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You are enjoying yourself too much! Considering you have taught yourself how to code in Z80 assembler, and use the Cromemco assembler Dazzler Library I would call that a success. Well done! To be clear though (and for education), Cromemco did not actually 'modify' their BASIC to support the...

A signal that starts at +5V, then goes to 0V for a short while, then goes to +5V again has two 'edges'. One goes from HIGH to LOW and the other one goes from LOW to HIGH. The edge that goes from HIGH to LOW signifies the START of the vertical flyback period. The edge that goes from LOW to HIGH...

No problem. Dave

First of all, what test equipment have you got? What is the peak-to-peak measurement of the ripple? Also, when you say ripple, what do you means? Do you know what frequency it is at? I would resist the temptation to swap anything before we know what we are dealing with! Dave

There is a test program for the IEEE-488 port on the PET written in BASIC. Type it in and RUN to test the port (without anything connected to the bus of course). This code was in a book, and it is available online as a PDF. Unfortunately, I am on a business trip this week, otherwise I would...

I am in the UK, so the default programming for the CRTC is 50 Hz... You haven't quite configured your oscilloscope according to my specification. Move the cyan probe from the VIDEO signal to HDRIVE. Make sure the YELLOW VDRIVE signal triggers on the LOW to HIGH part of VDRIVE. Adjust the...

I am not sure what you are doing... I would use the VDRIVE signal as the trigger point for the oscilloscope (say channel 1 configured to trigger from LOW to HIGH) - then adjust the oscilloscope timebase to get a few pulses of the HDRIVE signal (say 10 pulses) displayed on channel 2. Select...

It replaces the EDIT ROM (base address $E000). The reason for this is that this device is generally in a socket - whereas the others had a habit of being soldered into the board. The EDIT ROM takes care of the localisation for the keyboard and display etc. (in conjunction with a replacement...

The video signal is transitioning from the black level to the white level. The black level could be either where the video signal should be blanked (i.e. when HDRIVE or VDRIVE is 'flying back' the electron beam) or black pixels on the screen. The white level is an illuminated pixels on the...

HDRIVE and VDRIVE look to be fine. There also appears to be some black and white video on the VIDEO signal. Dave

OK, well something is better than nothing... Dave

Post the photographs and the oscilloscope traces please. Let's get some data points to start with... Dave

If the HDRIVE, VDRIVE and VIDEO signals are present, there should be a picture on the display then. Yes? What results do you see with my PETTESTER installed? Dave

It might be better as a new topic to save mixing faults. But, if you have finished the last one - it is your choice... Dave

Well done! A bit of persistence pays off in the end. As you can see, it is vitally important to be very critical of your own repair work, otherwise you could introduce more problems yourself than you solve. Yep, have another crack at the MMD-1 again sometime... Dave

Ah ha... Dave

Welcome to VCFED. I have just done a search on that title and found the ISBN number. You might be able to track it down in a library that way. I will post the ISBN number tomorrow when I am on a real computer as opposed to my phone. Dave

Going to your other thread, XH0 is the DEQNA ethernet bootstrap. I assume the controller is missing? If you go into Map, you can change the default bootstrap device from XH0 to DU0 if you wish. Or you could try XH0 followed by DU0... Dave

Simples! Dave

Err, have you tried booting from DU0, DU1, DU2 and DU3? Dave