Help understanding testing of 30v Zener for SMPS B+ feedback.

I am up late trying to figure out why a Switch mode will not output it's high voltage. It puts out the +5 and + 48 ok but I think it might have seperate mini SMPS setup on board to do that. i say that because I see extra, smaller SMPS regulator 5M02659R.

http://dalincom.ru/datasheet/KA5M02659R%20(5M02659R).pdf

The main one that drives the large transformer is KA3525A which is on a little side mounted daughter board.

https://www.fairchildsemi.com/datasheets/KA/KA3525A.pdf

It is receiving 7 volts on it's pin 10 which is the shutdown pin. It does so immediately on power up. Did the light bulb trick across the fuse holder and it lights for about 1 second then fades out within 1.5 seconds.

My belief is that the chip is being told to shut down from high voltage. The board has some 5K resistors across the + 60 and -60 rails right before they go out of the connector.

I believe the +60 is also known as the B+ and is the most likely place a sample voltage would be taken. Sure enough, upon further investigation is see that only that side has a trace going into 3 Zener Diodes 1N4751A connected in series. They are rated for 30v.

https://www.fairchildsemi.com/datasheets/1N/1N4751A.pdf

SO My understanding is that if the voltage got above 90V (Tolerance blah blah blah) then they would reverse bias and the voltage would continue on down that trace which just so happens to head into the mess of components near and on the daughter board containing the main IC KA3525A.

I decided to test them in case one blew. This unit has obviously been worked on as it was missing screws etc. Had a blown shotkey on the secondary side as well as the main power fet was also blown. Fixed those already. It has presets and someone may have screwed with them causing the voltage to be so high it is shutting down on start up.

Now the problem. These suckers are rated at 30V and my supply maxes out at 31.X

So I set up a series circuit with the zener and a 1K Resistor to limit the current. The diodes do drop .7 volts when forward biased so I believe that rules out an open. Even at 31 volts reverse bias they do not seem to junction although one of them does start to drop about .5 volts across the 1K resistor so I think maybe I am just not quite there yet. I figure this means they are good and not breaking down early.

Sound right to everyone else? Also...Can I put 2 power supplies in series if I need a voltage over 30? I'm only going to be using 10 to 30 ma for these tests most likely.

Thank You

Without a schematic it will be tough to figure-out. Even with a schematic, switching power supplies can be tricky to troubleshoot, partly because of the feedback... It's hard to know what's a cause and what's a symptom in a feedback-system.

I am up late trying to figure out why a Switch mode will not output it's high voltage. It puts out the +5 and + 48 ok

There's supposed to be something higher than 48V?

Yes, + and - 60v which is the part that does not work. It's an 8 channel Mixer Amp. The +48 is "Phantom Power" for a mic.

jarrod0987: Also...Can I put 2 power supplies in series if I need a voltage over 30? I'm only going to be using 10 to 30 ma for these tests most likely.

Of course you can,

only if ...

the output of at least one is isolated from ground; neither negative or positive is grounded. Perhaps even better if they are completely isolated and do not have an earth terminal on the power cord - though not many "lab". power supplies are designed with double insulation.

Alternatively get a 12 volt sla or lipo battery and put that in series with your power supply. You don't need mains powered supplies to produce enhanced voltages. In fact if you ever need a variable voltage that is outwith your supply capability and you don't need to vary below a particular voltage, you simply use standard batteries to provide the base voltage and the variable supply to provide the variance required.