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Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 6:26 pm
by bxdanny
Sideburn,
The circuit that reads the video RAM creates the dot clock (or just passes it on from the main clock generator). That and the two sync frequencies are really the only timings needed to create the video signal. On a standard 600B, the dot clock is simply the crystal frequency. On the 600D in 48-character mode, it is actually double the crystal frequency. I don't know the details of the video mod on your board. Is there just one crystal? It's frequency should be marked on it. I don't think modulating the signal on an RF carrier would do any good, it would still be the same signal when it was demodulated. But who knows?
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 6:43 pm
by Sideburn
I took some measurements.
The crystal is showing 7.81mhz
HS (u70 pin 13) is showing 15.43 kHz
HV (u70 pin 12) is showing 60.24 Hz
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 6:47 pm
by Sideburn
bxdanny wrote: ↑Tue Mar 26, 2024 6:26 pm
Sideburn,
The circuit that reads the video RAM creates the dot clock (or just passes it on from the main clock generator). That and the two sync frequencies are really the only timings needed to create the video signal. On a standard 600B, the dot clock is simply the crystal frequency. On the 600D in 48-character mode, it is actually double the crystal frequency. I don't know the details of the video mod on your board. Is there just one crystal? It's frequency should be marked on it. I don't think modulating the signal on an RF carrier would do any good, it would still be the same signal when it was demodulated. But who knows?
Only one crystal on my board and it’s just blank can.
Photos of its current state attached:
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 8:13 pm
by ronin47
Only one crystal on my machine, which is a 600D replica, It uses an LS93, U29, to split the clocks up IIRC.
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 10:02 pm
by bxdanny
Sideburn,
It looks to me like there are some numbers stamped on the end of the crystal's can, opposite the leads, but I can't make them out. Are they not actually there?
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 10:33 pm
by Sideburn
My mistake.
Yes it says 7.86342
Re: Improving signal strength on Superboard II
Posted: Tue Mar 26, 2024 11:16 pm
by bxdanny
7.86432 = 2 * 3.93216 (MHz), or exactly twice the frequency of the standard C1P/SB II crystal. Try replacing it with a crystal of 8.0 MHz even. (I guess you'd have to order one.) That would bring the horizontal frequency closer to the NTSC spec. And while it would also push the vertical frequency away from spec, the result could be good enough for your LCD monitor to display without problems.
Re: Improving signal strength on Superboard II
Posted: Wed Mar 27, 2024 12:20 am
by Sideburn
Ok I’ll order one. Thanks.
You saw I was reading 7.9 mhz off the crystal right?
This good enough ?
https://a.co/d/jjziXi1
Re: Improving signal strength on Superboard II
Posted: Wed Mar 27, 2024 1:10 am
by bxdanny
I guess so, yeah.
Re: Improving signal strength on Superboard II
Posted: Wed Mar 27, 2024 1:21 am
by Sideburn
bxdanny wrote: ↑Wed Mar 27, 2024 1:10 am
I guess so, yeah.
Will have some results otmorrow...