shoulda quit while i was ahead. … trying to juggle the two probes was more than i could deal with.
One of the ground clips from the probe popped off and shorted something on the main board. It worked a few minutes after that, but then died.
Now there is no screen display, nothing on the control board lights up

the power light is on, and the 15 v test point are fine. The AMPL and AMPR channels just look like messed up noise pulses.

very bummed. Time to look for a replacement board?

I want to express my great appreciation for the time and effort you put in to help. And apologize for this giant step backwards…

 

Sorry to hear about the setback.

I was supposed to mention that extra care was required when working with open circuitry because a slight slip with the probe can make things worse. I have seen this happen too often here on AAC. Then I forgot to say it.

Let's take one step at a time to assess the situation. Hang in there.

 

Hanging in there. next steps?

i was very confused by the ‘slow death’ of the circuit- it worked for a couple of minutes after i shorted something , suggesting a residual charge on a capacitor keeping things alive…

 

Start by measuring +Vcc and -Vcc on the power supply board.
Then measure +15V and -15V.

 

I pulled out the B (Asub) board to get the probes in. slight hum from instrument.
on CN6, measured the following

“-15A”. -25mV
“+15A”. 0
-Vcc -.43V
+Vcc +180mV

on the main board, next to the diodes,
Vcc are +|- 40V dc

 

Post sharp photos of this section of the MAIN board, from different angles so that we can see the writing on the board.

 

Sorry. We cross posted. Hold on for a moment.

 

Your tests points are here on the board.

 

main board corner

 

thanks for the reminder .
those “15” s are reading +/- -14.8

 

main board corner

This photo was cropped from the photo you already posted.
I need to see another view at an angle taken from the right so that I can indicate to you your next test points.

 

photos:

 

Here are your two test points.
Expected voltages are +3.3 V and + 5 V.

 

measured voltages
0.75 V (3.5 expected)
0.092 V (5 V test point)

 

These three tests points are all VCC. They should show +40V.

 

all seem to be at
0.238 V

 

BTW, make sure that your DMM is set to measure DC voltages. 200 VDC range is good enough. One decimal place is good enough.

Measure voltages at both legs of R28 and R29 power resistors. Expected voltage is +40 V.

 

R28a +.24 V dc

R28b .24

R29a .24
R29b 40

there were voltages present before i turned on power…as much as 20V????

 

Ok. Next test.
Unplug from power.
Measure the resistance across R28 and then across R29.
Expect to find R29 is open circuited.

 

Ok. Next test.
Unplug from power.
Measure the resistance across R28 and then across R29.
Expect to find R29 is open circuited.

R29, as predicted, is open circuited
looked like 2W, 2 ohm; closest ceramic resistor i found was 5W.


R28 measures 2 ohms