Oscilloscope HP 1740A, how to fix it?
 

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Hello,
i have a HP 1740A. Unfortunately it stopped working while it was showing
the 1V calibration signal and i was changing the VOLT/DIV scale to
increase its size on the screen.
The screen stopped showing the signal and also i noticed that it is not
anymore illuminated (changing the scale illumination doesn't change
anything).
I opened it and i can see that the cathodic tube shows an orange light
and its brightness changes when i change the "scale illumination" but
the screen keeps being "black" [without illumination].

Any idea about what might cause this?

I checked all the Test Points where i could read the supposed voltage
and all seems fine, unfortunately i do not have a service manual (i
believe this is how you call those manuals for reparation purposes) so i
could not test many other test points.

Does anyone have the service manual for this oscilloscope and could send
it to me by email?

Thank you,
~ Luca

P.S.: Sorry for my bad english.
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"Luca T." wrote in
:

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Hello,
i have a HP 1740A. Unfortunately it stopped working while it was showing
the 1V calibration signal and i was changing the VOLT/DIV scale to
increase its size on the screen.
The screen stopped showing the signal and also i noticed that it is not
anymore illuminated (changing the scale illumination doesn't change
anything).
I opened it and i can see that the cathodic tube shows an orange light
and its brightness changes when i change the "scale illumination" but
the screen keeps being "black" [without illumination].

Any idea about what might cause this?

I checked all the Test Points where i could read the supposed voltage
and all seems fine, unfortunately i do not have a service manual (i
believe this is how you call those manuals for reparation purposes) so i
could not test many other test points.

Does anyone have the service manual for this oscilloscope and could send
it to me by email?

Thank you,
~ Luca

P.S.: Sorry for my bad english.

Basic modules of any scope;
1. power supplies;*first to be checked*
2. vertical deflection circuits
3. triggering
4. sweep(horizontal) deflection
5. CRT unblanking and intensity control

If you can access the vertical and horizontal deflection CRT pins,you can
short them (vert-vert,H to H) and see if you get a trace or spot on screen.
If so,then you have a problem in the corresponding circuit.(short verticals
and get trace=bad vertical amp).short horizontal pins and get spot=bad
horiz or triggering)

--
Jim Yanik
jyanik
at
kua.net

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Jim Yanik wrote:

| Basic modules of any scope;
| 1. power supplies;*first to be checked*

This seems to work, at least the values that i could read on some
test-points were correct, unfortunately i do not have a service manual
or anything that would tell me what the correct voltages of other
test-points are (i could only test the ones where their supposed voltage
values is written on the board).

| 2. vertical deflection circuits
| 3. triggering
| 4. sweep(horizontal) deflection
| 5. CRT unblanking and intensity control
|

Sigh, i am absolutely ignorant about oscilloscopes and without a manual
to tell me exactly what to do i am lost :(

| If you can access the vertical and horizontal deflection CRT pins,you can
| short them (vert-vert,H to H) and see if you get a trace or spot on
screen.
| If so,then you have a problem in the corresponding circuit.(short
verticals
| and get trace=bad vertical amp).short horizontal pins and get spot=bad
| horiz or triggering)
|

I wish i knew where those pins are, i do not really feel like to
short-circuit random pins. Sigh... anyone knows where i could find the
service manual of this oscilloscope without having to buy a copy?

Thank you for your answers,
~ Luca
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I am not sure if you found your issue as yet, but I had the exact same issues. No trace, no screen illumination. The schematic, service and users manual is located at the Agilient web site. No Charge and in PDF format.
I studied the guide and found that if 15 volts of any flavor is an issue, the 120 volts will go awayt shutting down the HV cage. This is where the illumination and trace are provided. (HV Supply). So i printed off just the LV power supplu schematic. and found that the 122 volts was at 30 Volts. That told me shutdown. So I measured 43, ok 5 volts OK. -15 volts NOIPE .5 volts POSITIVE. Certainly knew this was wrong, so on the schematic I see a 3500MFD cap at 30 volts. Right across the output of the bridge rectifier. Checking this it was shorted at 1meg ohm. I am going to get one from someone now. To provce it was the -15 volt supply, I removed the triple connector board and measured the 120 volt line. Sure enough it came back. The -15 had risen and the IC3 got hot to touch. Testing the transistor, it was OK. I hope the IC is OK. I will just get a new cap now. Don't plug in the scope with the cap out of the circuit. There is 20+ volts there that will probably take out the IC or the diode zener for 10Volts. Brad