Audio problems on VIC-20

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Audio problems on VIC-20

Ethan Dicks
Hi, All,

I set up a retro-gaming event at our hackerspace over the US holiday
and I brought a couple of VIC-20s (since I hadn't fired them up in a
while and didn't know if one or both had problems).  Turns out, it's
good that I did.  One was fine, the other was not.

The one with the problems - the biggest issue I had, as an example,
was while playing Gorf from cartridge, the sound cut in and out while
the game played.  I can easily swap out the 6560 VIC chip in case
that's where the problem is, but are there any other diagnostic things
I can do?

Another symptom is the colors weren't quite as "bold" as they were on
the working VIC-20.  There was still color, but on a no-cartridge
bootup to BASIC, the border wasn't quite the same color, and when
playing Gorf, the colors weren't as saturated.  Could be related,
could be two different problems.

It's been a while since I've debugged a VIC-20, so pointers to
checklists and such are most welcome.

Thanks!

-ethan

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Re: Audio problems on VIC-20

Francesco Messineo
On Wed, Jul 5, 2017 at 8:03 PM, Ethan Dicks <[hidden email]> wrote:

> Hi, All,
>
> I set up a retro-gaming event at our hackerspace over the US holiday
> and I brought a couple of VIC-20s (since I hadn't fired them up in a
> while and didn't know if one or both had problems).  Turns out, it's
> good that I did.  One was fine, the other was not.
>
> The one with the problems - the biggest issue I had, as an example,
> was while playing Gorf from cartridge, the sound cut in and out while
> the game played.  I can easily swap out the 6560 VIC chip in case
> that's where the problem is, but are there any other diagnostic things
> I can do?

The audio signal is buffered with a single transistor
emitter-follower, so you would first check
it (solder joints and the 1uF electrolytic decoupling the DC after the
emitter-follower).

>
> Another symptom is the colors weren't quite as "bold" as they were on
> the working VIC-20.  There was still color, but on a no-cartridge
> bootup to BASIC, the border wasn't quite the same color, and when
> playing Gorf, the colors weren't as saturated.  Could be related,
> could be two different problems.

Inside the VIC metal box there's a 10k potentiometer that sets the
video signal level. Again, this signal is then buffered by another
emitter-follower. A DC-blocking electrolytic is placed before the base
input to the emitter follower, so this one might also be dried out and
have
high  ESR.
Your best friend is an oscilloscope in both cases.

Best regards
Frank IZ8DWF

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Re: Audio problems on VIC-20

Ethan Dicks
On Wed, Jul 5, 2017 at 2:23 PM, Francesco Messineo
<[hidden email]> wrote:

> On Wed, Jul 5, 2017 at 8:03 PM, Ethan Dicks <[hidden email]> wrote:
>> Hi, All,
>>
>> I set up a retro-gaming event ...  couple of VIC-20s
>> ...the sound cut in and out...
>
> The audio signal is buffered with a single transistor
> emitter-follower, so you would first check
> it (solder joints and the 1uF electrolytic decoupling the DC after the
> emitter-follower).

Perfect.  Thanks for the info.

>> Another symptom is the colors weren't quite as "bold" as they were on
>> the working VIC-20...
>
> Inside the VIC metal box there's a 10k potentiometer that sets the
> video signal level. Again, this signal is then buffered by another
> emitter-follower. A DC-blocking electrolytic is placed before the base
> input to the emitter follower, so this one might also be dried out and
> have high  ESR.
> Your best friend is an oscilloscope in both cases.

Awesome advice.  I'll keep your words in mind as I check the
schematics.  Thanks!

-ethan

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Re: Audio problems on VIC-20

Francesco Messineo
On Wed, Jul 5, 2017 at 9:09 PM, Ethan Dicks <[hidden email]> wrote:
> On Wed, Jul 5, 2017 at 2:23 PM, Francesco Messineo


>>
>> Inside the VIC metal box there's a 10k potentiometer that sets the
>> video signal level. Again, this signal is then buffered by another
>> emitter-follower. A DC-blocking electrolytic is placed before the base
>> input to the emitter follower, so this one might also be dried out and
>> have high  ESR.
>> Your best friend is an oscilloscope in both cases.
>
> Awesome advice.  I'll keep your words in mind as I check the
> schematics.  Thanks!

just don't quote me on the 10k trimmer. It could be anything from 1k
to 1M actually (but it does set the video level). I don't come to see
and repair many VIC-20s lately :/

Frank

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