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CBM Pet 4032 DOA

dave_m

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PR,
We're only allowed to discuss him in that other section, but briefly he was tossed out last December. Yes, we liked to poke fun at each other and race to solve a PET problem. I miss the old Commodore Curmudgeon. He acted gruff sometimes and then would send the person he yelled at some free needed parts to fix a Commodore machine.
-Dave
 
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mjnurney

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Well some news

The pet has passed the ram check ok with pettester. my sockets are not as good as I thought ..

It now chirps every time and will display READY. no commodore for some reason

however with the 6520A's

With the upper (UB16) fitted I get the READY
with the lower (UB12) I get the single block and no writing.

Perhaps both 6520s are dead or faulty, same fault if I swap them around.
 

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Pet Rescue

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Just another question on the sockets, did the CPU, PIA's, VIA and ROM's etc have sockets already and if so have you replaced them,
if they are the original ones the continuity test will check their integrity.
Regarding the PIA's I'd expect the fault to follow if bad.
 

mjnurney

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hi

ive replaced all the sockets , I was getting strange faults one moment and not the next, so all changed.

the ram sockets I have are some Chinese things - not the best but they work.

I expect the 6521 will work as a PIA ? 6520A are a little thin on the ground.

I wonder why the Commodore logo - writing isn't appearing?
 

Pet Rescue

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hi

ive replaced all the sockets , I was getting strange faults one moment and not the next, so all changed.

the ram sockets I have are some Chinese things - not the best but they work.

I expect the 6521 will work as a PIA ? 6520A are a little thin on the ground.

I wonder why the Commodore logo - writing isn't appearing?

Yes 6521 should be an equivalent to 6520, do you get a cursor flashing or just the ready text?
I don't like how sockets can throw up weird faults and behaviour.
Put the Pet away working and turn it on next day and get garbage!
It definitely looks to be nearly working.
 

mjnurney

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I've ordered a couple of 6521 chips let's see :)

I just get

READY.

No cursor or keyboard control obviously.

It's been a battle , probably the most difficult computer I've had to repair yet (8 bit anyway). The pet offers no clue if you have a blank screen.
We should really have a fault guide to work through on pets.

Power
CPU
Rom
Ram
Logic/via/pia
Crt

When my pet is working properly I'll compile something

so here is the result so far.

http://youtu.be/MZYn0HAGLjk
 

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dave_m

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I've ordered a couple of 6521 chips let's see :)

It may be cheaper to find 6820 PIAs.


so here is the result so far.

http://youtu.be/MZYn0HAGLjk

The random data is not right. It changes from normal to inverse video every 128 bytes instead of being random. The inverse video is controlled by the D7 being set in the video RAM.

Check for high impedance short between A7 and D7 at the video RAM chips. They should Ohm out as open; if you see < 500 Ohms, there is a problem with solder splash, socket, etc.
 

mjnurney

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not sure I follow Dave?

UC4 A7 (pin17) and D7 ? where is that ?... OSD7 - pin11 ?(UC5)


thanks :)
 

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dave_m

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UC4 A7 (pin17) and D7 ? where is that ?... OSD7 - pin11 ?(UC5)

Mike,
Well I was actually thinking about the main Address lines (BA7) and the Data bus (D7) as they would effect the boot process in addition to affecting the contents of the Video RAM data. But you are right, the problem could be just at the Video RAM,

So with power off, first check Ohms:

UD6-pin1 (BA7) to UD6-pin17 (D7) and then from UC5-pin17 (Screen Address 7) to UC5-pin11 (Output Screen Data 7).
 

dave_m

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OK, good try. I'm guessing the CPU is still stuck with the SYNC low? We may have to consider a NOP Generator to check all address lines.

But first can you measure the voltage of the upper Address lines to see what area of ROM space we are stuck at. Measure A15, A14, A13 and A12. That will tell use what ROM chip the CPU was trying to use, and we can look carefully at the solder joints of that chip.
 

mjnurney

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Crackling? ?

Like audible snapping?


It sounds like the flyback sparking but its only for a second until the signal is received from the PET and then it stops, the funny thing is that the PETTESTER rom makes it do this for a little longer, it doesn't seem to initialise the video as quickly as the commodore kernal.
Hence why i never got the pettester to run before, i didnt like the noise so i shut it down.

