• Please review our updated Terms and Rules here

Pet video board 320064-02 bad

Desperado

Desperado

Veteran Member
Joined
Nov 25, 2017
Messages
6,827
Hello guys!
Please can someone help me?
I've a 2001-N Cbm Pet with 320064-02 video board...when i turn on also without pet board,
resistor R4 (10 ohm) of monitor circuit burn immediately....
What could be the problem? I have already changed resistor, but the new one is also burned.
Thanks!
1.jpg2.jpg
 
The number looks like 320084-02 to me.

However, I can't find a circuit diagram for that particular model number.

However, I am looking at the schematic for video-1 (form the Zimmers website) and found a 10 Ohm resistor in the +85V boost circuit. I am hoping that it is the same beast... Look at http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif and locate R42 to see what I am talking about.

If it is the same resistor - I would check capacitor C27 (3.3uF 200V) for a short circuit. That would draw a high current through R42 and cause it distress.

However, there are a few other 10 Ohm resistors dotted about (e.g. R41) in which case I would check C32 (3.3 uF 200V) (internal -30V supply) also for a short circuit.

If you know the whereabouts of a schematic diagram for that particular monitor board - I could give you some more definitive advice.

In fact, the recommendation would be to leave the monitor overnight to let all the capacitors discharge (!) and check all of the polarised capacitors for a short circuit.

Dave
 
Ahhh. Makes more sense now :)!

In that case I would initially look at C17 for a short circuit (47uF 16V).

Dave
 
Yes,

Set to Ohms.

With the power switched off and disconnected from the PET totally. Allow time for the capacitors to discharge.

Keep your hands well away from anything that could potentially have a high voltage on it (e.g. the tube).

Measure the resistance across C17. A capacitor should start off with a low resistance, and then the resistance should start to increase as the capacitor charges up. If the resistance stays low - then either C17 is a short circuit - or something else has gone short circuit.

Dave
 
ilpaninaro said:
well!but when i check resistance on C17 i must turn on ?
Click to expand...
No! When checking resistance the power should almost always be off; it's also generally a good idea to short out the terminals you're measuring (C17 to ground in this case) before you measure in order to discharge any capacitors. You might also measure the resistance across Q13 collector to emitter (i.e. junction of Q13 and T1 to ground).

Damn! Dave beat me to it; hi guys!
 
Folks, when R34 on a PET monitor smokes, it's usually a problem with Q13 (also probably shorted by now) and generally Q11/Q12 circuit not working properly. Yes, it might be C17 shorted, but in my repair history it's the least common occurrence.
In these cases, you must first of all check Q13, if it's shorted, remove it from the circuit, then check Q11 and Q12 for correct B-E and B-C junctions with the diode probe function of the multimeter. If all looks good, without installing a replacement for Q13, look at the horizontal sync pulses that go through Q11 and Q12 with an oscilloscope and check where they go missing or have bad levels (so that maybe resulting in Q13 being constantly on). Only when Q11/Q12 circuit works fine, one can install a replacement for Q13.
Another thing that can happen is Q14 B-E short, but that usually takes more time for Q13 to give up.
Tip for the newbie repairer: Never power a PET monitor before checking all transistor and diodes junctions with a multimeter.

Frank

PS: you can't test reliably C17 and Q13 for shorts while in circuit since if either is shorted, the short will be "visible" on both C17 and Q13. You must lift either C17 or Q13 to be sure where the short is (I bet on Q13).
 
Last edited:
iz8dwf said:
Folks, when R34 on a PET monitor smokes, it's usually a problem with Q13 (also probably shorted by now) and generally Q11/Q12 circuit not working properly. Yes, it might be C17 shorted, but in my repair history it's the least common occurrence.
In these cases, you must first of all check Q13, if it's shorted, remove it from the circuit, then check Q11 and Q12 for correct B-E and B-C junctions with the diode probe function of the multimeter. If all looks good, without installing a replacement for Q13, look at the horizontal sync pulses that go through Q11 and Q12 with an oscilloscope and check where they go missing or have bad levels (so that maybe resulting in Q13 being constantly on). Only when Q11/Q12 circuit works fine, one can install a replacement for Q13.
Another thing that can happen is Q14 B-E short, but that usually takes more time for Q13 to give up.
Tip for the newbie repairer: Never power a PET monitor before checking all transistor and diodes junctions with a multimeter.

