PET Alive! - Pet Hospital

[tezza]

During the PET Alive! event some of our resident PET veterinarians have volunteered to assist with diagnoses of PET problems. If your PET seems a little off-colour or perhaps shows no life at all, post the symptoms in this thread.



Before you start, please read the below...

• A picture paints a thousand words. Showing an image of the symptom can often assist.
• Before posting, please check the postings which have come before. Someone may have had a similar symptom to yours and suggested fixes or tests might have already been given. If this is the case, just say you have a similar symptom to <whoever> and that you’ll check out the recommendations.
• Our PET veterinarians are small in number and are part-timers. They also have lives outside the VCF. They will respond when they can but it might not be immediately.
• While they will do their best to give folk pointers to how they might proceed for a fix, our PET experts can only go so far. Please understand they are under no obligation to fix the machines posted or take the diagnosis through to the end.
• Recommendations are not limited to our resident interns. It you have technical skills with the PET please offer suggestions to the problems posted!

Good luck and here’s hoping for a successful diagnosis!

 

[harry]

Great stuff Tez! looking at those photos, some of you guys have been polishing those casings!! meanwhile, I need an appointment at your hospital please, the case is a dead 8032, well not completely dead, but at least nothing appearing on screen? I have checked voltages as per schematics, and they all seem ok, quite a few of the ic's are getting warm, and that is where I am, wondering what should be checked out next, to try and narrow things down. I am not so technically minded as most of you guys, but really willing to learn. thanks Harry

 

[cosam]

I need an appointment at your hospital please, the case is a dead 8032, well not completely dead, but at least nothing appearing on screen? I have checked voltages as per schematics, and they all seem ok, quite a few of the ic's are getting warm, and that is where I am, wondering what should be checked out next, to try and narrow things down.

Ah, our first patient! ;-)

My first step would be to try to determine whether it's the monitor or the main board that's causing this. Have you checked voltages in the monitor? Is the neck of the CRT glowing? Yes, there are high voltages in there, so be careful, but you should be able to check the lower ones without getting near anything dangerous.

What kind of equipment do you have access to? A logic probe or scope would be handy to check the main board, but failing that even a volt meter will tell you something. What do RDY, /NMI and /IRQ look like? Is /RST working correctly? (Should be pulled low for a second or two at power up, then go high.) How about the clock pins? Is there any activity on R/W and the data and address busses?

 

[MikeS]

Great stuff Tez! looking at those photos, some of you guys have been polishing those casings!! meanwhile, I need an appointment at your hospital please, the case is a dead 8032, well not completely dead, but at least nothing appearing on screen? I have checked voltages as per schematics, and they all seem ok, quite a few of the ic's are getting warm, and that is where I am, wondering what should be checked out next, to try and narrow things down. I am not so technically minded as most of you guys, but really willing to learn. thanks Harry

Does it "tweedle" when you turn it on?
What tools do you have? Oscilloscope? Multimeter? Logic probe? Another PET?
First thing is to wiggle the connectors in case there's some oxidation.

<edit>: Looks like Steve beat me to it while I was typing; how's that for service ;-)

@Tez: love those PET pix of yours; really cute!

 

[cosam]

First thing is to wiggle the connectors in case there's some oxidation.

Oh yes, do that first! And reseat anything socketed.

Looks like Steve beat me to it while I was typing; how's that for service ;-)

Heh, don't these posts normally come in threes? No doubt Dave is also replying as we speak... ;-)

 

[harry]

Hi doctors! thanks for quick replies, the head consultants I presume... well I removed the crt casing, and the incoming connector has the 4.9von horiz. and vert, and ac on the first two pins on the plug,
the crt heaters are glowing so things have power but no signal.
I do have a RS logic probe, and access to a scope, and I'm armed with my digital,and analogue multi meters. I have removed and cleaned and reseated all ic's , and I quess I'll find the pin outs of the 6502. and give you some more info its readings asap,
Unfortunately Mike, it makes no sound at all when switched on, I checked the voltages on the board at diodes cr1...-5v cr5....11.98v cr6...4.98v cr7....5.06v and there is 9.16v on j11

well I guess its time for bed, but tomorrow I'll see what the cpu is doing and let you know great stuff lights out! ....Harry

 

[wmmullaney]

Reset is the top right pin, iirc. Do you have another PET you could swap chips with?

