Recent content by Robin Elvin

Any idea what type it is?

Redoing the power connector seems to have sorted it out. Next problem is the brightness control. It was very dodgy so I took it apart and cleaned the contacts and the carbon(?) strip. This seemed to make things better but now if the machine has been on for a few minutes the display will just go...

The Harris H100 photos look very familiar. I think my college had more cabinets so perhaps it was a more expensive model. In fact, it says the H100 supported up to 32 users and I know we had several rooms of around 30 terminals hooked up so it must have been a bigger model.

When I went to college in 1989 they were still running a Harris mini with serial terminals (not sure of the standard - VT100 I assume). The OS as I recall was VOS. I remember hacking together some kind of chat room and a file management utility in JCL. In about 1990/91 it was decommissioned and...

This is the cable to the main board that comes straight off the transformer. If you have a link to where I can get one I'd be grateful. Thanks

I checked the board everywhere for shorts and found none. Scratching my head I plugged it back in and there was a brief flash of life from the screen. After a lot of odd symptoms like the screen flashing up occasionally, machine resetting, and no 5V it turns out it is the power connector. It is...

Fresh PETTESTER ROM burnt and I get the rolling screen again. After a bit of RTFM I realised I had to edit the ROM for the correct CRTC parameters. New ROM burnt and I watched it get to the 32K RAM test. Made a cup of tea, came back and everything's dead. There is no 5V on the board any more...

After a long, long time I'm returning to trying to fix this PET. On first power up the screen started with some garbage in the same places are before then the whole screen filled with random characters. Subsequent power cycles just kept the garbage to the usual garbage in the lower half of the...

I've just put the ROM I burnt previously in and it does nothing so I expect it's gone bad. I can't burn another one as to do the original I had to supply a separate 21V as my TL866 can't do that and my bench supply had to be returned under a safety recall! When I manage to burn another one I'll...

It's the second character so that sounds like the odd RAM addressing. However, I can't fathom what's going on. Looking at the PETSCII table it appears that whatever character I type on an odd RAM address the 2x version of it appears in an odd RAM address lower down the screen. e.g. 1,a,q->...

Ok, wet bank holiday weekend so I finally have some time to look at this PET again. Power is all good again after a clean of the contacts. Hopefully that was all it was. I'm getting consistent garbage printed in the lower half of the screen depending on what is appearing in the upper half of...

Checked again this morning and the PET is still dead. So many issues with this machine!

After life getting in the way I've finally got round to looking at this. Looking at your suggestion, Dave, I put my scope on UC8, UC9 & UC10 and all the outputs seem to be outputting something. Nothing looks stuck. I'm trying to get my head around what is going on. When I use the PETTESTER ROM...

CRTC is in a socket. I'm not sure what you mean by TAn signals - the datasheet I've found doesn't mention that name: https://pdf1.alldatasheet.com/datasheet-pdf/view/139476/HITACHI/HD6845SP.html Anyway, I'll give everything a probe and see if anything looks wrong.

Thanks for that, Dave. It passes the tests - I think! Putting the old SRAM in halts the test at the first step but with the new ones it goes through to the DRAM test. Seems as soon as I fix one issue with this machine another one crops up. Now the vertical hold has gone and so I get a repeated...