Help Troubleshooting Millenia Media HV-3B PLEASE

[sonolink]

Hi again,

I have measured voltages of all opamps and compared both channels. Here they are:

U8
Both channels measure the same:

Pin 1 0
Pin 2 0
Pin 3 0
Pin 4 -24
Pin 5 0
Pin 6 0
Pin 7 0
Pin 8 +24

U3
First reading is ch1 (faulty)

Pin 1 16 / 15
Pin 2 5
Pin 3 5.8
Pin 4 -17.8
Pin 5 -17
Pin 6 -16.2
Pin 7 18
Pin 8 16 / 15

U4
First reading is ch1 (faulty)
Pins 5 and 6 are mismatched. Would that be the problem?

Pin 1 15.5 / 16
Pin 2 5 / 5.8
Pin 3 5.9 / 5.9
Pin 4 -17.8 / -17.8
Pin 5 -17 / -8.7
Pin 6 -16 / -8
Pin 7 17.8 / 18
Pin 8 15.5 / 16


Thanks for your help!
Cheers
Sono

 

[ccaudle]

Pins 5 and 6 are mismatched. Would that be the problem?

That is probably a good clue to where the problem exists. Don't forget that for some of the op-amps, they are in a feedback loop with discrete transistors.
So is U4 part of the input circuit with transistors in the loop, or just part of the diff-amps that remove common-mode from the first stage?
see above:

you can make your own [schematic] by tracing out connections on the PCB

 

[sonolink]

Ok, I'll try my best to trace it and report back....

Thanks!
Cheers
Sono

 

[moamps]

Can you repeat the measurement on U3? Voltage on pins 6 of both input single opamps U3 and U4 should not be -16V.
Please also send a picture of the underside of the PCB, and the voltages on the pins of the 16-pin brown element that most likely contains the input transistors.

 

[sonolink]

Can you repeat the measurement on U3? Voltage on pins 6 of both input single opamps U3 and U4 should not be -16V.
Please also send a picture of the underside of the PCB, and the voltages on the pins of the 16-pin brown element that most likely contains the input transistors.

Thanks for chiming in Moamps.
Here goes:

U3 ch1(faulty)

15
5
6
-17.8
-17
-16
+17.8
15

U3 ch2

16
5
5.8
-17.8
-17
-16
18
16

I'll post the element voltages in a sec.

Thanks
Cheers
Sono

 

[ccaudle]

single opamps U3 and U4

Are you sure they are singles? Isn't pin 8 usually a no-connect on single op-amps?

Although if they are dual op-amps, pins 5 and 6 should be the + and - inputs, and I would expect those pins to be a lot closer together than the 1V difference measured even on the good channel. Maybe not a standard pinout? Maybe the "good" channel is a little bit broken as well? Not sure what to make of it.

 

[sonolink]

These are the voltages at the 14-pin brown element:

5
0
-0.67
-3.3
-0.67
0
5
5
0
-0.67
-3
-0.67
0
5


Thanks again
Cheers
Sono

 

[ccaudle]

14-pin brown element:

Aren't there 16 pins?

 

[moamps]

Are you sure they are singles? Isn't pin 8 usually a no-connect on single op-amps?

I concluded this based on the voltages on pins 4 and 7. Sonolink can confirm this by checking the connection of these pins to to the power supply ICs (when preamp is off, is there a short connection between pins 4 of both ICs and also for pins 7)

 

[sonolink]

Aren't there 16 pins?

It's weird, the element mentions 16 pins on the top but there are only 14 pins underneath (you can check on the underside pic.

 

[sonolink]

I concluded this based on the voltages on pins 4 and 7. Sonolink can confirm this by checking the connection of these pins to to the power supply ICs (when preamp is off, is there a short connection between pins 4 of both ICs and also for pins 7)

Sorry Moamps, I'm not sure I understand: do you mean that when power is off I should be able to read continuity between pins 4 of both U3 and U4 and the same goes for pins 7?

EDIT: I confirm that is correct. When power is off pins 4 of both opamps are connected and pins 7 between them too

 

[moamps]

Yes. Pins 4 of all three IC should be connected. Pins 7 on input ICs (3 and 4) with pin 8 of the output IC too.
edit: Output IC (8?) is powered with separate +/-24V so
Pins 4 of IC3 and 4 shoud be connected, as pins 7 too.

 

[sonolink]

Do you think the problem is a faulty opamp (or several). And if so how can I find out what the replacement is?
If not, any other ideas?

Cheers
Sono

 

[sonolink]

Yes. Pins 4 of all three IC should be connected. Pins 7 on input ICs (3 and 4) with pin 8 of the output IC too.

Checked! They are!

 

[moamps]

Yes, I see traces on the bottom side.
Could you check the preamp with two 5534 for IC3, 4 and 5532 for IC8?

 

[sonolink]

I don't have any 5534 right now. Can I use TL071CP instead?

 

[moamps]

Most probably yes, for a test.

 

[sonolink]

Excellent!!! I'll test it and report back. Thanks Moamps!!!

Cheers
Sono

 

[moamps]

Do not leave the preamp in on state for a long time when the NE5532 is in the output opamp position, because it is not very happy with the +/- 24V power supply. It can overheat. But it can be used for testing.

 

[sonolink]

I just tested it but no go
Same noise and no gain....