i am an audio amp tech and a customer brought me this amp and i am stumped when i turn volume up slightly i get massive distortion in the whole amplifier even at the input (1khz 200mv) and into my speakers on bench not connected to unit under test in any way except for ac line (powered speakers)
please review schematic and post suggestions and questions thank you

 

Wow, that is one convoluted unit. In any case, if it is giving you interference/noise in a set of powered speakers not connected to the unit, and you are seeing distortion reflected all the way back to the input, then you most likely have something oscillating in the amp. Beyond that, I would need some bench space and a scope to trouble shoot further.

 

Wow, that is one convoluted unit. In any case, if it is giving you interference/noise in a set of powered speakers not connected to the unit, and you are seeing distortion reflected all the way back to the input, then you most likely have something oscillating in the amp. Beyond that, I would need some bench space and a scope to trouble shoot further.

thank you for looking at it its been interesting none of the components are designated on circuit board its really difficult to navigate hopefully the manufacture will have more info for me aside from that I'm about to get out the expensive shotgun and start replacing things

 

Begin with no input signal to the amp. Set GAIN to 0.
Turn up the VOLUME slowly until you hear the noise.
Describe the noise. Is it a high frequency noise or low frequency (60Hz maybe)?

Plug a cable into the Power Amp In socket (nothing on the other end of the cable).
Is the noise still there?

 

Is anything plugged into the 'Loop send' and 'Loop return' jacks? Any positive feedback there could cause oscillation.

 

Begin with no input signal to the amp. Set GAIN to 0.
Turn up the VOLUME slowly until you hear the noise.
Describe the noise. Is it a high frequency noise or low frequency (60Hz maybe)?

Plug a cable into the Power Amp In socket (nothing on the other end of the cable).
Is the noise still there?

thank you for looking mr chips i will get it back on bench tomorrow and check
to answer your question my out put sine wave (when volume applied 15% or so )turns into a nasty square wave meaning not symmetrical and has high freq squiggles on the negative slope (i wish i could draw it for you )reminds me of some rf triggers i used to get on the navy f-14 bench lol doesn't belong here both you and alec seem to be around u3 with your questions ill check it over for sure

 

Is anything plugged into the 'Loop send' and 'Loop return' jacks? Any positive feedback there could cause oscillation.

thank you for your reply alec ill get it back on bench tues i think your hinting around u3 op amp ill have a look at it for sure

 

Can we assume you set your audio signal generator at the equivalent levels when using either the A or B inputs?

 

Can we assume you set your audio signal generator at the equivalent levels when using either the A or B inputs?

good question i use typically a 200mv 1khz sine wave from my hp 201c (with tubes!) but now due to the crazy noise prob i switched to my plastic 9volt battery handheld unit to isolate generator sig from bench power still use 200 mv in b input because it has a trim pot to match instrument level to preamp level to avoid clip or in this case not to activate overdrive noise which is desirable for some players (metal)
thank you for your question however i am more interested in the link to the neets modules ;back in 1990 i had all the mods from avi school and c school in millington tenn. but lost them all in a flood at my storage in fla. i was needless to say pissed its cool you have that link ; invaluable info those are.

 

you can take pictures with your camera (phone) and upload them to this thread. do you have a load (other than a speaker) so you can test without killing your ears?

Input A was for musical instruments without preamps. I assume that to be way lower than 200 mV, probably more likely in the 20-50 mV range. Input B was for musical instruments with preamps. I assume that to be closer to line levels. That's what I ascertained from the available publications.

If you can test without killing your ears, can you narrow down where the signal begins to distort ? Which stage in the amplification ... we already know the output distorts.

Remember your six step troubleshooting ... it still works.

 

Can I assume you have these documents as well as the schematic?

 

Can I assume you have these documents as well as the schematic?

