Fixing my Mastech HY3005D-3

[TeraHz]

Hi guys, I've had this PSU for few years and recently as I was charging my lipo batteries (via charger, not directly) on both psu outputs at 17.8V @2A, my PSU went berserk and flipped one of the outputs to 50V and the other to 0. Needless to say, the charger on the 50V output died.

So I changed the input diodes, the power transistors, a bunch of caps (while I was at it) with good brand ones and managed to bring one of the channels (I think the secondary) back to life. It lost some of its upper range (only goes to about 26.4V), but I'm guessing because it needs recalibration from the new parts. However the other channel lost quite a bit of its upper range, doing 0V-0.02V

I've checked the voltage multiturn pots and they are still doing their 0-6.8kOhm range. Changed almost all of the transistors and regulators. Checked the big power caps, the diodes etc. I can't find what's busted.

I'm attaching the schematic of the PSU with some test voltages that I've done. I tried to trace the control signal from AC to the transistors which control the load. The green numbers are from the working channel, the red from the non working, as you'd expect.

I'm sure a better educated than me person can probably spot the issue right away, so I figured I'd ask.

Thanks

[Paul Price]

Looks easy enough for me. Need the V readings on the of the schematic:  N2, V22, V21, V13, V14  to give you a verdict

[TeraHz]

Here are more measurements.

[Paul Price]

I think I've localized the problem to a few parts.  V13 is the most suspicious, with V14 coming in as a close second.
Check the base-emitter, base-collector junctions carefully.

If they somehow seem to check out ok, I want you to momentarily connect a 1K resistor to the + output of the bridge supply to the base of V13 and look at the output voltage of the supply.  Whadja get?

[TeraHz]

Ok, just waking up . The two transistors check out. Desoldered them and tested them (I had already replaced them).

By bridge supply are you referring to the 7812?

[Paul Price]

The 18V measured high power V7-V10 bridge + point.

[TeraHz]

With 1K resistor to the base of v13, the relays all click and the output voltage goes to 40+.

[Paul Price]

Ok, set  output voltage control to 1/2 range of travel. Connect 1K resistor from v5 cathode to pin3 of N2

[TeraHz]

Nothing happens. Tried from 12V as well. Dead opamp?

[Paul Price]

If you disconnect V16 and the circuit behaves the same, the op-amp is kaput.

[TeraHz]

V16? That's the CC mode led connected to the output of N1?

[TeraHz]

Removing V16 makes no difference, other than when I turn the CC pot all the way down it doesn't enter CC mode.

Removing V15 (the CV led) sends the supply to 42V+

[TeraHz]

Well, it is not N1 or N2. Just swapped them between the channels and nothing changed.

[TeraHz]

I think I figured it out! The schematic isn't 100% match with the board, but it was very close. There is a big resistor on the board, which I don't see on the schematic, which appears to be between RP4 and the 0-32V line (what I wrongly called GND in the first post). This resistor was about 15kOhm on the non-working board and 470Ohms on the working board... no idea what happened there but it would explain why my output had such a tiny range. The multi turn pot is 10K so adding 15K to that line would mess things up. Changing it to 470 fixed the output. Now to do some more testing in the full range with a DMM see how accurate everything is and to put this piece back together.

Thanks Paul for the time and help!

[Paul Price]

I cannot understand the your explanation on how the resistor values changing from 4700 to 15k would  limit the output close to 0, and you talk about something way out of these values "470", but congratulations, it works! Perhaps the resistor was  actually closer to open-circuit in operation.

Success needs no explanation.

Better to test the output with a real load (and watch the action on a scope if you have one) before you put the screws back, instead a just a quick voltage test using a multimeter.

And before the screws go in, what about the other channel?

You're welcome, glad to be of help!

[TeraHz]

I changed it from 15K to 470. Not from 15K to 4.7K. I think the issue was that the circuit works over 500-10Kohm range on the  multiturn pot. A 15K resistor in series with that would make it ~15k-22K range instead, so turning the pot makes no difference it is always at its lowest voltage. Does that make sense?

Both channels work now. I'll be testing with load and will watch the scope. Unfortunately have to come up with something to do a proper load. I don't have an electrical, or even a CC dummy load.

[Paul Price]

Test Load Ideas for your power supply.

