help troubleshooting HP1740A oscilloscope

Dodgydave

Joined Jun 22, 2012
11,285
Looking at the PDF manual on page 111, the inputs go to a series of interlocking switches labelled "p/o A2 chanA / B attenuator" these switches are A to E and are moved by the rotary amplitude knobs on the front( CHA & CHB)

they are probably dirty and may need cleaning


failing that were going to need a "Scope" to look at the signals ( poor joke);)
 
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Thread Starter

pastinsain

Joined Aug 25, 2012
163
Dodgedave you said
Looking at the PDF manual on page 111, the inputs go to a series of interlocking switches labelled "p/o A2 chanA / B attenuator" these switches are A to E and are moved by the rotary amplitude knobs on the front( CHA & CHB)

they are probably dirty and may need cleaning
""

your talking about the two main channel ac=gnd-dc switches correct?
I will take another look at that and If its something I could tackle.

Why will it show a sine,square wave at high frequency? if the switch is dirty?
thank you
 
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Dodgydave

Joined Jun 22, 2012
11,285
Yes, and the rotary switches also,

the AC/DC switch A is possibly the problem, as it has a 33nF capacitor on the input

(at high frequencies the capacitance has a low resistance to signals, so at low frequencies the capacitor behaves as a high value resistor, at high frequencies it behaves as a low value resistor, so it does not attenuate the signal.)
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
So That 33nf cap could be the problem? what it I took a 33nf and placed it parrallelinline with the probe? As a quick way to test? the cap should be parallel one leg to ground and the other to center probe
 

MrChips

Joined Oct 2, 2009
30,720
For AC stetting, there should be a capacitor in series with the signal.
To allow DC through, you want to short the capacitor.
 

MrChips

Joined Oct 2, 2009
30,720
Let's back up for a moment.
You say your function generator and BNC cable is OK. That is you can feed a square wave output from the function generator to another scope and the display is OK at all frequencies.

Next take the same signal from the function generator and feed it via the same BNC cable (no probe) to the HP1740A Channel A input. You get spikes on the transition edges i.e. the derivative of a square wave. At what frequencies do you get a reasonable square wave? Below what frequency do the spikes begin to appear?

Switching the AC/DC switch makes no difference?

Next you put the same signal into Channel B and the same thing happens.
It is not likely that both Channel A and Channel B would have the same problem. But then again, never assume anything.

It is not a bad 33nF capacitor. This symptom is because the switch that is supposed to bypass the capacitor is not making contact.

Clean both Channel A and Channel B AC/GND/DC switches with contact cleaner and work them quite a bit.

If that does not fix the problem we have to look somewhere else.
 

Dodgydave

Joined Jun 22, 2012
11,285
It wont be the capacitor, it will most likely be the contacts on the switches, or it will be the circuit connections to the switches.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
MrChips, the square wave starts to get corrupted at 10K and below. The sine holds in there on down on both channels.

I cannot measure any dc or ac volts at the 1V test . Or with a 9v Battery.
However, These measurments all work with my TEK 475A oscope.

Im thinking the Vertical Input board is at fault.
Also if the A vs B button is pressed, that does not work as well. You only get a
couple of stationary marks on the screen.
 
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Thread Starter

pastinsain

Joined Aug 25, 2012
163
When I turn the function generator down to X100 and below, the triangle wave looks like the square wave and the square wave looks like the example I gave in the beginning. You see a deteriation of the triangle and square wave as you go from
1mhz and below .
 

Dodgydave

Joined Jun 22, 2012
11,285
RIGHT, can you get your working scope and probe,using your sig gen inject a signal at 1khz square wave 1V, into the faulty scope chA,

and look at the signal with the working scope on the gate of Q2A and TP9 on page 111 of the pdf manual, to see what the signal is like?
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
If I can fix this Oscilloscope with a group effort on this forum, that would be a moment that will live in infamy! Im convinced any electronic device could be salvaged with this forum.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Ok Dodgydave, It may take me a while to do this. "do to my lack of experience"
How do you inject a 1kz 1v signal using a TEK 475A ?

Do you mean inject the 1kz 1v signal with the function generator?
 
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Thread Starter

pastinsain

Joined Aug 25, 2012
163
Ok Dodgydave, I found the Q2A on page 111 of the schematic.
Where is this location on the board?
Is it located on the pics I uploaded ?
Is this a test location on the board? Or a gate of a transistor?
This may take some time to find.
This is where I may get lost.

The Test Point 9 or tp9 is lt labled on the board?
 
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Dodgydave

Joined Jun 22, 2012
11,285
Ok Dodgydave, I found the Q2A on page 111 of the schematic.
Where is this location on the board?
Is it located on the pics I uploaded ?
Is this a test location on the board? Or a gate of a transistor?
This may take some time to find.
This is where I may get lost.
yes on your post #22

TP9 is listed.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Ok Dave I found it. It looks like 3 or more components are junctioned
in a common solder area. its labled TP9

1) So take the oscope and hookup a the test probe clip "red and black connectors" take the red and clip it on one of the components
at the test area.

2) the take the black ground clip and hook it to the chassis ground?

3) turn on generator .

does it matter which test probe I use on the 475a oscope?
 
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