I've looked at user manual yes i use a 500 watt dummy load resistor big gold f****r and i have some 100 watt green ones i use during trouble shooting (i don't push amps until i do a final test to see if they make their specified wattage) i have one of my bench meters on it to measure rms voltage a little ohms law and poof i have the wattage and i have a probe on it from my tek tds 2002 also i view input and output sine waves on it and trace signals with my tek 2446a i should post a pic of my bench then you know what I'm working with to answer your question the signal freaks out at u3 pin 1 pin 3 has good input and pin one is 10 times+(off scope) the amplitude of noise and looks like rf pulses -lll--lll--lll--lll-- pretty weird (because u3 is a small op amp and shouldn't be capable of this amplitude) I'm gonna replace u3 of course but have funny feeling something else is causing this just can't put my finger on it i emailed Udo the engineer to see if he will give me the board layout really difficult to navigate this one there no lables or designators for components or test points on board pictures to follow soon have a happy thanksgiving

 

Ok ....

I got tired of flipping those two pages so I created a B5 sized drawing from both those pages. It might be easier to follow. I did zoom it out 200 percent to read it easier.

Post a pic of your bench and the waveforms you get. What is the frequency of those pulses?

It's important for you functionally test this. For instance, I don't know your preliminary setup. You can press the tune switch, S3 and mute all after the preamp. You should ensure there is nothing in the two loop jacks; nothing in the insert and insert return jack, and the equalizer is off. Hopefully, you will not see anything at pin 3 or pin 1 of U3A. Pressing the tune switch again would place your test signal back in the circuit, but not through the loop or insert or equalizers. If you see that noise again there, put a plug in the power amp in jack to disconnect the power amp from that circuit.

 

Ok ....

I got tired of flipping those two pages so I created a B5 sized drawing from both those pages. It might be easier to follow. I did zoom it out 200 percent to read it easier.

Post a pic of your bench and the waveforms you get. What is the frequency of those pulses?

It's important for you functionally test this. For instance, I don't know your preliminary setup. You can press the tune switch, S3 and mute all after the preamp. You should ensure there is nothing in the two loop jacks; nothing in the insert and insert return jack, and the equalizer is off. Hopefully, you will not see anything at pin 3 or pin 1 of U3A. Pressing the tune switch again would place your test signal back in the circuit, but not through the loop or insert or equalizers. If you see that noise again there, put a plug in the power amp in jack to disconnect the power amp from that circuit.

 

will do ill be back at shop fri morning thats awesome you did that with schematic it is easier to navigate ill try all your isolation suggestions and document wave forms really appreciate you staying with this I've never used one of these sights before its nice to get a fresh set of eyes on this one cause I'm really tired of this one i don't usually have much difficulty with audio power amps they usually tell you whats wrong right away they are like my specialty fixing modding and restoration of anything guitar, bass, music related tube amps are my favorite but this one has kicked my ass and i have a lot of other work in between again thank you for making this more fun i have a luthier at my shop he can wire a guitar and he's a very good luthier but doesn't understand this stuff the way military trained people do I'm assuming you are i was in navy my rate was AQ worked on f-14 fire control radar bench(AIMD)and vhf and iff gear enjoy the rest of the holiday

 

and please try to use the CAPS and the period more often otherwise the grammar police will be after you

 

and please try to use the CAPS and the period more often otherwise the grammar police will be after you

O.K.
I will try to do better with that.
And try not to be so long winded !
Happy Thanksgiving !!!

 

@hoop I see a mistake or two on that schematic. There are multiple full wave and bridge rectifiers, from +/-5V supplies, +/- 15 volt supplies, and I suspect a +/- 48V supplies for the final power amplifier.

I have "simulated" a number of the circuits as I work my way through, but the schematic is lacking in some details. I'm thinking there is a transformer with multiple secondaries,

Most of those 1/4 inch jacks are used as to switch signals. The Loop Send jack was improperly wired as the center pin should go down to the potentiometer that they have connected to the input wire from the equalizer switch. The original drafter should have connected the wires using a dot, vice just crossing them. That alone slows the process down.

What I intend to do, should I finish this, is to have a block diagram of that equipment, to ease the troubleshooting processes. Taking copious notes always help.

 

I have been wondering myself if schematic is complete. I know these don't show incoming power and its transformer i think they just show the taps at upper left coming into the board. I emailed Udo the engineer there and asked for more info (like a f***ing service manual) L.O.L.
Tests points are all over board and not labeled; Or show up on schematic. None of the components are either. That is really my main issue.
I have so much other work ; I set this off to the side fix like three things and then come back to it. have even been tempted to have customer come and get it . But now its a challenge ; whether I make money or not I'm gonna fix this!