However there is no sparking with the cover off and in darkness, there is lighting no or firing sparks as I've seen before with cracked flybacks. i had to repair an Apple monitor years ago with a liquid cement stuff. worked fine for years after.


PS

The PET screen is also fitted in the Amstrad GT65 (green screen sold with Amstrad CPC computers)

but i'll at the CRT later.....

the mission is the PET at the moment.

mike.
 

Pet Rescue

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If you left the kernal rom in place and pulled the basic roms would the computer boot into monitor.
That way you could pin point to a rom problem and see if keyboard etc works because we get chirps etc
Could chip select not be reading the roms fully.
 

mjnurney

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hi Dave

With Petester ROM fitted

The cpu sync is LOW and pulsing - according to my logic probe set to cmos.

12- 3.11v high/low & pulse on logic probe
13-3.11 high/low pulse
14-3.11 high/low pulse
15-4.25 stuck hi

So A15 is the problem?

All other address are pulsing as though working except A15 (pin25)

With commodore kernel fitted / ready or random graphics displayed

12=0.04v hi
13=0.13v hi
14=3.13v hi
15=3.14v hi
 
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Pet Rescue

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PR,
It would not boot at all. The screen is utilized in the $E000 ROM and the monitor is in the $D000 ROM.

Ah sorry Dave getting mixed up with the B series repair I did where it would boot without the basic ROM's to monitor.

I don't know how you keep all that knowledge in your head!
 
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dave_m

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It sounds like the flyback sparking but its only for a second until the signal is received from the PET and then it stops, the funny thing is that the PETTESTER rom makes it do this for a little longer, it doesn't seem to initialise the video as quickly as the commodore kernal.
Hence why i never got the pettester to run before, i didnt like the noise so i shut it down.

Mike,
When I patched Eudimorphodon's petest.bin code to add the 6545 initialization, I did not have a 6502 assembler handy so I just added the initialization routine at the end of his program by hand and looped back to the start. The result is that a complete RAM test must be run before the screen it initialized. I never knew how much delay it would cause because I could not run the program myself as the kernel ROM is soldered in my PET.

Thanks for letting me know. I will warn people in the future. And I will fix it someday and also add a BEEP at start. Below is a sort of source file I have from disassembling the original code and adding the my patch.

