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Old 13th May 2022, 11:12 am   #41
MelJon66
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Default Re: HP1743A Oscilloscope Rescue

Thanks again PJL,

I have attached a photo of the A12 Gate Output at test point A12TP1 with the 1743A timebase set at its fastest 0.05usec setting. The time/div on the Iwatsu monitoring 'scope is 0.5usec (500nsec) and I was using a 10X probe. This shot is with the brightness set at its mid point with a P-P waveform height of around 25V. The peak of the trace appears doubled, possibly due to oscillation. If I reduce the brightness to the minimum visible level, the waveform height is around 12V and the oscillation disappears.

At the 25V level the P-P rise time is around 300nsec. At the 12V level the P-P rise time is around 150nsec.

That's probably not very good is it?

Cheers
Mel
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Old 13th May 2022, 11:15 am   #42
WME_bill
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Default Re: HP1743A Oscilloscope Rescue

HP1743 gate response.
I am glad you making progress and confirm my suggestion of slow front edge to bright-up pulses by your picture on #41.
Have you looked at the Gate Response Adjustment (para 5-43 on HP1740 manual). I think you may find this will make a difference. View results on TP1, waveform 4 on the Gate Amplifier Assembly diagram 3, PCB A12.
Otherwise check the diodes on the +120v section of the EHT supply: CR2, CR3.
The diodes CR5,CR6 on the high voltage section to the right of C4,C5 you will have to check with the unit switched off as they are at EHT voltage, though I would be surprised if you need to do this, as the waveforms are pretty clearly checked at the gate output.
You still seem to be worrying about the Delayed Gate pulses. I do suggest you just test with the single Main time base, and switch the delayed TB off. You will learn nothing more at the moment, and merely get confused by the repeated bright-up pulses.
wme_bill

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Old 13th May 2022, 12:22 pm   #43
MelJon66
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Default Re: HP1743A Oscilloscope Rescue

Thanks Bill,

Para 5-43 in the 1740A manual corresponds to Para 5-16 in the 1743A manual. I have done those adjustments but there wasn't enough adjustment to eliminate the overshoot on the waveform at A12TP1. The best I could do was to set pot R12 at its minimum and the result is the waveform you see in the photo in #41.

The delayed TB is switched off. I only switch it on when the manual indicates that I should so that I can compare waveforms on a like for like basis.

A15 diodes CR2, CR3, CR4, CR5 & CR6 all test fine in circuit with good voltage drops A-C and no shorts. Will I need to remove them to check more thoroughly?

Cheers
Mel
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Old 13th May 2022, 12:34 pm   #44
Radio Wrangler
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Default Re: HP1743A Oscilloscope Rescue

1740 series scopes are normally a bit dim at the leftmost beginning of fast sweeps, probably the blanking being slow to come off. So, it's worth having a look at a few other examples to see if you are chasing a real fault.

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Old 13th May 2022, 2:55 pm   #45
MelJon66
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Default Re: HP1743A Oscilloscope Rescue

Hi All,

Problem solved

On HV board A15 there is a trimmer pot A15R2 which is the Intensity Limit Adjustment. It seems to have been set as per the manual so that when the brightness control is at minimum the trace is just extinguished. I made a little adjustment so that when the brightness control is at minimum the trace remains just visible and the trace brightness is now even across the screen at all timebase settings. The slightest movement back the other way and the start of the trace is lost at fast timebase speeds.

It was a long road to find the simplest solution but along the way I have learned a little bit about what a gate amplifier does.

Thanks to everyone for their help and I'm sure I'll be back with another query soon. There are one or two other oddities I need to investigate further.

Cheers
Mel
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