22 Apr
2020
22 Apr
'20
3:09 a.m.
On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:
Meantime reading the manual I found an interesting
test: If you short emitter to base on Q4 (easiest way is to jumper diode D10) the voltage
on the -12v supply goes to .4 volts. They're saying it's E2, R15,R17,R14.
Is there a way I can test the op-amp in circuit? Maybe it's dead.
The three regulators are all referenced via D8 (~ +5V).
Circuit-wise, for the -12, 0V/GND is the reference input at E2.3.
The D8 5V ref and the targetted output V are divided via R15 and R17,R18 to provide the
sense input at E2.2.
If one does the R ratio of the three resistors, it comes out, as would be expected, to ~
0V.
I'm a little surprised there's that much difference between E2.2 & E2.3 (
(-0.022) - (-0.8) ) without sending the E2 output off to +V, but not sure how much device
variability to expect normally.
You might look for the on-board values of R15,17,18. If they have been changed from those
values specified in the schematic, then the -12 may have been changed to -15 (could do the
ratio calc).
Also what is the V at Q4.E (should be ~ +2.6V), also Q4.B & C.
On 4/21/2020 5:36 PM, Chris Zach via cctalk wrote:
> Ok, some test results below.
> 1) Are we sure J2-2 should be -12? It really seems to like being -15.
> 2) The voltage across the zener diode there is a nice solid -3.8-3.9v. It appears to
be a germanium diode (.1v drop across, both directions) I could disconnect it and see if
that makes things better.
> 3)
> E2.2 is -.8 volts referenced to ground.
> E2.3 is -22mv referenced to ground
> E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down to
.60something. Note that is a positive voltage wrt ground.
> Q12 was reading
> -24.4v emitter
> -15v collector
> -23.8v base
> Q12 does not have a CE short
> What is this using as a reference to get the -12v from the transistor?