Author Topic: Astec LPQ154 PSU out of R&S SML01 signal gen., high frequency ripple!  (Read 1257 times)

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Offline alpherTopic starter

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Hi, I've got this R&S sml01 signal gen. on the bench, it came with flickering LCD and supposedly bad attenuator.
Powered it on and sure enough unit seems to be working but the LCD display flickers quite badly, so before doing anything else I decided to check the psu.
It's a separate  4 output Astec LPQ154 modular unit, i've attached a specs sheet for it. Measured voltages are fine
5V,12V,-12V and 24V all within specs but, when I hooked up scope to the 5V output the ripple was just over the top.
With the psu running inside R&S sig. gen. the noise on 5V line was ~4Vpp !.
When i removed the psu from the generator, hooked up on the bench with a resistive loads on the outputs (1 \$\Omega\$, 12 \$\Omega\$, 12 \$\Omega\$, and 24 \$\Omega\$ respectively) the ripple on 5V line came to be ~900mVpp, on 12V lines ~1.7Vpp similiar on 24V line.
Here are a cople of screenshoots from my scope:
5V loadded @5A :


and here 12V line loadded 1A:


Now normaly I don't repair PSUs, have very limited experience with these, but what I noticed is that it seems to be rather high quality unit, electrolytic caps are all Nichicons, there is no leaked electrolyte no bulging and meassured with my Peak ESR meter all the caps seem to be OK ( granted I meassured in circuit) ESR < 0.1 \$\Omega\$ .
Before I blindly start changing the caps, I wonder if there's something obvious that I don't know?
What may cause the excessive high frequency noise on all of the outputs at the same time?
Could it be something on the primary side of the transformer? or may a shorted diode somewhere in circuit cause these symptoms?
As I said I have little to none experience with the se switching PSUs, especially this one  :-// .
Seems way overengineered to me, schematic not available of course (at least I couldn't find one),
here are some pics of it:






Any pointers?



 

Offline Armadillo

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Re: Astec LPQ154 PSU out of R&S SML01 signal gen., high frequency ripple!
« Reply #1 on: January 16, 2019, 04:39:44 pm »
Yoo!, I don't think that is ripple, it's RF feed thru or simply RF noise. With the casing open, it can propagate everywhere.

What is the BW of the oscilloscope you set? Limit it to 20MHz and try again, yes / no?
 

Offline alpherTopic starter

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Re: Astec LPQ154 PSU out of R&S SML01 signal gen., high frequency ripple!
« Reply #2 on: January 16, 2019, 04:48:15 pm »
Thanks, the bandwith was full 100mhz, will measure tonight with 20Mhz limit, but at first when measured wth the cover on inside the generator noise was even higher.?
What gives, it's suppossed to be a high quality unit, I don't think that even with the cover removed this amount of noise is normal?

 


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