Frank

PS: you can't test reliably C17 and Q13 for shorts while in circuit since if either is shorted, the short will be "visible" on both C17 and Q13. You must lift either C17 or Q13 to be sure where the short is (I bet on Q13).
Click to expand...

Thanks so much Frank ;)

Now i try!
 
Hi, i ve tested Q13 and is in short!
Thanks!! I buy new one and soon i ll update post ;)
 
Hello guys,
i've changed q13 transistor and now r34 don't burn again! Thanks
Unfortunately Pet show black screen....i've changed all 16 chip rams....i've programmed all Roms with new eproms and new one 6502 cpu.
On 555 reset chip pin 3, when i turn on i've 5,2v for 2 seconds and after 0v.....on 74ls04 pin 2 and on 6502 pin 40 i've always 5,2v....
Reset signal are always high.....
 
ilpaninaro said:
Unfortunately Pet show black screen
Click to expand...

I don't know the Pet but basic principles probably still apply.

Did you mean to say black or blank ?

Your remark is insufficient to determine two very important differences:

Is the screen black because there is no video signal coming into the monitor circuits, and if there is no video signal, are the H and V drive pulses arriving at least ?

Is the screen black because the monitor is still not working, despite the replaced parts. The video signal may still be there. Do you have a scope to check that ?

To help answer this, turn the brightness control to max, can you see a scanning raster on the CRT (does the CRT face light up ?). Is the CRT's heater glowing ? Can you hear the sound of small electrostatic discharges when the CRT powers up and off indicating the CRT has an EHT supply ? Your type of monitor relies on the H drive signal being present or there will be no EHT and the CRT will not light.

To check if the H scanning and EHT generator are likely working, you can test the 85V boost voltage or the -30V rail with a meter (DVM).

If there is no video signal you need to investigate the cause of that with a scope in the video generating parts of the computer. Check initially that the CPU is being clocked and the power supply voltages are good.
 
Last edited:
Without the reset signal, it is highly unlikely that the CPU will start-up correctly.

You have checked the reset pulse at the NE555 pin 3 (device A2), but what about pins 1 and 2 of the 74LS04 at A3 and pins 13 and 12 of the 7417 at A10 (i.e. Follow the reset chain through the logic). The signal will probably fail at one point, where?

I would have thought you would have had 'garbage' on your screen if the CPU was faulty, so you may be chasing multiple faults with this PET.

As Hugo has stated, either check the video, horizontal synch and vertical synch signals from the PET main board to the monitor with a scope if you have one, or measure the d.c. Voltage on these signals with a multimeter set to indicate 5V full-scale and post the results.

Dave
 
daver2 said:
Without the reset signal, it is highly unlikely that the CPU will start-up correctly.

You have checked the reset pulse at the NE555 pin 3 (device A2), but what about pins 1 and 2 of the 74LS04 at A3 and pins 13 and 12 of the 7417 at A10 (i.e. Follow the reset chain through the logic). The signal will probably fail at one point, where?

I would have thought you would have had 'garbage' on your screen if the CPU was faulty, so you may be chasing multiple faults with this PET.

As Hugo has stated, either check the video, horizontal synch and vertical synch signals from the PET main board to the monitor with a scope if you have one, or measure the d.c. Voltage on these signals with a multimeter set to indicate 5V full-scale and post the results.

Dave
Click to expand...




Thanks!
On j7 video connector i ve checked with multimeter and these are results:

pin 5= 0,17v
pin 3 and pin 1? 5,2V
 
J7 pin 1 (video), and pins 3 and 5 (synchronising pulses).

I think those readings indicate no video and/or synch signals.

Need dave_m to weigh in here to confirm my suspicions.

How many of your chips are in sockets? I am thinking of pulling things like the PIAs and VIAs as they can be a source of problems and aren't needed for normal operation.

Do you have access to an oscilloscope? If you do, it will be much easier to look at the clock divider chains and signals.

Dave
 
daver2 said:
J7 pin 1 (video), and pins 3 and 5 (synchronising pulses).

I think those readings indicate no video and/or synch signals.

Need dave_m to weigh in here to confirm my suspicions.

How many of your chips are in sockets? I am thinking of pulling things like the PIAs and VIAs as they can be a source of problems and aren't needed for normal operation.

Do you have access to an oscilloscope? If you do, it will be much easier to look at the clock divider chains and signals.

Dave
Click to expand...


in socket i ve all roms,rams,cpu,6520 and 6522 chips...
I don't have scope now :(
 
You must log in or register to reply here.
Back
Top