Search found 174 matches

by rune
Sat Jun 10, 2017 7:56 am
Forum: Hardware
Topic: Create ULA for €5?
Replies: 84
Views: 33135

Re: Create ULA for €5?

I'd like one too Paul and am happy to prepay.
by rune
Fri May 26, 2017 10:50 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Hi Mark, Im going to post a couple of pics to make sure I know what pulse/rapid pulse etc looks like: http://www.weegieboard.co.uk/Images/Pulse.jpg normal pulse eg Z80 Pin 16 .5us 5V http://www.weegieboard.co.uk/Images/Rapid.jpg rapid pulse eg Z80 Pin 19 .5us 5V http://www.weegieboard.co.uk/Images/A...
by rune
Thu May 25, 2017 10:19 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

A little more work, replacing "equivalent" transistors with the correct ZTX313s and finding a short now means I have three boards all showing good signals at ULA P14, TR2 and Z80 P6. Also got decent back porch at ULA p16. On the three boards I think I have one with a good CCB Composite board, one I ...
by rune
Tue May 23, 2017 1:58 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Thanks Charlie, Heinz and Mark I've checked the connection between the C7/R6 and the transistor base which is good. Checking between both and all other pins nearby, this doesn't show a shorts. Tried without the Z80. The emitter is attached to ground. R6 is 2.20 K Ohms. The only thing that is differe...
by rune
Sun May 21, 2017 10:42 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

I've selected an issue 3 board and got a bit more testing done. The oscillator left pin puts out a nice low amplitude sine wave, the right pin puts out a meandering wave. I can get a very similar meandering wave at Pin 14 on the ULA. Tracing the circuit to the ULA side of C7/R6 I get the same wave a...
by rune
Sun May 07, 2017 9:43 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Hi Mark, I did try some of the pins today and they did show signals. I never got any photos though as I didnt have a lot of time. My plan is to get all the information here printed out plus the chips pin outs and then pick a board and work on that. Thanks for your advice, its greatly appreciated. Dave
by rune
Sun May 07, 2017 10:32 am
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Thanks Mark, there's a lot of info to digest there. I'll dig out the printer later and get all as hard copy. As you have probably guessed, Im a novice at this electronics thing, but trying to learn.

I'll stick to one board now for tests and get pics of the pins you mentioned.

Cheers

Dave
by rune
Fri May 05, 2017 8:55 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Cheers Darren, I've tried another brand new RAM pack. No difference - Also tested across 3 ZX81s. Hi Mark, heres some results of testing. Apologies for it being so long, but I tested all three boards. There are two issue 1s and an Issue 3. I tested Pin 14 & 16 on the ULA and Pin 6 on the Z80 plus al...
by rune
Thu May 04, 2017 9:38 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Re: Resurrecting a ZX81 - Step by step help appreciated.

Hi Mark At least two of these boards did work at one point, but with the usual problems - cracked membranes, old ULA (very dark picture). I had other ZX81 that did work so I concentrated on those. Now its playtime and I'd like to try getting these working again. The PSU is good. I got the diodes cha...
by rune
Thu May 04, 2017 8:00 pm
Forum: Hardware
Topic: Resurrecting a ZX81 - Step by step help appreciated.
Replies: 26
Views: 11766

Resurrecting a ZX81 - Step by step help appreciated.

Hi guys, Im looking to try and resurrect 3 ZX 81 boards - 2xIssue 1 and an Issue3. I have an oscilloscope, a decent multimeter and a bit of time to do this now. All 3 boards have had the RAM desoldered and a 32K RAM socket added. At the moment I'm testing using a Sinclair RAM Pack. I have two boards...