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CBM Pet 4032 DOA

dave_m

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I've unsoldered the 74SL04N UD2 and put a new old one in and now i get this.

chequred pattern with video ram out

solid screen with character rom out

These are the correct patterns! So the the UD2 inverter was bad? It supplied the 8 MHz clock to the video data shift register circuits and would explain the loss of video.


why are two legs cut on the UD2 ? any idea?

They are not cut on my 8032. Sabotage??

What about burning smell on video board?
 

mjnurney

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i don't know dave , the legs are properly cut level with the IC. I never noticed before until removed the chip. i wonder if it was a mod for a graphics board or something? The guy who owned this Pet, was a hardware developer for Apple i believe , so who knows....

The burning smell has subsided now - maybe its just heat from being on all day - its not been run for twenty or more years :)

I have noticed the 6502 gets pretty warm , is this normal ?

i uploaded a little vid to show the start up , no keyboard control and no basic logo on screen yet ....but its getting closer :)

mike.
 

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dave_m

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i don't know dave , the legs are properly cut level with the IC.

Must be an unused gate.

I have noticed the 6502 gets pretty warm , is this normal ?
Yes, NMOS devices run a little warm.

Reinsert the video RAMs and character ROM and do we get a beep and random data on the screen or just that one square? Maybe we try the PETTest2 2532 EPROM in the UD6 socket now that we have video?

-Dave
 

mjnurney

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just one square with the ram back in - i tried different ram too

Pettest 2 and pettest 50z don't initialise the video although the voltage increases to around 4 volts on pin7
 

dave_m

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just one square with the ram back in - i tried different ram too

Pettest 2 and pettest 50z don't initialise the video although the voltage increases to around 4 volts on pin7

Let's forget about PETTEST2. Pin 7? Of J7? Do you mean pin 1 (Video Data)? Pin 6 is missing (Key).

If the 2 PIAs and VIA are on sockets, try to remove the chips and see if you get the Commodore sign on message.

It that is no help, on the last ROM socket (UD6), measure voltage on all the eight data lines (D0 through D7) looking for an open circuit (1.4 V).

Don't despair; We are getting close.
-Dave
 

mjnurney

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Yes sorry that's what I mean :)

It won't boot without the via in but with pia x 2 removed I get a few different charectors on screen.

@ and !

I'll check the voltage

D0 1.41
1.41
5.09
1.46
1.53
1.54
1.53
D7 1.60v

Some interesting screen nonsense too with pia's removed
 

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mjnurney

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That is with rom out , below with rom in

D0 .21
.82
5.3
0.3
0.3
4.48
4.3
D7 4.55


With original basic Roms in i get READY
 

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dave_m

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D0 1.41
1.41
5.09
1.46
1.53
1.55
1.54
1.53
D7 1.60v

Most of these data lines are pulsing, but unfortunately they have a DC average close to 1.4 VDC open circuit so it is hard to tell anything. I may need to give you a quick lesson in using the scope to tell which lines are pulsing and which may be open.


Some interesting screen nonsense too with pia's removed

Hey, the system almost made it to the machine language monitor program that is built into the UD8 ROM! Let me think if there is a clue there somewhere.
-Dave
 

mjnurney

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The big capacitor next to the transformer has just started leaking , so I've turned it all off until tomorrow. I have a new cap. It arrived this morning actually so that's an easy fix.

Cheers
Mike
 

mjnurney

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Capacitor changed this morning

Soldering touched up in a couple of places under the board and my proms fitted back in

I now get a jumbled screen all the time but Pettester2 finally works , yay

here are the results - bad ram?
 

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Pet Rescue

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mjnurney you look like you are getting closer, keep up the good work!
It may be worth checking that each chip leg in sockets are actually making contact to the solder side.
The intermittent faults sound like a pet I had with an address line on the cpu not making proper contact
due to a dirty socket. Just use the continuity test, a ball ache but at least you know they are ok.
 

mjnurney

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hi and thanks , these are new sockets but i'll do another test ...

we almost have it

then on to the crackling screen ....
 

dave_m

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The intermittent faults sound like a pet I had with an address line on the cpu not making proper contact
due to a dirty socket.

You are on to something there about address lines. Notice that the 'garbage data' in video RAM is switching from normal to reverse video every 128 bytes. Something about address line A7 is screwy. Perhaps it is making contact with Data Line D7?
 

Pet Rescue

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You are on to something there about address lines. Notice that the 'garbage data' in video RAM is switching from normal to reverse video every 128 bytes. Something about address line A7 is screwy. Perhaps it is making contact with Data Line D7?

Hi dave just checked my notes and it was pin 33 on my 6502 which was D0 on the data bus that was the problem.

mjnurney did you changed all the existing sockets including cpu etc
 

dave_m

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Crackling? ?

Like audible snapping?

KC,
That doesn't sound good. Also the wavy lines. Someone on the forum fixed a similar problem years ago but I can't remember the details. If you remember MikeS, he was good a fixing this kind of video problem. Too bad he wouldn't kowtow to a certain moderator. There is a discussion on the General Off Topic section to un-ban him, but it was vetoed. A real loss to the Commodore community.
-Dave
 
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