Hello! Help needed

Introduce yourself here!
User avatar
Dave
SandySuperQDave
Posts: 1997
Joined: Sat Jan 22, 2011 6:52 am
Location: Austin, TX
Contact:

Re: Hello! Help needed

Postby Dave » Fri Mar 02, 2018 11:56 pm

realpeterjack wrote:But it would run, at least, wouldn't it? That would let me know the video generator and processor are OK, at least.


The 8301 can generate a video image without a CPU. It would just be black because the CPU can't put any data in the RAM, but it would be a valid video signal.

realpeterjack wrote:I've just started reading other forum posts and found something about Minerva. Do I have to make any changes to the PCB to get it running?


The ROMs on the QL are quite like EPROMs, except the /OE signal is inverted. On one it is active high, on the other active low. So Minerva on an EPROM comes on a little carrier board with a 7400 or similar to invert the ROMOE signal.

IMG_1884.jpg
Minerva bare board, Minerva EPROM right behind it.


realpeterjack wrote:Thank you for the offer. Right now my biggest concern is having no spare parts to check it for problems. I want to have it booting to the prompt, at least, then I may pursue fixing the drives (it had only one installed). I think trying to randomly replace RAM chips would be a waste of time and possibly good ICs, so I basically have two options:

I could ask the friend who gave me this QL to test the socketed ICs on his working QL;

I could get a Minerva rom and perform more testing on my own.

So, it safe to swap the possibly bad parts into the working QL?


It is safer to test the questionable components on the working QL than to take known good parts and risk them in a potentially harmful faulty QL. If you determine you have any faulty ICs, I have spares here I could include in the package.

realpeterjack wrote:I think I'll have to keep from investigating the problem any further until I get either the Minerva ROM replacement or authorisation to test my chips inside a working QL. Even if I replace all memory chips that would not ensure having it in full working order...


Well here's some things you can do...

Tie the data pin of each RAM IC to GND and see if/how that affects the output.
Check RASL, CASL and WEL are being generated and distributed properly.
Check the 74LS257s are being switched properly, and are functioning correctly.
Check X1 and X2 are both making 15 MHZ, and you see 7.5MHz on CLKCPU (8301 pin 7, CPU pin 34)

Reference clock pic:

38______.jpg
7.5MHz clock.
38______.jpg (110.27 KiB) Viewed 451 times


realpeterjack
ROM Dongle
Posts: 7
Joined: Fri Mar 02, 2018 3:29 pm

Re: Hello! Help needed

Postby realpeterjack » Sat Mar 03, 2018 12:36 am

Dave wrote:The 8301 can generate a video image without a CPU. It would just be black because the CPU can't put any data in the RAM, but it would be a valid video signal.

Sure, but I already got to that point, even further: despite the fact nothing meaningful is being shown, it clearly is a stable video signal. Having numbers and concrete information being shown, though, would add extra validation, and would demonstrate the processor is OK, as well.
Dave wrote:The ROMs on the QL are quite like EPROMs, except the /OE signal is inverted. On one it is active high, on the other active low. So Minerva on an EPROM comes on a little carrier board with a 7400 or similar to invert the ROMOE signal.

Thank you for the information and picture. It looks quite easy to install, and very convenient.
Dave wrote:It is safer to test the questionable components on the working QL than to take known good parts and risk them in a potentially harmful faulty QL. If you determine you have any faulty ICs, I have spares here I could include in the package.

I thought so. I'll borrow the working QL and test the suspect chips on it.

Dave wrote:Well here's some things you can do...

Tie the data pin of each RAM IC to GND and see if/how that affects the output.
Check RASL, CASL and WEL are being generated and distributed properly.
Check the 74LS257s are being switched properly, and are functioning correctly.
Check X1 and X2 are both making 15 MHZ, and you see 7.5MHz on CLKCPU (8301 pin 7, CPU pin 34)

Reference clock pic:

38______.jpg

Shall I check these before trying to check the socketed ICs on the good QL? I don't know an easy way to test the 74s, so I'll probably leave it for later.

Thanks once again


User avatar
Dave
SandySuperQDave
Posts: 1997
Joined: Sat Jan 22, 2011 6:52 am
Location: Austin, TX
Contact:

Re: Hello! Help needed

Postby Dave » Sat Mar 03, 2018 12:58 am

realpeterjack wrote:Shall I check these before trying to check the socketed ICs on the good QL? I don't know an easy way to test the 74s, so I'll probably leave it for later.


On the 74LS257 (IC19/20): pin 15 is /OE and pin 1 is SELB so you can check for presence or absence of the high impedance state from A0/B0 to Q0, A1/B1 to Q1, etc.

On the 74LS245 (IC21, wrongly labeled as '257): Pin 1 is DIR, pin 19 is /E, so if you ground /E and toggle DIR you should be able to check the direction control operates correctly for each pair of pins 2<>18, 3<>17, 4<>16, etc.

Many people who do repairs replace these ICs and put the new ones in sockets.



Return to “The Welcome Area”

Who is online

Users browsing this forum: No registered users and 2 guests