 

[tezza]

Good to see the first patient checking in. Looks like Harry has all the necessarys to tackle the job too.

Mike: Re: the photos. Yes, I must give credit to my lovely wife Annette for those PET costume ideas. She's a designer so go figure!

I'm at a work conference for the next couple of days so I won't be as active on the boards during that time as I would like to be, but I'll be reading the new posts when I can get a chance.

Tez

 

[harry]

Ah, our first patient! ;-)

My first step would be to try to determine whether it's the monitor or the main board that's causing this. Have you checked voltages in the monitor? Is the neck of the CRT glowing? Yes, there are high voltages in there, so be careful, but you should be able to check the lower ones without getting near anything dangerous.

What kind of equipment do you have access to? A logic probe or scope would be handy to check the main board, but failing that even a volt meter will tell you something. What do RDY, /NMI and /IRQ look like? Is /RST working correctly? (Should be pulled low for a second or two at power up, then go high.) How about the clock pins? Is there any activity on R/W and the data and address busses?

Hi, back with some results, I have done a check on the pins on the 6502, using my logic probe. the reset works exactly as you describe, and stays high .NMI is high at switch on and stays high.
IRQ.is high at s/on and stays high, RDY is high at s/on and stays high, pin3 .. cl out gives all lights hi.lo. and pulse. the address lines 9- 14 seem to have contention with hi,lo, and pulse,
pins 15 &16 are High, pin 17 , hi/lo and pulse pins 18, 19 20 all low. all data lines except pin 29 are high, pin 29 is low, r/w pin 34 is high. I hope this gives the picture of what the cpu is doing or not doing! look forward to further tests .... Harry

 

[cosam]

well I removed the crt casing, and the incoming connector has the 4.9von horiz. and vert, and ac on the first two pins on the plug, the crt heaters are glowing so things have power but no signal.

Yes, you should have AC on pins 1 and 2. What does the other side of the voltage regulator (TP1) look like? It should be a steady 18V. Did you measure the rest just with the volt meter? There may be some kind of signal on there which would show up on the logic probe. If you can get a scope on there, there are waveforms on the schematics to compare against. If there's really nothing there, there's something wrong on the main board.

Hi, back with some results, I have done a check on the pins on the 6502, using my logic probe...

That all sounds reasonable at first glance. Not sure what to make of the data bus, but it certainly looks like the CPU is at least doing something.

 

[harry]

Yes, you should have AC on pins 1 and 2. What does the other side of the voltage regulator (TP1) look like? It should be a steady 18V. Did you measure the rest just with the volt meter? There may be some kind of signal on there which would show up on the logic probe. If you can get a scope on there, there are waveforms on the schematics to compare against. If there's really nothing there, there's something wrong on the main board.


That all sounds reasonable at first glance. Not sure what to make of the data bus, but it certainly looks like the CPU is at least doing something.

Hi Steve, thanks for help, TP1 on my board is just a small hole, no actual test pin, but I got a small lead, and yes voltage is ok, But now when I switch on the monitor produces a
intense green dot in the center of the screen? could I have moved something? or made a bad connection work? The dot is so intense I fear leaving the machine switched on in case of damage to the monitor. well at least something is happening? where can I go from here? thanks Harry

 

[cosam]

But now when I switch on the monitor produces a intense green dot in the center of the screen? could I have moved something? or made a bad connection work?

The dot could indicate a lack of horizontal and vertical drive signals, so at least it'd be consistent. Have a gander at those lines with the logic probe. If they're really flatlining, I'd trace back through pins 3 and 6 of UC2 (a 7486) and if there's still nothing, pins 1 and 4 of the same chip.

