speaker signal looking fine).

[Enzo]

Not sure what to tell you, if the amp sounds healthy, then the signal at pins 12 and 14 kinda has to be OK. What level is your scope set to display? The FX loop and power amp jacks are at line level, so I expect maybe a volt for a strong signal.

Do the experiment, run a cord from preamp out to some othe amp, sound OK there? Then you knkow the signal is an appropriate level. Now scope the end of that cord. That would be what we expect coming across at pin 9.