Help to repair Velleman PSU PS3005D 0 - 30 V/DC / 0 - 5 A dual led display

[clearscreen]

Greetings,
first let me thank EEVBlog owner, this is a grat source of inspiration.

I didn't see a presentation area, so here i'm.

I'm a computing engineer (that's my main job) working in software architecture and now in information security. I did also physics but i didn't do my job with that :-).

Although i'm doing a lots of numeric, i stayed in vintage and analog stuff (i'm a strange beast for my colleagues). I love to restore vintage computers (cp/m one, old pc and unix stations). I love also reel 2 reel tape (i've restored a revox A77 and a studer A80 RC MARK II - see my avatar).

I hope to learn a lot and to have great discussions on this forum.

I open this topic to ask about PSU repair because i've not so many luck with my lab instruments :-)
After my Ox8022 metrix oscilloscope i bought (fixed !) it's now my velleman PS3005D (0 - 30 V/DC / 0 - 5 A dual led display) which troubleshoot.

There is no problem in current and voltage delivery but the first digit of V and A are anormally weak (so imagine you read 1V and you sent 21V...). Neither velleman nor the seller will provide help.

So i ask your help to try to fix that because i don't like to trash things and if it's only display i think that i can perhaps solve this with your help.

Many thanks for answering.

[LateLesley]

From what I can find, it seems to be a rebadged Korad KA3005P. (source -  https://sigrok.org/wiki/Velleman_PS3005D)

I can't seem to find a service manual for it though.

Dave did do a teardown of this type of supply, but I don't know how useful that would be.

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Greatings LateLesley,
many thanks for answering.

That's funny the velleman is a Korad rebranded :-) (i don't know the reputation velleman have on the forum ? )

I've seen the video of Dave and i'll try to analyze it before put my hands in the PSU.

I'm always afraid about  power supplies (although i repaired with success the PSU of my studer a80) :-)

But in my case, the power supply part seems to be ok. The strange thing is that it's the first digit on V and also the first of A which are faulty.

I cannot see on Dave's video if each digits is single led sector display or if there are a group for A and V.

Perhaps there is also somme connection for each row ?

And the more i wait, the more the 2 digits (1st V and 1st A) are bright...  So perhaps a capacitor ?

Best regards,
Ludovic.

[LateLesley]

I'm thinking it may be a faulty transistor. I cannot find the circuit diagram for the display, but I'd bet that it is a form of 7 segment multiplex display. It'll have switching of some sort on a common anode or cathode on each 7-segment. And I suspect the first digit may have faulty switching.

I found a site which explains the technique, and might help you find the common connection on each 7-segment display, and let you concentrate on why the first digits are not working correctly.

http://embedded-lab.com/blog/lab-11-multiplexing-seven-segment-led-displays/

[SpecialK]

https://elektrotanya.com/korad_ka3005p_velleman_labps3005d_power_supply.zip/download.html#dl

This should help.

[LateLesley]

Well, Thanks to SpecialK, we can see the circuit is indeed a multiplexed display, with transistors on each digit. So on the display board, you want to check Q8 and R12.

The board connects to the main board through connector CON12, a 12 pin connector, to U10 on the main board. Since you have just one digit dim on each display, it is conceivable that it is just a bad connection on pin 9. It may be worth just trying re-seating the connector first, before digging deeper. As all the other digits are ok, a bad connection is highly likely.

[SpecialK]

Well, Thanks to SpecialK, we can see the circuit is indeed a multiplexed display, with transistors on each digit. So on the display board, you want to check Q8 and R12.

The board connects to the main board through connector CON12, a 12 pin connector, to U10 on the main board. Since you have just one digit dim on each display, it is conceivable that it is just a bad connection on pin 9. It may be worth just trying re-seating the connector first, before digging deeper. As all the other digits are ok, a bad connection is highly likely.

I see that too. I would think that maybe check that R12 reads 100 ohms. Maybe it's out of tolerance?  If the transistor needs to be turned on a little harder, maybe try jumpering a 330 ohm across the 100 ohm?  If that works then maybe a 68 ohm or 82 ohm would be something to consider?   

