Vectrex white dot fix and no buzz mod
3 September 2021
Commodore TV Game 3000H
18 October 2021
**** CMB BASIC V2 ****
510 BYTES FREE
READY.
Finally I managed to get a Commodore Max, it was a long hunt but in the end I managed to find a specimen cataloged as garbage, I couldn't not get it !.
Now all that remains is to try if it works and add the composite signal.
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As it appeared in the beginning
From what you can see in the photo, it was not in bad shape externally, but it was clear that it had been placed in a humid and very dusty place.
The RF video switch is totally rusty and unusable, but thankfully it's not necessary. Inside there is a lot of dirt but the motherboard is well preserved, no damaged tracks or oxidized sockets, only rust marks on the protection shields and on the RF connector.
Well, now let's go and try if it works! But where to start?
The Max does not have an internal rom and therefore without a cartridge it will not display anything on the screen and the cartridges for the C64 are not compatible for this computer, but vice versa yes, that is, you can use a cartridge of the Max on the C64.
As usual I start to test the power supplies, I downloaded the schematic and checked that there is the correct voltage on the integrated, everything seems ok from this test and now?
Not being able to do much more, I used a generic cartridge for C64, the Versacart (https://github.com/bwack/Versa64Cart)
On the side you can see what I have and the settings I have set to make it work (working is a big word), obviously do not look at the last switches 3,4,5 which are used for the selection of the rom and therefore to be set differently in based on the location where you have the cartridge image.
Well, I said "work", let's say more visualize something !, yes because this computer has an internal ram of 2500Byte and not enough for many games, so these will immediately stop at startup.
I am only interested in verifying that the computer is working, and that something appears on the screen, so for me it went well, in fact I was able to start the Clowns game with these settings and I was able to verify that the SID and the VIC2 were working correctly, well if the game starts then the processor and ram are working too !.
And finally here is the Multimax, the cartridge for the Max machine that also includes 1500bytes of ram necessary to run all the programs written for this computer. In its ROM we already find all the games and programs created for the Max, plus the mini basic and the max basic.
Why 2 basic?
The mini basic is a smaller version made on purpose to be able to run without the additional ram and therefore with a lower cost, with this version we cannot load or save our program on cassette. With the Max basic instead we can use a recorder to save our programs and be able to reload them.
Composite editing
Fortunately, this signal is already present in the computer, so a wire is enough to bring it externally.
If we look at the computer from the back we notice, starting from the left, we find the power switch, the power connector (compatible with the C64) the audio jack, because in the RF output we only have the NTSC video format, the cartridge port like the C64 (but not compatible with 64 carts), the RF channel selector, the RF output (without audio) and the cassette holder.
Without drilling the case or passing external files, we can use the audio connector because it is stereo and the computer has the audio in mono, the other channel "should" be free.
… Why do you say it should?
Yes, it should because in the rev.A of the card we only find the audio output on one channel and the other is empty, therefore usable without problems.
In the rev. B instead the other channel is connected to the SID as an audio input that has not been exploited on this computer, so we can disconnect it and use it for our purposes. In the case of rev.B the solutions are 2, either the track under the jack is cut in order to free it and bring the composite signal, or following the track we will arrive at a signal decoupling capacitor, just lift a pin to free the track and be able to use it without damaging your computer.
Here's where to go! take the signal from the modulator pin (in the picture the one on the right of the 4 pins) and take it with a cable to the stereo jack on the pin not used or previously disconnected (in case of rev B).
End! now just use a simple 3.5 to rca stereo jack cable (usually red and white).
