Sunday, July 30, 2017

Found bad custom chip part in 7854 scope and replacement part is amazingly available


I can see that the Display board is generating the XYINH signal that should block the analog scope signals from being displayed during the digital interval, however the characters are smearing because the analog signals are not inhibited.

Further, in looking at the way the characters are distorted, it is strictly in the horizontal direction. The top and bottom of the character is never affected, but the left to right placement of the pixels is impaired. That suggests that the flaw is in the Horizontal Amplifier circuitry, which is not acting on the XYINH.

Looking at the horizontal circuitry, I see one place where XYINH is fed into a custom chip that is the channel switch. This chip is available on ebay as New-Old-Stock just in case it appears that problem is there. It will have to come from the island of Rhodes, meaning several weeks of transit time, but is reasonably priced all in all.

When checking out the rest of the scope, I observed that only the B horizontal plug-in is working, the A plug in does not cause traces. Swapping the actual plug-in modules made it crystal clear that the problem is in the horizontal switch circuitry, just where my other problem is manifesting.

The circuitry in question is on board A9 inside the main body of the scope, not on the plug in boards on the side. It is behind the plug-ins, which suggests that access a real pain. However, when I began disassembling I found that it was brilliantly designed for easy access.

Card A9 exposed behind metal plate
Card A9 seen from underneath
A metal cover plate blocks each of four possible card positions, with the card itself designed to fit through the opening exposed. I only had to remove a bunch of coax cables and one small connector, take out two screws and it slide right out.

Card removed through opening
Access from behind plug-ins
The switching chip is right at the beginning of the circuit path on the board, routing two sets of horizontal signals from the A and B plug-in positions, switching with the Benable signal and blanking with the XYINH signal. This is pretty clearly the part that is fried, so I bought the NOS spare from ebay.
Suspected chip circled at left
New Old Stock chip to replace on card

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