Standel preamp module

[jvm]

The low volume condition is not intermittent, though and I've had the tranny out and tested with a small ac voltage and got identical readings on both secondaries. I don't think the transformer is the issue. Kind of stumped here. Time to step back and stew on it for a while.

[jvm]

I unhooked the orange secondary wire from the circuit and have signal on it. Obviously, the transformer is not the problem.

[g1]

It has voltage but maybe no current capability. Try running it into a resistor.
Otherwise, the only thing that could kill the signal there would be a defective (shorted or leaky) Q3 or Q6.

[jvm]

I subbed a pair of MJ15015's for Q3 and Q6. No change in behavior at all. I did recall replacing a blown C14 filter cap initially. It supplies +30 volts to the primary of the drive transformer at R13. Might this favor a damaged transformer? Mercury will build a clone but with shipping it will be about $150. I'd like to be more confident that the transformer has a problem.

[g1]

I'd like to be more confident that the transformer has a problem.

Me too .
That's what I was getting at when I suggested lifting the secondary and running it into a resistor, simulating driving a load.
Or it may be easier to just swap the 2 sides of the secondary and see if the problem moves to Q2 & Q5 base.

[52 Bill]

The schematic has some very specific ac voltage figures at different points in the circuit. Input a 50mV, 300hz signal and what ac signal voltages do you get at the driver transformer?

[jvm]

Bill, I don't have any type of signal generator that will get me down to 50mV. Lowest signal I can get is about 250mV from my signal generator. My variable died and I haven't replaced it yet. I also can't get a 300 Hz freq setting. Any suggestions for an online generator?

[52 Bill]

Bill, I don't have any type of signal generator that will get me down to 50mV. Lowest signal I can get is about 250mV from my signal generator. My variable died and I haven't replaced it yet. I also can't get a 300 Hz freq setting. Any suggestions for an online generator?

Even if you cant get down to 300Hz, you can add a small volume control to the output to get the signal down to 50mV.

I'm sure somebody here will have a suggestion for an online or phone app signal generator.

[J M Fahey]

Couldn't find a 300 Hz tone, except on a You Tube video, not very useful.

But here: Download Audio Tone Files you have high quality 100/250/440/1k/10k MP3 and even better WAV (if your system can play it).

I'm quite certain odd 300 Hz was chosen because standard (needle) multimeters in late 60's were rated good up to 400Hz but today you can choose either 250Hz or 440Hz with no big difference, if any.

Play 250Hz, measure at some early stage (say after your first Blue module) , then repeat at 440 Hz.

If measurement is the same (or there's a very small change, say 5 to 10%) use it, much nicer

As of signal level, add a fixed attenuator.

If you have, say, 250mVAC ; 10k in series with 2200 ohms to ground will give you 45mVAC , close enough within 10% .

And so on.

FWIW I always carry with me an MP3 player with the 5 frequencies I mentioned above, a stereo 1/8" to 1/4"cable .... saved my bacon tons of times.

FWIW it's also loaded with wideband pink noise, some guitar test tracks: clean/distorted Strat ; overdriven Humbuckers, some real clean Steel guitar played through .... guess what? .... a Standel , a Metallica guitar-only track, clean as straight from the guitar pickups so as to test and abuse amps, and a couple AC DC songs to play amps all-knobs-on-10 to overheat them and destroy speakers.
Last but not least, some classical piano recordings, which absolutely hate crossover distortion or speaker scratching or buzzing or mechanical vibrations or anything other than pure sound, so it's used as a final test.

Incredibly useful "set of test instruments" which weighs and ocuppies nothing.

[jvm]

Per g-one's suggestion, I switched the secondaries. Incredible squeal! In fact, the cat that was laying in front of the amp is still pasted to the ceiling! lol Is there a polarity or phase issue here that I'm unaware of? I don't see how just switching secondaries would cause this.

[52 Bill]

Remember that most modern amps use a lot of negative feedback in their designs. You changed the negative feedback to positive feedback, creating a signal oscillator.

[g1]

Disconnect R18 & C8. Hopefully it will no longer oscillate. You shouldn't need the speaker connected for troubleshooting until the final tests.
Now you should be able to check if there is signal getting to Q3 & Q6, and not Q2 & Q5, opposite of what you found in post #28.
If the problem has moved, then you have verified that the transformer is bad.

[jvm]

Unhooked C8 and R18. Switched secondaries. Still no signal at Q3. Just a hum like before. I do have signal at Q2 so I think we can rule out the transformer at this point. Any ideas?

[52 Bill]

You have been testing different things trying to find the problem, have you actually isolated the problem to a specific section of the amp?

Take a cord from the signal out jack and plug it into another power amp do you get a strong signal? Have you tested C6? If not just jumper it with another cap, any change in the problem? Have you tested Q1?

You have told us that you get signal here and there, but not having the amp in front of us to see and hear, unless you can help us understand how much signal is present at different locations, we have no idea of how to quantify your statements. That's why I suggested testing ac signal voltages per the schematic. At least that way we could get an idea of what we are dealing with.

[jvm]

Bill, I don't yet have a meter that will accurately measure AC mV. I am sitting down to order one right now. I am going to walk away from this amp until I have the equipment necessary to use the schematic info. I appreciate your input and don't want you to think I am ignoring your advice. I have been trying to find the fault without having the proper tools available. When my new meter gets here, I'll post the results of my measurements compared to the schematic voltages. I do appreciate all the help from everyone. ( I did sub a new cap for C6 and tested Q1 with the diode test on my meter. Q1 tests good and changing C6 made no difference.)