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Cbm 2001 Pet strange boot

Do you have a spare 6502? I think it is worth swapping at this stage.

I am also a bit concerned about the fault not occurring immediately... perhaps we have a short of some kind.. and the voltage regulators cannot keep up.

Do some parts get hot? (other than the previously mentioned regulators)
 
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>>> Can otherwise testing with logic probe or scope ??

The trouble is we are not looking for something that just isn't there.

99.99% of the time the PET is probably working. We are statistically looking for the 0.01% of the time when it is not working and goes wrong.

Logic probes only tell us whether there is a steady logic 0, 1, pulse or open circuit. I don't believe we are looking at this type of fault. Hence, the logic probe is no good.

To use an oscilloscope you need to have at least a theory of what is happening and you need to set up some sort of trigger situation to capture whatever you think the problem is. Plus the fact that your current oscilloscope doesn't have sufficient bandwidth for the job at hand.

You may be lucky by randomly poking about with the oscilloscope probe to identify some logic signal(s) that do not look correct. However, you need the experience to stop and say to yourself "that isn't right".

Ask Nivag where to purchase one from. I think he sells them on Tindie?

Use Google to search for "romulan ramulator". It's the top hit...

Dave
 
So it appears now we are looking at two faults. One is intermittent where the CPU fetched all zeros and did a BRK instruction, the other is probably a fault in RAM found with the PETTESTER.

I think one thing to try is to remove both the PIA's UC6 and UC7. Boot the computer, it will come up without the cursor. Then leave it running and see if it does that BRK thing again, or not. If it doesn't insert the PIA's back in one at a time and do a soak test in each case to find out which of the two of them carries the intermittent fault. If the intermittent fault occurs without the PIA's at least they could be ruled out as suspects.

Then try to sort out the RAM problem after that.
 
Hugo Holden said:
So it appears now we are looking at two faults. One is intermittent where the CPU fetched all zeros and did a BRK instruction, the other is probably a fault in RAM found with the PETTESTER.

I think one thing to try is to remove both the PIA's UC6 and UC7. Boot the computer, it will come up without the cursor. Then leave it running and see if it does that BRK thing again, or not. If it doesn't insert the PIA's back in one at a time and do a soak test in each case to find out which of the two of them carries the intermittent fault. If the intermittent fault occurs without the PIA's at least they could be ruled out as suspects.

Then try to sort out the RAM problem after that.
Click to expand...
I have faul also without PIA and VIA Hugo...

Nivag Swerdna said:
Do you have a spare 6502? I think it is worth swapping at this stage
Click to expand...
I already changed 6502 with a new one but it'same..... :(
 
Hugo,

Faulty RAM can also account for a BRK instruction (or a KIL instruction or random effects).

The stack is held in RAM from $0100 to $01FF.

A CALL (JSR) instruction results in the return address being stored on the stack (RAM) - where it can become corrupt - and the subsequent RETURN (RTS) instruction disappears off to ‘somewhere’. That ‘somewhere’ may contain a $00...

On a 6502, 16-bit pointers are generally formed in page 0. These pointers can become corrupt as a result of faulty RAM also.

Dave
 
daver2 said:
Hugo,

Faulty RAM can also account for a BRK instruction (or a KIL instruction or random effects).

The stack is held in RAM from $0100 to $01FF.

A CALL (JSR) instruction results in the return address being stored on the stack (RAM) - where it can become corrupt - and the subsequent RETURN (RTS) instruction disappears off to ‘somewhere’. That ‘somewhere’ may contain a $00...

On a 6502, 16-bit pointers are generally formed in page 0. These pointers can become corrupt as a result of faulty RAM also.

Dave
Click to expand...

Good, that means then there is likely only one fault left in the RAM (though possibly more than one RAM IC or support logic) and it looks like the PIA's and VIA and the CPU have also been excluded now. So we must be getting closer to a full repair.

Generally I think the 74LS support logic is more reliable than the RAM IC's.

