Bob Rall Mod

Bob Rall “Pro-Fifty” TBC proc amp mod for the Panasonic WJMX50 video mixer

If you just stumbled on to this page, these are instructions and documentation for adding proc-amp controls (brightness, contrast, hue, and saturation) to both channels of a Panasonic MX50 video mixer. Bob Rall devised these modifications, and offered them as a service throughout the late 90’s. Then he disappeared from the face of the earth.

Thanks to Benton C. Bainbridge for donating a modified MX50 and making this possible.

Disclaimer: This documentation is untested and incomplete. It has been published for educational purposes only. If you break your stuff, it’s your own fault. Do not contact me with technical support questions for performing this modification.  If you use this information for personal profit, you have to live with yourself and your bad karma. If you are Bob Rall, sorry dude — I tried to contact you repeatedly.

UPDATE: contrast mod successfully replicated.

Black level, Chroma & Hue pots are 5k linear, center detent.

Contrast pot is 250Ω linear, center detent. 



Analog Board JRZMX50P2C connection points (underside of the board):

A: Tiny hole to the left of the R211 marking

B: Left leg of R208
NOTE: The trace between A & B has been cut.

A & B.jpg

C: Pin to the right of R223

C copy

D: Lower leg of C153

D copy

E: Pad for upper leg of r214

F: Pad for lower leg of R214

NOTE: R214 has been removed.

E & F copy

G: Left leg of C174

H: Pin above C164

G & H copy

I: Tiny hole to the left of the R302 marking

J: Left leg of R299
NOTE: The trace between I & J has been cut.


I & J copy

K: Pin to the right of C250

K copy

L: Upper leg of C179 (ground)

M: Lower leg of C179

L & M copy

N: Pad for upper leg of R255

O: Pad for lower leg of R255

NOTE: R255 has been removed.

N & O

P: Left leg of C200

Q: Pin above C190 marking

P & Q copy

43.2kΩ SMD resistors have been placed on the previously empty pads at R314 & R366:

r314 - 4322

r366 - 4322

On the top of the analog board, 270 pF ceramic capacitors have been added across the top two legs of VR15 & VR18 (hint: those are the ABUS and BBUS Y-gain trimmers, smoothing maybe?)



This chip (below) is connected to the level pot for the BC generator.


I found this entire daughterboard living in the digital shield. There go my dreams of cloning this thing! Red wire is going to 5v, blue to ground (if memory serves). Green and yellow go to the IC in the above pic.

edit: after some more consideration and hitting the service manual again, I’m almost positive that this board has nothing to do with the proc amp controls. As the legend goes, the Mx50 resolution was factory crippled so as not to compete with broadcast gear, but Rall figured out how to bring it back up to spec. Pretty sure that’s what this funny business on the digital board is all about.