Author Topic: Another Yeehaa 3010d 30V 10A PSU fault.  (Read 839 times)

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Offline BradC

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Another Yeehaa 3010d 30V 10A PSU fault.
« on: January 09, 2018, 07:55:22 pm »
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.
« Last Edit: January 09, 2018, 09:32:56 pm by BradC »
 
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Offline Electro Detective

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #1 on: January 12, 2018, 01:59:21 pm »
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

 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #2 on: January 12, 2018, 03:58:02 pm »
Hope the sucker works great again after putting in some decent pots  :-+

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.

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

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.
 

Online SeanB

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #3 on: January 12, 2018, 06:09:21 pm »
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.
 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #4 on: July 08, 2018, 09:48:22 pm »
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.

 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #5 on: July 14, 2018, 10:28:40 pm »
Fitted 4 new pots, 4 new trimpots, new tl431s, re worked the bias on the tl431, upgraded the reference divider resistors and a bit of extra cleanup.

I'm astonished at a couple of things :

Carbon pots change value under significantly varying voltage (much like old carbon comp resistors I suppose).

Conductive plastic pots have a huge TCR (like up to 1000ppm/K). That might not be a problem when using them as a potentiometer, but when used as a variable resistor it's massive.

The manufacturer of these nasty power supplies make all sorts of component value tweaks to cope with the horrible tolerance of the factory floor reject parts they use. Every part I replaced with a quality product using the values screened on the board meant I needed to change another part that wasn't the same value it was supposed to be ( like the 91k resistor supposed to be in series with the 50k current trimpot was actually 68K. After replacing the trimpot and current setting pots it had to be changed back to 91K to get things in-spec).

I can not believe how much this unit now drifts under temperature change (like over 200mV over a 10C ambient change). That is double what it use to do with the carbon pots.

I also knocked up a nasty temperature control circuit for the fan, which has worked a treat.

At this point, on a parts and time basis I think I would have been well ahead buying a quality 10A psu. Still, it's something to do I suppose.
 

Offline wasyoungonce

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #6 on: July 15, 2018, 08:23:35 am »
How are the pots being measured ? By digital multimeter on ohms?  Many just don’t have sample rate to check pots correctly.

This is where an older moving coil meter comes to the fore.

Yeah I’ve seen bad cheap Chinese trim pots they are at almost “electrolytic capacitor plague proportions”.

In fact I’ve seen a lot of poorly made for fake Chinese parts it’s becoming a bit of a nightmare especially on fleabay


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Offline coromonadalix

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #7 on: July 16, 2018, 03:54:07 am »
my way of testing the pots, i have an dmm who has a tone following the values, very sensitive, fieldpiece hb71, had it on wavetech hd160 / beckman industrial hd160 

if the pot scratches i hear it ...
 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #8 on: July 16, 2018, 09:30:37 am »
This wasn't a case of the pot being noisy or scratchy. The track changes value depending on the position of the wiper and the applied voltage across the track.

If I put the meter across both ends of the pot (so the wiper isn't actually connected) the track varies in value as the wiper moves across it. It's repeatable in that I get ~55K with the wiper at one end and ~47.5K at the other. Putting 40V across the pot sees the track value drop below 40K. It's bizarre, but it is certainly not an artefact of my meter.

Anyway, replaced now and of course after doing all that work on the unit it slid off a stand last night and snapped a banana post, so I have to strip it down again to replace that now.
 

Offline amyk

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #9 on: July 16, 2018, 10:15:47 am »
It's repeatable in that I get ~55K with the wiper at one end and ~47.5K at the other.
The wiper shorts out a small part of the track.
 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #10 on: July 16, 2018, 10:57:34 am »
It's repeatable in that I get ~55K with the wiper at one end and ~47.5K at the other.
The wiper shorts out a small part of the track.


True, but in a linear pot wouldn't the effect be symmetrical? ie the resistance change should be the same at both ends? Rather than none at one end and significant at the other? And that still doesn't account for the value change with varying input potential.
 
 

Online xavier60

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #11 on: July 17, 2018, 09:57:36 pm »
The conductive inner ring that provides the wiper connection to the center terminal is printed on with some type of potentiometer. 
I have seen leakage develop between this inner ring and the resistive outer track or directly to the outer terminals.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu, New! FY6600 function gen.
 

Offline BradC

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #12 on: July 17, 2018, 10:28:05 pm »
The conductive inner ring that provides the wiper connection to the center terminal is printed on with some type of potentiometer. 
I have seen leakage develop between this inner ring and the resistive outer track or directly to the outer terminals.

Yeah, I just pulled it to bits and popped the wiper off. There is something funky going on with the wafer. The bare resistance is drifting up and down 5K with no interference at all. Once I try and place the wiper back on it's all over the shop. I can't measure any definitive leakage between the inner ring and the outer, but in some places it's awfully close and would not take much contamination to cause a problem.

This power supply spends 100% of its life outside. It's exposed to all sorts of humidity and contamination and these cheap pots don't seem to like damp. Hopefully the conductive plastic replacements last a bit longer.
 

Offline coromonadalix

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Re: Another Yeehaa 3010d 30V 10A PSU fault.
« Reply #13 on: July 18, 2018, 07:31:36 am »
I had pots who had a badly "punched" press fitted pins on the wafer pcb, it was intermittent, the pins where rotating a little with some apparent slack

I removed it from circuit and gently punched (pressed ) it again and my problem was gone ???
 


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