LeCroy 9314L with dead PSU

[Fuwo]

Hello everyone,
I need a tad bit of input for repairing a 9314L.
I got it for a couple bucks untested from a scrap yard and not entirely unexpected, it doesn't turn on.

So I opened up and checked some voltages. And oh see there, the PSU is dead. No Voltage drift, no milivolts, just absolutely nothing at any of the outputs.
Obligatory mentioning: the fuses are all fine, none of the Caps or RIFAs >look< blown, nothing obvious.

Is there any known "usual faults" with those PS1715 PowerSupplies?

I'll obligatory replace all the caps and the nasty RIFAs.
What I've noticed is that Resistor R6 reaches over 100°C, closely followed by R11 and R3 (checked with a thermal camera).

[benj38]

The service manual for the 9314 is easily available online.
It contains a component-level schematic of the power supply, as well as specs and a block diagram.

Usually, diagnosing a completely dead supply is one of the easier repairs (though repairing a SMPS can always be non-trivial).

I think its time to start getting your hands dirty with good old fashioned troubleshooting

[shakalnokturn]

The PS1715 schematic is far from the most readable one, the PS1724 is one of the best to view, although general layout is the same some details vary as well as component names.

In any case and if you are able to read component values on schematic beware some of these schematics have errors, they do not match reality perfectly.

If your green hybrid PWM module is in good condition I'd advise doing some preventive protection by adding a 1W Zener diode with cathode to pin 8 of module (count the missing pin), anode to primary ground (pins 1/19). This can save you a lot of bother if you slip while probing.

The most common problems are:

Auto voltage selector (black box) on primary.
High value resistors going open on primary. You should measure about 15V on PWM module pin17, 14V on pin13, 18V on pin18. (Don't slip!!!) If not, check high value resistors to start with.
Optocoupler.
Leaky capacitors on secondary. (Check/Replace at least the two 2.2µF/50V ones near the trimpot on secondary, those are usually past it.)
Defective LM339 on secondary.
Heated interconnect between PSU and acquisition board.

If you aren't familiar with SMPS this one isn't the easiest to learn on...

[shakalnokturn]

Looked at schematic:

Forget about black box. Auto voltage selector, it is not encapsulated on this PSU but on board. R11 is part of it. Seems normal it should get hot.
R6, R3 run hot also, that's normal and also means they haven't gone open.

That leaves you with R4 (750k) resistor to check.

[Fuwo]

Thanks for the input so far shakalnokturn and benj38.

Replacing Caps is on the list. As for the Zener, which voltage would be the best?
I'll have to test R4 tomorrow, as it seems I have to solder one leg free to get a final result.
Depending on the range I set on my handheld meter, it either slowly counts up or it sets itself on 3.4 meg-ohms,
so to get a conclusive, I'm gonna loosen a leg to get it out of circuit and also use a better bench DMM.

[SiliconWizard]

For LeCroy gear, I highly recommend the following: https://groups.io/g/LeCroyOwnersGroup/
you'll find useful info there.

[Fuwo]

Hi everyone, sorry I've kept you waiting, had a work filled week and just now got to take care of everything.

@shakalnokturn Pin values are way down to what it should be. They changed a tad after I replaced R4, which was all over the place in values. Even after being removed from the board, when measuring, it starts with like 30kOhm, then jumps to 150megOhm, slowly creeps towards 200meg and then my meter bottoms out.
Checked with a second meter, had the same, so R4 went to retirement.
Caps where all replaced.

             original R4 replaced
PIN13 -   2,6mV  -  175,8mV
PIN17 -   8,73V  -   9,26V
PIN18 -    7,8V   -   8,35V

According to schematics, PIN17 gets power directly from the rectifier through R2 and R3. Both Resistors are fine, and measuring from a rectifier pin to PIN17 gave me 39,9 kOhms, which is what it should (40 kOhms) be.
So my guess is the rectifier has gone belly up somehow? On 232V~ the rectifier puts out 312V-. If I remember correctly, It should be more in the ballpark of 325V-?

PS: I also added the zener.

@SiliconWizard
Thanks for pointing the group out. They have quite a collection of Schematics, files, manuals and a lot of knowledge.

[shakalnokturn]

For the added Zener I use 15V 1W.
Diode bridge should be OK as measured.

Have you checked primary side basics:
Switching transistor not shorted?
Continuity from diode bridge "+" to transistor collector?
Continuity from diode bridge "-" to transistor emitter?

[Fuwo]

Have you checked primary side basics:
Switching transistor not shorted?

I admit, no, I haven't. Should have done that, it's apparently really not my week (nor my month for that matter)

Continuity from diode bridge "+" to transistor collector?
Continuity from diode bridge "-" to transistor emitter?

It's screaming and yapping at me from all sides in continuity check.
A meager 0.4 Ohm resistance on both lines....

And for Zener I too used a 15V 1W

[shakalnokturn]

Check pin8 (count including missing pin4) on the green PWM.
You should measure close to 0V (must be < 0.6V), if not that could explain having a low voltage on pin13.
In such a case opto is the most likely cause.

[Fuwo]

You should measure close to 0V (must be < 0.6V), if not that could explain having a low voltage on pin13.

Well, PIN8 is running on 4.2V

That should be Optocop U2 then.

[shakalnokturn]

Yep, hopefully... If you're really out of luck the green thing is fried.

[Fuwo]

Well, at this point I can only hope.
I can pick up the optos on Wednesday as conrad first has to get them from central storage, so I'll give feedback at the end of the week.

Although, greeny being dead is not necessarily the end, the LeCroy group managed to reverse engineer it, and there are gerbere files and part lists to build a replacement.
A single greeny comes at roughly 11bucks, although due to shipping costs of digikey (and free shipping above certain order costs) and minimum order at JLCPCB, it costs the same to just get the parts for 5, and probably afterwards sell the others in the group to other people in need of it.

Edit: it seems when I go through PCBway and take the 5 buck welcome bonus and assambly option I can get away with 30€ for 5

[Fuwo]

Aaand optocoupler U2/Type CNY17F2 it was.

After switching with the new one, all voltages are spot on and the scope is purring once again.

Thanks you all for the help.