Sharp MZ

Hi Everyone first post.

I got an MZ-711 model that let off the magic smoke on first switch-on. After recapping the PSU (the stuffed RIFAs and the filters), I finally switched it on, but the computer is not booting.

The red light on the keyboard is on. The tape drive is working. However there is no boot beep sound or video. I have tried half a dozen other Z80's that I have with no result.

Before I start to randomly check things, are there specific things in a specific order that I should test first?

Thanks!

Hi

No beep, therefore no execution of the monitor code by the Z80.
This may be due to the 2732 monitor ROM eprom referenced IC5H.
Maybe start by testing it.
Does the screen display some sort of mosaic of random, color characters

So basically, you have no beep (even if volume set at max) and no video? if you unplug the Z80, do you have a static random characters on display?

When Z80 plugged, do you get the clock pin around 3.5MHz? Do you have 5V at VCC and 0V at GND? Do you have /RESET at 0 then 1 when booting? Do you have /M1 set to 0 every 4 cycles or a little more?

Thank you both.

Pacman, the screen displays nothing. Is the IC5H you refer to a circuit diagram reference or is it noted on the motherboard?

hlide, yes no beep at any volume and no video. Also no video with the Z80 removed. I'll test the chip shortly. What do you mean by /RESET and /M1 please? I'm not versed in these Sharp machines sorry.

Thanks!

/RESET and /M1 are pins on Z80. /RESET = 0 resets the Z80. /M1 = 0 means "opcode fetch in progress" (the first byte of an instruction read from memory) so if you see a sequence of /M1 pulses, chance the Z80 is running instructions (using oscilloscope).

You also have VCC and GND on Z80 pins, they should be respectively around 5V and 0V.

No beep -> 8253 not programmed -> check M-ROM (IC 5H) pin /OE (pin 20) whether there are activities. Check VCC and GND on ROM (pins 24 and 12).
No video -> video circuit -> check pins /CSYNC, /CVIDEO, /VSYNC, /HSYNC on connector P-9 (pins 2, 3, 4, 5) whether there are activities.

Have you tried pressing the manual reset button on the rear of the machine?
The Z80 requires a short low pulse to reset, and is generated on power up by charging a capactor which may have failed.
The video circuit runs independantly and desplays 1000 bytes of the VRAM.
The Z80 only overwrites the data in the VRAM, hence the static random characters displayed when the Z80 is missing or not running.
Both need the 17.53MHz clock signal to work.
The LSI chip divides this down to give the 3.5MHz clock for the Z80 and /CSYNC, /CVIDEO, /VSYNC, /HSYNC for the video.
So check the clock signal into the LSI and the 3.5MHz, /CSYNC, /CVIDEO, /VSYNC, /HSYNC signal from it first, because without them the MZ700 is dead.

This is great, thanks everyone, I suspected that this will be a scope job. I'll get cracking on all of these items next weekend and will keep you posted.

On a slightly different track, I downloaded the service manual but yet to look at it. Ahead of time, can someone please quickly confirm what the two separate Mitsubishi ROM chips are responsible for, and also which one on the board is the LSI chip.

IC 7A is the LSI with its long spider-like legs.
IC 5H is the ROM containing the MONITOR code, usually called M-ROM.
IC BC is the rom containing the characters bytes to output on display, usually called CG-ROM.

Well, I tried checking these things with a scope, but regardless of which pins I touch (for all components on the board), all I get is the 5v sine wave and that's it!

Very weird!

Or do I need a logic analyser or a frequency counter for these simple tests?