Help Troubleshooting Millenia Media HV-3B PLEASE

[sonolink]

I am sending again points A, B, C, and S in a video. The preamp wasn't actually on previously. I apologize

I'll check U3, U4 pins 6 now

 

[sonolink]

This is pin 6 of U3 and U4 of channel 2 (working one)

 

[sonolink]

Here are the pics

 

[sonolink]

Just to clarify, all these readings I'm taking them from the working channel.

Should I do the same with the faulty one. If so, should I put the original opamps in the sockets or should I use the TL071s and the NE3352 I used for testing purposes before?

 

[moamps]

Measurements of this correct channel do not look completely correct.
Yes, go through all these points on a faulty module, you can leave TL071 for U3 and U4, U6 is not important for now. Send points A, B, C and D.

 

[sonolink]

But I need to put an opamp in the U6 slot right? An NE5532? Or the one that and with the unit? Or shall I measure without any IC in U6?

 

[sonolink]

So I put in an NE5533 in U6 and here are the readings on the faulty channel
First spots A, B, C, D

 

[sonolink]

U3, U4 pin 6 of faulty channel using TL071s

 

[moamps]

Try changing the positions of the gain switch and see with an oscilloscope if the AC voltage changes at pins 6 of U3 and U4

 

[ccaudle]

Is there no one local to you who can sit and show you basic troubleshooting techniques? This is the type of learning which is really best hands-on rather than back and forth on a web forum. Or at least real time on chat, or better over Zoom, Hangouts, WebEx, etc. with a video connection. I think those participating here will still help you, and I encourage your learning process, this is just a very inefficient way to do it.

That said, I will encourage you again to spend an hour tracing out the connections and drawing a schematic with the reference designators noted. It is hard to give advice about what signal to expect at U4, U6, etc. without any reference to what functions those devices provide in the circuit. If you can draw something crudely I can help get it into standard schematic form.

 

[sonolink]

Is there no one local to you who can sit and show you basic troubleshooting techniques? This is the type of learning which is really best hands-on rather than back and forth on a web forum. Or at least real time on chat, or better over Zoom, Hangouts, WebEx, etc. with a video connection. I think those participating here will still help you, and I encourage your learning process, this is just a very inefficient way to do it.

That said, I will encourage you again to spend an hour tracing out the connections and drawing a schematic with the reference designators noted. It is hard to give advice about what signal to expect at U4, U6, etc. without any reference to what functions those devices provide in the circuit. If you can draw something crudely I can help get it into standard schematic form.

Ok, I will trace the board to make life easier for everyone, especially Moamps who is struggling with the info I'm providing. I will spend probably more than an hour since it's my first tracing but I'll report back with a verified schem with as much info as possible.
Cheers
Sono

 

[sonolink]

Couldn't make it tracing the board, sorry.


BUT, I did my homework and re-did all the readings that Moamps kindly requested to diagnose the unit and here they are. I think these are more accurate.
I have also renamed each pic file with a descriptive name

Here goes Ch1 (faulty) points A, B, C and D

 

[sonolink]

Ch2 (working) points A, B, C and D

 

[sonolink]

Ch1 (faulty) pin6 U3 and U4

 

[sonolink]

Ch2 (working) pin6 U3 and U4

 

[sonolink]

I hope these readings make more sense Moamps

Thanks a lot for your help
Cheers
Sono

 

[Ike Zimbel]

Have you verified all of your power supply voltages? That flat part on the bottom half of the sine wave is common to both channels and looks wrong to me. A faulty negative PS rail might be to blame.

 

[sonolink]

Have you verified all of your power supply voltages? That flat part on the bottom half of the sine wave is common to both channels and looks wrong to me. A faulty negative PS rail might be to blame.

Ok. I'm getting -24, 24 and 48 at the PS.
U7 pin4 -18 and pin8 +18v
U6 pin4 -25 and 8 +24
U3 and U4 pin1 -18, pin2 6v, pin3 6v, pin4 -18v, pin5 -18v, pin6 -8, pin7 18, pin8 0v

Voltages on regulators and on test points seem ok

 

[moamps]

Thanks to the measurement results, it seems to me that the faulty channel now looks good (unlike the "correct" channel where something is wrong with U3). You should now finally measure the voltages at pins 1 and 7 of the IC6 on the faulty channel for the different gain switch positions.

@Ike
Opamps are DC biased to about -8V, so for excessive input voltage and high gain, the negative half-wave is limited first, the limitation is not symmetrical.

 

[sonolink]

Thanks to the measurement results, it seems to me that the faulty channel now looks good (unlike the "correct" channel where something is wrong with U3). You should now finally measure the voltages at pins 1 and 7 of the IC6 on the faulty channel for the different gain switch positions.

@Ike
Opamps are DC biased to about -8V, so for excessive input voltage and high gain, the negative half-wave is limited first, the limitation is not symmetrical.

I will do the measurements today. I guess that if the faulty one looks good now it can be due to one or both of 2 things:
1- it has the TL071s and the NE5532 installed, so if there was a fault with the original ones it's been fixed.
2- to make the measurements I had to desolder and resolder the cables between the board and the gain pot. If there was something wrong there it could have been fixed.

I'll report back asap
Thanks again for your precious help, patience and good humour

Cheers
Sono