Fixing Power Supply HY3005D (was Modding a powersupply's DMM)

[Wh1sper]

Well, I could simply test it myself. But because I have to do some mechanical work which is not my best discipline I would be glad if someone could second my thoughts.
The calibration of the DMM modules is done with the cheap 200R trimmer.
In the Front are holes which I plan to use for a 200R multi turn trimmer cemented just behind and connected with some small awg's.
My question is now would this lead into a more unstable last digit display?
I fear because the long cables.
What do you mean?
Just trying only to end up to reverse it because the last digit noise would be annoying.
Thanks.

Please look down, the objective of the Thread has change to a repair attempt:

[SeanB]

If you replace them with good quality 200r multiturn pots you will only need to adjust once, and then you can simply just solder the 10 turn units in place of the single turn ones, adjust and be done pretty much for years.

If you are wanting to do the front panel mounted presets you can get panel mounting kits for regular 10 turn pots and they mount with a single hole. then you simply take the 3 wires and solder them to the pot, then twist the 3 wires together and lead it up to the board and solder it there. Might be more noisy but in this application probably alright. If you want use a shielded 4 core cable and connect the shield to pin 26 of the COB ( negative supply voltage for the 7106) on each board, and use 3 cores for the pot. Shield at the pot end is not connected so you have less noise. Separate shielded cables for each meter to the pot, and no connecting of wires from one board to another.

[Wh1sper]

If you replace them with good quality 200r multiturn pots you will only need to adjust once, and then you can simply just solder the 10 turn units in place of the single turn ones, adjust and be done pretty much for years.

Yes, this was my first thought, but then I saw the unused holes. ...
What to do with them...
I think these was used for analog instruments for the zero point adjustment?

If you are wanting to do the front panel mounted presets you can get panel mounting kits for regular 10 turn pots and they mount with a single hole. then you simply take the 3 wires and solder them to the pot, then twist the 3 wires together and lead it up to the board and solder it there. Might be more noisy but in this application probably alright. If you want use a shielded 4 core cable and connect the shield to pin 26 of the COB ( negative supply voltage for the 7106) on each board, and use 3 cores for the pot. Shield at the pot end is not connected so you have less noise. Separate shielded cables for each meter to the pot, and no connecting of wires from one board to another.

If definitly would this give a try.
The best would be to cement the pots  at the module and not cement the pots behind the front panel. May be I'll give this a try.

[Wh1sper]

I tried several options and end up with simply replace the single turn 200R trimmer with a multi turn trimmer.
The hole is somewhat to big and center the multi turn trimmer exactly behind was slightly overdrive for my fine mechanics skills 
And to tell the truth. One time calibration will be enough for years. 

[Wh1sper]

I'm reactivating this thread just to let you know that I discovered a huuuuge failure with this Power supply.
The first Picture shows both power supply output without any load!
WTF! I thought. This gear is oscillating!
Just a teaser, I will post more photos and my solution so far and asking later for some suggestions :-)

[Wh1sper]

Looking at the PCBs I found a TL 431 with a single Opamp 741 beneath and 2 cer. 100pf Capacistors.
I gave an handy 0.82nF C a try and bingo! the oscillation was stopped immediatly.
So I unmounted the PCB and replaced the 100pF with 390pF. This lead to an stable system.
The only issue that now persists is an short oscillation while turn up the Voltage Poti fast.
So i.e. 5V turn within 1 second to 20V the scope shows the well known 200Khz 0.5V Curve.
Not sexy but I can live with it.
I tried a few other Capacistors in parallel to other small Cer. in the hope I cold improve the situation but had no success with it so far.

The Powersupply has 2 nearly identical pcb for Out A and OUT B. So I fixed the second one the same way.

Now comes my question for more advanced girls and guys.
The regulation seems damped, now. But can I fix the unwanted short oscillation while upscaling the voltage too?
I mean surely this behavior seem not apply to an perfect designed Professional Power supply, do it?

[Wh1sper]

Ohh, must regret my "I can live with it"
It turns out, that the massive oscillation is back when some load is connected, no matter pure ohms or something real electronics 
I think I have to investigating this deeper.
If I tune the current limiter just to begin work, the oscillation is stopped, so in this part seems nothing to do.
I'll try the other capacitors that I left original, as there was no effect @ 0 load.

[Andy Watson]

There is something wrong with this area of the schematic. The emitter of V26A should go to ground, or a negative supply rail. The -ve input of N1 should be connected to somewhere else! I'm guessing that it should be connected to positive output terminal, i.e. the points labelled with the Earth/Chassis symbol.

