Hello Marion
On 07-Aug-00, you wrote:
> Hey,
>
>
> I've got a pair of sickly Vic-20s and I was wondering if someone (Tony?
heh --
> you helped me with this before) could give me some ideas about fixing one
or
> both of them.
>
>
> The first one is fine except for having no sound. I've checked the pin
> assignments and continuity, and according to the info I was given when I
built
> the cable, everything should be great. But there's no sound with any of
the 6
> or 7 cartridges I've tried, or with no cartridge inserted. And the
monitor's
> sound is known-good. (I am using a Commodore 1084 monitor.)
Audio and video both source from the 6560 VIC chip:
Pin 19 is the sound output, which drives a 2N3904 emitter follower (Q5)
through a .1 mfd cap (C16) to pin 3 of the 5 pin DIN connector.
Video output is on pin 3 of the 6560. It goes through a 1 meg pot to ground
(R7). Wiper goes to 2.2 mfd 10v (C25) to base of PN2222 (Q7) as an emitter
follower. R7 sets video level and R32 sets pedestal via CR2. This emitter
follower feeds a voltage divider consisting of a 220 and a 270 ohm in
series. L1 connects the emitter directly to pin 5 of the DIN connector, and
L2 takes a divided signal to pin 4. Either of these should give you
composite video. Use a scope and see if the sync and video are there okay.
Also check 10mfd (C24) and .1 mfd (C27) bypasses.
One more thing to check is the external 14.31818 oscillator and UB9, a 7402.
output I think is 7.16 mHz into pins 38 and 39 of the VIC chip. This
timing signal is what all sync and color information is derived from.
I would bet the emitter followers are shot, or the caps in the circuit have
dried up. If it seems the 6560 chip is gone, the computer has gone too --
those chips used to be available from Jameco until they phased them out of
inventory.
FYI, early VIC-20's had the 5V regulator built into the motherboard with a
large heatsink attached -- these had the two prong 9vac input. Later models
had the C-64 power supply with an external regulator. These had a DIN power
connector on the side. The later version is the desireable one.
Also remember that numbering on a 5 pin DIN is goofy -- 1-4-2-5-3 seems to
ring a bell. And some had an 8 pin DIN which the 5 pin would plug into with
no modifications, identical to a C-64. Look carefully at the numbers in the
plastic if you can see that fine print.
>
>
> The second Vic powers up and appears to send _some_ kind of signal to the
> monitor (because the scan "whine" and screen color change) but there's no
> actual video, with or without a cartridge inserted. I took it apart and
looked
> for obvious things like unplugged cables, burnholes, etc. but didn't find
> anything like that. I check the continuity on the monitor port (where it's
> soldered to the board) and those seemed fine. I am using the same cable as
> before, so I know (?) it's not a problem with the cable.
>
>
> Any ideas?
>
>
> Thanks in advance,
>
>
> -- MB
>
Regards
--
Gary Hildebrand
Box 6184
St. Joseph, MO 64506-0184
816-662-2612
or
ghldbrd_at_ccp.com
Received on Mon Aug 07 2000 - 23:43:53 BST