Author Topic: hp1740a no trace issue  (Read 2271 times)

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Offline ApplemacTopic starter

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hp1740a no trace issue
« on: November 15, 2015, 09:44:00 am »
Hi,

Was wondering if anyone could help, I was checking the rail voltage test points of my oscilloscope, which was producing a trace. All rails  matched voltages shown in the manual. Then I was checking a capacitor, and after taking the probe out heard a bang. Rechecked the rails and voltages no longer matched what was shown in the manual, and the trace on the crt was gone. The power l.e.d does not light up although I hear a hum/buz from the mains transformer.

Any ideas?
 

Offline wn1fju

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Re: hp1740a no trace issue
« Reply #1 on: November 15, 2015, 04:29:11 pm »
Chances are a capacitor popped and the power supply is in over-current or over-voltage shutdown mode.  If you heard a loud bang, it is likely that the component is visibly damaged.  Look carefully and you will probably spot it easily. 
 

Offline ApplemacTopic starter

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Re: hp1740a no trace issue
« Reply #2 on: November 15, 2015, 08:43:40 pm »
The capacitor popped because I discharged it by touching it with the dmm probe, there doesnt not seem to be any visable damage, this is whats troubling me. I was expecting to see some visable damage.
 

Offline wn1fju

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Re: hp1740a no trace issue
« Reply #3 on: November 15, 2015, 10:35:07 pm »
Oh, it sounded from your description that something banged a while after removing the probe.  Just a guess, because it's not too easy to fix test equipment remotely, but I assume the cap was one of the main filter caps after the diode bridges?  I would check the diodes and see if you blew some of them if you inadvertently shorted the cap to ground.  If there is some DC voltage present (on a particular supply), look for excessive ripple.  120 Hz means cap and 60 Hz means diode.
 

Offline ApplemacTopic starter

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Re: hp1740a no trace issue
« Reply #4 on: November 16, 2015, 03:02:45 pm »
I need some help here, I am a newbie to repairing test equipment and when you say look for excessive ripple, on tp5 the rail should be 15 volts its o.5 but how do I measure the ripple?
 

Offline wn1fju

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Re: hp1740a no trace issue
« Reply #5 on: November 16, 2015, 06:10:03 pm »
If you have another scope, set it to AC coupling and adjust the V/div setting so that you can see it (nominally 120 Hz on a standard USA P/S).  Alternatively, use your multimeter on the AC setting.  Typically, the ripple should be small, like 10-50 mV, but that is hugely variable on the P/S supply design.  Having said that, you have another problem having nothing to do with ripple - namely, your 15V measures 0.5V.  At least on the service manual I looked at (from ko4bb.com), the schematic is annotated with lots of voltage measurements.  Work your way back from the test point until you find a spot where the schematic's indicated voltage is good going into the spot and bad coming out of it.  Could be something simple like a shorted output capacitor or a fried transistor.  Or it could be much more complicated if the P/S is
actually OK and is going into current-limit shutdown mode because something in the scope circuitry is loading it down.  For the latter, you need to disconnect the P/S from the rest of the circuitry to see if the P/S returns to normal voltage.

Also, since you say you are a newbie, be careful when working on this.  There are some nasty voltages lurking around.  Also, don't go anywhere near the CRT tube since there are even nastier voltages there.
 
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Offline ApplemacTopic starter

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Re: hp1740a no trace issue
« Reply #6 on: November 16, 2015, 09:54:46 pm »
I will check it in the bext couple of days and report my findings. Thks
 

Offline ApplemacTopic starter

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Re: hp1740a no trace issue
« Reply #7 on: November 20, 2015, 11:09:42 am »

Ok seems like I have a trace now, but the trace instead of being horizontal is vertical.
 

Offline ApplemacTopic starter

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Re: hp1740a no trace issue
« Reply #8 on: November 20, 2015, 04:09:44 pm »
Problem solved I have a trace, things seem to be ok. Just need to figure out what I need to calibrate it and how to do it.
 


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