Unobtanium (next to the Thatwouldbehandium)

Some of you may remember the Interstate console that my awesome Dad gave me a while ago. It was, when presented, a bit dusty. But a good clean and one composite mod later and the thing sprang into life. Almost.

Pardon my (Dad's) Dust

The joysticks were a bit iffy and to try and improve things I ordered a super cheap donor console from eBay. These joysticks were physically the same but, when I dismantled them, I found that they were a few conductors short in their cables and they only had four of the potential six internal connections.

In any case, the joysticks were a suitable donor so I managed, after MUCH swearing and soldering, to get the attached to the correct cables and installed on my now cleaned console.

Joysticks Fully Repaired - About Time!

Pong. Yay!

So, the games on this thing are actually in the cartridges. While this may sound like a newsflash from the Ministry of the Bloomin' Obvious, the cartridges contain pretty much everything. The only thing in the console is the circuitry required to interface the controllers and marshal the video outputs from the cartridge into a coherent video signal. Everything else is done by the chip(s) on the cartridge i.e. the CPU is on the cartridge!

In this case, the '610' cartridge has an AY-3-8610 which contains 10 games. 

8610 - 10 Games on a chip

Other cartridges were allegedly available, but I had never seen them and I seriously began to doubt that they actually existed. Imagine my surprise then, when one of my eBay searches turned up this:

O. M. G.

Three cartridges, two that I had never seen before! I had to have this and, as luck would have it, the seller sent me an offer that I couldn't refuse. For some reason they only showed the Moto game in one picture which I queried. Apparently, the Moto game wouldn't work but the seller very kindly agreed to throw it in the box in case I could get it to work. Challenge accepted.

After a minor delay in despatch (lockdown and all that), the box arrived. It does mean I have two consoles but that was fine. More spares. But most importantly, all the cartridges were here. 

Squeee!

Hmmm - 604 has been opened... 

I have to be honest, I plugged in Roadrace to check it worked (it did) and then turned my attention to Moto Jump. First switch on attempt gave nothing, as if there was no cartridge inserted. I gave the contacts a good clean with some isopropyl alcohol and tried again. Partial success!

Signs of Life..

Something appeared on screen but I couldn't make anything move. The picture was pretty terrible too. Well, at least there's something to work with, so I cracked open the cartridge.

70's Goodness.

First things first. Change that socket for the AY-3-8765. It was really crusty and horrible. At this point I discovered that forty year old circuit boards are quite fragile. I had to turn the de-soldering station down, just a tad, to stop the tracks from lifting and shrivelling.

Caution - Fragile

I avoided any damage removing the socket and installed the new one without incident, although I didn't have any of the right width. I had to butcher a narrow socket with the correct number of pins, but it worked. :)

Hmm - somethings not quite right...

I won't tell if you don't..

I also noted that there was a logic chip too and it seemed only fair to give that one a socket too.

New socket - invaluable later.

At this point I tried again but ended up with the same problems. Really crappy picture (my display thought it was SECAM) and nothing happening, although I noticed that I could change the display slightly by pressing the 'Program' button. I decided to replace the caps on the board. There were no electrolytics but there were a few ceramics. But this didn't really change much although the picture did seem to become more stable.

So, I decided, what the heck, and I replaced all of the other passive components except for the one variable dude that I didn't have a replacement for.

No change.

I asked for some assistance from Twitter in that I wondered if the 8765 chip in the cartridge could be faulty but still work for some things i.e. changing the game type. The helpful response was that one of the pins, pin 18 to be exact, should have a signal of around 50 to 250khz to indicate the 'throttle' of the bike. If the signal was good there then the chances were, the 8765 chip was toast. Out came the sillyscope.

Well, that's not right.

There was no signal as such at pin 18 which indicated the issue was not the chip but something else in the cartridge. Phew! 

The only other thing I hadn't changed was the logic chip which was a CD4069. This is a CMOS Hex inverter. To determine if this was at fault I built a circuit (shamelessly copied from Google) to test it. Basically, I sent a voltage into one of the inputs and checked if the output was inverted. Pressing a switch would invert (or ground) the input. So, with the switch not pressed, the voltage into the input was 0v and the output would be 5v so the LED would light. If the switch was pressed the input voltage would be 5v and the output would be 0v so the LED would go out.

No switch press, 0v in, 5v out.

Switch press, 5v in, 0v out.

Doing this I managed to identify that pin 11 input to pin 10 output was open circuit so the LED stayed off all the time. By this point I'd spotted that the 610 cartridge also has a CD4069 so I nicked that to try. But I thought I'd test it in my contraption first. Would you believe that pin 9 input to pin 8 output was short circuit so the LED stayed on all the time. Arggghhh!

So, on to eBay to order some new ones. I managed to get three ceramic ones as NOS for a very reasonable £3.95. 

I love the smell of new chips in the morning.

Will it work?

Looks like a throttle signal to me.

Yes. Yes, it did. The game now works as expected. The operation is quite tricky as it needs the joystick to be pushed to the left (no throttle) and then to the right to accelerate. Kids today have it easy with their games. Back in my day etc. etc..

This has to be the rarest purchase I have ever made on eBay. I have had a search on 'Interstate' since my Dad handed over the console about 18 months ago and these are the ONLY cartridges I have ever seen. I'm pretty chuffed to have them in my collection.

Yay!

Post Script - I did the composite mod on the seller supplied console and it was this one that I did most of the testing for the Moto Jump game on. I was quite surprised when I plugged it into my original console to find that the picture was dark green rather than blue. I'm not sure why this is.

Green? Whatever..

Also, I worked out that the reason the picture kept going bad and switching to SECAM was not because of the cartridge. My re-purposed laptop display appears to be very intolerant to slightly out of spec composite signals... A normal TV has no issues.

Finally, I added a potentiometer to the speaker on my original console so I can reduce the volume. The speakers in these things are freakishly loud!

Shhhh!