Help identifying part - diode ?

[phlegeton]

Hello,

I'm trying to fix a PSU from a Tektronix PB200 (BERT tester).

The unit doesn't power up, and after inspection of the PSU (Which is a real pain to get out) I noticed a fuse 250V 6A has blown. I also spotted a burn mark on the PCB. Upon further inspection I noticed a diode was blown to pieces.

The PSU type is : LWQ-200-5FF5

The only thing I can read on the diode is: G1 and possible the letters RGMK. or RG1K

When I search for RG1K the following datasheet is popping-up :

https://datasheetspdf.com/pdf-file/1229090/Vishay/RG1K/1 

It looks like the same part.. but how to be sure ?

I hope someone is able to help me identify the type of the diode.

[Gyro]

That looks like an excellent candidate as it's a high voltage fast switching diode, the sort of thing that you would expect on the primary side of an SMPS. There's no possible confusion about it being a diode or a Zener either, as Z4 next to it is clearly marked on the silkscreen. It looks as if it may be part of a snubber circuit.

Your next challenge is to find out why it blew - a shorted MOSFET is a likely suspect. They don't blow apart like that for no reason.

The first step is to locate a service manual, or if not available, draw out the schematic for that part of the PSU.

[phlegeton]

Thank you for the fast reply :-)

I can't find any service manual for it. And I also suspect the mosfets. I'll try to draw out a part of the schematic.

As I lack experience in repairing PSU, what is a good replacement for the RG1K G1 diode ? I tried searching for a replacement, but cannot find it. Can I simply replace it with a fast switching diode with a VRRM = 800 V and 1.0A Maximum average forward rectified current  ? Or does this kind of diode has a special feature ?

[Gyro]

You're welcome. 

No, I had a look in my usual places and couldn't find a manual either.

It looks to be a pretty average fast 1A diode, and doesn't appear to be avalanche rated. So yes V_RRM of 800 - 1000V, IF_AVG of 1A, I_FSM of 30A and T_RRM of 150-200ns.


Edit: On a quick distributor search, BYV26D (or E) and UF4006 / UF4007 look like possible matches.

[phlegeton]

Thanks for your help so far. I'm trying to get a fast switching diode with these specs. And hopefully I get this PSU going again :-)

[Gyro]

I just added a couple of possible candidates to my post above. 

While you're drawing up your schematic, you can check for any shorted semiconductors at the same time.

[phlegeton]

Thank you for adding diode types. This helps me a lot, and may help others as well :-)

Yes, I'm now trying to follow the the traces from the blown fuse up to the blown diode. and going to check the mosfets. And will check any semiconductor in the path as well.

 

[bdunham7]

You've identified the diode correctly, but as others have said, there's more trouble in there!

I just happened to notice that a US-based eBay seller has a number of similar PSU's dirt cheap:

https://www.ebay.com/itm/Brand-New-Nemic-Lambda-LWQ200-5225-Power-Supply-200W/233467454975?hash=item365bbea5ff:g:Q54AAOSwHwleIR-h

The parts in the center section are most likely all identical.  I don't know if it is worth it for you to purchase a whole new unit like this just to cannibalize it for parts to fix yours, but the price is very, very low and you would be able to do an en masse parts swap if you like.  Just a thought!

[phlegeton]

Yes, this sounds like a good idea! Thanks for the tip. The shipping costs are somewhat high, but looking at the parts.. this could be worth it. And As I compare both PSU's they look indeed very similar.

(And I really like TEA ) So.. I can't help myself    went ahead and bought one

[bdunham7]

I think the only difference is that the +/-12V split on this one is +/-15V on yours.  Perhaps figuring out how that voltage is set might be even easier then swapping parts.

[phlegeton]

yes I saw that difference as well. Yes that may be an option. At least I can compare, and swap parts if I need to.

[phlegeton]

Quick follow-up: I tried to repair the primary side of the PSU, and also tried to draw a schematic, but got stuck in the process. I swapped out two mosfets, but that didn't fix the problem. When I measured across the two filter caps behind the full bridge rectifier, no voltages at all. To desolder other components I had to desolder some large heatsinks.

Luckily the supply I ordered from Ebay arrived, and decided to go for plan B: Adjust the voltages rails so they PSU from Ebay puts out 15V instead of 12V. As it turns out, I only needed to swap some zener diodes, and one resistor. And the Tektronix PB200 is back to live :-)

So thank you all for helping!