Author Topic: HP 1715A PSU repair  (Read 391 times)

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Offline JKKDev

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HP 1715A PSU repair
« on: December 11, 2018, 06:54:13 pm »
It's me again :P I just got my HP 1715A scope and since the ad stated tested and not working I decided to peak under the curtain before I even attempted to connect it to the mains voltage.

It didn't take more than a solid minute to spot a nicely charred pair of diodes. Looking at the schematic its a pair of diodes on the 5V rail used as a bridge rectifier across a center tapped transformer winding. As I was probing for the connections that were supposed to be present on said 5V rail (at this point 9.2V before regulation) I realized it was short circuited to the chassies. Since the only 2 connections on this trace were a 2600 uF cap and a power transistor I checked and sure enough the cap was shorted out.

Is it reasonable to assume that the cap shorted out and blew the diodes or should I still be concerned with what's going on down the 5V rail? (Ofc I'll check but I'd rather have another persons opinion on this)

Also on this topic... Since I'll have to replace the cap I was wondering what to get? (sure same type, same rating, 105°C) I can't seem to find any caps with this type of layout anymore. Should I just plop another elco in and wire the leads to the appropriate terminals? (the cap is one of those yellowish gray ones on this
) Should I also replace all the caps on the PSU board? (I think the answere is yes but I'd rather ask :)) EDIT: I did find the Epcos axial ones but the price is something else. 4-5x the price of the radial ones.

Another thing... The diodes are according to the manual some 400V 750mA power ones but since they operate on a 9.2V AC line I was thinking of inserting some 1N4001 just to test the PSU (no connection to the other parts of the scope so no actual current draw). Am I gonna have another explosion xD?

Thank you for all the help and have a nice day/night :)
« Last Edit: December 11, 2018, 07:09:47 pm by JKKDev »
 

Offline coromonadalix

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Re: HP 1715A PSU repair
« Reply #1 on: December 11, 2018, 11:40:50 pm »
in4001 should do the job    but check after the diodes if you have low impedance, shorts ...   if everything is fine after the cap change, i think you can try to power it up again ?

service manual : https://literature.cdn.keysight.com/litweb/pdf/01715-90903.pdf?id=1863106

Not sure about the schematics quality ?  maybe you have it already
 

Offline JKKDev

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  • Country: si
Re: HP 1715A PSU repair
« Reply #2 on: December 12, 2018, 10:32:33 am »
in4001 should do the job    but check after the diodes if you have low impedance, shorts ...   if everything is fine after the cap change, i think you can try to power it up again ?

service manual : https://literature.cdn.keysight.com/litweb/pdf/01715-90903.pdf?id=1863106

Not sure about the schematics quality ?  maybe you have it already

Since I was already ordering all the caps for the PSU I decided to add the diodes aswell. I got some 1N4004 which I hope will do the job just fine. I already have the service manual and as you said some of the schematics are garbage but it's better than nothing :) At this point I'm just praying that the transformer wasn't too abused by the fault (fuse on the primary is intact but who knows it might have been swapped...)
 


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