RE: Malvern CBM 8296 Update
Date: 2006-11-16 08:33:56

Hallo Bill,

> The 6545 chip (Cathode Ray Tube Controller) in this picture 
> is hot to the touch, probably is bad. I am currently searching
> for a replacement.

I know some ICs can become very warm but IMHO the 6545 is not one of them. I don't have a spare either but this could do the trick: Two years ago I have been experimenting with the Motorola 6845 wondering why that one doesn't work inside a 8032. And to be honest, I still haven't any idea. But during these experiments I took a Hitachi 6845 out of the box and suprise.... that one worked fine! And so did the UMC version. All three worked fine on a PC MGP card but all Motorola 6845's refused to work on a 8032. Having a reliable source of 6545 replacements, I quit the research. 
Remark: I never tested it on a 8296. 

> Note also that the processor is a 6502 A, not a "B" like other 
> 8296's.  Would be slower?

No. The A means it is the 2 MHz version, the B means 3 MHz version. This means they can run reliable at these frequencies. The 8032 is only a 1 MHz system so the 6502A should run very fine. 

Remark: I wonder if the A, and especially the B, is made for these frequencies or are they justed tested. A 80486-33 running on 25 MHz is much cooler then the original 80486-25. One idea of reducing the heat output in, for example a 1541, could be replacing the parts with their A or B versions.

> 2) could the display be causing the problem?

You failed to mention the bee-de-bee-de-bee beep at start-up. So I presume that it didn't occur. And that isn't a good sign either.

> 3) Any diagnostic ideas?

All the brooken 8296's I met so far had all the same major problem: bad PLA's. Your luck: replacements exist, IIRC  Jens Schönfeld sells them. There even exists an EPROM replacement, no URL at hand.

A not so hi-tech solution: start swapping parts with another 8296. In some cases a 8032 will do as well (6522, 6502, 6545, 6520). The B700 won't do as it is 2 MHz IIRC.

For the next one you need an osciloscope: replace the upper ROM with an EPROM filled with NOP's and JMP $F000 at $FFF7. This should produce square waves at the address lines A0..11. Taking this idea a step further: use a EPROM filled with a diagnostics program, one that outputs the result at the userport. Problem: I don't have such a ROM and if others cannot help you, you must program your own one. 

A hi-tech solution: I myself repaired quite some 6502 machines using my debugger,<>. I simply compared what the 6502 was doing against the source code. That enabled me to detect bad RAM, bad 6522's etc. A logic-analyser will do as well of course but I don't think you have that laying around somewhere (me neither).

Good luck!

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