Dick Smith Super 80 computer kit

[Peter]

Most of those signals check out OK on the CRO although there's a lot of noise, very short negative spikes that's fooling the frequency meter and I'd say is responsible for the odd behaviour of the video display.

 

One really problematic test point is U28 pin6 the manual says it should output bursts of 6mHz at 750kHz. I can't see how this can possibly be correct as one input is constant HI (derived from BusAck#) which comes from the CPU, do you have anything on your diagram? I'm wondering if U28 Pin 6 should be connected to U28 Pin 10 which is a 6MHz input? I can't see how Busack# could ever produce a 6MHz signal if the CPU is runnng at 2MHz.

 

My super 80 has an interesting mod... instead of 4116's it has 4164 (64k x 1) RAMs. It meant I could get rid of the -5volt supply and only have 8 ram chips, and I get 52k of RAM.

 

That sounds interesting, I've got a bundle of 4164's I've read about a few mods on arcade game PCBs that involve this.

[Robert]

I hope you have a scanner or photocopier, there's a few things you may wish to copy that i'll bring with me.

 

The spikes are absolutely no surprise, considering the mishmash of unbalanced gates with no consideration of propagation delays whatsoever.

 

U28 pin 6.. hmm I couldn't say exactly what it is, there are 12mhz and 3mhz signals going through U76 (a pair of flip flops) which goes to U28 pin 5. The BUSAK signal will be at the vertical frequency (48.8hz) and will only be present when the screen is ON, at U28pin4. U28pin6 will be these two signals NANDed - whatever that results in. Perhaps bursts of 3mhz at 48hz. Whatever, it is the signal that allows the video circuitry to read the RAM during screen refresh times. Btw, this exact part of the system is what needs a mod I will bring to you. Once things are running, you MUST add this mod.

[Peter]

I was wrong about the u28 pin6 connection I should've said pin4 as pin 6 is the output. Anyhow I have a clear picture of @s and ?s alternating on the screen (with the bars from faulty 2513) the poor picture quality was a dodgy socket, wiggling u28 cleared that up. Here's what I get:

 

1. @s and ?s alternating on the screen.

2. verticle solid bars over the top of each char (faulty 2513)

3. top 3 rows are snakey

4. there's some induced hum scrolling from top to bottom and mysteriosly it's at 50Hz (now what could that be from? need I ask? ) I see it on the scope to.

5. The reset circuit is totally shoddy, the 4.7K pullup isn't doing it's job and there's this 50Hz hum again coming out of the ground signal. I suspect the CPU is in a constant catatonic state as a result.

6. I can't get the VDU to blank, probably related to the hum.

 

That's as far as I've got so far. Need to socialise some.

 

I reckon if I can eliminate the mains hum from the ground circuit which is leaking into the reset and video circuit a least the video will be as near to perfect as it can be. And then the CPU may get a chance to clock a few more cycles.

 

BTW: I have a scanner. things are looking up.

[Robert]

Hopefully the big filter capacitors are doing the job. If they are original ones they possibly need replacing. Make sure the ground circuits are properly connected throughout the board. The transformer windings must be correctly phased.

 

The video display is not likely to improve much, the design of that area is a total abortion.

 

We just need to get the CPU to start working.

[Robert]

Visiting Peter today, first time I ever meet anyone from the Internet.

 

I'll report any progress when I get back. Wish us luck!

[Garos]

Visiting Peter today, first time I ever meet anyone from the Internet.

 

I'll report any progress when I get back. Wish us luck!

 

Hi guys

can't wait to hear how it went, did the S80s sing? (is Peter a mass murderer?)

 

I have just begun a 5am shift at Parkes observatory, but we return to Sydney this morning.

 

Gary

[Robert]

We removed the RF modulators (yes 2 of them!) because they are useless, just using composite video instead. Verified that Peter's Microbee monitor with amber screen works. Inserted all chips except for those to do with RAM and the S100 bus. I supplied a replacement 2513 character generator as the original was faulty. Found that at least 8 chip sockets are faulty. Peter will replace these, very carefully, as the board has hundreds of incredibly thin tracks on it. Showed him my super 80 working. He took a photo of it. Had a nice salad lunch. He scanned a number of my documents into his pc.

 

After the sockets are replaced, hopefully some improvement should be noticed, and then I will return to continue the battle.

 

An enjoyable day, I must say! <_>

[Peter]

An enjoyable day, I must say!

Thanks Robert, I'm glad you enjoyed the day. I've started removing the sockets and now have a stable video image. I did find a chip with a missing leg whcich had been causing some problems. Not much time to do anything else this week apart from domestic duties.

 

The ROM is now being selected by the memory decode circuitry although not as one would expect. I have 1 good monitor ROM without BASIC, that should bring me closer to seeing some results. The original ROM has been damaged. I've bought a larger solder sucker and that should make removing the bad sockets much easier.

 

I'll report on progress as it's made.

 

Cheers,

Peter.

[Robert]

Sounds good, I await your progress reports.

[z80]

Sounds good, I await your progress reports.

 

 

It is interesting to see this machine sparking some interest.

 

I remember building one of these along long long time ago.

 

A number of us were mainframe engineers and we also built a microbee, trs80 etc etc.

 

We wirewrapped all of them. I still have the microbee complete with floppy controller.

 

 

The original super80 had quite an excellent installation troubleshooting guide that you guys could follow.

It tells you exactly what signals should be where and what chip is likely to be at fault.

 

The most common issue was the cheap sockets budled with the units, they only contacted the pins on one side, hence very unreliable.

 

We had a users group in Melbourne to which I attended on ocassions.

 

Do you guys have software for this machine?

[Robert]

Welcome to 1emulation, z80!

 

The original super80 had quite an excellent installation troubleshooting guide that you guys could follow.

It tells you exactly what signals should be where and what chip is likely to be at fault.

That is the technical manual and we've been using it.

 

The most common issue was the cheap sockets budled with the units, they only contacted the pins on one side, hence very unreliable.

Actually the sockets pack was an option (otherwise you solder the chips straight in), but you are correct, they were very poor quality sockets.

 

We had a users group in Melbourne to which I attended on ocassions.

I have a few of their newsletters.

 

Do you guys have software for this machine?

Yes but MORE IS ALWAYS WELCOME.