BK Precision 4051 Func. Gen Power Supply (SMPS) Repair/Changes

[BobbyK]

Hi, I need some help with the switching supply on a BK Precision Function Gen/Lab Supply I got a while back in the US. After our move to Europe, I checked, and on the back the device claimed to be 120/220 50/60Hz compatible, so I went ahead and plugged it into 220v, and nothing! No bang, no sounds, no smoke,....just silence - the device will not turn on.
I opened her up, and there is a switching supply that has a bunch of jumpers and selection points  - turns out, it was factory selected 110, and that I probably would have needed to do some mods before plugging in 220.

I connected the supply up to 110 (using a 500W step-down transformer), in its original configuration, and the output voltages are all very low (about 0.4volts, 0.2 volts, and 0.05). On the input side the voltages (120vac) are there.

The next step was to disconnect the supply board and inspect for blown fuses, burned, shorted, open resistors, charred tracks, etc. but it looks like not much power flowed through there at all - no signs of damage. Checked all the caps with DE5000, and all seemed OK with reasonable ESR. No bulges, etc. either. The board has no documentation that I can find, and no markings that show how to modify it for 220 volts. Neither can I figure out what is wrong with it in its current state (maybe it's just shut down when not connected to a load?) Any help regarding possible damage and failure modes when connecting to 220 Volts in 110 volts config, and ways to fix/reconfigure the supply for 220 would be greatly appreciated.

In the attachments, the last couple of photos show the jumper, and some pot (RJ) that might need changing. I am also thinking that the input bridge diode configuration might need changing to half the incoming power (to switch between 220/110?)

[tautech]

You may have exceeded the TL494 controller IC's max supply voltage and possibly other devices as well.

Normal fault finding for SMPS is to power the controller IC ONLY with another PSU and look for correct operation with a scope.

Time to start downloading some datasheets ...... look for typical application circuits for guidance.

[thm_w]

The blue transformer appears to power the control circuitry.
Its marked ?20V (1-5) and ?V (7-9) 1.5W. Try measuring resistance of those windings to see if any have blown. If you are lucky there is a separate 230V winding.

I'm not seeing any fuses at all on this board. Is there one on the chassis thats ok?

[BobbyK]

@thm_w: Thanks! I missed that entirely. Will check as soon as I get home.  I will check to see if there is  an easy way to change the taps used on the blue transformer so they are in series.

There is a fuse in the power line connector, and it was indeed blown. Replaced it with a properly rated 220 fuse.

Any idea if the rectifier section on the switching HV section needs changes?


@TauTech: thatnks - ouch, I hope not! Although, it should be easy to find a replacement. After I am done adjusting the control circuitry supply, I will power it separately to see if there is any life in it. Downloading the sheets for the TL494 was actually what I did rightaway - it helped identify some of the components that are surrounding it, and most designers don't stray too far from from datasheet designs when it comes to switching ic's. Thanks for the tips.

[Shock]

It's marked on the PCB 120V and 230V. In the manual it says voltage is set by a jumper. If you move the mains input to the 230V pin and install a 1A fuse does it work?

Before you do that though I'd confirm it runs on 120V first, with a 2A fuse. You should use some current limiting method like a dim bulb circuit, or ramp up voltage slowly with a fused variac so you don't kill anything when powering it up.

[BobbyK]

Quick progress report:

  I checked the line fuse, and it was blown! the weird thing is that I had checked it before and it was fine, which led me to believe that I had damaged something on the supply. A bit of wiggling of the fuse end-caps proved that the thing was intermittent...never had a fuse blow on me that way. Anyways, replaced it with a 110V 2A fuse, and it works fine on a step down transformer.

@Shock: Thanks. I'll buy a bulb (100Watts OK?) and whack it in series (with the jumper change of course) and see what happens - need to draw out some parts of the circuit first - not sure if that one jumper would do the trick...

[Shock]

Since you have it working, send an email to support and ask them.

To use the dim bulb tester you can go down to a 50W. Worst case scenario is a smaller one will glow too bright loaded to see if the inrush current has settled. But it if glows bright then dim perfect. The other thing is the SMPS may not start up anyway with a dim bulb tester, but it will at least serve it's purpose and avoid immediate fireworks.

[BobbyK]

BTW, for anyone visiting this thread in the future:

   The jumper on the power supply board of the BK Precision 4051 sets/unsets a simple diode-based voltage doubler. That way 110V is converted to 220 if needed, and everything else works fine afterwards. After initial tests with the bulb, and seeing that everything was OK, I just connected it to 220 and it works fine now. Thanks again to everyone who commented.

[Shock]

Good to hear it's back up and running.