HP1741A scope need some help

[dondakin]

Hello There,

First post on this forum needing a little help.


I picked up an HP1741A scope that is in very nice condition. It did not have a tace when I got it.

I have been poking around with it and the LVPS all seem good.

So I moved on to the HVPS. I don't have a HV probe so I fashioned a voltage divider out of 5 100K resistors.

I measured the cathode and it was at about -2160 volts so that looked ok. Then I measured the grid. As soon as I put the resistors on the grid test point (to ground) the trace popped up. If I remove the resistors then it disappears.

My guess is that the grid and cathode are at the same potential and when I ground the grid thru the resistors it pulls it up enough to cause the beam to run.

I have looking into the schematic but I don't have any clue of what is causing this despite the excellent description done by HP on the scope operation..... 

I don't really have any background in scopes and I don't have a strong background in Electronics but I am an Electrical engineer (doing mostly software...)

Anyways I would need a mentor to guide me to bringing this guy back to a usable state. It's a nice scope and whenI can get the trace a lot of things seem to be working fine.

Any help would be appreciated.

Thanks in advance

Don.......

[basinstreetdesign]

" As soon as I put the resistors on the grid test point (to ground) the trace popped up. If I remove the resistors then it disappears."
There seems to be something up with the DC bias network for the CRT grid.  This goes through R9, R8, R10, CR7 and CR8.  It makes absolute sense that if you put a 500K resistor between the grid and ground that the trace would brighten up because that would yank the grid far above the -2200 V that the cathode sits.  One or more of those components might be open (?).  Have you fiddled with intensity limit adjust, R2 pot?  The front panel Brightness control.  I guess you have.
The other possibility is that there is a problem somewhere in the "Gate Amplifier".  If you have a second scope you might probe around there to see if the signal "Gate Output" that arrives onto A15 board as in the top left corner of the schematic on page 8-21 is good.
One thing is for sure you need a probe with impedance much larger than 500K to measure the voltages on the cathode or grid if this CRT.
f I have time in the next couple of days I will take the cover off my 1741A and see what the signal "Gate Output" looks like.
Cheers,
Tim

[dondakin]

Hi Tim,

Thanks for the reply.

I have played with the intensity control and the R2 intensity limit pot. no dice. But when I ground the grid as I mentioned and the trace appear the intensity control works I can change the beam intensity with it.

If the intensity control works when I pull up TP5 then does that validate the path from the gate ? I.e. R7 C5 R17.

I do have another scope a real (old dependable PM3230) oldie but still working. I will check TP3 and the gate input coming in on xa12 pin 10. I already checked these but I don't remember off hand what they were. I can post pictures of the traces.

BTW just wondering what are the 5 neon lights for in the circuit ?

I don't have a high voltage probe but would a voltage divider with 5x2 mega ohm resistors in series work ? Or do I need to go higher in resistance then that ?

Thanks again for the direction. It really helps to discuss these things with someone who has the experience and background in the area.

Don....

[dondakin]

I did some more testing last night.

I connected the b channel to the test squarewave generated by the scope for reference. and had the scope in chop mode.

The Intensity limit pot seems to work fine and I did traces of

TP3 I said tp2 on the video but it's actually TP3 just beside R5 and the gate input  coming in on pin 10.

https://photos.app.goo.gl/6XucRjBTVWGaaaT67


https://photos.app.goo.gl/41i3Q8q9dvbVUimGA

So it looks lke there is ~ 100 volts comming out of the gate input. I did not measure any HV as I don't have the resistors to create a voltage divider yet.

Don.......