Results 1 to 3 of 3

Thread: Spectravideo SVI-318, saved from the dump, revived, and upgraded.

  1. #1
    bmamo's Avatar
    Join Date
    Oct 2018
    Country
    Malta
    Region:
    Xghajra
    Posts
    32
    Feedback
    1 (100%)

    Default Spectravideo SVI-318, saved from the dump, revived, and upgraded.

    Hello everyone, sorry for the long post, but I was thinking it might be better if I write all I did somewhere, and this seems to be a good spot just as any considering I don't have any blogs or youtube channels (or video editing skills) or stuff like that.


    Ok, so here's something I wanted to share which I think might be of some help to other users in the future. I know I tried to find information online to help me work with this, but information was too sparse and not directly related to the issues or hardware that I needed to fix. This way it will be maybe cached by google or by the internet archive. Please keep in mind I am no expert in this field, I never ever had any power or electronics training. Ever. All I know is what I looked up on the internet, through trial and error, and through disassembling stuff trying to understand what's going on in the background. So please try to keep the criticism to a level where people can learn from if you really feel the need to indulge in some I'd be very happy with any constructive criticism you might come forward with.



    I got hold of a Spectravideo SVI-318 from my friend who said he apparently saved it from outside, on the side of the street waiting for the garbage collecter to pick it up. I had never ever heard of this computer in Malta, let alone used one, but decided this is something I want to experience, look at, and if necessary fix. So when he offered it, I knew I just couldn't give up the opportunity. There was the outside box, half the polystyrene, the unit, and miraculously a small red joystick! There was no power supply, no datasette unit, no cables. Nothing else.



    First problem I needed to fix was to see how I will power this on. I searched online and found out that the power supply as some may know is a strange dual voltage AC with an unusual power connector. I knew I needed to see if this thing at least powers up first, so I don't waste hours going round in circles for nothing. I searched high and low for a second hand power brick, saved ebay searches, scoured foreign auction sites, etc, but it seems like searching for a gold nugget under the sea might be an easier feat! After finding the schematics on samdal website and on archive.org, I decided (as per a previous post here) that I would power it up by feeding 5v and GND directly to the expansion port as an initial test. I opened it up and looked at the mainboard, confirming that there is a direct connection between the expansion port and the 5v output from the regulator, thus resting my mind that the expansion port can be used as both an "in" and an "out". So I connected using alligator clips using a powerful USB power source, and I saw the first sign of life - the power and caps lock came on. Thanks to this I knew at least there is some kind of life in this. Now I had a seed of hope planted



    Next I decided to power this up correctly, by providing the AC voltages. I looked around for some transformers I could use with dual windings or maybe 2 seperate transformers. I did find some called R-core which are used for audio applications, but their price was more than I wanted to spend on something which I have no idea if it works, so I looked at other avenues. I was browsing online and found a forum with details about the spectravideo power supply. Someone mentioned that the Commodore 1541 drive has a similar transformer, and I happen to have recently bought a commodore 64 with a 1541 included! Bingo! I opened the drive, disconnected the transformer from 1541 internals, switched on power and tested the outputs on the 1541 transformer, and it seems like it was giving out the voltages I required plus or minus a volt, which I figured wouldn't be too much of an issue considering they are being regulated inside the machine. To work around the power connector issue, I soldered wires directly to the mainboard from the bottom to where the power connector connects to the mainboard. I switched on the 1541 drive, then switched on the SV-318 power side switch, and power LED came on, and caps lock LED came on but never went out in a second or so as it's supposed to. Not ideal, but at least i know power is going in.



    Now, the next issue was connecting the computer to a display. This wasn't that much of a problem as such since this computer is directly compatible with commodore 64 video cables, albeit with the early 4 pin DIN type. I had some blank 6 pin DINs and a spare AV cable, so I found out the pinout online, removed the extra pins from the DIN I had available, and wired up the required cable ends. Finally I was gonna be able to test out everything and see something! So again I started the 1541, switched on the display, and powered on the SV-318, but to my disappointment nothing came on screen at first.



