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Interact Home Computer Repair


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I have an Interact home computer that I'm trying to repair. I've had it for several years now, and I'm just getting around to testing it. The main reason I haven't tested it before now is that it only has an RF out connection, and I don't currently have any monitors or TV's in the house that support that. But I found that someone has posted the service manual for this baby on-line and I thought, "Here's my chance". Besides, looking at the schematic, I'm pretty sure I could do a component video mod to this machine without too much effort. Anyway, I decided it's now or never so I dug out an old 13" TV from the garage and connected everything up. The computer does power up and does produce a picture, but I'm pretty sure the display in the first photo is not what is supposed to show up on the screen. I've also included pictures from inside the machine. That's quite an RF shield, reminds me of the the VCS and the Atari 800. Looks like this machine was upgraded to 16K or perhaps it shipped with 16K, but it's the early keyboard so I'm guessing it was an upgrade.

 

I checked the +5 and +12 and they looked pretty good. The 12 volts was a little low at 11.89, but neither supply had any significant ripple. The socked chips had significant oxidation on the leads so I decide to clean them up and re-seat them. This was going pretty well until I broke off one of the legs on a 4116 RAM chip. I didn't have any 4116 in my parts box. I briefly considered digging out an old TRS-80 Model one and stealing the part from that, but Jameco still sells 4116's, so what the heck. I've got the RAM's on order, but in case that's not the problem - does anybody here have any ideas where to look next? Even I get this guy to boot, I'm not sure I can get the old cassette deck to operate. And on these machines that was the only way to load software. Even BASIC had to be loaded from cassette. I suppose could tap in to the audio input.....

 

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Edited by mutterminder
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4116s are sensitive to power.
If you can replace the 4116 chips with 4164s, the machine will be more reliable.
4116s require +5v, -5v, and +12v applied in a specific sequence when powered on and they tend to go poof if the power supply has any issues.
4164s only need +5v.
Doing this will require cutting the power traces for -5v and +12v where they enter the block of RAM on the circuit board, and you will need to add a jumper wire to supply +5v to the RAM in place of one of those.

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I might try to upgrade the RAM later. But for now I have replaced the 4116's and the machine boots! The RF output doesn't look so good, but I think now I'll go ahead and try to do the component video mod to it. Once I have that going I'll be able to use it on my more modern TV's. Then I have to see if I can get the cassette deck working again. It was very sticky and none of the keys worked right. A little oil took care of that, but the belt is sort of misshapen and perhaps a bit stretched out. Now that it boots though, the motor now works when I press L to load. So things are looking better.

Edited by mutterminder
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Looking at the schematic for the video output, I decide where I wanted to tap into the video for the component video output. The schematic is hard to read being a scan of what looks like a 1980's copy, but it's all I have. I tapped into the video where it goes into pin 12 of the LM1889. R-Y, I pulled off of R98 and B-Y, I got from R85 (I think, at least it's the resistor that connects to pin 4 of the LM1889.) The signal was too weak at the LM1889, so I ran all three video signals through simple transistor amplifiers. Actually, I used little circuit boards intended for an Atari 2600 composite video mod. The video signal works great connected to a composite video input, nice and clean, no tearing. Black and white only. However when I connect it, along with the other two signals to a component video input, no signal is detected. I don't have an o'scope handy at home to look at my signals so I'm not sure what's going on. It could just be that it's a 240P signal and my TV's don't like it on the component input. I haven't really tested them with 240P yet. I tapped the sound across R110 on pin 13 of the SN76477. At least a couple of times, I heard the tape motor come on when I press L on the keyboard. Now that I can see what I'm doing, I don't hear it any more so I'll have to see what's going on there as well.

 

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I've been looking at the data sheet for the LM1889. If I'm reading this right, the R-Y and B-Y inputs are not standard component video type signals, but are instead, variable DC levels used to change the phase of the chroma subcarrier generated inside the LM1889. This means that a component video output will not be that simple. Even getting a color composite signal will require a more complicated solution.

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After looking at the circuit for the LM1889 a little more, and seeing that it has several mods of operation, it looks like pin 13 is both and input and an output for chroma. In this case the signal is use internally to drive the RF modulator, but the chroma is still present on pin 13 so I used it as my chroma for S-video and pin 12 as luma. Now I'm seeing "DEPRESS L TO LOAD TAPE" in white text on a blue background. Unfortunately pressing "L" doesn't seem to do anything.

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After a little more testing, it is starting to look as though the keyboard isn't working. I suppose it could be just the "L" key. I can't find any information on the keyboard matrix yet but I should be able to check the keyboard switches from the bottom of the keyboard circuit card. If my switch is okay and path to the mother board is good, that mean that one of the keyboard buffers is bad. Since the tape motor doesn't come on, I soldered a phono wire directly to the input of the cassette audio circuit. When I play back the cassette I can clearly hear the audio on my monitor so I'm sure its getting in there. That's why I'm thinking it's not reading the keyboard, because not only does it have to turn on the tape motor, but the it has to switch the A/D input to tape audio.

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It is the chicklet key version. Thank goodness for oldcomputers.net. If they hadn't posted a copy of the service manual, I'm not sure I would have gotten this far. I really don't have a lot of extra time too work on this machine, but I would be nice to see it in action after all of these years. This computer was concurrent to my first computer, the TRS-80 Model I, Level 1, 4K that I got way back in 1977, but I don't remember even noticing that it existed. I may have just dismissed it out of hand, when I noticed that it had "only" an 8080A, no real keyboard, and no real expansion options. Although it does have color and sound, and joystick ports, somethings the TRS-80 didn't. On the other hand I do remember lusting after the Compucolor II which also had an 8080A.

 

Also I also always wanted an Exidy Sorcerer, which had a Z-80, and hi-res graphics. Actually, I do have one now, and when I get a chance, I plan to try to get it working as well.

Edited by mutterminder
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The Interact was one of those machines that was closed out by COMB or some other similar outfit.
I remember the ads. You got a computer and several tapes pretty cheap.
I remember a music maker tape, hangman(?), and a sub game or something like that in the ad.
Not sure what year that was though.
If I remember right, a French company bought the rights and came out with an updated version with a decent keyboard.

I liked the Exidy. Faster than the TRS-80, better graphics, and and program carts that looked like 8 track tapes.
The graphics still weren't a bitmap, but at least you had some programmable characters.
*edit*
It was Protecto, not COMB.

Edited by JamesD
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  • 2 years later...

Found this thread when researching fixing up my Interact Micro Video. I'm actually looking to see if there is any info about the main tape belt. Mine worked initially but soon stopped playing tapes. The main belt is rather loose and should almost certainly be replaced. I could measure it and try to order a belt from somewhere, but a more precise bit of info would be nice. I don't know much about measuring belts by thickness, etc. 

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