Re: 6561-101

On Tue, Oct 25, 2016 at 10:38 PM, Hegedűs István <hegedusis@t-online.hu> wrote:
> Hi,
>
> It has black screen of course. I know that the Kernal ROM or VIC used to be
> the guilty ones. I have burnt a 27c128 with Kernal but still have to build
> an adapter so I cannot 100% rule out Kernal issue. The guy who sold it said
> that the 6502 is also sick (he has tried the CPU in an 1541), though with
> oscilloscope I have seen e.g. proper phi2 signal coming out. I have bought
> some Rockwell 6502s to replace it. Anyway even if it turns out that VIC is
> good it is better to have a spare IC.
> I should hook up my logic analyzer to it but so far I had no time (or I was
> just too lazy).

An oscilloscope is more than enough most of the times. I repaired a
VIC-20 with black screen (it's on youtube) and it turned out to be the
basic ROM. I'd check the data lines on CPU side and on RAM side
(there's a 74LS245 separating RAM from CPU/ROM data bus). The ROMs are
usually much more likely to go bad.
Given the symptom and that you have the CPU clock output from the VIC,
I'd bet in a ROM gone bad :)
Of course, first replace the 6502 if it's sick.


Frank

>
> -----Original Message----- From: Francesco Messineo
> Sent: Tuesday, October 25, 2016 10:23 PM
> To: cbm-hackers@musoftware.de
> Subject: Re: 6561-101
>
> Hi
>
> On Tue, Oct 25, 2016 at 10:07 PM, Hegedűs István <hegedusis@t-online.hu>
> wrote:
>>
>> Hi,
>>
>> I have a broken VIC20 which seems to have a bad VIC. Does anyone know
>> where
>> can I buy one for a fair price?
>
>
> what's the symptom? The 6561 was reliable enough. I can't remember
> changing any in the '80s (but VIC-20s were already almost estinct when
> I started repairing commodore stuff, so don't flame please).
> As far as I remember, the 6561 was only used for the VIC-20.
> Some IC traders might have them in stock, but I'd say it's a question
> of luck to get good chips instead of bad/fakes nowadays.
> I can help in troubleshooting your VIC-20 (as surely many others here)
> if you're unsure.
>
> Best regards
> Frank
>
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>
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