    I power cycled it a few times and tested the voltages at the pins, and i could see that for some reason the 5v rail was only at 4.2v, the 12v was only at 10v, and so on. Since the transformer did not have a label on it with rating, I assumed that it was not big enough to provide me with the amount of power needed so I scrapped the idea before I did any damage to it.



    I decided then to go back to my first plan. From the schematics it's very clear that what I needed was +12V, -12V, +5V, -5V, and GND. All these are available in a standard ATX PSU - or so I thought. Nowadays -5V is not normally included, so I had to search for a PSU from the Pentium 3/4 era. Luckily I got a good one from the computer graveyard at work which works fine. So now I did the same process as before. I located the connection points on the bottom of the board, soldered some wires directly to them, and prepared to connect to the PSU. For 5V I used what looked like 2 vias close to the expansion port, for 12V, -12V, and -5V I connected to the regulator output legs, and for GND I just found a random GND point. Plenty of GND to choose from. Once all was ready I matched up the wires with the various voltages to the corresponding pins in the 20 pin PSU connector, shorted the power-on pin for PSU, connected monitor cable, and switched all on. The disappointment at this point was getting to a frustrating level. All voltages now were at correct levels when testing at RAM chips, CPU, sound chip, I even tested at the logic chips such was my hopelessness.



    Now I had to start troubleshooting the WHY this is not loading. Many reasons could be and I was afraid that there was some case of cold solder joint which is hard to find, or a discontinued chip which would be impossible to replace. Some searching online led me to this site https://sites.google.com/…/repairing-the-spectravideo-svi-3… (are you on here Mr.Greg Newton?) but this was for a 328 MK2, while I had a 318 MK1. I still could get some pointers, but I most definitely did not have the skills or equipment this guy has. I started off by checking if there was a stable voltage on the CPU with a cheap €20 oscilloscope. It looked fine, so then I checked if the CPU reset (pin 26) was being held high or not. It seemed to be working correctly as it stays low for a split second on startup, then goes high and stays high without any ripples. After this I tried to see if there was a clock signal (pin 6) on CPU. I did connect the same oscilloscope but to my dismay, I realised that for €20, you get a toy not a real device. It only reads up to 200khz (dso-shell, I suspect a clone). This was not good for me since the clock on the SV from what I read goes at around 3.6 mHz. At this point I remembered I also have in hand an even cheaper logic analyser I got from the internet (€16 or so haha) which analyses logic up to 24 mHz. This would be perfect for me, so I connected it to the clock, vcc, reset, and some data lines. All seemed to be working correctly - Reset was high, Clock was there and constant, and there was changing activity on the data lines. From my very limited knowledge on these things, I took this as an indication that there was no problem with the CPU.



    Next I turned to the ROM, which I was very wary of since I have no EPROM burning facility at home should I had the need to burn a new one. Nor the hex reading know-how, or the binaries that I should burn. I took the plunge and connected the analyser so that maybe I could see if at least there is data coming out of the chip, and to my relief I saw nothing out of the ordinary, data seemed to be passing through. I left that be, and started exploring other avenues. Not much use looking at something which I couldn't do anything about at the moment!



    As an aside, each time I powered up the computer it seemed like a roll of dice of whether the caps lock will go out or not. I never heard any beeps on startup, so I suspected that whatever was wrong was intermittent and bad enough to sometimes work a bit, and sometimes fail completely. Since all I checked up to now was working in a reasonable way, I turned my attention to the RAM. I switched on the machine and left it on for a few minutes, so I could physically feel for unusual temperatures on any of the RAM chips. The system ram was only very faintly warm, while the video ram was hot to the touch, but not unbearable. I had no idea what this means, I was trying to see if I could find one of the chips that was significantly different than the others on the same bank. For example all system ram chips at warm level, and one ram chip that's super cold or unbearably hot. At this point I know I'm arriving at the end of my knowledge, and decided to order replacement ram chips and sockets for them all from aliexpress. I wasn't going to start testing logic chips yet, since I read somewhere those 4116 chips are quite unreliable so I was suspecting they were messed up.