Q8 can be seen in this video, but not enough to see the markings:

https://youtu.be/AkbgkZZIA74?t=313 

[eblc1388]

1. Why would the designer use NPN transistors this way instead of the normal emitter to GND connection?

2. Why is the common cathode drive transistor base resistor differs(100R and 1K) in two of the four digits?

3. Why is the top group of LEDs without any transistor drive to GND? What is Q7 doing connecting GND to GND?

[SpecialK]

1 - Not sure.  To me the transistors are drawn wrong.  Wouldn't that symbol be PNP?  Or at least shouldn't the emitter be tied to ground assumng +V voltages?

2 - The TRX bench video seems to have 102 (1k ohm) for R17, R18, and R19.  But it looks like 331 (330 ohm) for the R12 in question.

3 - I wonder if Q7 grounds all ten of those LEDs.

I may look at my unit (KA3005D no PC interface) tomorrow.  Maybe I can identify some components and trace some connections.

[eblc1388]

It looks more likely to be a "doctored" schematic, especially created/prepared for the customer. Like rebranding a 74HC595 as 74HC95 to throw people off.

I respect intellectual property rights but multiplexing LEDs isn't something worth such effort to protect it. 

[floobydust]

I noticed someone put a copy of the KA3005 display board as a shared project at OSH Park. It seems to fix that schematic's errors. The two sets of five annunciator LED's common-cathodes go to Q7 collector.

[djedditt]

I noticed someone put a copy of the KA3005 display board as a shared project at OSH Park. It seems to fix that schematic's errors. The two sets of five annunciator LED's common-cathodes go to Q7 collector.

Ah, that someone is me. Sorry for replying to an old thread, but just in case someone stumbles upon this topic in an effort to repair their Korad KA3005 display: I recently uploaded the schematic, KiCad project files, gerbers and bill of materials to GitHub as well.

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Sorry i cannot properly attach picture. They are in order welow on thumbnail


Greetings,
i've taken the part from the shelf and try to fix it but i did not suceed (perhaps it's worse :-)

When i've got the problem the 2 left digits were low illuminated


I've opened the power supply and found in the K002 board (with display) that there was a "solder bridge" instead of a wire. Perhaps that was the problem (below is the board when i opened the case - see the led)
[ Specified attachment is not available ]

I've checked all the components (Q, R, D), seems fine, and i've re heated all the components (and that's my fault i think i made a very big mistake ...)

And after that i soldered a wire with trying to see where (thanks for the picture of the pcb) the connexion should be
[ Specified attachment is not available ] [ Specified attachment is not available ]

Well ... :-)
i've the 2 first digit with high illumination !!! but i've lost some other segments :-)
[ Specified attachment is not available ]

i've seen on the board that there is written on the 7segment led "QJ4403AS". I've seen the answers of LateLesley (thanks). Is the led display are the same ? I've searched with part number but i cannot find the same led display.

Many thanks for your help.
Best regards.

[LateLesley]

oh!!!  yes, I think you need a better soldering iron. It looks like you've been cooking! :-)

Ok, things I think you should check.  Q4 looks like the bottom and middle pin may be bridged.

That wire bridge looks messy, I'd remove it and retry with a little bit of solid core telephone wire. Assuming U3 isn't fried after being melted like that.

The through-hole pin connector where the wire links to - all those solder joints have too much solder, except the top left pin which looks like the pad has been ripped from the board. You can probably fix this my using a scalpel (exacto knife) to scrape some of the green solder mask away from the track to expose copper, then soldering between the copper and the pin.

I would be tempted to tell you not to touch any more, and see if you can find someone local experienced in fine soldering. I think you need some teaching and practice before tackling fine soldering like this. the pin connector needs the solder removed, then resoldered with fresh solder, using half the amount of solder. each pin has about twice what it should have. You will need to get a desoldering tool, or some solder braid, to clean up what is there, then resolder. Also try and use the heat sparingly, don't hold it on components for a long time. the goal is to heat the pin and the pad with the iron, then add just enough solder for a nice cone shaped joint (not bulged), and as soon as the solder has flowed nicely, remove the iron. it should be done in 2-3 seconds, unless you are doing large connections that suck the heat away.

I would maybe get some prototyping board, and some resistors, and practice soldering first, so you get a feel for it, before tackling this again.

[aqibi2000]

Your soldering is ok it just needs a bit of flux which would help it flow and excess would come off easily and stick to the iron.

Q4 looks fine too

No need to find someone else

[clearscreen]

Greetings All,
i made a promise to reply when the power supply will be repaired... it's done.

It will be first the thanks session !
Many thanks for all your advices. Thats right i had no experience in CMS soldering. I've not also mounted small soldering iron pan and never used flux...
That's why the first try on the card i made was ugly. 

