help troubleshooting HP1740A oscilloscope

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Just picked up a HP1740A oscope and need to fix it. The problem is that
it will not calibrate . I have the repair manual but you need a phd to understand it. So maybe someone could give some advice

IMG_2865.JPG

this is the image I get with volts set to .5 and time set main div .2
I tryed to adjust the prob resistor but it has very little effect.
When connected to a wavetek 180, can get a sine,triangle and square wave but only at high hf 1meg setting.
Also the dc volt measure will not work. For example if connected to a nine
volt bat it will not show the image. or any dc voltage. however, it will
show the amplitude of a sine on hf.

thanks for any help
 

MrChips

Joined Oct 2, 2009
30,714
There is a trimmer capacitor in the 10x probe, not a trim resistor.
Make sure the probe is switched to 10x and adjust the trim capacitor.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Ive tryed 4 different probes all the same. One of the probes is just a bmc with two
clips. all wire no caps. all the same
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
The scope is set on dc but ac or dc same result.

But when its connected to the function generator, it works on sine,triangle and square at 1mhz settiing but the square is lost as you turn the freq down. and no DC output at any setting. however, the sine stays displayed as you slow the freq and the triangle will get distorted as well at low freq.

the generator works perfect with the Tektronix 475a. so its not that
 

Dodgydave

Joined Jun 22, 2012
11,285
Well , unless you have the settings wrong , it must be the scope at fault?

Can you input a sine wave at different frequencies,at 1vpk, 1Khz, 10Khz, 100Khz, 1Mhz into both channels separately AC then DC,
then try it on square,
 
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Thread Starter

pastinsain

Joined Aug 25, 2012
163
Absolutley the scope got a problem. now its a boat anchor. I will have to wade though the service manual on this one.

I did take the covers off and its got old original power caps. and one the the power board capacitors had been replaced at one time .Also, a rectifier was replaced looked somewhat of a hack butcher job. I have not taken the test voltages as of yet
 

Dodgydave

Joined Jun 22, 2012
11,285
Absolutley the scope got a problem. now its a boat anchor. I will have to wade though the service manual on this one.

I did take the covers off and its got old original power caps. and one the the power board capacitors had been replaced at one time .Also, a rectifier was replaced looked somewhat of a hack butcher job. I have not taken the test voltages as of yet
OH DEAR! have you got the schematics?
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
dogdydave I have tryed every conceivable configuration but still no resolve.
It will not take low volt dc test That distortion you see in the example screen shot
is the dc and ac result. I just hope its not a proprietary chip thats gone south!
 

MrChips

Joined Oct 2, 2009
30,714
Without studying your situation more carefully, I suspect your scope is measuring AC and not DC. Hence the problem is in the AC-GND-DC switch. Try switching it back and forth many times to try and clean the switch (or use a contact cleaner).
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Yep I got all the schematics and service manual in pdf format.

I need to know what board it need to look at?

Power supply?
it could be anyone of five or so boards.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
The ac -gnd-dc switches on both channels are the same so I dont think its the switches. The interior and boards are very clean with no visible problem. The scope looks in excellent condition.
 

Thread Starter

pastinsain

Joined Aug 25, 2012
163
Mrchips I will try a litlle CRC contact cleaner on the ac-dc switches and see what
happens.

Well I took the cover off and there is no way to get to the back ac-gnd-dc switches. without major surgury.
It looks like it would take days to get to them. providing I did not damage somthing else in the process.
 
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Dodgydave

Joined Jun 22, 2012
11,285
Yep I got all the schematics and service manual in pdf format.

I need to know what board it need to look at?

Power supply?
it could be anyone of five or so boards.
look where the inputs go to, there will be a common circuit that these inputs use , any chance you can post the pdf?
 
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