Code:
                 * = F000
F000   78         SEI
F001   A9 0E      LDA #$0E
F003   A2 00      LDX #$00
F005   4C 1A F0   JMP $F01A
F008   A9 55      LDA #$55
F00A   A2 00      LDX #$00
F00C   A0 00      LDY #$00
F00E   4C 3B F0   JMP $F03B
F011   A9 00      LDA #$00
F013   A2 00      LDX #$00
F015   A0 55      LDY #$55
F017   4C 5A F0   JMP $F05A
F01A   9D 00 80   STA $8000,X
F01D   9D 00 81   STA $8100,X
F020   9D 00 82   STA $8200,X
F023   9D 00 83   STA $8300,X
F026   9D 80 80   STA $8080,X
F029   9D 80 81   STA $8180,X
F02C   9D 80 82   STA $8280,X
F02F   9D 80 83   STA $8380,X
F032   E8         INX
F033   30 03      BMI $F038
F035   4C 1A F0   JMP $F01A
F038   4C 08 F0   JMP $F008
F03B   95 00      STA $00,X
F03D   9D 00 01   STA $0100,X
F040   9D 00 02   STA $0200,X
F043   9D 00 03   STA $0300,X
F046   95 80      STA $80,X
F048   9D 80 01   STA $0180,X
F04B   9D 80 02   STA $0280,X
F04E   9D 80 03   STA $0380,X
F051   E8         INX
F052   30 03      BMI $F057
F054   4C 3B F0   JMP $F03B
F057   4C 11 F0   JMP $F011
F05A   B5 00      LDA $00,X
F05C   C9 55      CMP #$55
F05E   F0 0B      BEQ $F06B
F060   A9 02      LDA #$02
F062   9D 00 80   STA $8000,X
F065   E8         INX
F066   30 0E      BMI $F076
F068   4C 5A F0   JMP $F05A
F06B   A9 07      LDA #$07
F06D   9D 00 80   STA $8000,X
F070   E8         INX
F071   30 03      BMI $F076
F073   4C 5A F0   JMP $F05A
F076   A9 00      LDA #$00
F078   A2 00      LDX #$00
F07A   A0 55      LDY #$55
F07C   4C 7F F0   JMP $F07F
F07F   B5 80      LDA $80,X
F081   18         CLC
F082   C9 55      CMP #$55
F084   F0 0B      BEQ $F091
F086   A9 02      LDA #$02
F088   9D 80 80   STA $8080,X
F08B   E8         INX
F08C   30 0E      BMI $F09C
F08E   4C 7F F0   JMP $F07F
F091   A9 07      LDA #$07
F093   9D 80 80   STA $8080,X
F096   E8         INX
F097   30 03      BMI $F09C
F099   4C 7F F0   JMP $F07F
F09C   A9 00      LDA #$00
F09E   A2 00      LDX #$00
F0A0   A0 00      LDY #$00
F0A2   4C A5 F0   JMP $F0A5
F0A5   BD 00 01   LDA $0100,X
F0A8   18         CLC
F0A9   C9 55      CMP #$55
F0AB   F0 0B      BEQ $F0B8
F0AD   A9 02      LDA #$02
F0AF   9D 00 81   STA $8100,X
F0B2   E8         INX
F0B3   30 0E      BMI $F0C3
F0B5   4C A5 F0   JMP $F0A5
F0B8   A9 07      LDA #$07
F0BA   9D 00 81   STA $8100,X
F0BD   E8         INX
F0BE   30 03      BMI $F0C3
F0C0   4C A5 F0   JMP $F0A5
F0C3   A9 00      LDA #$00
F0C5   A2 00      LDX #$00
F0C7   A0 55      LDY #$55
F0C9   4C CC F0   JMP $F0CC
F0CC   BD 80 01   LDA $0180,X
F0CF   18         CLC
F0D0   C9 55      CMP #$55
F0D2   F0 0B      BEQ $F0DF
F0D4   A9 02      LDA #$02
F0D6   9D 80 81   STA $8180,X
F0D9   E8         INX
F0DA   30 0E      BMI $F0EA
F0DC   4C CC F0   JMP $F0CC
F0DF   A9 07      LDA #$07
F0E1   9D 80 81   STA $8180,X
F0E4   E8         INX
F0E5   30 03      BMI $F0EA
F0E7   4C CC F0   JMP $F0CC
F0EA   A9 00      LDA #$00
F0EC   A2 00      LDX #$00
F0EE   A0 55      LDY #$55
F0F0   4C F3 F0   JMP $F0F3
F0F3   BD 00 02   LDA $0200,X
F0F6   18         CLC
F0F7   C9 55      CMP #$55
F0F9   F0 0B      BEQ $F106
F0FB   A9 02      LDA #$02
F0FD   9D 00 82   STA $8200,X
F100   E8         INX
F101   30 0E      BMI $F111
F103   4C F3 F0   JMP $F0F3
F106   A9 07      LDA #$07
F108   9D 00 82   STA $8200,X
F10B   E8         INX
F10C   30 03      BMI $F111
F10E   4C F3 F0   JMP $F0F3
F111   A9 00      LDA #$00
F113   A2 00      LDX #$00
F115   A0 55      LDY #$55
F117   4C 1A F1   JMP $F11A
F11A   BD 80 02   LDA $0280,X
F11D   18         CLC
F11E   C9 55      CMP #$55
F120   F0 0B      BEQ $F12D
F122   A9 02      LDA #$02
F124   9D 80 82   STA $8280,X
F127   E8         INX
F128   30 0E      BMI $F138
F12A   4C 1A F1   JMP $F11A
F12D   A9 07      LDA #$07
F12F   9D 80 82   STA $8280,X
F132   E8         INX
F133   30 03      BMI $F138
F135   4C 1A F1   JMP $F11A
F138   A9 00      LDA #$00
F13A   A2 00      LDX #$00
F13C   A0 55      LDY #$55
F13E   4C 41 F1   JMP $F141
F141   BD 00 03   LDA $0300,X
F144   18         CLC
F145   C9 55      CMP #$55
F147   F0 0B      BEQ $F154
F149   A9 02      LDA #$02
F14B   9D 00 83   STA $8300,X
F14E   E8         INX
F14F   30 0E      BMI $F15F
F151   4C 41 F1   JMP $F141
F154   A9 07      LDA #$07
F156   9D 00 83   STA $8300,X
F159   E8         INX
F15A   30 03      BMI $F15F
F15C   4C 41 F1   JMP $F141
F15F   A9 00      LDA #$00
F161   A2 00      LDX #$00
F163   A0 55      LDY #$55
F165   4C 68 F1   JMP $F168
F168   BD 80 03   LDA $0380,X
F16B   18         CLC
F16C   C9 55      CMP #$55
F16E   F0 0B      BEQ $F17B
F170   A9 02      LDA #$02
F172   9D 80 83   STA $8380,X
F175   E8         INX
F176   30 0E      BMI $F186
F178   4C 68 F1   JMP $F168
F17B   A9 07      LDA #$07
F17D   9D 80 83   STA $8380,X
F180   E8         INX
F181   30 03      BMI $F186
F183   4C 68 F1   JMP $F168
F186   A9 00      LDA #$00
F188   A2 00      LDX #$00
F18A   A0 00      LDY #$00
F18C   4C 8F F1   JMP $F18F
F18F   BD 00 80   LDA $8000,X
F192   BD 00 81   LDA $8100,X
F195   BD 00 82   LDA $8200,X
F198   BD 00 83   LDA $8300,X
F19B   BD 80 80   LDA $8080,X
F19E   BD 80 81   LDA $8180,X
F1A1   BD 80 82   LDA $8280,X
F1A4   BD 80 83   LDA $8380,X
F1A7   E8         INX
F1A8   E0 FF      CPX #$FF
F1AA   F0 03      BEQ $F1AF
F1AC   4C 8F F1   JMP $F18F
F1AF   C8         INY
F1B0   C0 FF      CPY #$FF
F1B2   F0 03      BEQ $F1B7
F1B4   4C 8F F1   JMP $F18F
F1B7   A2 00      LDX #$00
F1B9   A0 00      LDY #$00
F1BB   A9 00      LDA #$00
F1BD   4C C0 F1   JMP $F1C0
F1C0   8A         TXA
F1C1   9D 00 80   STA $8000,X
F1C4   9D 00 81   STA $8100,X
F1C7   9D 00 82   STA $8200,X
F1CA   9D 00 83   STA $8300,X
F1CD   E8         INX
F1CE   E0 FF      CPX #$FF
F1D0   F0 03      BEQ $F1D5
F1D2   4C C0 F1   JMP $F1C0
F1D5   A2 00      LDX #$00
F1D7   A0 00      LDY #$00
F1D9   A9 00      LDA #$00
F1DB   BD 00 80   LDA $8000,X
F1DE   BD 00 81   LDA $8100,X
F1E1   BD 00 82   LDA $8200,X
F1E4   BD 00 83   LDA $8300,X
F1E7   BD 80 80   LDA $8080,X
F1EA   BD 80 81   LDA $8180,X
F1ED   BD 80 82   LDA $8280,X
F1F0   BD 80 83   LDA $8380,X
F1F3   E8         INX
F1F4   E0 FF      CPX #$FF
F1F6   F0 03      BEQ $F1FB
F1F8   4C DB F1   JMP $F1DB
F1FB   C8         INY
F1FC   C0 FF      CPY #$FF
F1FE   F0 03      BEQ $F203
F200   4C DB F1   JMP $F1DB