[Wh1sper]

I do not sure about the correctnesses of the schematics not even if it is matching my PS.
So I investigated the supply further and I connect a 4700µF Capacitor in parallel to the output to see, if it helps.
Yes, this worked it out.
Now it was time to search for the C20 in the Power Supply. After some unscrewing I found it beneath the output conectors. See Pictures.
I unsoldered the C20 (2x because of double PS) and measured them with my Component tester.
Uhh, 1 has a loss factor >9% the other >7% ESR bad, too.
I did have spare capacitors at hand with old Date code '97 but they measure quite well so I gave it a try and soldered them in and made a test.
Whow! what a difference. Almost flat line on my Scope no matter what I tried.
So I hope the failure is fixed now.
I examined a few other Electrolyte C, but they all were in a good state.
Does anybody can interpret the build code for the old C's?
Interesting, that my replaced Cs are longer than the other, I do have the hope they will lasting a few years, now 

[SeanB]

Binfeng ones are made in 2008, probably around June.

[Wh1sper]

Binfeng ones are made in 2008, probably around June.

Uhh, thats surprising new!

[SeanB]

Cheap Chinese no name manufacturer, so not surprising they died fast.

[saturation]

From what I see either Mastech quality has deteriorated a lot since I bought one unit in 2011 or many folks assemble for Mastech with varying quality, or Mastech alone is a brand that is used by many makers, also of varying quality.  The parts and build quality don't look like their olders models.

https://www.eevblog.com/forum/reviews/lw-dqgs-power-supply-review-and-teardown-cautionsafety-risks!!!/msg58005/#msg58005

For cheap Chinese PSUs, it helps to tear it down before its placed in service; its good to fix what needs repairing before it fails when being used.

It may be better to buy 3 separate single channel PSUs and stack them, so the chances of getting a bad one are reduced, and if it died it won't kill one's need for power while one or more PSUs are serviced.  Single supplies are easier to service too as they are less complicated.

[Wh1sper]

From what I see either Mastech quality has deteriorated a lot since I bought one unit in 2011 or many folks assemble for Mastech with varying quality, or Mastech alone is a brand that is used by many makers, also of varying quality.  The parts and build quality don't look like their olders models.

https://www.eevblog.com/forum/reviews/lw-dqgs-power-supply-review-and-teardown-cautionsafety-risks!!!/msg58005/#msg58005

(...)

Thank You for the link, will read it later.
Yes @your photos I see unwanted soldering pearls and such.
Thought in my case it was due a former repair attempt before I catch the device.
 It seems the final control for the solder girls (and boys) isn't very exact 

[saturation]

Yes, even the better brands suffer this problem, but not as bad as the lesser brands, consider Rigol products discussed on this forum.

It seems the final control for the solder girls (and boys) isn't very exact 

[Wh1sper]

Once again with an observation.
The data sheet for my PS says

Code: [Select]

Ripple noise: CV <= 0.5 mV RMS, CC <= 3 mA RMS;This may be valid for some new gear.
I found at my PS a ripple 1.22mV rms for an load of 0.3A - 2.5A
So I'm fine with it, should I?
If Current limiting is reached it goes up a bit but not dramatically

[Wh1sper]

Even if nobody cares, here comes just an Plot of switching PS On ; Off and On.
An  impressive / destructive overshoot takes place!

[saturation]

Sorry to see, I forget now but did you also replace the filter capacitors from the full wave bridges and those around the regulators, such as C1a, C2a, C3a etc., ?

[Wh1sper]

No, I tried some and they seem to be fine.
It is not a critical one.
The beest way to connect Powersupplies is always via plugs after the PS is online...
But who knows, some day when I feeling bored I'll replace all of the C's and will see then :-)
 

[Refrigerator]

Hello.
There's this guy on YT ( electronupdate ) who has had this same issue with a similat chinese PSU and it seemed like a pretty easy fix.

[Wh1sper]

Today, I have changed the C3, which is the out 220µF to 470µF.
The 220 was quite absoltuly Ok, incl. ESR and Q were fine, the bigger µ brought a somewhat better behavior.
It is hard to measure because my Voltmeter can only measure 10 time a second. I do not have faster ones.
So I tried it a couple times.
I could measure the negative voltage, the tl431 and the output in a row. When I only have had a four channel scope or a multichannel Voltmeter..
The I'm sure I could hunt down the faulty part.