Search results

BTW, did the same with a bunch of 6502 CPUs. First tested them in the NOP generator (clearly a 6502 NOP generator) and then in a VIC-20. Interestingly, one 6502 that would work in the NOP generator had a very flaky Sync output (pin-7). With the CPU cycling through the addr lines, Sync would stay...

Yup, agree.

I tested them in the NOP generator at 10KHz and 1MHz. In the SBC they run at round 3.6MHz.

This is *EXACTLY* why I come to this forum!! :love:🙏

You mean, like in using an actual computer?? :oops: That would be cheating though! :ROFLMAO:

Indeed. Done this already with a Grant Searle Z80 SBC. The test in this case is to boot the SBC, load CP/M, load BASIC, then load and run a Mandelbrot program. That's how I found the 3 faulty Z80s. But it's not quite the right answer. For example, no testing of IO or memory refresh. :)...

I quite like this. A complete overkill (I've got loads of Z80 systems in my collection for easy testing), but exactly what I'm thinking. :) In a sense, I love designing and building the test kit even more than fixing the actual target systems. :)

Recently got a batch of Z80 CPUs and wanted to test them for functionality. I first used a Z80 NOP Tester to test them all. This basically straps the Data lines to 00h and then monitors the Address lines as the CPU cycle through the full memory map. The clock is only about 10Hz, allowing you to...

A trick that can help for people who can't program old EPROMS, is to write PETTESTER to a 28C16 EEPROM. For me it worked, so far, on all the different boards I've repaired.

While drawing the circuit in KiCAD to make a PCB, I realised that you can put 5 video images in the ROM in successive Data bits (D0 - D4) as they all share the same H-Drive (D5) and V-Drive (D6) signals. Switching between them, gives you 5 different test images within the same data file. . ...

Picking up on this again to add support for the 8296 and 8296D. . This is the circuit diagram on Zimmers stating 8296, 8296-D and 700. I suspect it's maybe not applicable to the 8296 (non-disk version), as my unit have normal AC running to the monitor, ala the 8032. . Q1: Is the 8296 monitor...

Thanks Hugo, . Interestingly, in both my 9" CRTs, I don't get the "white spot" problem. I do intend to increase the cap you identified in any case as it can have only a positive benefit.

Will try and see if I can test them as leaky. This makes me wonder if I should not just replace the other 2 diodes in the V-Drive circuit. And potentially any others, in the rest of the circuit. I'm not normally for replacing components, "just because" (this trend in the vintage computing...

That's a Freudian slip of epic (Double D??) proportions!! :biggrin:

Bottle of SA's finest. Which I'll happily consume with you! :) :)

. You win the bottle of SA red! :) . Been wanting to come visit your part of the world for a long time. Next time I'm there, I'll personally deliver it to you. :)

With the reference measurements from a working monitor, it really felt like there is something wrong, right on the input of the V-Drive circuit. It's like the 5V TTL input is directly driving Q6 making one think there is a shorted component. But I've already removed and tested the components...

Ah, yes. But that is because the Vertical size is so big that that line is painted off the top of the display area. . Just found the %$^&* culprit, pictures incoming. First going to pour a glass of red. In the mean time, I'll take bets on what it was. :)

Managed to connect another 9" monitor up and do some measurements around Q6, 7 and 8. Board is still in the casing, so a bit difficult to measure. . The signals are significantly different. . Q6: In the faulty monitor, there are significant levels around Q6, with a 3V pulse on the base and 600mv...

Indeed. The thing with every second V-drive pulse looking different is telling us something, just no idea what. :) The only bit that seems (potentially) related is in the push-pull output stage. The different width / amplitude pulse things is consistent and there must be a fixed cause for it...