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Image on the left was posted in the groups.io forum 2021-04-23. by Lance.
Below, I have an image, nearly identical from May-10. from the
M1000- Rev-7a that came with the MPA-10 keyboard on my desk.
First, Lets document what capacitors, resistors, etc are missing from the motherboard, Then take a look at what has been added.
C49 & C9 removed,
R5 was removed, there is a TAP soldered to the bottom, leading to a new 47pF capacitor.
C6 is still there.
C8 is still there.
C7 was removed.
Was R27 Removed? - No.
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There is a new capacitor, 22pF in place of the old C9. - C9 used to be 15pF, but the new 22pF capacitor strattles over the top of the IC's, MC1372 ... it is green in the above photo - - from my notes" pin-10 of MC1372, then the other lead of the cap is "Bottom of C9". Looking at the schematic, it electrically replaces C9 - but the stray parasitics of it being hair-wired might create some odd behavior (or might be nothing). - this one capacitor couples the 4.5MHz Audio subcarrier into the video going into the RF Modulator of the MC1372.
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There is also what appears to be a filter from the 6847 "Y" Video-Out . It consists of an RF-Choke (analysis shows it to be 10 uH), three resistors, and a 47 pF capacitor (from the Chroma-Out) -- These feed into the pin-9 "Luma-In" of the MC1372.
Here is the filter circuit , the 10uH on the RF-Choke (coil) was un-readable in my Revision-7a that came with the MPA-10. (right-click and "open image in new tab" for a larger view.
A quick parametric sweep in QUCS-S ( I used a Raspberry-Pi for this ) shows the coil at 10 uH will do the usual 3-dB drop just above the 6-MHz that NTSC Video before 2006 digital-TV.
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