Looking for help troubleshooting a PET 4032 with no video

[rglenn]

The power looks good +5 -5 and 12v
The CPU tests good
There is a faint glow in the filament of the CRT
Using a Tynemouth Software ROM/RAM board - still no video

I attached a pic of the main board.

Any help would be appreciated.

 

[daver2]

Do you have an oscilloscope?

If so, measure the VIDEO, HDRIVE and VDRIVE signals from the logic board to the monitor.

If the signals are present, the problem is with the monitor - otherwise the problem is with the logic board. Divide and conquer...

If the problem is with the monitor, measure the DC voltage rail after the voltage regulator on the monitor PCB, then look for fusible resistors that have gone open circuit.

Note that there are two (2) off +5V regulators. Are BOTH +5V regulators working?

Dave

 

[rglenn]

Not sure I set/did this correctly. I measured J7 Pins 1,3 & 5

 

[rglenn]

I'll look for the voltage regulators.

 

[daver2]

You need to speed up the oscilloscope timebase for pin 5 to get 4 or 5 pulses on the screen. The frequency should be about 15 kHz.

You also need to slow down the oscilloscope timebase for pin 1.

Pin 3 looks fine.

Dave

 

[rglenn]

I'm getting 5v on c4, c6 & c9. Does that mean the VRs are ok?

I think I got PIN5 ok... having trouble getting PIN 1 to look right.

 

[daver2]

Yep, spot on.

HDRIVE and VDRIVE are OK.

So you may, or may not, have a video signal.

With the timebase setting at the same as in post #6, transfer the probe to pin 1 (video).

Dave

 

[daver2]

>>> I'm getting 5v on c4, c6 & c9. Does that mean the VRs are ok?

Unfortunately not...




Assuming I have identified the same schematic as belonging to your specific machine, C4, C6, and C9 are off the output of VR3 (so you are still only measuring the output from one (1) voltage regulator.

Try across C10 - or one of its siblings...

Dave

 

[rglenn]

Thanks. Sorry for the delay, we had severe storms and lost power several times.

C10 and C13 are at 5v.

The first pic is of PIN1 at boot and the 2nd pic is PIN1 shortly after - which is where it stays.

 

[daver2]

The voltage regulators on the logic board seem OK then.

Just to confirm, with pin 1 in that state - that pins 3 and 5 are still 60 Hz and 15.625 kHz respectively.

A logic 'high' on the video signal is BLACK on the monitor - so your problem may be on the logic board and not the monitor.

It would be worth checking IC G11 pins 1, 2, 4, 5, and 6 for a HIGH, LOW or PULSING.

Dave

 

[rglenn]

Will do and post shortly. Thank you.

 

[daver2]

No problem.

You have storms, we have a heat wave!

Dave

 

[rglenn]

We have both.

 

[rglenn]

Just to confirm, with pin 1 in that state - that pins 3 and 5 are still 60 Hz and 15.625 kHz respectively. Yes they are.

A logic 'high' on the video signal is BLACK on the monitor - so your problem may be on the logic board and not the monitor.

It would be worth checking IC G11 pins 1, 2, 4, 5, and 6 for a HIGH, LOW or PULSING. They are all HIGH.

 

[daver2]

>>> They are all HIGH.

Hmm, not possible...

Can you tell me what part number IC G11 is on your board (just to make sure we have the correct schematics)...

Dave

 

[rglenn]

It is labeled UG11 and is right next to the J7 connector - SN74LS20N

 

[daver2]

An SN74LS20 is a dual 4-input NAND gate.

With all of the inputs HIGH, the output should be LOW. So, all the pins identified in post #14 should not be permanently HIGH.

I assume you are measuring the specified five (5) pins with your oscilloscope?

Dave

 

[rglenn]

Sorry, I used a logic probe

 

[rglenn]

I'll check them again after my next work meeting.

 

[rglenn]

Is using a logic probe ok?