[RESOLVED] VC20 (no serial number)
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- mrr19121970
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Re: VC20 (no serial number)
Socketed & replaced UD8 & UE8. No change black screen only. No blind disk commands, no 'reset' signal received by floppy
- mrr19121970
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Re: VC20 (no serial number)
Maybe onto something now. Desoldered UD2-UD6 and UE1-UE6. Tested them in the colour ram of this machine and found 1 out of 11 that was bad (the rest are all good). Will socket tomorrow and try a spare RAM.
Also socketed UB4, UB6, UC4, UC5, UC6, UD9 & UF8 today. I might as well finnish off UB9, UB10, UC1, UC3, UD1 and then I have a fully socketed board if the RAM turns out to be the cause here.
Also managed to break my 1084, had a dodgy on-off switch that didn't always go off. Now it won't go on anymore.
Also socketed UB4, UB6, UC4, UC5, UC6, UD9 & UF8 today. I might as well finnish off UB9, UB10, UC1, UC3, UD1 and then I have a fully socketed board if the RAM turns out to be the cause here.
Also managed to break my 1084, had a dodgy on-off switch that didn't always go off. Now it won't go on anymore.
- mrr19121970
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Re: VC20 (no serial number)
Well screen is still black, tested all memories again. Still all ok. Last one to try is UD9 (I ordered some 133 logics yesterday). After this I think I can't save this one from the landfill. Will have to become a 'spare parts' donor with so many low dated & working components.
- mrr19121970
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Re: VC20 (no serial number)
Anyone recommend anywhere else to be testing with oscilloscope?
- Mike
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Re: VC20 (no serial number)
If the drive still doesn't get a reset signal, the 6502 in the VIC-20 most probably won't either.mrr19121970 wrote:No blind disk commands, no 'reset' signal received by floppy [...] Also socketed UB4, UB6, UC4, UC5, UC6, UD9 & UF8 today. [...] Anyone recommend anywhere else to be testing with oscilloscope?
Do you see a low-level pulse at pin 40 of the 6502 directly after power-on? Probe the signals around UB6 (555) and UB4 (7406) with extreme scrutiny. Compare the charging of C10 and C13 with a working unit. If in doubt, replace these two capacitors, R9 (220K), R14 (1.5M) and also R3 (1K).
- mrr19121970
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Re: VC20 (no serial number)
I was mistaken here. Was using a SD2IEC. Tried with a real drive it got the resetMike wrote: If the drive still doesn't get a reset signal, the 6502 in the VIC-20 most probably won't either.
I will check this.Mike wrote:Do you see a low-level pulse at pin 40 of the 6502 directly after power-on? Probe the signals around UB6 (555) and UB4 (7406) with extreme scrutiny. Compare the charging of C10 and C13 with a working unit. If in doubt, replace these two capacitors, R9 (220K), R14 (1.5M) and also R3 (1K).
I now tested the original locic chips and all are working
UB4 : 7406
UD8, UE8 & UEF9 : 74LS245
UE4, UC5 & UC6 : 74LS138
UD9 : 74LS133
UB6 : LM555
- mrr19121970
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Re: VC20 (no serial number)
I compared the 'pattern' on a working machine with this one. PIN40 6502 looks the same, small ripple for a few seconds then flat 5v
- mrr19121970
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Re: VC20 (no serial number)
I believe the capacitors and resistors are also fine.
- Mike
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Re: VC20 (no serial number)
Can you try to probe the address bus activity of the CPU?
Normally, while the VIC-20 is idle in the "READY." prompt, the PC is in a tight loop at $E5E8 .. $E5F0:
This pattern repeats every 13 Cycles, and should produce a discernible waveform on each bit of the address bus:
Now just read off the columns to probe the signal for a certain address bit.
During the keyboard interrupt, the PC will of course be located at another place. But if the 6502 is completely somewhere else, that should also give a good hint where the problem might be located (and in that case, please tell the suspected PC range).
Normally, while the VIC-20 is idle in the "READY." prompt, the PC is in a tight loop at $E5E8 .. $E5F0:
Code: Select all
.E5E8 A5 C6 LDA $C6
.E5EA 85 CC STA $CC
.E5EC 8D 92 02 STA $0292
.E5EF F0 F7 BEQ $E5E8
Code: Select all
$E5E8 = %1110010111101000
$E5E9 = %1110010111101001
$00C6 = %0000000011000110
$E5EA = %1110010111101010
$E5EB = %1110010111101011
$00CC = %0000000011001100
$E5EC = %1110010111101100
$E5ED = %1110010111101101
$E5EE = %1110010111101110
$0292 = %0000001010010010
$E5EF = %1110010111101111
$E5F0 = %1110010111110000
$E5F1 = %1110010111110001 (taken branch, internal cycle)
During the keyboard interrupt, the PC will of course be located at another place. But if the 6502 is completely somewhere else, that should also give a good hint where the problem might be located (and in that case, please tell the suspected PC range).
- mrr19121970
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Re: VC20 (no serial number)
I Don't know how to do this
- Mike
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Re: VC20 (no serial number)
You do it the same way as you did with the clock signals: connect the ground clamp of the probe to a ground plane somewhere near the 6502, and then hold the probe tip onto each of the address bus pins of the CPU at a time.
A13, A14 and A15 for example (at pins 23, 24 and 25, respectively) then should all show the following waveform for most of the time:
A13, A14 and A15 for example (at pins 23, 24 and 25, respectively) then should all show the following waveform for most of the time:
Code: Select all
110111011111011011101111101101110111110 ... (starting the list at $E5EA, and then repeating):
or
__ ___ _____ __ ___ _____ __ ___ _____
_ _ _ _ _ _ _ _ _
- mrr19121970
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Re: VC20 (no serial number)
This is what I see
- mrr19121970
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Re: VC20 (no serial number)
The pattern is different to a working 6502
- Mike
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Re: VC20 (no serial number)
Can you confirm the pattern in my posting above on a working VIC-20?
Those three flat-lines could indicate, that the CPU spins somewhere in the RAM address range ($0000..$1FFF) - do you see *any* activity in the lowest address bit (bit 0 = pin 9)?
Please also check the logic level at the /IRQ and /NMI pins of the CPU (IRQ = pin 4, NMI = pin 6) - a stuck-on-0 on the IRQ, or a flakey signal on the NMI (NMI is edge-sensitive) could put the CPU into an eternal interrupt loop.
Those three flat-lines could indicate, that the CPU spins somewhere in the RAM address range ($0000..$1FFF) - do you see *any* activity in the lowest address bit (bit 0 = pin 9)?
Please also check the logic level at the /IRQ and /NMI pins of the CPU (IRQ = pin 4, NMI = pin 6) - a stuck-on-0 on the IRQ, or a flakey signal on the NMI (NMI is edge-sensitive) could put the CPU into an eternal interrupt loop.
- mrr19121970
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Re: VC20 (no serial number)
I do this tomorrow after work. Just reassembled both VIC20s and packed away.