; Initialize CRTC 


F203   LDX #00    ;Clear X register
F205   LDA $F220,X    ;Load CRTC constant from table
           STX $E880    ;Select 6545 register starting with R0
           STA $E881    ;Store data into 6545
           INX    ;Increment Register number
           CPX #$12    ;loaded last register?
           BNE $F205    ;No, continue loading
           JMP $F000    ;Yes, jump to start of test


F220   .BYTE              31 28 29 0F 20 03 19 1D
           .BYTE              00 09 00 00 10 00 00 00 
           .BYTE              00 00 

; Don’t forget to insert the RESET Vector in the EPROM binary at $FFFC to cause power up to start at $F000


    * = FFFC
FFFC    .BYTE    00F0
        .END
 

mjnurney

Experienced Member
Joined
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Messages
199
Location
UK
great stuff Dave,

it works fine but i takes a little while to initialise the screen. i'll video it so you can see it.

its about 5 seconds or so but after that its fine.

mike
 

dave_m

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Messages
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Location
Southern California, USA
Mike,
Yes, please when you get a chance. Now I know why some people would report the Pettest2 did not work for them. It takes too long to run properly.

When you turn off the PET, do you still get a dot in the center of the screen, slowly fading? Someone reported that it is caused by a leaky capacitor on the video board, but I don't remember which one...
 
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