Electronics > Repair

HDL Research lab military power supply LCD replacement

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Just got the result of some spontaneous action from ebay- a linear power supply, military version. Seller says it is basically working, but the LCD displays do not display very well, so it was sold as defective and i got it for a decent price. From the markings inside it has been handled decently, not beaten to death, and built in 1987- some dust inside.

This is something I also want to learn some stuff about linear power supplies, so I probably will ask some questions for the sake of understanding the underlying concepts....

But: As I could not find schematics- here the obvious question: Does anyone know some source of schematics for this baby? Or would it only be the manufacturer itself, that is probably with military equipment not eager to give them out?

Also I would be thankful for some anecdotes or deeper info of this military stuff- some looking showed that the NSN Number has been assigned around 1982 for this Power Supply.

Next weekend I will probably have some time to take care of it- like cleaning it first, getting the dust and stuff out, and then checking the cabling for damages.
After that, I will assess the issues with the LCD displays, and maybe look for replacements. Last pic is the driver PCB from the current display.

first thing i would do is replace the biege tantalum caps that are in the last pic,would check all the electrolytics for high esr also and replace if high.

Disassembled most of it for cleaning- as the PCB are in good condition, only some compressed air and wiping of metallic surfaces were needed.
Did some measurements of the conductivity of the grounding and rough checking of isolation of the high-voltage parts with a multimeter- as I could deem it good i switched it on and checked for stability of voltage at the output.

The 10-turn pots do nicely, very sensitive, but not overly jumpy, LCD display correct values, with the Voltage LCD display there are most letters correct, only some bars in those 7-segment displays seem to be completely broken, other are working fine- so for example the number behind the decimal point would be displayed wrong, like an 8 would bekome a 3...

Voltage output seems stable at open clamp situations, and LCD reading correspond with what my DMM says.

Decided to ramp it up a bit, hooked up my electronic load (rebadged Korad KEL102) and tried to go for a CC load scenario. Put the Power supply to 6V, and went on in 10mA steps.
Current display displayed nicely (without missing bars, at least what I could notice) and displayed a bit higher current than what my electronic load would note.
Current capping also seemed to work nicely, but when I went above 330 mA, no matter how far (to the end of 10 turns) the current limiter was turned, voltage wouild drop to nearly zero, and the electronic load also noticed a max of 330 mA flowing through it.
As this is way apart from the rated current of 1 A, I suspect some defective parts...

So my findings so far:
- LCD display for voltage has issues, so I will have to determine if the display itself is broken or if the managing electronics behind it (on a separate detachable PCB) are defective. As the PCBs behind those LCDs are coded differently with jumpers, I will have to see if this is due to the different stuff they have  to put out, or if they really are the same, so I could do some cross-swap
- The 230V plug is probably not original- and as there is only one fuse on the primary side of the transformer, I really would it like to be on phase
- There are no dedicated buttons or switches for turning the PSU on/off and output on/off- and even after turning it off, voltage at the output will slowly go down over several seconds to nearly zero
- secondary of the transformer puts out several voltages- 8.7V AC are going to the display units at front, at the main PCB  there will arrive approx 35V AC to ground
- at the blue ELNA electrolytic cap i measured about 47V

Further steps now would be to disassemble the heatsink and maybe draw some rough schematics for better understanding and sharing.

I am working on some hand-sketches for schematics.
First I am to go for the primary side of the transformer, the secondary electronics probably have to wait a bit- it seems that the hex/allen Screws that hold the heatsink are US standard, not metric like my tools...

Anyway, I could need some help with the transformer- it seems that is double-tapped and has multiple voltage levels on its secondary outputs.

 Rough going of wiring as follows:
Cable to wall outlet->single Fuse on line, after that to the switch in the front that switches line and neutral, this goes to a connector board. Nicely done, as it allows for some bridging of connectors. From this board on it goes to the transformer.

This routing board has 12 terminals. I numbered them 1,3,5,7,9,11 for the side coming from the main switch, 2,4,6,8,10,12 for the side facing the transformer.

1: Black wire from Switch/Line potential
3: not connected
5: Bridged to 1
7: bridged to 9
9: bridged to 7
11: light brown wire from Switch/ neutral potential

2: not connected
4: brown wire to transformer
6 grey wire to transformer
8: white wire to transformer
10: black wire to transformer
12: other black wire to transformer

Removed the secondary wiring from the pcb and powered the system up. Voltages measured are like the following:

1/5 (L)-> PE 235 V
3 ->PE 229 V
7/9 ->PE  117V
11  (N) -> PE  0 V

On the secondary side it looks like:

3 connectors coming out. Two of them providing 8.7 VAC as supply voltage to the LCD displays. Each of them is isolated against each other.
The third connector goes to the PCB board, is manufactured from Panduit and looks like an IDC connector type on the cabling side. 6 Pins, one of them PE.
Pins and voltages against PE:
1-orange 4V
2-orange  5,4V
3-black 9,5V
4-yellow 3,9V
5-yellow 15,1V
6- PE green 0V

For curiosity, I checked some voltages against line potential:
1-orange 17V
2-orange  15V
3-black 30V
4-yellow 35V
5-yellow 24V
6- PE green 235V

Conclusions I drew from this:
a) the primary side of the transformer is probably double-tapped to allow adjustment for a supply voltage of whether 110V or 220V (given its date of manufacture somewhere in 1987 according to the date code of the parts)
b) output is not floating (therefore some caution is needed, as a isolation transformer and differential probe for my oscilloscope are still on my shopping list)
c) the transformer is having multiple windings, especially for the secondary side

Question: As this is my first approach to this kind of transformer- could someone point me in the direction for some literature and how to draw it in a nice way in schematics?

Voltages here:

Neomys Sapiens:
I would not expect much problems with the manufacturer, if it still exists and if they still have something on it. It is not military technology per se, it is not even ruggedized, as you can, for example deduce from the large standing (radial) electrolytic cap. Under rugged use conditions, a no-go due to acceleration forces, as are the unsecured axial elkos. Further, the drop-shaped tantalum caps would be absent (hermetic tantalum caps with metal case and glass feedthroughs instead) and there would not be a plastic DIL on board. MAYBE it has some changes to fit into a test environment where the power supply could come from multiple manufacturers or that is easier to maintain.
So basically you are looking on a minor variation of a commercial lab power supply.


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