I guess with the limitation of piggybacking RAMs as a diagnostic approach, it might require ultimately that they are removed and sockets fitted to accommodate a batch of new 4116 RAM IC's. I guess it would be good to replace only the defective one/s but that might be difficult if they are only intermittently failing.

These ones in Italy look pretty good:

www.ebay.it

4116 Dynamic ram STC 16K x 1 DRAM SINCLAIR ZX SPECTRUM 8PCS + Socket base (NEW) | eBay

Le migliori offerte per 4116 Dynamic ram STC 16K x 1 DRAM SINCLAIR ZX SPECTRUM 8PCS + Socket base (NEW) sono su eBay ✓ Confronta prezzi e caratteristiche di prodotti nuovi e usati ✓ Molti articoli con consegna gratis!
www.ebay.it

This appears to be a very economical RAM tester:

www.ebay.com

Dynamic RAM (DRAM) Tester 4027/4096/4116/4516/4816/4532/3732/4164/4128/41256/etc | eBay

This DRAM tester supports 1 bit DRAM devices from 4Kbit to 256K bit (eg. MCM4096, M4027P,4116, TMS4532NL3/4, M3723L/H, 4164, 4128, 41256, etc. ). 4256 or 41256 256 Kbit Right. Features of this tester not found in other DRAM testers.
www.ebay.com
 
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I have been looking around for economical sources of 4116 RAM IC's

I came across these ones that are very well priced. They appear to be USSR equivalents of the 4116, from a seller in Bosnia. They look beautifully made with ceramic bodies. So I bought a number to try out.

www.ebay.com

Vintage - Dynamic RAM DRAM 4116-3 (8 pcs) | eBay

Find many great new & used options and get the best deals for Vintage - Dynamic RAM DRAM 4116-3 (8 pcs) at the best online prices at eBay! Free shipping for many products!
www.ebay.com
 
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This is a completely off-topic post - but I got paid once for a job in 4116 DRAMs...

My little Z80 machine suddenly gained an extra 32KB (that was a lot then).

Dave
 
I also found these, while hunting around. Clearly good with large sales numbers and 100% + feedback, so I bought a swag of these too for my spare parts inventory (it is that darned spare parts disease I suffer from) but oddly like the person with an OCD for lawn mowing or house cleaning, at least the lawns always look good and the house nice & clean:

www.ebay.com

10 pcs TI TMS 4116-15NL TMS4116 16 Pin, DIP 16, IC Chips Texas Instruments RAM | eBay

Find many great new & used options and get the best deals for 10 pcs TI TMS 4116-15NL TMS4116 16 Pin, DIP 16, IC Chips Texas Instruments RAM at the best online prices at eBay! Free shipping for many products!
www.ebay.com
 
I have a tester Ram and some 4116 spare...i'll try to desold one for time the lower bank rams ic to test...right?
 
We already suspect data bit 3 is faulty (post #590) so you may as well start with that.

However, if you are going to buy one of Nivag's boards, I would seriously resist the urge to do anything until you install his board and give it a run...

Remember the fate of one of the Vectrex machines...

Dave
 
I don't know. I am on a business trip for a few days and do not have access to much until I get back.

Look at the RAM schematic for your machine...

I told you a few posts ago how to identify the low 16K bank of RAM. Look for the RAM that is connected to data bit 3. At the intersection of the two is your IC. Simples...

Dave
 
I tried with ram tester but i have ram error message...
 

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Either the RAM is faulty or the RAM tester is faulty then...

Did you try a good RAM in the RAM tester first?

Dave
 
Also with new spare ram ics same fault....i don't know why.... i am desperate
 

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Have you read the instructions for the RAM tester?

First rule of electronics - don't plug ANYTHING into ANYTHING unless you know that the parts are compatible- hence all the wasted time and effort with the 2532 and 2732 saga.

In that case, no damage was done. In another case, you might not be so lucky - especially with +12 and -5 Volts around the 4116s.

Look up the data sheets on the parts and check the pinouts, voltage levels and (if nothing else) the part description text.

Dave
 
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