Electronics > Repair

Another Yeehaa 3010d 30V 10A PSU fault.

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BradC:
G'day all,

I have another one of the Yihua clones. It has a YH3010 PCB number. One of these :



I bought it in 2012. My notes indicate it exploded in 2013 and received a new set of relays, rectifier, output banana sockets, some internal reinforcement/re-wiring, An extra cap across the op-amp supply to stop the turn off spike and new screws.
Since then it has served duty running motors or charging batteries, has been subject to quite some abuse and has taken all I've thrown at it, including 10A @ 28V for hours on end.
Recently it has only seen a bit of work charging 12V lead acids, but last weekend I wanted to charge some 24V packs. I turned up the voltage output and it stopped at 19.5V. Cue swearing and sad face.

I grabbed a schematic and got into it. After about 3 hours of not being able to see any logical thing wrong I decided to take a look at the pots. Now this was a close enough to linear output from 0 to 19.5V and showed no signs of a damaged potentiometer.
The Voltage setting on this unit is a 50K + 5K Coarse/Fine, and when I dropped a meter across them 41K was all I could get. The 50K linear pot has become a perfectly (or close enough) linear 40K pot, and the 5K pot has become a 1K pot (again, close enough to linear). I pulled the plug from the pot board and poked a 56k resistor into it and was rewarded with about 32V.

I've seen pots drift high, get noisy, develop dead spots or just up and die, but I've *never* seen a potentiometer drift consistently low like that. I *would* never have suspected the pots (which is why I spent hours checking everything else first!).

So there you go, another Chineseium mystery solved.

Now to dismantle the pot and have a look.

Electro Detective:
That sounds like one of my typical adventures when troubleshooting shiney cheap 'TooHungLo' gear  |O 

going through the switches and pots first, before going down the cap and component track

Hope the sucker works great again after putting in some decent pots  :-+

and we await patiently to see the photos of the dissected pots  :popcorn:

and freak out at the thin, cheap inner carbon trackwork mess, broken/bent/roadkill contact arm thingies, and/or seized/dried cheap lube   :o

BradC:

--- Quote from: Electro Detective on January 12, 2018, 02:59:21 am ---Hope the sucker works great again after putting in some decent pots  :-+
--- End quote ---

It does. Back to battery charge service. Interestingly the original pots were obviously _way_ out of spec ex factory as I had to significantly adjust the internal trimmers to set the voltage and current limits.


--- Quote from: Electro Detective on January 12, 2018, 02:59:21 am ---and we await patiently to see the photos of the dissected pots  :popcorn:

and freak out at the thin, cheap inner carbon trackwork mess, broken/bent/roadkill contact arm thingies, and/or seized/dried cheap lube   :o
--- End quote ---

Honestly nothing to see. The pots look well built and equally as good as the replacement pots I bought locally. No track damage, no corrosion to note, lubricant was still present and in relatively good condition. The wiper has 3 equally spaced contacts and there are no signs of any physical deformities or damage. Just a matter of the track resistance dropping significantly.

In fact, I have pots from Futurlec (Chinese source) & Altronics (local source but probably from China) that are identical in every visual physical respect down to the stampings on the case.

Maybe they were just 'D' grade factory rejects that were "good enough" at the time they were installed. It has been so long since I used this unit anywhere near its voltage or current limits they've probably been drifting for the last 5 years and I just hadn't noticed. Although now I think about it, I'm pretty sure I was charging 24V banks a year or so ago.

SeanB:
Probably were simply releasing the binder with time, and thus compacting the resistive material into a denser film, lowering the resistance as the wiper compacted it in travel. Those cheap SRBP based pots are pretty variable in terms of stability and tolerance. 20% is pretty typical as to tolerance and stability.

BradC:
So it has done it again. Again it's the 50K pot, but this time the rest are fine.

The failure mode is different, in that the resistance of the pot varies as the wiper is moved.

So with the pot out of circuit, the track resistance varies from 47.5K (usually) to 55K (every time) relatively smoothly as the wiper is moved across the track, even though the wiper isn't connected to anything. With the pot in circuit, as the wiper is tied to one end the pot maxes out at ~47.5K, but that changes slightly depending on the way I hold my mouth and I can't find a repeatable mechanical way of altering the ultimate resistance. The only thing that is constant is with the wiper all the way anti-clockwise the track measures 55K (which is about what it measured when I installed it).

This time I'm not farting around with more cheap carbon pots (from the local Electronics emporium). I've ordered some quality TT Electronics conductive plastic pots to replace them from Mouser.

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