Vic 20 2114 chips. How compatible between versions?

[tezza]

Hi,

I'm slowly reparing my (later-version) Vic-20. I've found out it actually has more than just one problem. First, I've found the 74LS04 chip was faulty, so it was replaced with a new socketed chip.

This stopped all the random bars and lockup, but I'm not out of the woods yet. The character colours are still all wrong. I strongly suspect one of the colour SRAM 2114 chips (at UE1) as this gets very hot.

Anyway, I have no source of these exact chips. However, I do have an old Vic motherboard Terry Yager gave me. It's the first version of the Vic, so it's not the same as mine. However, it does have a whole bank of 2114 chips.

The chips in the old board are stamped MM2114N-3L (used for the 4k RAM)
The chips in the newer Vic are stamped MM2114P25 (used for Colour control somehow)

My question is, are these interchangable? If so, I'll just whip on out of the old board and use it in the newer one.

I suspect they are not interchangable as they seem to perform a slightly different function (In this newer board, conventional RAM is facilitated by two large chips). Maybe the 2114s are interchangable though?

Do any chip jockeys out there know?

 

[Druid6900]

Well, they are close enough so that they will probably work.

The main difference is speed, the ones you need are 250mS and the ones you have in the other board are 300mS.

However, just because the chips are 250mS doesn't necessarily mean that the data will be on the buss in that amount of time.

Try one, socket it, if it works, great, if not, you can pull it out and I'll start looking around for a 2114-250.

What's the processor speed on the one you're fixing, BTW? You can calculate the required speed by taking the reciprocal of the processor speed (and hope it's not divided down too much).

 

[tezza]

Well, they are close enough so that they will probably work.

The main difference is speed, the ones you need are 250mS and the ones you have in the other board are 300mS.

However, just because the chips are 250mS doesn't necessarily mean that the data will be on the buss in that amount of time.

Try one, socket it, if it works, great, if not, you can pull it out and I'll start looking around for a 2114-250.

Hmm...Ok, I did just that. Once the older chip replacement was inserted, the screen display improved. Letters seemed better formed. It seemed that original chip may have indeed been faulty. However, the improvement was only 1/2 way towards being correct. Colours are still mixed up (although nowhere near as much).

I thought perhaps the second colour SRAM chip was also faulty so I extracted another 2114N-3L from the old board, snipped out the existing chip and clipped the replacement chip over old pins. It didn't seem to improve matters. Tomorrow I'll socket it into the board and see if that makes a difference.

It could be that it was just the first 2114P25 that was faulty and the reason the symptoms weren't completely cured was due to the speed difference? Is this a possibility?

What's the processor speed on the one you're fixing, BTW? You can calculate the required speed by taking the reciprocal of the processor speed (and hope it's not divided down too much).

I think the processor in both boards is 1 MHz.

 

[Dwight Elvey]

Hi
Old 2114s are notoriously bad. You need to check several
to see which are good and bad. I bought a bunch from
Jameco once and found that about 50% were bad. Jameco replaced
bad ones I sent back but it was still a pain.
I don't really blame them, they have no way of testing the
quality of NOS that they get in.
Dwight

 

[tezza]

Yes, I'm de-soldering them from an old board which was in working condition. However, I'm still learning the art, so I wouldn't be surprised at all if I do cook some of them.

Luckily I have eight of the chips and I only need three. Hopefully the odds will be in my favour.

Tonight I intend to continue on, socketing the 2114 I'v just extracted. If there is no improvement I'll replace the remaining 2114 (there are three that deal with colour SRAM). If this also fails to improve things I'll try my final throw of the dice, which is to replace the CD4066 chip (colour interface logic).

If that fails, I'm stuffed! Maybe not...problems COULD now exist because of the lower SRAM speed with these old chips, but if it's not that I really don't know what it could be.

 

[Druid6900]

Not likely a speed issue or you'd, most likely, have a problem on one section of the screen (depending on how they work the video). So, seeing as you're getting good at it, replace the last VidRAM and see what happens.

You might want to do a search for the manual and, if you can find a Sams ComputerFacts on it, it would give you the most likely causes.

Hell, if you have a logic probe, it'll give you what the reading should be on every chip on the board.

 

[carlsson]

On Zimmers.net FTP there are very detailed schematics and other cruicial documents on the VIC-20 and other Commodore computers. I think you'll find most of what you need there.

 

[tezza]

Vic-20 repair job. Another patient saved!

Vic-20 repair job. Another patient saved!

Well… finally!

With the aid of Ray Carlsen’s DIAGNOSTICS AND REPAIR GUIDE, Terry Yegar’s beer-money board (providing the replacement chips), help from this forum and a fair whack of time I’ve managed AT LAST to repair this Vic-20.

In the end it seemed three separate chips were faulty. A 74LS04 Logic chip at UC2, an SRAM 2114 chip at UE1 and the colour interface logic chip CD4066 at UD1. I don’t know what happened to this unit in an earlier life, but it mustn’t have been pretty!

In the end I replaced 6 chips during the diagnostic/repair process. In hindsight three of those that were clipped and snipped away (two 2114 chips and the Character ROM) were probably perfectly ok. Anyway, the replacements are socketed now which is a good thing. I’ve attached an image of the board showing the replaced chips. The red arrows show which ones turned out to be faulty and the blue arrows show those that were replaced anyway. I’ve also attached a screen shot of the monitor screen that greeted me after that “last roll of the dice” I mentioned in the earlier post. I was happy to see that!

Unlike the Osborne I and SX 64 repairs, where the patient leapt back to life after a chip replacement, this was more of a slow awakening. Each time a faulty chip was replaced, the patient looked a little better. Full health was only restored after the CD4066 was replaced.

Anyway, I’m now a happy man. Thanks to everyone who guided me in this. All my machines now work, the only remaining issues being the faulty RS423 on the BBC (b) and the suspected dud Dallas chip in the SLT-286.

Time for a well-earned beer before bedtime!

 

[Druid6900]

Feels good, don't it?

 

[tezza]

Yes. It does feel very satisfying. As I said in a previous post somewhere, the diagnosing and repairing aspect of vintage computers was one I never considered when I started this hobby. In fact, I first viewed it as a intimidating chore. It's actually very rewarding.

And frustrating. I just wish I could get that BBC (b) serial port working.

Anyway, there will be no vintage computer dabbling for a few days now. Tomorrow I have to guide a friend up one of our mountains (Mt Taranaki). The next few days after that I'll be recovering!

I should take my TRS-80 Model 100 up to the peak. It would make a good but bizzare photo (-:

 

[nige the hippy]

The next "Beeb" I find at the dump, the ULA is yours! (I need a spare keyboard switch for one of mine!) - that might be another year though so encourage your antipodean brothers, sisters, cousins & second cousins to keep looking in the meantime.

 

[tezza]

The next "Beeb" I find at the dump, the ULA is yours! (I need a spare keyboard switch for one of mine!) - that might be another year though so encourage your antipodean brothers, sisters, cousins & second cousins to keep looking in the meantime.

Thanks Nige! I'm not holding my breath to find one here. BBC's were never used in schools here and were hidiously expensive. Hence they are not that common.

Also our dumps are way less productive than yours!