 

[dave_m]

Harry,
From your signal information, things are confusing. As cosam says the data lines are suspicious struck at hex 10, a conditional jump instruction. The address lines seem to be in a loop which may be OK. Give us the states of the upper four address lines, A15 (p25), A14 (24), A13 (23) and A12 (22). They should indicate the ROM area of the loop.

Check Sync (p5); this shows instruction fetches and should be pulsing. We may have to get lucky to solve this quickly as your PET is giving us very little clues so far.
-Dave

 

[harry]

The dot could indicate a lack of horizontal and vertical drive signals, so at least it'd be consistent. Have a gander at those lines with the logic probe. If they're really flatlining, I'd trace back through pins 3 and 6 of UC2 (a 7486) and if there's still nothing, pins 1 and 4 of the same chip.

Steve , there is nothing on horizontal or vert signals. I probed uc2 3 & 6 are high, 1& 4 are low,
no pulse on either, green dot now gone? nothing on monitor? wish I could be more helpful
thanks ...... Harry

 

[dave_m]

Steve , there is nothing on horizontal or vert signals. I probed uc2 3 & 6 are high, 1& 4 are low,
no pulse on either, green dot now gone? nothing on monitor? wish I could be more helpful
thanks ...... Harry

Harry,
The CRT controller may not be initialized as the PET seems to be getting stuck very early in the boot process. Do not worry too much about that until we get the PET system board running better with interrupt and I/O activity, etc. However I'm not sure about how harmful the bright dot is. Does anyone have thoughts about this? Should he unplug power to the CRT?
-Dave

 

[harry]

Harry,
From your signal information, things are confusing. As cosam says the data lines are suspicious struck at hex 10, a conditional jump instruction. The address lines seem to be in a loop which may be OK. Give us the states of the upper four address lines, A15 (p25), A14 (24), A13 (23) and A12 (22). They should indicate the ROM area of the loop.

Check Sync (p5); this shows instruction fetches and should be pulsing. We may have to get lucky to solve this quickly as your PET is giving us very little clues so far.
-Dave

Hi Dave, ok I have some more results for you, p22.. initial pulse when switched on then stays low, p23 hi/lo lit and pulsing,... p24 same as 23,.... p25 same as p23 p 5 stays HI
hope this helps......... thanks....... Harry

 

[dave_m]

p 5 stays HI

Harry,
This is not good. I do not think a working CPU will do this (stay in instruction fetch forever). You stated you already re-seated the chips. So do you have a spare 6502?
-Dave

 

[cosam]

As cosam says the data lines are suspicious struck at hex 10, a conditional jump instruction.

Wouldn't that be EF hex? As far as I can make out, that'd also be a condition jump, but it seems to be a three-byte instruction for the 65C02 only. Maybe a bad ROM?

However I'm not sure about how harmful the bright dot is. Does anyone have thoughts about this? Should he unplug power to the CRT?

Not sure, but you'd probably end up with a burnt-in dot. It can't do any harm to unplug the monitor for now to be on the safe side.

 

[cosam]

This is not good. I do not think a working CPU will do this (stay in instruction fetch forever).

That certainly looks to be the smoking gun: I'd expect that to be flapping around too. It would of course be the very pin I didn't ask for a reading of! ;-)

 

[dave_m]

Wouldn't that be EF hex? As far as I can make out, that'd also be a condition jump, but it seems to be a three-byte instruction for the 65C02 only. Maybe a bad ROM?

Steve,
You are right! I got it inverted (dyslexia?). Harry stated only D4 was zero. So this would make it an invalid instruction! Curiouser & curiouser. Possibly a bad ROM, but will that explain the Sync struck high? I can't remember what the CPU will do with a bad instruction. Trap to NMI or just go on to the next instruction? If it goes on to the next instruction, it may be possible that sync will stay on...I need to do some research on error conditions. This is fun ... well maybe not for Harry .
-Dave