Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe


If I remember correctly there is a separate small 5V rail especially for the lamps and lights. You have to add the connections for a 7603. As it does not have it. Most likely this is what is being dragged down. It wont effect the scope. I have bad news on the trigger light though If I remember correctly that is driven from a custom tech IC. I had one of those burned out in an A plugin it did share the IC with a 485 of which I had a parts unit for. If you are looking for shorts on the main 5V supply I do not think you will find them. At least for the lamps.


-----Original Message-----
From: <> On Behalf Of Dave Daniel
Sent: Sunday, February 7, 2021 1:43 PM
Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ummm... I’m not sure hot-plugging a 7xxx plug-in is a good idea since they were not designed for that. The result will depend on the sequence of contact between the plug-in board fingers and the 7904 connector contacts when the plug-in is inserted (hint: they will not all make contact at the same time).

All hot-plug systems of which I am aware have different length pins/fingers which serve to control how the circuits on the hot-pluggable board are activated as well as active circuitry to protect things during power-up.


On Feb 7, 2021, at 12:07, wrote:

It's been a while since I've been inside a 7904, but I'm sure it has some accessible power-supply test points. With power off and the plug-in not installed, measure the resistance of each test point to ground. Then, with the power still off, insert the plug-in and measure the test points again. They may measure under 100 ohms, but should not be under 10 ohms. This should identify which supply the plug-in is shorting and suggest which caps on the plug-in to look for. Tantalums don't always show external distress when they short so dont rely on a visual check.

Bob Haas

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