Is that scope shot in post #206 supposed to be showing the crossover notch?  Because there is none there.

Quote from: g1 on November 27, 2024, 06:50:29 PMIs that scope shot in post #206 supposed to be showing the crossover notch?  Because there is none there.

That is a pic of the sine wave at idle with the trim pot FCCW. All controls except the red channel vol and gain are set to zero. The red vol and gain were only turned up just enough to give me a sine wave.

Quote from: Miyagi_83 on November 27, 2024, 05:45:36 PM

Quote from: RG100ESROX on November 27, 2024, 03:04:53 PMQ: Is there a reason my sine wave is so jittery??

What do you mean?

I have to pause the live sine wave because it is jittery on the screen. It's not steady.

Quote from: RG100ESROX on November 27, 2024, 07:34:14 PMI have to pause the live sine wave because it is jittery on the screen. It's not steady.

In what sense is it not steady? Does it float up and down? Does it shake?
Please remember that English is not my native language and, although I do speak it quite fluently, some things may not be obvious to me.

Is the return jack grounded? I'm not sure if it matters, but perhaps it's good to know.

In what sense is it not steady? Does it float up and down? Does it shake?
Please remember that English is not my native language and, although I do speak it quite fluently, some things may not be obvious to me.

Yes. Both. Snake AND float.

Is the return jack grounded? I'm not sure if it matters, but perhaps it's good to know.

The return jack IS grounded when nothing is plugged into it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMThe return jack IS grounded when nothing is plugged into it.

Ok, another question.
By 'ungrounding' the return jack you meant plugging something into it, right? Not disconnecting the return jack ground by, for example, unsoldering it?
If that's the case I'd like to clarify something.

The return jack is very often (usually?) a switching jack configured so that when a cable is plugged into it, it breaks the connection between the preamp and the power amp. It's as if you put an SPDT switch in series with the preamp out and power amp in and flipped it off. The return jack, unlike the PREAMP input jack, does not ground its input signal with nothing in because that would shunt the preamp out to ground.

Now, if your signal generator is connected to the PREAMP input AND there is some plug in the loop return jack, you will only get a faint signal at the output, if any at all. Simply because the connection is broken, as if our imaginary SPDT switch were open.
*Unless you've got a parallel loop which doesn't seem to be the case here.

Quote from: Miyagi_83 on November 28, 2024, 01:31:22 AM

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMThe return jack IS grounded when nothing is plugged into it.

Ok, another question.
By 'ungrounding' the return jack you meant plugging something into it, right? Not disconnecting the return jack ground by, for example, unsoldering it?
If that's the case I'd like to clarify something.

The return jack is very often (usually?) a switching jack configured so that when a cable is plugged into it, it breaks the connection between the preamp and the power amp. It's as if you put an SPDT switch in series with the preamp out and power amp in and flipped it off. The return jack, unlike the PREAMP input jack, does not ground its input signal with nothing in because that would shunt the preamp out to ground.

Now, if your signal generator is connected to the PREAMP input AND there is some plug in the loop return jack, you will only get a faint signal at the output, if any at all. Simply because the connection is broken, as if our imaginary SPDT switch were open.
*Unless you've got a parallel loop which doesn't seem to be the case here.

Aw, great explanation. Now I know why the amp made a pretty big humming noise when I ungrounded the return jack. Good info. Thank you.

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Exactly. When you plug a cable into the loop return, most likely, there is nothing to attenuate the input noise, like a gain pot, tone controls etc. because probably also the master volume potentiometer is in the preamp section.

Now that you know that your preamp is good, I suggest you do the following for signal testing your power amplifier (correct me if I'm wrong, someone):
- get a spare jack socket, a patch cable, and a 10-22k linear (B taper) potentiometer;
- connect the pot so that the wiper is tied to the hot terminal of your socket and one of the extreme lugs is tied to the ground terminal. Breadboard, solder, alligator clips, whichever you find convenient. This way you will be able to attenuate the signal from your generator to low levels, which g1 suggested earlier;
- plug the cable into the jack socket;
- hook up your generator's signal clip to the free lug on the potentiometer and the ground clip to ground of the whole setup;
- finally, insert the patch cable into the return jack. This way the preamp won't be in the signal path and so it won't introduce its distortion.
EDIT: In case I'm being unclear, I've drawn a diagram for it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

I don't know if this is going to make any difference, but why not try? Have you tried adjusting your scope's parameters, such as volts per division or milliseconds per division?

Hope this helps,
M.

Quote from: Miyagi_83 on November 28, 2024, 02:31:50 AM

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Exactly. When you plug a cable into the loop return, most likely, there is nothing to attenuate the input noise, like a gain pot, tone controls etc. because probably also the master volume potentiometer is in the preamp section.

Now that you know that your preamp is good, I suggest you do the following for signal testing your power amplifier (correct me if I'm wrong, someone):
- get a spare jack socket, a patch cable, and a 10-22k linear (B taper) potentiometer;
- connect the pot so that the wiper is tied to the hot terminal of your socket and one of the extreme lugs is tied to the ground terminal. Breadboard, solder, alligator clips, whichever you find convenient. This way you will be able to attenuate the signal from your generator to low levels, which g1 suggested earlier;
- plug the cable into the jack socket;
- hook up your generator's signal clip to the free lug on the potentiometer and the ground clip to ground of the whole setup;
- finally, insert the patch cable into the return jack. This way the preamp won't be in the signal path and so it won't introduce its distortion.
EDIT: In case I'm being unclear, I've drawn a diagram for it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

I don't know if this is going to make any difference, but why not try? Have you tried adjusting your scope's parameters, such as volts per division or milliseconds per division?

