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E4421B Display Sorted

 

About a year ago one of my E4421B signal generators developed a display contrast problem. I left it for a while and then decided to see what info i could find about the problem.
After a while i decided i needed the services of Milan for the CLIP details, this was neatly done (thanks Milan). This info also was left for a while and then i finally took the cover off to see what was happening. As previously reported Q601 had an open BE junction. Mmm strange. But prior to this i measured some weird voltages on the LT1170 regulator and even worse an unusual to me configuration, the ground tab is connected to the 1mH switching inductor.
So i confirmed the DAC was supplying a 0 to -10V control signal from the front panel-good but the output from the TL074 was jumpy as was the output from the LT1170 regulator with supplies of up to -46V being measured.
Q601 was replaced (not the best thing to do) and the display was back for a minute and then it all went wrong again. Working at the LT1170 was not easy due space and depth issues so i rebuilt the supply on stripboard to emulate the problem. So i couldn't get these weird varying voltages even thought the control signal was fine. Almost there so it was the Schottky diode CR607 that was the culprit.
The excess voltages explains the demise of Q601, C651 the 47 uf output capacitor was also replaced. I would suggest to any one with this problem to immediately connect a 24V zener across R625 the 2 watt 1K5 resistor and remove the 15 way flat ribbon going to the display? to avoid damaging it whilst affecting repairs as $350 is a high price for a new one
cheers
Geoff
GI0GDP
 

I'm glad you solved the problem and that I was at least a little bit helpful!
Milan

On Sun, 23 Mar 2025, 16:06 geoffrey pike via , <gi0gdp=[email protected]> wrote:
About a year ago one of my E4421B signal generators developed a display contrast problem. I left it for a while and then decided to see what info i could find about the problem.
After a while i decided i needed the services of Milan for the CLIP details, this was neatly done (thanks Milan). This info also was left for a while and then i finally took the cover off to see what was happening. As previously reported Q601 had an open BE junction. Mmm strange. But prior to this i measured some weird voltages on the LT1170 regulator and even worse an unusual to me configuration, the ground tab is connected to the 1mH switching inductor.
So i confirmed the DAC was supplying a 0 to -10V control signal from the front panel-good but the output from the TL074 was jumpy as was the output from the LT1170 regulator with supplies of up to -46V being measured.
Q601 was replaced (not the best thing to do) and the display was back for a minute and then it all went wrong again. Working at the LT1170 was not easy due space and depth issues so i rebuilt the supply on stripboard to emulate the problem. So i couldn't get these weird varying voltages even thought the control signal was fine. Almost there so it was the Schottky diode CR607 that was the culprit.
The excess voltages explains the demise of Q601, C651 the 47 uf output capacitor was also replaced. I would suggest to any one with this problem to immediately connect a 24V zener across R625 the 2 watt 1K5 resistor and remove the 15 way flat ribbon going to the display? to avoid damaging it whilst affecting repairs as $350 is a high price for a new one
cheers
Geoff
GI0GDP

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