Schematic incorrect and some solid state audio amp questions

[gooseEL34]

For this discussion, click here:

http://www.music-electronics-forum.com/t12368/

the second response has 4 thumbnails and the schematic which I am asking about is the first thumb (Ampeg BA115/BA210)

1.  My brother is troubleshooting this amp.  The Test Note.  Indicates that you inject a 170mV signal into the 0 dB input.
Test Point 1 (TP1), seems to be incorrectly placed.  It indicates that he should see 700mVRMS on pin 3 of the TLO-72.  However, at that point I am not sure where the gain is coming from?   We believe this is a schematic error and TP1 should be post OP-amp (at PIN 1 of the TLO-72).  Wouldn't that make more sense?   (mind you, we both fool with tube amps regularly and solid state is befuddling to both of us).

2.  He has a HP209 (I think) which he says only gives him (at lowest) a 1 volt sine wave.  If we need to get a 170mV sine wave, is there any reason you could not build a voltage divider between the output and the amp (simple pot should do, right?)

3.  What is the function of D10 and D13 (diodes from the input to the rails)?

Thanks... We are learning solid state by asking questions....

Thanks
Tim

[nickm]

1. Yes it looks like you're right and you should expect to see the 700mV on the output of the opamp not on the non-inverting input.  It's configured to have a gain of about 4.2 so 170mV * 4.2 = 700mV so that makes sense.

2. Yes you can use a voltage divider to reduce the output of your signal generator.  You need to be sure to take the input impedance of the amplifier into account but it looks to have a really high input impedance so it shouldn't affect it.

3.  Those diodes clamp the input voltage to the opamp from exceeding the rails.  The 22kohm in series makes the outside world have a high impedance so it will take a huge amount of voltage to push the opamp input past the rails and damage it.

[gooseEL34]

Thanks so much!

[gooseEL34]

Ok.
Another follow up...

My brother did some tests and the +16v/-16v rails were low (2-3 volts total).
He asked me to pose this question to the board:

1.  He was looking for a short or near short and measured the resistance from pins 4 and 8 of a TLO-72 (V+ and V-)
and got 55 ohms (with the chip still "in circuit")  -- this is U3
2.  He got the same reading on U4 (which is a NE5532 (which makes sense as they are tied together.

Does this seem like a chip has failed, or are we missing something?  (mind you, were are solid state newbies).

[nickm]

Not necessarily.  To measure resistance you are applying a DC voltage and measuring a current.  If you are applying the DC voltage in opposite polarity to what it is expects you can conduct through some PN junctions that would normally never turn on.  Similar to those two diodes you asked about earlier.  It would then look like a much lower resistance than it normally would.  So try measuring the resistance and reverse the multimeter leads.

On your schematic there is a +16V and +16V1 which are separated by 100ohms.  Measure both of these points to ground.  If +16V1 is much lower than +16V then your short is after the 100ohms.  If they are around the same voltage then you have a short before and maybe after the 100ohm too.

If the opamps are broken and acting like shorts they will likely get hot so merely feeling the case could indicate whether they are bad or not. 

[gooseEL34]

This is the latest:

>>>>
Potentially good news!

 I lifted the jumper wire(more on that later*) and was able to isolate U3s pin 8 from U4s pin 8. 56 ohms across 4&8 of U4 not U3. So the next thing I did was to cut pin 8 off of U4 in such a way that I could test from pin 4 to pin 8 on the chip or pin 4 to the stub of pin 8 still connected to the board. 56 ohms across pin 4&8, not across pin 4 and stub!!!
I'm convinced it's the chip. Now I'm charged with the unenviable task of removing the chip without damaging the board. YIKES!!!

      BUT.... on the schematic it looks like U3s output is sent down the chain([NEXT PAGE] on schem) and also to  U4s input(through R77). It looks to me that all that U4 is used for is the headphones and the balanced line out.

   
Thanks for all the help folks!
Seems like he is on to something.