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Sol-20 Personality Board Diagnosis

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    Sol-20 Personality Board Diagnosis

    Currently I have my Sol-20 outputting video to a TV. Its a cheap digital flat screen I had lying around connected with a homemade coax to rca cable.

    When the machine turns on I get the ">" followed by a "#" instead of a solid black cursor. I am wondering if this perhaps means I have a bad ROM or maybe it is custom software running? All the commands I have tried such as EX DU and EN all make the first character turn to a "?".

    My personality board consists of four 5204 eproms, so not stock but not completely out of the ordinary. Any ideas or advice would be great thanks.



    Try pulling and re-seating U89 (74LS367) a few times. If that doesn't fix the problem, replace it.

    It looks like writes to video RAM are working since the ">" and everything you type is correct. However, the cursor is set by inverting a space (20h). SOLOS does this by reading video memory and toggling the MSBit. However, instead of reading 20h, the processor appears to be reading 23h ('#'). Also, SOLOS does not use a keyboard buffer in regular RAM for commands, instead, it just pulls the command you typed from video RAM. Instead of reading "DU" for the command, the processor is probably seeing "GW" since bit 0 and 1 are always set. This could be what's giving the command rejection "?" you're seeing.

    Video RAM bits 0 and 1 are buffered back to the processor through U89. They do not go through this buffer for video display, so this also makes it seem like U89 could be the problem.



      I had a similar but not identical problem in my SOL and I wondered initially if it was rom rot, but it wasn't. One bit in the video RAM was stuck low due to a faulty video ram IC. If Mike's suggestion doesn't fix it, one thing you can quickly try is to swap the IC's, one at a time, between the general ram and the video ram that are in rows nearby on the main board and see if the fault condition changes.

      PS: Also, in my SOL & probably most, they used TI brand IC sockets that grip the IC pins side to side. Corrosion develops between the dissimilar metals of the IC pin claw and the IC pin itself over the small surface area where they make contact. Often its possible to see a grey or a dark line on the sides of the IC pin. This is oxide that needs to be scraped away and the pin sides smoothed with 2000 grade paper and the pins & IC socket lubricated. I made a "test pin".....a pin taken from a defunct IC soldered to a piece of wire to make a small handle to push into the socket holes(claws) one by one to check(feel) the claw tension and help clean them. Do not put anything other than a standard geometry IC pin into them, as it could stretch the claws apart and they will lose spring tension. My Sol became reliable after I cleaned every IC pin and checked every socket pin, it was a very big job. I found a few socket claws that were defective and had lost their tension, I think they might have been damaged when IC's were very roughly inserted when the board was assembled and the IC pin missed the central claw grip and went beside the claw and the plastic housing. In the case of the TI socket, the plastic top can be removed to gain access to the socket claws. Re-seating the IC's can help for a while, but if it is an intermittent (high resistance) socket connection issue, the problem will likely return later.
      Last edited by Hugo Holden; April 11, 2020, 10:49 AM.


        Thank you Mike and Hugo. It ended up being an issue with U89, it had a pin that was a little bent that was definitely my fault when I was reversing the 65k mod.

        I want to be able to run some programs and test the machine, but I am still waiting for new foam for my keyboard. It seems like a lot of people use the serial port on the back of the computer. Can I buy a pre-made DB-25 to USB adapter, or do I need to wire up my own null-modem style cable?

        I would love to eventually be able to use a disc drive as I have about 20 various 8-in discs for the Sol with software such as CP/M, some C compilers, wordstar, supercalc, micro-cobol, pascal and others. I do not have a 8-in drive unfortunately but I do have a dual 5 1/4in drive from DEC that I use with my TRS-80 and I also have a dataspeed "The conductor" S100 board. I know software of this era normally uses 8-in but could these be used together?

        I also have a Wameco Mem4 S100 board that is populated with 60k of m58725p RAM. This board appears to be uncommon, it appears to work properly though.

        I am just looking for some general advice. Thanks