6522 broken. Is it a common issue on Vectrex?

Hi! I'm trying to re-live two Vectrex boxes. I don't know if the
infamous LM379 issue is present in any of these units, but prior to
deal with it, I have to give life to the logic board.
No one of them makes sounds when powered up. I've opened one of them
and I have disassembled the logic board, and powered with a 5V
stabilized power supply.

To check for CPU, RAM or ROM damage, I've replaced the ROM (actually,
a 2764 EPROM) with a Flash EPROM pin compatible with 2764, with a tiny
test program. This program does not make use of RAM, so even if RAM is
damaged, this program should work (assuming that the CPU is alive).

The program is this:
ORG $E000
Start:
Init_VIA: LDA #$D0
TFR A,DP
LDD #$9FFF ;Port A=all output
STD <VIA_DDR_b ;Port B=10011111
CLRA
STA <VIA_int_enable ;Deshabilitamos
interrupciones en la VIA
STA <VIA_port_b

LDA #$05
Bucle: STA <VIA_port_a_nohs
ROLA
BRA Bucle

The effect of this program is to write 00000101 to port A on the VIA
6522. Then, 00001010, then 00010100, and so on, rotating the value one
bit to the left on each loop, so I can see using a scope two 5V
adjacent pulses, followed by a 0V region, then two 5V pulses again,
and so on. This should be seen on every pin at port A.

When I power up the Vectrex logic board, I can see this pattern but
only in some pins. Others show always the same logic level, and some
others show some noise, with the pulses overimpossed and atenuated.

If I plug the VIA 6522 from the other console, it shows more or less
the same, or worse (no pulses at all).

So, was the VIA 6522 a common issue on the Vectrex? Did this chip get
damaged frequently?

Thanks!!

PS: I ran a more complex program to test the RAM and use working pins
on VIA to show me the result: the RAM is OK.

On Dec 21, 3:08 am, "McLeod/IdeaFix" <rodri... DeleteThis @gmail.com> wrote:
> No one of them makes sounds when powered up. I've opened one of them
> and I have disassembled the logic board, and powered with a 5V
> stabilized power supply.

>
> The effect of this program is to write 00000101 to port A on the VIA
> 6522. Then, 00001010, then 00010100, and so on, rotating the value one
> bit to the left on each loop, so I can see using a scope two 5V
> adjacent pulses, followed by a 0V region, then two 5V pulses again,
> and so on. This should be seen on every pin at port A.
>
> When I power up the Vectrex logic board, I can see this pattern but
> only in some pins. Others show always the same logic level, and some
> others show some noise, with the pulses overimpossed and atenuated.
>
> If I plug the VIA 6522 from the other console, it shows more or less
> the same, or worse (no pulses at all).
>
> So, was the VIA 6522 a common issue on the Vectrex? Did this chip get
> damaged frequently?
>

Sounds to me like the AY-3 sound chip could equally be to blame,
especially as there is no sound. Did you try swapping those chips
between your boards? You could try just removing it and repeat your
port A test, in case the AY is shorting some data lines to a rail.

Your method for testing the system is very thorough. I think I would
have probably just replaced the ROM in a spare cartridge with an eprom
and tested it that way. Mind you, I actually acquired a 6809 ICE for
helping with my Vectrex development. Lovely bit of kit, and makes
things very easy!

Neil