Repairing a Motorola M7000 23" Black and White Monitor
By, William Stephens
(clicking any picture in this article will bring up an enlarged version)
A 1977 Midway "Boot Hill" videogame came in with a working game board and power supply, a pristine 23" black and white tube, but no monitor chassis. A little looking around the shop and I discovered that I had a Motorola M7000 chassis and a Motorola M700 chassis (with yokes). The M700 was a 13" coctail monitor and the M7000 was a 23" Black and White monitor just like what was missing.
There is very little information about Motorola monitors from that era and I never found a schematic for the M700 chassis but I noticed something peculiar about the two chassis, while a bit different from one another, they were virtually identical. The circuit boards were the same, as were the parts as far as I have been able to determine, even the flyback transformers were interchangeable. I did have a schematic for a M7000 so, with these things in hand, I figured that I had at least one working chassis out of all this.
I decided to work on the M7000 chassis using the M700 chassis for spare parts. Back then, Motorola marked all their components with their own part numbers making easy modern substitution a bit difficult. You would have to derive the parameters from inference and tasking in the circuit, and then you could only guess (hopefully intelligent) about the beta or hfe. It's nice to have spare exact parts available.
I first checked the fuses and found they were O.K., then I hooked the monitor to an isolation transformer and used my old Heathkit vectorscope as a test generator for it. I then clipped my multimeter up to the B+ (+73V) and turned the monitor on just long enough to read the B+ voltage and then clicked it off. The B+ was at +98 volts. Obviously, the voltage regulator was kaput! I unbolted the regulator from the heatsink, checked it and found that it had a collector emitter short. This is kind of rare, they usually fail open. When that happens the monitor just goes black and silent. With a shorted regulator, all the unregulated raw D.C. voltage gets applied where once a clean and stable +73 was. The voltage is higher than needed before the regulator and so everything also gets a boost in voltage which will cause things to pop. I replaced the regulator and checked the associated transistors. All looked fine.
I hooked things back up and this time in addition to the multimeter checking the B+, I connected a high voltage probe to the CRT's anode. I power things up and this time I see a nice steady +73 volts at B+, so I leave it on a bit longer. The high voltage probe shows nothing, which I suspected especially after noticing a hole blown in the side of the flyback transformer. Another indicator that things were not yet well was the horizontal output transistor getting a bit hot. I power off and replace the flyback transformer and check the horizontal output transistor to make sure it was not damaged.Once again all looks fine.
This time when I power up the monitor I hear the old familiar charging crackle from the CRT, I think we have a winner!!! I'm getting about 18KV from the flyback and soon, the monitor lights up way out of wack. I let the monitor run for a bit now to make sure there are no more fire startes in there. While this takes place I start twisting knobs to straighten out the picture. I can get the picture pretty stable except there is no vertical sync and I can't adjust the brightness at all. Once again I power off and start checking transistors, my first suspect is the spot killer, because it's collector is hooked to the brightness potentoineter and then off the +150V from the flyback. Just as I suspected, the spot killer has a base-emitter short. I check the components in the spot killer circuit and find no other damaged parts so I replace the spot killer transistor.
Upon power-up I now could adjust the brightness of the monitor, actually it looked real good except for that vertical rolling. It appears that I have to take a look at the sync section of the monitors circuits. The M7000 (and M700) boast a unique 4 stage sync section, so I check all 4 of the sync transistors... the sync seperator, sync amplifier, vertical sync integrator and the vertical sync amplifier. First I take voltage reading at the base, emitter and collector of each transistor and compare them to the voltages on the schematic. Some match and some don't, some are really off. Now I hook up the oscilloscope and check the waveforms at test locations against the waveform images in the schematic. They all conform pretty much, I see raw video and then see vertical sync pulses get stripped off and see the pulses get amplified and inverted and see them get integrated and so on. None of the scope readings looked wrong. Even the timebase was right at 63.5 milliseconds as it should be.