Hope this helps,
M.

Wow! I don't know what to say. Thank you so much for investing to such a degree in a problem that isn't even yours. I really appreciate it.

Unfortunately, it will be Sunday or Monday before I am home again because of today's holiday.

However, once I return home I will get right on this as I am very eager to see what your suggestion reveals.

Hopefully you will be available Sunday or Monday to respond to my replies regarding my findings.

Again, thank you very much for taking the time to help me in my quest in having a functional amp again.

I look forward to further correspondence come Sunday/Monday.

Enjoy your holiday today and we'll be talking soon.

Happy Thanksgiving to you and your famil.

-Jay

PS: I will try your suggestion regarding the adjustment to the signal generators output voltage to see if it improves the stability of my sine wave.

Jay, as regards your sine wave dancing around the screen, read this post and Loudthud's reply just under it.

QuoteThank you so much for investing to such a degree in a problem that isn't even yours.

That's how I understand a forum - a place where people discuss things, including trying to solve other people's problems. Although, actually, the thanks should go to g1, IMO. After all, he's been involved in this thread the most.
*This is not to suggest that you have ever forgotten to thank him.

QuoteHopefully you will be available Sunday or Monday to respond to my replies regarding my findings.

I should be available, but remember that I reside in a timezone that's 8 hours later than yours What's more, solid-state power amplifiers are terra incognita for me, but I'll try to help you to the best of my knowledge, hopefully, with supervision from the more experienced people here.

Happy Thanksgiving to you and yours!
M.

Quote from: Miyagi_83 on November 28, 2024, 04:41:37 PMJay, as regards your sine wave dancing around the screen, read this post and Loudthud's reply just under it.

QuoteThank you so much for investing to such a degree in a problem that isn't even yours.

That's how I understand a forum - a place where people discuss things, including trying to solve other people's problems. Although, actually, the thanks should go to g1, IMO. After all, he's been involved in this thread the most.
*This is not to suggest that you have ever forgotten to thank him.

QuoteHopefully you will be available Sunday or Monday to respond to my replies regarding my findings.

I should be available, but remember that I reside in a timezone that's 8 hours later than yours What's more, solid-state power amplifiers are terra incognita for me, but I'll try to help you to the best of my knowledge, hopefully, with supervision from the more experienced people here.

Happy Thanksgiving to you and yours!
M.

Good to know. As I am often up late and up early. So, I will work around our 8 hour difference.

And yes. there are a few members here that deserve a lot of credit. So, thank you to ALL that have chimed in trying to help me in my quest to fix my amp. Your input and efforts are more appreciated than you'll ever know.

Lastly, thank you for the Thanksgiving well wishes.

I'll follow up with you late Sunday or early Monday when I get back home.

Cheers!!

Quote from: Miyagi_83 on November 28, 2024, 02:31:50 AM

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Exactly. When you plug a cable into the loop return, most likely, there is nothing to attenuate the input noise, like a gain pot, tone controls etc. because probably also the master volume potentiometer is in the preamp section.

Now that you know that your preamp is good, I suggest you do the following for signal testing your power amplifier (correct me if I'm wrong, someone):
- get a spare jack socket, a patch cable, and a 10-22k linear (B taper) potentiometer;
- connect the pot so that the wiper is tied to the hot terminal of your socket and one of the extreme lugs is tied to the ground terminal. Breadboard, solder, alligator clips, whichever you find convenient. This way you will be able to attenuate the signal from your generator to low levels, which g1 suggested earlier;
- plug the cable into the jack socket;
- hook up your generator's signal clip to the free lug on the potentiometer and the ground clip to ground of the whole setup;
- finally, insert the patch cable into the return jack. This way the preamp won't be in the signal path and so it won't introduce its distortion.
EDIT: In case I'm being unclear, I've drawn a diagram for it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

I don't know if this is going to make any difference, but why not try? Have you tried adjusting your scope's parameters, such as volts per division or milliseconds per division?

Hope this helps,
M.

Is this what you had in mind???

Quote from: RG100ESROX on November 29, 2024, 12:48:37 PMIs this what you had in mind???

Almost. There should be no red wire between the pot and the jack plug.

Quote from: Miyagi_83 on November 29, 2024, 12:56:47 PM

Quote from: RG100ESROX on November 29, 2024, 12:48:37 PMIs this what you had in mind???

Almost. There should be no red wire between the pot and the jack plug.

You are correct. Thank you for pointing that out...

How important is it that the pot be a log (B) taper??? All I have at home are audio taper pots.

Quote from: RG100ESROX on November 29, 2024, 02:22:57 PMHow important is it that the pot be a log (B) taper??? All I have at home are audio taper pots.

It's not going to explode if you use a log (A, audio) pot. The difference lies in the rate at which the resistive divider is formed as you turn the knob. In an ideal linear (B) potentiometer the resistance value will change, as the name suggests, linearly. A log potentiometer will reach about 10% of its value at 50% rotation. Do a search for 'log pot graph' / 'log pot curves' or something along those lines. A picture is worth a thousand words.