My CBM8032 adventure started in August 1980. My father was invited to the introduction of the Commodore CBM8032 at Carl Reese Bürotechnik in Kiel. Since he wanted me to have a look at those computers, too (we had been playing with programmable calculators), so I also got invited.
Bürobedarf Carl Reese an der Rendsburger Landstraße 196-198 in Hassee
by Magnussen, Friedrich (1914-1987) (Source: Wikimedia Commons)
Actually, it was a cool event. A part was like a BASIC course and I have programmer my first little program there. This day had changed my life for the next 15 years (before I started to work in my job as an electrical engineer).! In December 1980 my father bought such a Commodore 8032 and I have learned to programm it quickly. First I typed in some programs from magazines, then I wrote some simple games and finally, I wrote some business software for my father. It was a program for real estate appraisal. Not a big thing, actually, just calculating the value from a couple of inputs and printing out the results.
Me, while the winter holidays 1980/81. With the CBM8032.
From that on, I have spent my school holidays on programming. I wrote software for the good old Commodore probably until 1987 or 1988. Then the company switched to PCs and DOS programs. I was doing pretty tricky things with the 8032. Most of the programming was in BASIC, but there were some machine code parts to pimp the user interface.
At school, there was an 8032, too. One! For all students. Actually, only the "older" students of grade 12 and 13 were allowed to access its, but I got a special permit to use the computer, as long as no other student had to. Well, the interest of the others was limited, so I accessed it a lot.
At the early age of 14 year, I knew, that I wanted to become an electronic hardware developer, but for not a short while, there has been a great disturbance in teh Force and I considered software engineer as a carreer. My father said, it is nothing decent, probably he was right, so I finally became a hardware developer :-)
In 2020 I was offered a CBM8032 and I thought, I don't want it. Too big! My collection was not really small, already. But I felt, I wanted it. Temptations, temptations!
The lady, wjho offered it to me, wanted to sell it. It was her father's computer. I asked her, if it would switch on. She said, there was no sign of life. I offered her to repair it and we could share the price. So in August 2020, a huge box arrived. An 8032 packed in stuff, that I immideately dumped that day :-)
The CBM8032 after arrival
It was dirty, it was smelly and the space bar was missing. Fortunately, I have found it in the box, before I threw away the rest of the "packing material". It was dead when I have tried to switch it on. My first repair attempt is always cleaning the pins of the ICs in a socket and also putting some tuner spray at the sockets. That has repaired quite a couple of old computers, that I had on my workbench. The IC sockets, that Commodore has used, have traditionally been bad. I have learned from a fellow Commodore technician about this, back in the early 1980s (even then, the sockets have been a major source of problems).
Signs of life!
It was alife again! I was so glad. I could hear the chime adn see the Commodore BASIC message. Wow... this moment really touched me, because I had spent so many hours in my life with this kind of computer. It was the first time, that I have heard and seen it since the 1990s.
Unfortunately, I could only see it for about 10 minutes. Then the display got darker and darker and finally faded away. I could hear the chime and when I typed in "?chr$(7)", I could hear it again. So I knew, the computer is working, but there is a problem with the monitor.
As a youngster, I was repairing TVs, that I had collected from the junk, a couple of times. I knew, there was high voltage, but I was not afraid. But now, the 20kV and more, that I might find in a CRT monitor, did impress me. Also, I wanted to measure the voltage to find out, if something was wrong. So, I have bought myself a high voltage probe. It is a pretty big probe (actually, I was surprized, how big), that can be connected to the multimeter, it also helps monitoring the state of discharge of the CRT.
HV-Probe HVSP HSTK-40
From the Facebook group "Commodore PET / CBM Enthusiasts", I have learned, that there are two 56R resistors, which are usually the source of this kind of problem.
In my monitor, R752 had failed, it looked a bit roasted. So E400 has not been 412VDC, like noted in the schematics, but about 100V, which was of course not enough for the CRT's anode voltage.
The resistor is specified with 1/4 Watt, there must be a reason, while it fails so ofgten, so I have replaced it with a 1W resistor with a maximum voltage of 500V. I have also replaced R753.
And, yeahh...the monitor was working again.
[...to be continued...]