Standel preamp module

[jvm]

Ok. New meter and function generator. I input a 300Hz, 50mV signal and have approximately 20mV at R11 and approximately 23mV at R12. 13.9 mV at the input of the green module. Output of the green module starts at about 20mV or so and drops to 0 within a few seconds. With the green module output wire (orange) unhooked from the circuit, I saw the same quick drop to 0.

[52 Bill]

Ok. New meter and function generator. I input a 300Hz, 50mV signal and have approximately 20mV at R11 and approximately 23mV at R12. 13.9 mV at the input of the green module. Output of the green module starts at about 20mV or so and drops to 0 within a few seconds. With the green module output wire (orange) unhooked from the circuit, I saw the same quick drop to 0.

Leave the orange wire unhooked and inject a signal into the Signal out jack J3. Does the power amp play loud and strong?

[jvm]

I plugged a cd player into the signal out jack with orange wire unhooked and the power amp is working fine. Loud and strong.

[52 Bill]

Loud and strong.

Congrats, you have found the problem. Time to carve out the green module or build a replacement.

[R.G.]

Here is a way to tell that the transformer does or does not have a problem to within 99+% certainty.
(1) Set your meter to ohms. With the transformer out of the circuit, no leads connected, test each winding for whether it's open or not. If it's open and should not be, the transformer is faulty.
(2) Again with the meter for ohms, test each winding for any resistance to every other winding. This takes N!-1 tests, so for a three winding transformer, it takes 5 tests. If a winding should be isolated from another winding and shows resistance to another winding of less than 1M (just to have a boundary), then the transformer is faulty.
(3) Check continuity from each winding to the core/mounting feet. No winding should show continuity to the core or mounting. If they do, the transformer is faulty.
(4) Do the inductive discharge test shown here: Neon Bulb Transformer Tester. This test determines whether any winding has an internal short. If the light does not blink, there is an internal short.

For a transformer that ever worked, these tests tell you the windings are good and not shorted. What this sequence misses are the thermal-intermittent (i.e. shorts when it gets hot) and voltlage intermittent (too much voltage makes it short) conditions, which are quite rare.

[jvm]

Thanks everyone for your help and advice. I'm already having fun with my new test equipment. Nothing like having the proper tools for the job! R.G., I've printed off your test procedure and will keep it handy for any future transformer issues. Thanks

[jvm]

Update: I built a green module replacement and installed in the amp. The new module behaves exactly like the original. Good input readings but the output signal starts at about 20 mV and quickly drops to 0. Same results with Orange wire connected to the power amp or not. ????

[g1]

Monitor the connection marked "C" (-24V) and see if it holds up when the signal fails.

[jvm]

g-one, "C" holds steady at -26.4V.

[g1]

The only other possible suspects (aside from the new module) are the ground connection and the input signal.
Does the signal at the input of the module hold up when the module output fails?

[jvm]

yes, the input signal remains steady at about 9mV. I have also made a new, clean ground connection with no change.

[g1]

You are using AC volts range? While you are reading that 9mV, turn the volume down to see if the reading goes to zero. If not, it is just measuring "noise" and you will need to check it with a signal tracer or scope.

[52 Bill]

I don't have the green module schematics open now, but which version did you build? There are two transistors in both versions, if you follow the signal through the module, what happens after the first transistor?

[jvm]

I'm using AC mV range, g-one. With volume turned all the way down, I measure 1.3mV at input of green module.

[jvm]

52 Bill, I built the first version from the Standel website. My signal starts dropping off after the output of the first transistor. I built this circuit using NTE 159 for the first stage and NTE 129 for the second stage. greenmodule.pdf