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Old 24th Nov 2018, 11:58 am   #10
mole42uk
Nonode
 
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Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,591
Default Re: Gould 407x display help

Updates from inside the 4074 on the workbench:

Have you ever had one of those days when every step makes two more? I started with a display on the screen, replaced a few failed components and finished with a blank screen and no further forward.

Thanks to Noopy2014 I checked the protection diodes on the tube base, three of the four were short circuit (well, ~170R in both directions). I tried the other tube base PCB that I have but that showed no trace at all, so I robbed two of the diodes which tested okay and now the previously working tube base doesn't let the tube display anything.,

I checked the Y amplifier outputs at the tube base, Y1 is 87.5V, Y2 is 87.3V so that seems good. I didn't try my other tube because I got into testing other parts. I did try out my other driver board which displays on the screen in a similar way to the other but it keeps the instrument in self-test because of corrosion from a battery leak some time ago.

I am still concerned that the brightness controls do nothing although I can measure that the trace brilliance control moves from -6V to 1.5V and the alpha control moves from 1V to 2.5V. The trace/alpha switch seems to work, believable voltages on the switch pin and the outputs and the display changes when I press the 'menu' button. The 245V supply to the Bright-Up Amplifier is present so I suspect that there's a failed transistor or diode in the Bright-Up circuit. That's where things start to get interesting, partly because it's all SMD and crammed in under the CRT and partly because the component layout in the service manual that I have is different to the PCB that I have on the bench.

I think the simplest way out might be to replace all the active parts of the Bright-Up amplifier, three transistors and four diodes. My reasoning is that the supply protection resistor (R922, 100R) was o/c and, although I replaced it, the circuit still isn't working. It was probably a failure in the driver transistor Q801, the emitter of which should move between 20V and 60V over the screen intensity range and it doesn't, that popped the resistor R922.

That's all for today, folks!
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