BK Precision 1665 variable SMPS

[GnatGoSplat]

I have a BK Precision 1665 0-20V variable SMPS. When I got it, it had 2 burned out switching MOSFETS, IRF840A and 2 shorted 1N4744A zener diodes on the MOSFET driver circuits. The driver transistors test good. I was unable to find any more shorted semiconductors.

Problem is, after and even before replacing those parts, when using a 100W bulb in place of the fuse and I turn on the PSU, the bulb blinks and the panel lights go out. The bulb blinks at full brightness and when it is off, it is completely dark. The front panel is lit when the bulb is off, dark when bulb is lit. It blinks at a steady rate, just like the turn signal on a car.

It was doing this even with the bad parts removed, and it does the same even with the new parts installed. New parts seem not to have been harmed.

Any ideas what I should check?

[tom66]

Re: BK Precision 1665 variable SMPS

It might be that you need a more powerful bulb in series for the supply to start, or two 100W in parallel.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

I see. So you think the bulb blinking might just be a repeating pattern of the capacitor charging up and then the PSU shutting down?
I'll see if I can pick up a 200W bulb over the weekend.

[budm]

Re: BK Precision 1665 variable SMPS

If you have shorted circuits, the bulb will stay lit at full brightness, it will never go dark. When you first turn on the caps will charge up and cause the lamp to light up and then it should stop once the cap is almost fully charged, then the lamp will dim down to almost dark, but if it stay fully lit then you still have short.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

Yeah, it's behaving a bit differently than when I've used the bulb on PC power supplies. Usually the bulb lights up briefly and then dims down and stays lit dimly.
This is going to 100% brightness, shutting off abruptly, then 100% brightness, and repeating. Like flipping a switch on and off. I wondered if it could be a protection circuit kicking in or something.

[budm]

Re: BK Precision 1665 variable SMPS

It sounds like it still detecting the shorts so it shutdown and restart and just cycle like that. When MOSFET shorted out, it will usually take out the driver output of the IC that drive the Gate of the MOSFET. Do you know the IC P/N for the IC that drive the MOSFET's?

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

I'll check when I get home.
From what I could tell, it's not quite as simple as a PC PSU. The MOSFET gates are driven by a pair of transistors which is driven by the output of a small transformer. The driver transistors themselves were okay, but the zeners near them were shorted.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

It uses a TL494CN for the main SMPS. However, there is something that appears to be another SMPS and large transformer with a large TO-3P package MOSFET and TO-220 rectifier. It almost looks as though the output of it drives the input of the 2nd transformer. I'm not positive though, as I get a bit lost following all the traces.

[tom66]

Re: BK Precision 1665 variable SMPS

Can you show us some pictures?

[ben7]

Re: BK Precision 1665 variable SMPS

It could be that an output rectifier shorted, causing the MOSFETs to blow. Now it could be complaining about the overload.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

Pics attached.
The part that looked like another SMPS to me is actually the APFC circuit. I didn't expect this kind of PSU to have APFC, but I ran the P/N of the 8-pin DIP (MC33262) through Google and it's a PFC controller, so now things make a bit more sense to me.

I checked all the output rectifiers and none were shorted.

Could bad input caps cause this? They are CapXon which I've had some bad experience with on PC PSUs. I had an APFC PC PSU that blew the MOSFETs when the CapXon input cap went bad. However, it didn't behave like this one with the blinking of the bulb.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

Decided to pull the input caps and test them. Capacitance and ESR test good.

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

Finally got around to picking up a 200W bulb. It powered right up, never even saw the bulb glow, so looks like that was it. I replaced it with a fuse, and all is well.

Odd thing is, being an SMPS and also having APFC, this PSU is not particularly efficient. It seems to be using 10-11W at all times even with no load. That made me wonder if something is not quite right, but I didn't notice anything getting unusually warm inside. Heatsinks all stay pretty cool, and regulation works fine when testing the voltage display against my multimeter. I've only been able to dig up a 10W load so far, but it seemed to work perfectly with that load. I guess this is normal?

[tom66]

Re: BK Precision 1665 variable SMPS

It's because it's a cheap ass Chinese power supply sold as a premium brand. BK Precision have really started to disappoint me, they used to make good gear but they've thrown in the towel now and just rebadge stuff adding a 10 - 100% margin depending on the item.

[budm]

Re: BK Precision 1665 variable SMPS

That means the surge curent at turn on was more than 1A and cause the 100W to light up (100W lamp will draw about 0.83A). What is the size of the main AC fuse for this thing?

[GnatGoSplat]

Re: BK Precision 1665 variable SMPS

Huh, I guess if it's cheap crap, I'm glad it didn't cost me much! My old linear unregulated PSU from the 1960s only uses around 7-8W with no load which I thought was a lot.

I actually couldn't find the main fuse. I don't remember if I just misplaced it or chucked it thinking the fuse rating would be on the rear label, but it wasn't. I tried a 1A fuse which it promptly blew, so I found a 5A fuse that works.

[budm]

Re: BK Precision 1665 variable SMPS

The power supply is rated at 20VDC 10A, that is about 200Watts. So I expect the fuse to be about 3~4A.