Fixed Sharp but not yet!

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gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Fixed Sharp but not yet!

Post by gregctaylor »

Hi,

After a long gap (moved house) am finally getting round to fixing all three of my Sharp MZs (K,A,B) as all have failed caps in the PSUs.

The K is now working almost as it should, screen, tape and board all working but all I get on screen is a mixture of checkerboard and - patterns. I suspect this might be a failed RAM but has anyone any better ideas and any way to test which RAM (of three full banks) is faulty? I have access to a logic probe and an oscilloscope so testing is not a problem. Have tried the service manual but it references a RAM tester I don't have.

Many thanks

Greg
hlide
Posts: 681
Joined: Thu Jan 25, 2018 9:31 pm

Re: Fixed Sharp but not yet!

Post by hlide »

What do you have as facility to load a program apart from the tape player? I guess it'll be harder with MZ-80 K because it has no external EAR jack.

There could be several issues:

-DRAM (16Kbyte/20KB/24KB/32KB/48KB): well, it has nothing with the screen but it may lead to bad code execution and some garbage with distinctive characters on the screen.
-VRAM (1 KByte): some garbage with distinctive characters though.
-CGROM (2 KByte): garbage with NO distinctive characters.
-ROM(1KB): same as DRAM.

If you think DRAM is faulty, I made a program: https://sharpmz.no/forum/viewtopic.php? ... 7c0ebcc825.

The issue is how to load this program. If you only have the inner tape player and have an MZ-700, use the MONITOR of MZ-700 to input the bytes and then save them on a cassette. And play this cassette with the MZ-80 K.
hlide
Posts: 681
Joined: Thu Jan 25, 2018 9:31 pm

Re: Fixed Sharp but not yet!

Post by hlide »

If you have a program to type bytes and save them :

Code: Select all

 	00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F	0123456789ABCDEF
D000	FF	BD	00	12	FE	00	11	10	0F	ED	53	71	11	2A	04	D0	..........Sq.*..
D010	7D	B4	F5	2F	CD	C3	03	F1	ED	4B	00	D0	2A	02	D0	C5	}../.....K..*...
D020	E5	E5	D1	13	77	ED	B0	E1	C1	ED	A1	20	0A	EA	29	D0	....w...... ..).
D030	21	04	D0	CB	06	18	CF	2B	E5	7E	2F	CD	C3	03	E1	CD	!......+.~/.....
D040	BA	03	C3	82	00	 	 	 	 	 	 	 	 	 	 	 	.....           
gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Fixed Sharp but not yet!

Post by gregctaylor »

Thanks. I do also have an MZ700 so shall try that. I am assuming that although the screen is garbage the tape will still load if I press L?

It is extremely regular garbage. If you imagine that # is the checkerboard pattern and - is the centre graphic line then it looks like

#-#-#-#-#-#-#-#-#-#-#-#-
-#-#-#-#-#-#-#-#-#-#-#-#

etc etc.

Greg
hlide
Posts: 681
Joined: Thu Jan 25, 2018 9:31 pm

Re: Fixed Sharp but not yet!

Post by hlide »

Normally, the program is loaded at the first bytes of the VRAM. If VRAM is not okay, it won't work. If CGROM is not okay, it won't work because the RAM test relies upon having a working screen. I suggest you to try another program which only outputs a music: I already tried it by curiosity upon ùy MZ-80 K. That's way you know if ROM (loading the program) and DRAM are working.
gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Fixed Sharp but not yet!

Post by gregctaylor »

Just got back off holiday to some new VRAM chips. Have socketed the board and replaced and ...... (builds up tension ......)
.
.
.
no change . *?!!!****???!!!

The next option must be the boot ROM (EPROM?) but I don't have an EPROM blower so a bit stuck. Anyone any ideas or can help me out?

Greg
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