    Since the SV-318 has 32K RAM, it uses 4116 instead of 4164 chips. 4164 RAM is much more readily available and appears to be more reliable. It also has the added advantage of only needing +5V to work unlike the stupidly designed (in my opinion) 4116 which need +12v, +5v, and -5V. There are various guides on how to exchange (upgrade) the chip to 4164, which is quite easy to do - you basically just raise a couple of the legs and connect one leg to another. You don't need to worry about the extra address line being held high at +5v, since it's never used.



    So I started desoldering and replacing the chips with sockets. At first I tried replacing a couple of the system RAM with the new 4164 chips, but nothing - until there was one instance on a random power cycle I did where the screen turned light blue and some white lines appeared in a box in the centre! Does this mean video RAM is good? I had only changed some of the system RAM and created some life after all. I was ready to give up after doing these changes, I had no idea what else to do other than logic chips, but this speck of life I got, got me excited, (admittedly probably a little "whoa" was uttered) and I got a new boost to my drive to resume with troubleshooting.



    I continued removing all RAM chips one by one, and once I was in the mood, I also desoldered the video ram and got it socketed. I replaced the video ram first with the modified 4164s, but still got nothing on screen. then I decided to change all system ram chips with the ones I removed from video ram, since they were different brands. Finally! I got the Spectravideo logo with changing colours! I got the beep too! This thing was finally fixed! So it was some of the pesky motorola system ram chips that was dead. So much head scratching just for maybe 3 or 4 bad ram chips.



    Next up, I will be trying to upgrade the RAM to 64K, as, as it is now with 32K, not much software can be loaded on this with the measly 12K you end up with, and I read online somewhere that most software was written for 328 upwards.


    Also, I will be needing something to load CAS files onto this. Maybe one day I will be able to buy a tapuino like device. I really don't want to build one myself. That would be too much for me to do for sure.


    And again, sorry for the very long post! Hope this will help someone in the future, as the help I found online was in the right direction, but in many different places. Comments welcome Questions too, but remember I'm no qualified person.

    Click image for larger version. 

Name:	IMG_20191021_005505.jpg 
Views:	5 
Size:	224.9 KB 
ID:	150774Click image for larger version. 

Name:	IMG_20191021_005513.jpg 
Views:	4 
Size:	198.6 KB 
ID:	150775Click image for larger version. 

Name:	IMG_20191021_005858.jpg 
Views:	3 
Size:	221.7 KB 
ID:	150776Click image for larger version. 

Name:	IMG_20191021_010547.jpg 
Views:	4 
Size:	202.8 KB 
ID:	150777Click image for larger version. 

Name:	IMG_20191021_010600.jpg 
Views:	3 
Size:	218.6 KB 
ID:	150778Click image for larger version. 

Name:	IMG_20191021_011003.jpg 
Views:	3 
Size:	221.9 KB 
ID:	150779Click image for larger version. 

Name:	IMG_20191021_013111.jpg 
Views:	7 
Size:	200.8 KB 
ID:	150780Click image for larger version. 

Name:	IMG_20191021_013359.jpg 
Views:	13 
Size:	206.5 KB 
ID:	150781Click image for larger version. 

Name:	IMG_20191021_005523.jpg 
Views:	9 
Size:	115.1 KB 
ID:	150782
    Last edited by bmamo; 21st October 2019 at 23:44.

  2. #2
    fragment's Avatar
    Join Date
    Feb 2009
    Country
    Finland
    Region:
    Helsinki
    Posts
    76
    Feedback
    17 (100%)

    Default

    Great post! Thanks for writing. I have some glitchy machines (NMS-8250 with random memory issues for example) that might need similar treatment. Maybe some day I will get to work on it..

  3. #3
    bmamo's Avatar
    Join Date
    Oct 2018
    Country
    Malta
    Region:
    Xghajra
    Posts
    32
    Feedback
    1 (100%)

    Default

    Thanks. Lots of 74 style logic chips on that machine! Try using a logic tester if you have the will to desolder everything. These chinese eprom programmers seem to have the functionality of testing them.

Similar Threads

  1. Replies: 0
    Last Post: 22nd March 2015, 15:33

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •