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NOT WORKING CBM 3032

It would be interesting to find out why that jumper was in the wrong state though. I don't like unanswered things like this.

The video RAM fault was also an interesting one, lots of randomness from that device!

Dave
 
Of the two PET boards I have repaired so far, I have found that the most unreliable IC's are the 4116 DRAM and the 2114 video SRAM. They seem a lot less reliable, or resistant to the effects of aging than the general 74LS series IC's. Since there are only 2 of the video RAM IC's, it would not be unreasonable to socket them off the bat for this sort of fault (assuming the soldering/de-soldering skills are good) and have known good & tested 2114's on hand.

I think, back on post #151, Desperado had a hunch that the other 2114 was defective. If one is , it does throw some suspicion on the other, since probably they were the same age and brand.

So well done Desperado ! Your fault finding skills are improving.
 
I think, back on post #151, Desperado had a hunch that the other 2114 was defective. If one is , it does throw some suspicion on the other, since probably they were the same age and brand.
I was suspicious about the other video ram because I had tried piggy back and noticed changes on the monitor!
It would be interesting to find out why that jumper was in the wrong state though. I don't like unanswered things like this.
Hi Dave, now i remember when i first opened the pet, there were two sewing needles stuck in the jumpers!
 
>>> Hi Dave, now i remember when i first opened the pet, there were two sewing needles stuck in the jumpers!

These 'observations' are important when you first look into any computer...

The link settings for the 'dynamic PET' are here: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320351-6.gif.

You should be building up a folder of all of this useful information. When you open up a computer for the first time, you need to 'eyeball' everything and check (and document) all switch and link settings.

Your setting for these links depends (largely) on the type and number of RAM chips fitted. In your case, if you have two banks of 4116 16K devices fitted, you should be looking at the 32K link setting.

However, if the links are set differently to the documentation, you shouldn't just blindly change them without further investigation. The original owner may have made some additional modifications that mean the machine may not be 'factory'.

I suspect your machine may have been 'home upgraded' at some point - hence the 'needles'?!

Dave
 
Links A, B and C define the 'type' of DRAM that is fitted. 16K or 8K devices. 8K devices had two options. I think early 8K devices were 'half functional' 16K devices - so required one address line strapping high (Link A) or low (Link B). For 16K devices, Link C should be fitted.

Only one out of links A, B or C should be fitted. But no links fitted is not an option! In this case, one of the DRAM address lines will be floating, and will cause mayhem...

Dave
 
I followed this pattern and it didn't correspond with my pet because one was open.. when I closed the jumper, the pettester worked completely!
 

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