Issue5 start problem

[timovo]

Hi,

I wonder if anyone would have suggestions on what might be the problem?
I got this QL unit a while ago - it booted Ok a few times except that one microdrive kept on running.
Then after power off and on it started to show just garbage on screen.... didn't boot anymore.
I now have the Minerva 1.98 ROM installed. I was hoping that it might give a hint on the RAM chips to check in case that might be the issue.
Unfortunately the screen is pretty messed up so, doesn't really help... Could it still be a RAM problem ?
Keyboard is disconnected currently.

Screen when micro drives were still connected

The 5V main regulator seems ok - Vcc measured, for example, at the RAM chips shows 4.98V.
I noticed that sometimes when I switch the QL on the regulator at the second microdrive outputs about 7.5V - then
after switch off and on again it outputs 5V. I replaced the regulator - but that doesn't seem to matter.... perhaps there is a capacitor problem in that microdrive...
I removed both microdrives for now and the screen now seems totally different - but still not working.

Screen when micro drives removed

Any ideas?
-Timo

[Pr0f]

could be a RAM problem - or it could be a problem with the bus being loaded - the Zx8302 chip sits on the same part of the data bus as the RAM chips in the issue 5. Could also be the 74LS245 chip or either of the 2 mux chips that sit near it. Clearly the ZX8301 can see some memory or it would not generate the display, and what looks like the Minerva reporting a RAM issue shows that the CPU / ROM appear to be working.

So you have a few potential suspect players in there...

[tofro]

Yep, looks like Minerva would want to tell you something but cannot - The four lines with garbled text look like the Minerva RAM failure message, but garbled - because of a screen RAM (or multiplexer/bus driver) failure.

[timovo]

Thanks very much for suggestions!

I've got the IC21 (74LS245), IC19 and IC20 (74LS257) socketed now... don't have replacements, though.
I've ordered some from the usual place but I'm not holding my breath - may take a while to get here.
I did get brave enough and installed the xgpro Windows software for my TL866 II Plus programmer since I saw in some youtube video it could be used
for some simple logic chip testing.
I don't know how good that is at testing those chips - but at least it claimed them to be ok.

[Pr0f]

Thanks very much for suggestions!

I've got the IC21 (74LS245), IC19 and IC20 (74LS257) socketed now... don't have replacements, though.
I've ordered some from the usual place but I'm not holding my breath - may take a while to get here.
I did get brave enough and installed the xgpro Windows software for my TL866 II Plus programmer since I saw in some youtube video it could be used
for some simple logic chip testing.
I don't know how good that is at testing those chips - but at least it claimed them to be ok.

The chip tester works really well - I was trying to debug the Gigatron I built - and it was one LS244 that was duff - the chip tester found the chip had one defective buffer. It will be my go to TTL checker from now on.

[timovo]

About the 4164 DRAM ICs in this Issue5 board... I wonder what the speed rating of these is?
It says "HYB4164 P1BD" on top of IC ...

[Pr0f]

According to the datasheet - the -P1 is a 120ns part, so should be fine for a QL

[timovo]

According to the datasheet - the -P1 is a 120ns part, so should be fine for a QL

Thanks Pr0f,
I'll try to buy some 120ns chips as replacements.