SVI-738 not booting

Expert (126)

16-08-2021, 16:44

I recently acquired an SVI-738 that unfortunately isn’t booting. Voltages from the PSU are okay but the CPU doesn’t appear to be getting a clock signal. Reset upon boot is very briefly low before going high. No activity on address lines and no sync signal to the TV on composite. Already tried changing the hex inverter on the reset line to no avail. The relay clicks on boot.

The machine appears to be dead as a doornail. The obvious step would be getting clock to the Z80, but the schematics I had available are a bit inconclusive regarding that.

What should I try next? I have an old analog scope at my disposal.

By NYYRIKKI

Enlighted (6095)

16-08-2021, 23:12

Yes, you definitely should have clock on test point TP3 / Z80 pin 6
If not, check IC29 pin 1 ... This also connects to V9938 (IC37) pin 8 (see sheet 6, 1B on service manual)
If you wonder... the inverted clock from middle is feeding IC35 (on sheet 2)
If your revision has V9938 or Z80 on socket, you may want to try if lifting one up makes the clock wake up again.
You may also want to try if you can wake up the clock by adjusting VC3 (see sheet 6, 3C on service manual)

By protek

Expert (126)

17-08-2021, 11:30

Thanks! The V9938 is in a socket. Removing that didn’t help with the clock signal, though. I’m assuming, based on my experiences with a Sinclair Spectrum, that the clock should show on a logic probe. Currently it shows just high. I’ll check the rest later this evening.

By protek

Expert (126)

17-08-2021, 15:42

About VC3, I can find only VC2 in 3C, and that’s not populated as I don’t have the RTC. V9938 pin 8 was low and IC29 pin 1 seemed to be either high or low.

By NYYRIKKI

Enlighted (6095)

17-08-2021, 16:00

Ok, I think you must have different revision than in the service manual... IIRC the crystal is somewhere near VDP as well as the related capacitors. For sure the machine will not get any further before you manage to isolate, what is problem with the clock... It can be ie. faulty capacitor or something like that. Obviously something is now preventing the clock from oscillating... Unless "IC29 pin 1 seemed to be either high or low" means that you have working clock there(?)

By protek

Expert (126)

17-08-2021, 20:54

It wasn’t oscillating between high and low. It just happened to be either one randomly when probed

By NYYRIKKI

Enlighted (6095)

17-08-2021, 23:15

protek wrote:

It wasn’t oscillating between high and low. It just happened to be either one randomly when probed

Well... In theory I guess you should see something like 3579545Hz signal in there, so I bet something somewhere near it is your problem, I just can't say how to solve it... Maybe you could try to provide some software clock from Arduino board or something like that to see if the Z80 it self walks the steps? Sorry, but I don't really have any reasonable off the shelve solution for you.

By protek

Expert (126)

18-08-2021, 15:42

Here’s a picture of the mainboard. The only crystal rated for 3,58 MHz is the one near the MC1377, but I gather that’s just for PAL. The other crystals are rated 8, 21 and 1,8 MHz. The 8 MHz one is the one closest to IC29 and the 21 MHz is next to V9938. Which one is the one that should be providing the clock for the CPU?

By sdsnatcher73

Enlighted (4318)

18-08-2021, 17:47

The 21Mhz serves as input for the VDP on it’s XTAL input (pin 63 and/or 64) which then is divided by 6 to create the CPUCLK on VDP output pin 8.

By protek

Expert (126)

18-08-2021, 18:22

Ah, so it’s the VDP that generates the CPU clock.

By NYYRIKKI

Enlighted (6095)

19-08-2021, 20:32

protek wrote:

Ah, so it’s the VDP that generates the CPU clock.

So it seems... Sorry for misleading you... Next obvious place to check would be clock input in VDP pin 63/64... Good thing is that if the problem is on VDP it is easy to replace.