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Old 2nd Oct 2018, 5:30 pm   #1
mole42uk
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Default What does a blanking signal look like?

I am still trying to fix a Gould 4074. The power supply is now behaving itself and the 'scope appears to do what I might expect it to do - except there's no on-screen display.

There was a display when I first got some life from the power supply but that disappeared with the cracking sound of a component failing a few weeks ago.

Using the service manual, there's a handy step-by-step guide to repairing the instrument which has been very helpful so far. Currently I'm working through Section 5/9 "No CRT Display" and have proved everything so far up to the instruction in 9.1 "Is the blanking signal present at the emitter of Q801"

There is a signal, it's a sinusoidal shape about 200mV p-p but I don't know if that's a blanking signal or just noise. It looks like noise to me.

Can anyone help to point me in a forwards direction again?
Thanks,
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Old 2nd Oct 2018, 5:56 pm   #2
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Default Re: What does a blanking signal look like?

It should be a square shouldered pulse waveform after all it is used to cut the crt off at a given transition point.
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Old 14th Nov 2018, 7:24 pm   #3
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Default Re: What does a blanking signal look like?

Some news just in: the tube heater isn’t glowing. That would explain the lack of display.

I tested the tube by connecting the heater pins to my bench 6.3v supply and the heater glows, so the power supply has to come out of the ‘scope again. I rather hope that one of the series resistors has failed rather than the transformer.
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Old 15th Nov 2018, 7:14 pm   #4
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Default Re: What does a blanking signal look like?

Yes, series resistors had failed open circuit - replaced with 15ohm ¼W. Test tomorrow.
I also discovered that the RIFA capacitor in the IEC socket had gone very smelly so I’ll replace that while it’s out …
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Old 15th Nov 2018, 10:06 pm   #5
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Default Re: What does a blanking signal look like?

Sounds like you’re making good progress , Richard, I’m pleased to hear it.
Hopefully you’ll tie it all up soon and it will be working brilliantly!
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Old 16th Nov 2018, 7:47 am   #6
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Default Re: What does a blanking signal look like?

Oh, I hope so. This 'scope has been un-clothed on the bench for far too long!
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Old 16th Nov 2018, 8:07 pm   #7
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Default Re: What does a blanking signal look like?

Yes, the screen now shows a display. It’s not yet a readable display but still means significant progress.
I’m happy that the power supply is behaving, I don’t even use the Variac any more.
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Old 18th Nov 2018, 7:58 pm   #8
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Default Re: What does a blanking signal look like?

Quote:
Originally Posted by mole42uk View Post
Yes, the screen now shows a display. It’s not yet a readable display but still means significant progress.
Sounds like steps in the right direction, Richard. I'm sure you'll get there!

Keep us updated!
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