Commodore 3016 scrambled screen

[ScottishColin]

When I have had that output before (post 417) it has been all three 3446s that needed replacing.

Colin.

 

[TheMatrix]

That's also a possiblity, but i don't expect that.
The 6520 is not responding to the poke's.
Or i am doing it wrong..

 

[ScottishColin]

That's also a possiblity, but i don't expect that.
The 6520 is not responding to the poke's.
Or i am doing it wrong..

Are you able to swap the 6520s or are they soldered in?

Colin.

 

[daver2]

>>> Or i am doing it wrong..

No idea, unless you post the commands you are using and tell us what you don't think is working ...

Dave

 

[daver2]

/NRFD in/out is purely driven by the VIA. So a faulty 6520 cannot influence this signal.

Dave

 

[daver2]

Use the commands:

POKE 59426,n

Followed by:

PRINT PEEK(59424)

The POKE will output an 8-bit binary pattern to port B of the 6520. You can check the value of the bits on the 6520 port B with a multimeter or logic probe.

The value of n is in the range 0 through 255 corresponding to all of the combination of bits in a byte.

I would suggest using the individual values of 0,1,2,4,8,16,32,64,128,255 because they test each bit out individually.

You should see these bit patterns as voltages on port B of the IEEE488 6520.

You should also observe the same value back from the PEEK.

By the way, don't have anything plugged into the IEEE488 port whilst you are performing this testing.

If the outputs on port B of the 6520 work fine, it is probably a buffer (or three), but we can test these as well using the same POKEs and PEEKs...

Dave

 

[TheMatrix]

for example:
10 POKE 59427, 60:REM SETS DAV HIGH
20 POKE 59427, 52.REM SETS DAV LOW
30 POKE 59427, 60:REM SETS DAV HIG

I don't see any changes with scope on de DAV output (Pin 19)

 

[TheMatrix]

Are you able to swap the 6520s or are they soldered in?

Colin.

Is soldered in.

 

[daver2]

It is starting to look like the 6520...

I would try a few other signals to make sure just before we call it.

 Dave

 

[TheMatrix]

If i use this --> POKE 59426,255
and then PRINT PEEK(59424)
I get as return 90 and see no change on 6520 output

 

[daver2]

What about a poke of 0 and then we will call the 6520 as dead?

Dave

 

[TheMatrix]

I tried a few pokes
poke 0 give 191 as output

 

[daver2]

That seems weird.

You have replaced the 255 in post #430 with 0? Yes?

Dave

 

[TheMatrix]

That seems weird.

You have replaced the 255 in post #430 with 0? Yes?

Dave

That's correct

 

[daver2]

I think it is likely the 6520 is toast then I am afraid.

Dave

 

[TheMatrix]

i will desolder the 6520 tomorrow and try a spare one.
Solder first a socket.

 

[Hugo Holden]

If you need the MC3446 port IC's there are (were) some excellent vintage new old stock ones for a good price at SH-Halbleiter in Germany.

Their website though has been under editing for a while, I'm concerned they might have closed. They have a massive array of vintage IC's & transistors. I bought a stack of TTL IC's with early 70's date codes from them and a number of rare parts.

In any case, here is their email:

info@sh-halbleiter.de​

 

[TheMatrix]

After changing the 6520 it's now working
@Hugo Holden Thanx for this info. I will try to contact them. I need a 6520