...another gray screen weirdness...

[WacKEDmaN]

yeah me again with another dumb one! 
firstly i see the other thread with the same sorts of issues, but i didnt want to hijack his thread....

im now getting the gray screen of death!... it started as just some glitching on bootup...would see a like a red pixel here or there on some boots... well i left the cpc running all night with a simple clock program.. today ive gone to use it, (it seemed to be fine) i reset it and loaded up a game.. a few seconds in..and the game would crash, or id get killed out of nowhere and the level would restart.. i reset and tried reloading a few times with the same sorts of results...
so then i started swapping out ICs,and now im getting constant gray screen

things ive tried to cure it
..ive always had some issues with IC115 (74LS244).. where i need to slightly lift it from the socket for it to work.. i did that, and even resoldered the socket, and replaced the IC... all giving the same results

ive swapped out the ram ICs for the originals (which worked fine last time i used them).. same results (put back newer ram after testing)

i swapped out the z80, crtc and GA.. all are giving same results

i have lower rom board with diagnostics...it gives 3 beeps and shows ALL ram is bad...(even with the original ram)

i have a strange feeling its something to do with IC115-74LS244...
but im also wondering if one of the mux ICs (74LS153) could be causing this?

[d_kef]

Or maybe it's a power issue.
Have you checked your power supply for a stable, adequate voltage?

d_kef

[WacKEDmaN]

Or maybe it's a power issue.
Have you checked your power supply for a stable, adequate voltage?

d_kef

i have a boost/buck converter..with variable voltage and current and display...  pushing 5.5v (no load 5.53v)..pulling 880mA (with lower rom board and ps2 adapter)..running from 12v 2amp wall wart... so plenty of power...
onboard voltage is 5.49v

im gonna test ram for continuity through the ram chips and the buffer ics..

[eto]

If you swapped several ICs already, make sure you don't have a bad connection or a short somewhere. While doing my 320KB CPC6128 every time I got a grey screen, I just had a bad connection or an unintended short. 

Have you also changed the socket? If an IC works if I lift it, I would expect that it's the connection there, not necessarily the IC. 

I did not yet have the situation that any other IC but a RAM chip failed. But I guess that any IC that is crucial to address lines or data lines could lead to the same situation. If a MUX IC is bad, probably all RAM access will fail. But also if IC114 is bad or the PAL (6128), RAM access probably is screwed up. Or something putting RAMDIS constantly high.

[WacKEDmaN]

well ive changed out IC114..
ive changed the mux ICs...
ive changed IC110 and IC113..(my other 74LS132 is bad.. it makes the board pull 1amp, and shows weird green pixels moving around)

all with the same results

checked for shorts.. seems ok...

im really starting to get pissed off with this whole thing... its driving me crazy...ill get it working, then somethin else screws up 

[Bryce]

The trick with pulling the 244 out of it's sockets would suggest that there is possibly a cracked trace on one of the lines from the 244.

Have you measured the voltage directly at the supply pins of any of the IC's? If they are really getting 5.5V then your CPC won't last very long anyway. Most TTL chips will start failing above 5.25V.

Bryce.

[WacKEDmaN]

The trick with pulling the 244 out of it's sockets would suggest that there is possibly a cracked trace on one of the lines from the 244.

Have you measured the voltage directly at the supply pins of any of the IC's? If they are really getting 5.5V then your CPC won't last very long anyway. Most TTL chips will start failing above 5.25V.

Bryce.

oh crap! youre right... why was i thinkin 7v was max... dropped to 5v..(4.98v on 244).. i had to run a bit higher for USIFACII to work...
..that could be why somethings failed...
ive just pulled all the ram n tryed one IC in each slot.. ..still cant tell whats up...
checked 244 for continuity to both pin2 and pin14 on the ram sockets and to 373.. seems fine..

gonna order a 244, 373, and 132...(i just broke some legs on originals SMH i suck!)

i guess im going to have to eventually change these sockets to machined sockets too...

[WacKEDmaN]

i built up this Arduino ram tester... https://github.com/randyrossi/arduino_dram4164_tester/blob/main/dramtester.ino

ran all 18 of my 4146s through it... 
ALL are good....

so its gotta be the logic chips...

i found a TTL tester i could build up, https://www.instructables.com/Arduino-IC-Tester/
it doesnt test 244 or 373...
but it will let me test the 32,132, and 153s

[Bryce]

i built up this Arduino ram tester... https://github.com/randyrossi/arduino_dram4164_tester/blob/main/dramtester.ino

ran all 18 of my 4146s through it...
ALL are good....

so its gotta be the logic chips...

i found a TTL tester i could build up, https://www.instructables.com/Arduino-IC-Tester/
it doesnt test 244 or 373...
but it will let me test the 32,132, and 153s

Lucky you, because that Arduino design is majorly flawed. If any RAM IC had an internal short it would destroy your Arduino outputs from over-current.

Bryce.

[WacKEDmaN]

Lucky you, because that Arduino design is majorly flawed. If any RAM IC had an internal short it would destroy your Arduino outputs from over-current.

Bryce.

how do you figure that?.... im genuinely interested... ive seen may others use it without issue....

[Bryce]

The i/o pins of an Arduino can handle 40mA and they have no protection. If you enable an output connected to a RAM pin that has shorted to GND (a common occurrence - We call it a "stuck bit"), then a lot more than 40mA is going to be pulled from that i/o pin and the MOSFET in the Arduino will fail.
To avoid this you should have a small resistor in series with each output (probably around 150R) so that the logic will still work, but a short to ground would still pull less than 40mA due to the resistor.

Bryce.

[WacKEDmaN]

The i/o pins of an Arduino can handle 40mA and they have no protection. If you enable an output connected to a RAM pin that has shorted to GND (a common occurrence - We call it a "stuck bit"), then a lot more than 40mA is going to be pulled from that i/o pin and the MOSFET in the Arduino will fail.
To avoid this you should have a small resistor in series with each output (probably around 150R) so that the logic will still work, but a short to ground would still pull less than 40mA due to the resistor.

Bryce.

hmm..i have come across that before ...but never had any issue..even running leds directly without resistor (only for a very short period tho!)
but ill keep that in mind for next time i build it up... 

gonna try building up the logic tester later tonight...

i also found a few pages on manually testing the 244 and 373..so im gonna build them up in the next few days (ill get new chips on friday)

[WacKEDmaN]

replaced the mux ics (153s) and the 244 buffer.. and 32 or gates...back working now!..at 5.00v!!

i ordered the 373 with the others but got given 273s ..SMH..


edit: im still having issues with usifac...screws the boot every time...even with slight voltage increase...