hp1740a no trace issue - Page 1
Forum section: electronics / repair

author: Applemac
author_rank: Regular Contributor
timestamp: November 15, 2015, 09:44:00 am
content: 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?

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 15, 2015, 04:29:11 pm
content: 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.

author: Applemac
author_rank: Regular Contributor
timestamp: November 15, 2015, 08:43:40 pm
content: 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.

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 15, 2015, 10:35:07 pm
content: 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.

author: Applemac
author_rank: Regular Contributor
timestamp: November 16, 2015, 03:02:45 pm
content: 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?

author: wn1fju
author_rank: Frequent Contributor
timestamp: November 16, 2015, 06:10:03 pm
content: 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 isactually 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.

author: Applemac
author_rank: Regular Contributor
timestamp: November 16, 2015, 09:54:46 pm
content: I will check it in the bext couple of days and report my findings. Thks

author: Applemac
author_rank: Regular Contributor
timestamp: November 20, 2015, 11:09:42 am
content: Ok seems like I have a trace now, but the trace instead of being horizontal is vertical.

author: Applemac
author_rank: Regular Contributor
timestamp: November 20, 2015, 04:09:44 pm
content: 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.