Bench PSU - Tenma 72-8345 Repair - Quick and dirty method ! By Fraser

[Fraser]

A few years ago I purchased a faulty Tenma 72-8345 mini bench power supply.

It looked nice and compact and useful as a LAB PSU in the house.

Long story cut short, my health failed me and it has sat on the 'to-do' shelf ever since its arrival. Well this week I decided it was time to fix it or put it in the bin. With this in mind I decided upon the quick and dirty repair method that some will likely disapprove of. What must be considered though, is the low value of this power supply. Spending much time on repairing it is not justified. If it cannot be repaired quickly, it goes into the bin, simple 

The power supply is compact due to its use of a switched mode power supply topology. Such permits high current capability in a small volume and light weight. As some may know, SMPSU's are not adverse to self destruction, especially if built down to a price with cheap components. A good mains spike and its Sayonara baby 

I will now provide my working process to repair this cheap unit. It ain't pretty and does not really need a brain !

1. Check the built in fuse. In my case it had suffered catastrophic failure indicating a very high current overload event.
2. Covers off.
3. Visual inspection of the PCB for obvious burn-up's, overheated PCB areas and physical damage. None in my case.
4. A quick test of the MOSFET's with the a multimeter. - In my case both were shorted all pins. Not at all uncommon.
5. Test the drive transistors for the MOSFET's (if used) - Mine were OK which was a surprise.
6. List the electrolytic capacitors used in the Power supply and order replacements.
7. Fit new electrolytic capacitors, MOSFET's, MOSFET drive transistors and the controller for the MOSFET's
8. Check the output of the PSU for a short. - No short in my case.
9. Switch on the power supply and hopefully enjoy it starting normally.

In my power supply the two switching MOSFET's were totally shorted, all pins to all pins = Catastrophic failure. The capacitors were all of cheap, failure prone, manufacture. The power supply uses a start up power chip and a switching controller. Both were considered potentially compromised and new parts fitted. They are very cheap. The drive transistors to the MOSFET's were undamaged and tested perfect on the transistor tested. It was decided to re-use them as the MOSFET's are cheap parts anyway. There was a risk in this decision but I did not have exact replacement transistors on hand and they were carefully tested by me first.

Once all the new parts were fitted, and any suspect dry joints within the power supply were cleaned up and re-soldered, a new fuse was fitted and the power supply tested.

It started up perfectly and is working as it should.

Why is this a 'dirty' repair. Well it is a known (and sometimes controversial) technique for repairing cheap SMPSU's. You basically replace all the likely failure points in the power supply. They are often cheap to buy and quick to fit. Tracking down the failed components on the PCB can take time and with a live SMPSU, can be hazardous. If a component is cheap and likely low quality, fitting a new good quality part makes sense for the longevity of the units service life. Proving the original component faulty is really a moot point if you are going to change it out anyway due to quality concerns. Changing just the switching MOSFET's can also be a mistake if a cascade effect repeats and just blows them again due to an undetected fault in the switching controller. I tend to fit a new switching controller as a matter of course to address this possibility. The stand-by power supply in the unit is exposed to the mains supply directly and so vulnerable to mains excursions and spikes. For that reason I always change these chips. Consider the possibility that the stand-by power IC was spiked, failed and damaged the controller IC that it supplies, leading to destruction of the switching MOSFET's. That is what I mean by a 'cascade' effect.

I repaired this power supply in less than 30 minutes with just some basic checks with my multimeter and solder rework tools.

For info the two IC's  and MOSFET's that I replaced were as follows:

TNY264PN Stand-by supply IC Qty 1
TL494 Switching controller Qty 1
IRF840A Power MOSFET Qty 2

Fraser

[Fraser]

PSU after repair.....working fine.
Removed components.

[Fraser]

Just realised I did not provide a complete view of the PCB, here it is.....

[geoffwood]

Similar circumstance to yours, started using it just after warranty over.  Mine fires up fine, but seems fairly random wrt when it decides to reacts to the push-buttons, or V/A encoders. Not the buttons themselves I'm sure. Maybe I'll start by checking the PSU quality to the micro-controller (?)

Manual is a little vague I find too ....

Appreciate any other suggestions.

cheers

geoff

[CJay]

Yeah, before I'd even read the component list that gate drive transformer had told me it's a TL494 based supply, it's the absolute bog standard design, I must have repaired *hundreds* with that sort of design back in the day and the absolute best way to do it is with the blunderbus approach, there really is no point wasting time with involved diagnostics on such a simple supply as component cost is so low.

[Fraser]

CJay,

Indeed, the low value of these power supplies makes anything but the quickest repair seem a waste of time. Hobbyists may spend their time working through the unit but I had to consider the units value to me, which is quite low.
Thanks to forum member Toploser, I have several of these light weight power supplies around the house and lab

Fraser