HP 1660AS XL130-3604E PSU problem

[modrobert]

I have not replaced C5 and C45, because those are not electrolytic (as far as I can tell). C5 is a white square shaped one between C22 and R7 on photo, and C45 is the red square shaped one on the right of Q2 (TIP41C).

This is the only scope I have at the moment, so no.

EDIT:

I have attached photos of C5 and C45, they are small.

[Toasty]

Okay.  Wish you had a 2nd one so you could check the input & output waveforms.

I'm sure you have already checked, but please verify any power resistors to the right of the transformers.

Also, what is the large white one R69?  It looks to be a fusible resistor.  Where is it connected to?  I don't see it in the schematic.

I'm wondering if Q2 or Q3 are damaged somehow.  Not providing enough "drive" to the transformer.  Or, (worst case) if the transformer is bad.

T

PS:  What are the numbers for the 2 rectangular blue caps next to Q3?

[modrobert]

I still have some resistors to check, and I will desolder Q2 and Q3 and do a transistor test on them.

R69 is labelled "5W120ohmJ" (I can't find it on schematics either), it is connected between ground and the positive of C34 (on page 6) which in turn leads of to pin 19 +15V on the output connector.

The rectangular blue caps close to Q3 are C20 and C21 (on page 5).

[Toasty]

It's R63(?) to the right of C34 in schematic.  Presents a small load to the 15.5V line; 0.13 amps, 2 watts.

T

[modrobert]

Yes, you are right, after checking some other digits in the schematics, what looks like '3' is his way of writing a '9'.

I'm going away from the PSU for the weekend, will be back on Monday. I can still reply here, but can't do any further testing on the board until Monday.

Toasty, thanks for helping me with this, really appreciate it.

[modrobert]

OK, after desoldering and checking a lot a components, I've reached the T3 transformer on page 6 in the schematics.

After removing the T3 transformer I have some questions...

Is it normal there is zero ohm for all windings (between pin 1,2,3 and between 5,6) on T3?

Also, after removing T3, the T2 transformer secondary winding on pin 7 to pin 8 are in effect isolated from circuit, also measuring zero ohm, normal?

[Toasty]

Zero ohms is not uncommon.  You are reading the DC resistance of the wire.  The transformer reacts to AC.

If you are reading anything from the primary windings to the secondary, then the transformer is bad.

To properly test it, you would need a ring tester.

My concern is that it is not oscillating correctly.  Coming from Q1, the UJT, should be a waveform that is in turn amplified (Q4, Q2) to the base of Q3.  If the gain on any of those 3 has been damaged, then that could be why the output voltage is low.

Without a scope it's difficult to tell.

Testing a UJT: http://www.learningelectronics.net/VA3AVR/gadgets/ujttest.htm   

2N2647 datasheet: http://www.solidstateinc.com/specsearch/specs/2N2646-47-ssi.pdf

On the second page, bottom right, is the basic circuit you have here that produces the switching oscillations.

T

PS:  I've been researching the power supply by itself.  Several are out there, but they want 3X as much as a used working scope -with- leads and manuals.  LOL   

[modrobert]

Never mind about the T3 transformer, I managed to test it on top of another transformer, and it was OK.

Today I followed up on an idea to check the -5.2V rail which is -0.01V (voltages are listed in first post) based on the assumption there is a short somewhere, and that causes the rest of the voltages to drop to half as well.

So without mains connected, I just measured resistance at the 20 pin output power connector on the PSU, and started to see a pattern where voltage output pins had a few k ohm resistance when measured against ground except -5.2V which was a dead short. With this in mind I started to desolder everything in the trace path from that -V5.2 pin (ignoring the schematics), regulation control board, transistors, electrolytic capacitors, regulators, T3 transformer (was out already), and so on, after removing each I checked for the short between -5.2V and ground pin, and the short was there even when the whole path was pretty much empty except for a few passives. Then I noticed that one of the pads for C19 (previously replaced capacitor) on the top side of the board was a bit dark from cap leakage, so I cleaned it up with rubbing alcohol and it became visible that the pad was isolated (circle shaped border) within another broad trace, measured again and now it was ~200k ohm between -5.2V pin and ground!

After that I soldered everything back, checking resistance between -5.2V and ground just to be sure the problem wasn't back. I powered on the PSU with an incandescent flash bulb in series with mains just in case (as recommended by George), measured +5V at the external control point and the rest of the voltages were OK.





Two weeks of repair finally over, learned a lot about power supplies in the process. Thanks for the help Toasty.

[Toasty]

 

Good to see it working.  Lesson learned!  Check your work!

Best of luck! 

T

[orin]

The capacitors do like to leak and cause all manner of grief in these supplies.

On mine (from a 54510A), the electrolyte affected a crowbar circuit, so the PS would start up, then immediately shut down.  The scope would make a ticking sound.

You have to give the board a really good scrub before reinstalling/replacing capacitors.  I use a DMM on ohms and put one probe on a capacitor pad and drag the other probe around the board on the solder mask; _anywhere_ that shows a reading needs scrubbing.  Repeat for all unique (i.e. not connected to one already done) pads.  If the contamination goes under a component, then you have to unsolder the component, clean or replace it and clean underneath it too.  In my case, I had to clean under one of the trimpots.  It is surprising how far the electrolyte will spray across the board.

I used IPA from MG Chemicals for cleaning my board.  It seemed to be effective enough.

[modrobert]

An update; had another problem with the same power supply (SMPS). The HP 1660AS scope/LA kept powering off sporadically, sometimes refusing to power on as well.

This time it was CR16, CR18, CR19, CR20 and CR21 zener diodes leaking too much current into the gate of A2 (C122F1) which is a reverse blocking thyristor in a "crowbar" configuration this resulted in reduced voltage on most rails except a few while A2 happily shorted 15.5V (VREF) rail to ground, also rendering A6 (LM339) useless with Vcc at 0.7V (it requires >3V).

What I did to fix this was lowering the resistance between A2 gate pin and ground from original 22 Ohm (R43) to 7 Ohm by adding another resistor between the cathode (ground) pin and gate pin of A2. Check the reversed schematics page 7 linked previously in this thread to understand what is going on.

EDIT:

Almost forgot, also had to remove S1 (MTS90A), the exotic curie effect over temperature switch reducing drive of the Q2 transistor (page 5 schematics). After desoldering to check S1 during first stages of analysing the problem it somehow got damaged, and now shorts to ground permanently, so it had to go, perhaps for the better, it couldn't take the heat in Bangkok.