[RESOLVED] VC20 (no serial number)

History and Preservation Issues

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mrr19121970
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Re: VC20 (no serial number)

Post by mrr19121970 »

Well it seemed I also had 74LS02 in my store box too. Didn't help. Still screen of blackness
Kakemoms
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Re: VC20 (no serial number)

Post by Kakemoms »

I haven´t really taken part of this discussion, but if it helps I would comment on the following:

You are probing at 1MHz doesn´t show much since the CLK cycle is low/high for 500ns per period. It would help if you probed the CLK/A0-15/D0-8 signals at >2MHz (10MHz preferably) so that it is possible to see the actual data there. The pictures you posted up til now does´t really help much except for the 6502 startup which shows its starting (e.g. flat address lines until some data appears). From that it looks like your reset network and crystal are ok.

It also looks like you tested most of the logic chips, so assuming you tested them well and did not miss a problem I would have focused on the capacitors and resistors. It may not be an actual component, its enough that you have a crack in a solder that prevents a good connection.

The actual garbled screen that you had on the first power-up is something I see a lot when I develop and rewire the address and/or databus. Capacitive load will mess up data on the bus and sometimes you get the coloured character, sometimes its just black and sometimes it goes all the way to "Ready". Its very indicative of a messy data or address bus. Reseating the 6561-101 chip would change the capacitance (the pins are probably covered in thick tin oxide) and increase the noise on the bus and/or signals into the 6561. As the 6561 is driven at higher frequency its a little more sensitive to such changes. You could even try to start the machine without the 6561 and try to see (see previous post of Mike) for the cyclic data on the databus that appears once you have the "Ready" prompt (you won´t see it without the 6561 there, but the CPU would still get there).

If you haven´t given up already I would have taken off all socketed IC´s, cleaned the IC pins and replaced the sockets. Then I would have
re-solder all the capacitors (just melt the solder with a little added new solder on the pin), then the resistors. If it still does´t work, check SRAM, 74LS245, 74LS138 and 74LS133. Replace Kernal and Basic ROM.

And please use anti static wrist band unless you want to add random damage to the components you are "fixing" :roll:
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mrr19121970
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Re: VC20 (no serial number)

Post by mrr19121970 »

I didn't give up just yet. Just thing a break....

This is the summary of what is known so far:

. All socketed ICs tested and working in another board
. All memory socketed + tested (1 piece bad and replaced)
. All logic chips socketed and tested (1 74ls133 and 74ls02 bad and replaced)
. 1 capacitor replaced (c42) . Now the pin 40 reset is stable
. All newly socketed ICs checked for continuity and shorts

Next try your tips for removing VIC-I and re flowing (maybe replacing all) capacitors.

The pdf cc12 is quite hard to read. I have bit the bullet and bought a NOS sams computerfacts cc12. I will scan in for the forum when it arrives.
Kakemoms
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Re: VC20 (no serial number)

Post by Kakemoms »

mrr19121970 wrote:I didn't give up just yet. Just thing a break....

This is the summary of what is known so far:

. All socketed ICs tested and working in another board
. All memory socketed + tested (1 piece bad and replaced)
. All logic chips socketed and tested (1 74ls133 and 74ls02 bad and replaced)
. 1 capacitor replaced (c42) . Now the pin 40 reset is stable
. All newly socketed ICs checked for continuity and shorts

Next try your tips for removing VIC-I and re flowing (maybe replacing all) capacitors.

The pdf cc12 is quite hard to read. I have bit the bullet and bought a NOS sams computerfacts cc12. I will scan in for the forum when it arrives.
Interesting. Did you reflow the sockets of the IC´s that were already there?

I´m wondering why the 74LS133 had failed.. It unusual to have such a component fail without an electrical problem. Since its only feeded by a few 138 components (+CLK if I remember correctly), I would suspect that the 74LS138 may have suffered some damage as well... unless the previous owner shorted the expansion port.
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mrr19121970
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Re: VC20 (no serial number)

Post by mrr19121970 »

Good point, remember this VIC20 has been dropped from a great height.

I have had in the past factory fitted (or aged) sockets with broken legs. I think the continuity meter needs to come out here too.
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Re: VC20 (no serial number)

Post by mrr19121970 »

It lives...


I revisited all my own soldering.

. Memory, still black
, 74LS257 + 74LS133 - startup screen

Image is black an white. Next issue.
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Re: VC20 (no serial number)

Post by mrr19121970 »

Twiddling the R7 seems to have introduced colour, but pretty washed out. Chroma mod, and it seems better.
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Re: [RESOLVED] VC20 (no serial number)

Post by mrr19121970 »

For reasons unknown to me, a few keys were yellowed. After having bad experiences loosing some of the printed fronts I decided to live with this. However after reading Ken's post and getting a tip from a buddy decided to try it.

Basically take a pipette and apply the emulsion only to the top of the keys. 45 mins sun and we are done. Like new again...
IMG-20160808-WA0014.jpg
IMG-20160808-WA0028.jpg
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mrr19121970
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Re: [RESOLVED] VC20 (no serial number)

Post by mrr19121970 »

The final results with before and after pics....

file-1.jpeg
20160808_173041.jpg
file-2.jpeg
20160808_173050.jpg
file-3.jpeg
20160808_173058.jpg
Kakemoms
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Re: [RESOLVED] VC20 (no serial number)

Post by Kakemoms »

Did you get nice screen colors eventually?

The retrobright looks astonishing. I am planning to do that on one of the spare Vic-20'ies I have to see if it gets that "new" feeling. But I am also planning to look more closely on the plastic surface before and after the treatment. With more closely I mean using an electron microscope. You may wonder why, but since the retrobrigth contains amounts of oxidizer (peroxide), that is something that usually eats plastic. So I want to see if it actually damages the surface (like ozone).
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Re: [RESOLVED] VC20 (no serial number)

Post by mrr19121970 »

Not really. No. I have 4 rev c boards and actually they are all as bad as each other other the 1084 so I decided not to pursue it further. I should have tested on a tv, as I had the idea there was no issue here.

After retr0bright I use the matt cockpit spray ArmourAll which hopefully seals and protects the case against uv and oxygen further.
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