I was able to do good things with throught the hole component like that (this is a ide cf adapter for 8 bit bus) but i've seriously to improve my skills (from zero) in SMD... (you were right LateLesley)



Then i've a discussion with djedditt who was very nice with me and explain me a lots of things, and finally i ordered 3 cards at OSH Park. The team is really pro and nice. Unfortunately the post hard the package which was broken on the weak point (i.e the display angle marks). Osh Park send me other card with free package.

Afterwards, i ordered the components on ebay and aliexpress because i've had some trouble to find the 1k Ohm, 1%, 1/8 W, 0805 : not in 1/8W, not in 0850... then i found them ...

And then the covid ... and finally i decided to make it.

So thanks youtube, for SMD tutorials .... So i bought a cheap ditial microscope (a mustool G 1200), some flux in a metal box, some ceramic pliers (very good choice indeed - just have to adjust with screw for one ).

This is my personal technic after i've seen some videos ....
1) put flux on the card
2) take big iron pan, put solder on it and gently brush pads on the card
3) put the component with the plier
4) briefly solder it on one side , adjust if necessary , then other side
5) put some solder

Below the story in photo :

The card with solder


All my instruments (i've a controlled iron solder with big and fine pan <= the one i've to mount afer lol ; and note that i've bought a desoldering gun. This one perfectly works to de solder some old components on vintage computer or tape i repair ...)


I'm putting a resistor (330r). Key point : always see right to spot where is the iron pan in order to avoid to burn the microsope end (i've set it as upper as possible and set numeric zoom)


I'm soldering the resistor after it was secured on both side.


Final result comparing old card and new one (violet)


Note that i've some spare LED and they were too big, so i've to short them with a dremel ...


And it works perfectly. i'm really astonished, it was my first SMD job


So, to conclude, really thanks for your help.
I'm not the best, but i really want to learn and try to solve problems.
To repair some vintage tape recorder (studer - see my avatar) it took 2 or 3 years but i finally made it (power supply , cards , mechanical parts ...)

djedditt, your project is very nice, and you are really nice to have put it in open download. I really do not like to trash objects (i repair everything my kids broke ... :-) ... i've five).

Best regards and take care.
Ludovic.

[LateLesley]

That's awesome! And your soldering skills have come a long way. It's not perfect, you will need to practice getting the components flat on the board. I use hot air, which makes SMD work easier, but if you only have a soldering iron, you should really clean off the pads on the board, set the component flat, solder one side, then solder the other, then you can tidy up the first side.
With hot air it's easier, as you can just heat both sides with hot air, while using tweezers to push the component down flat.

But looking at the board, as Dave would say, "it's good enough for Australia!" :-)

I'm glad you got it all working again.

Maybe you should now buy one of those DSO-138 kits, and build that. It would give you more soldering experience, and you'd then have an oscilloscope good enough for audio stuff. It does around 200kHz.
https://www.amazon.com/NooElec-DSO-138-Oscilloscope-ESD-Safe/dp/B072FW6ZJ8
https://www.aliexpress.com/item/32634533934.html
https://www.aliexpress.com/item/1005001587551095.html

Anyway, well done.

Kind regards

Lesley

[djedditt]

Well done, happy to hear it all worked out for you. And thank you for your kinds words!

[clearscreen]

Thanks Lesley and Thanks djedditt
Now that i have the microscope and tools i've to check what to do :-)

By the way i'm finishing to change the battery and some capacitor of my old persona 1600.


Configuration is wonderful 
- 8086 10Mhz + 8087
- 11 MB (yeah eleven megabyte lol) hard drive (i've serviced it with a low level formating) - it's an MFM you can do that (you can see the tracks on the platter)
- 640 KB
- a brand "old" 8/16 network card with packettool, 8bit recompiled driver and mTCP
- cf adapter in running

i've recapped this machine because had a lots of glitches on screen, remade the main video card (very heavy) all sorder points - fixed the problem.

Now well... i've learned that there is an IRC on EEvblog lol   

For my scope, i've repaired an old metrix OX 8022 (some mass contact with numeric board... it worked like that during years, and not harmed... woww, impressive). I really like this one because it have analog mode with cursors and numeric mode. And it's a differential.


but the kit you spoke is fun.

Take Care and i'll be very glad to exchange with you. Eevblog is a great forum - i'm a huge fan !.
Ludovic.