(1) One or more 3 to 10 watt 1 to 10-ohm resistors placed  in a can filled with ice and water...good for several minutes of entertainment. I always enjoy watching the tiny bubbles.
(2) A resistor from collector to base of any power transistor, even a small one on a large heatsink (a bench vise will work clamping any flat surface of the power transistor to a large piece of flat metal) will make a variable load you can use until your workbench catches fire or your little brother keels over from the smell and the smoke.
(3) Even T0-220 NPN or even PNP power transistors or power n-chan MOSFETS mounted to a 2-in sq. heatsink will survive many minutes when biased to handle 3-amps or so(100 to 1K from collector to base), if submerged in a robot's cocktail, made with plenty of ice and served in an old deep dinner plate. add enough water and chopped ice and simmer until done. Use a small PC cooling fan if you run out of ice.
(3)You can usually find one or two 3-15amp  High Voltage NPN power transistors aleady mounted  on a large heatsink in discarded apparatus, such as a discarded PC power supply which you can ravage and just rip the parts you want out quick with handy tools, for example, a chainsaw. If your pets are bothered by the noise, use use a small hammer, a large screwdriver and, if you like to get the job done fast, a hand held saber saw. The fan on it will cool the load if you power it with 12V from the power supply channel you are not testing.
(4) Car 12V lights, the higher wattage ones used in headlights or tail lights are both durable and blindingly good loads.
   

[Teneyes]

Hello, I picked up a HY3003-D3 on Ebay, and it was Blown apart, I ended up just paying shipping.
 I found debugging was better when voltage measurements were made using the Positive output (X2) as the reference point, Note that all the control logic is riding on top of the Pos, output.  In my PS the big Caps 4700uf were gone (cut out), but you may wish to also check C20 ,470uF , if you see(meas.) AC Ripple.  I my case I added a 3rd 2N3055 and convert to my HY3003D-3 into 5 amps like yours. See pics . I used a  bank of 100 watt ac lights (old style not LED!) for load
Is this a Better Schematic

[Paul Price]

It appears that the emitter of V26A wiring if definitely in error in your schematic.

[Teneyes]

It appears that the emitter of V26A wiring if definitely in error in your schematic.

  Yes Missing the connection, emitter of V26A to the output.  It was just one on the many I found.,  It was a fun mod for me, and learning experience

[Russ314]

I'm having trouble with this same unit. Can anyone tell me how to increase the current output? One channel outputs 5.7A, but the other only outputs 4.6A. Probing around, the "bad side" has a higher Vce on all the 2N3055 pass transistors, so I'm assuming I need to increase the base current somehow. The zener V11 also has a higher voltage drop on the bad side. I tried reducing R4 but that did nothing. Would a lower voltage zener help? I guess I can't really follow the circuit well enough to figure out what's happening. Maybe if the op-amp output a higher voltage, it would allow more into the base of the 2N3055's?

Anyway, as advanced payment, I'm going to teach the forum how to add fine adjustment control to this style of power supply. Simply remove the board-mounted trimpots and replace them with panel-mounted trimpots. I'm using a 10k pot for the voltage (which makes the main voltage knob go from zero to 5v, instead of zero to 30v), and a 500k pot for the current (which makes the main current knob go from zero to 100mA, instead of zero to 5A). With this simple modification, you can easily adjust the current to a tenth of an mA, which is essential for doing LED testing where you want to set it to exactly 3.1mA, or 8.7mA.

[Russ314]

I just realized the schematic in OP's post is quite a bit different than mine. My unit appears to be a knockoff (what the heck is HE Test?), but it's 15 years old, and the layout is identical to the Mastech original. As such, I've attached the schematic from Mastech's website. Notice it has the three 2N3055's in parallel, so they all have the same Vce. I figure I need to either increase the base current, or maybe replace the transistors... which I'm trying to avoid. The unit doesn't even get warm when I power a resistive load at 10A for several minutes, so I feel like I can definitely squeeze more current from it somehow. And I've never used the little standalone 3A output, so I'm hoping that means the main transformer isn't saturated. It'd be nice if I could squeeze that 3A from the main channels somehow.

[Teneyes]

Hi Russ... . I has been awhile since I modified my PS , and here are some comments.
In your testing at 10 amps .did use an independent ammeter?  .
It does take awhile for the heat sinks to warm up at 10 amps. 
   Reviewing the schematic it appears the adjustment of W5A, W8 pots set the limits of the current adjustments, along with W6 (front panel pot).  You may check those.  Also W3
For fine adjustment of voltage , I replaced front pot with a 10 turn Pot ,Voltage only.
I have a un balanced current situation in my PS (parallel mode) that I am looking into Also.
Good luck to you.

[floobydust]

I would do a current-limit calibration, three trimpots. Unfortunately, they all interact with each other and I've forgotten the sequence, so there is some back and forth and experimenting needed.

Trimpots W8, W5A interact the most and also changing front panel pot W6 (end-end value) would upset the CC calibration. I think how it works:
W3A sets the CC op-amp's offset when not in CC mode. In CV mode at minimum output voltage setpoint, I guess set W3A just above the point the CC LED lights or maybe around 0V at pin 6 is the goal. I thought it can also upset priority whereby CC does not overrule CV which is bad if you short the output.
W8 sets the PSU's maximum output current. Dangerous to go way over the design's max. for a long period of time = smoke.
W5A sets the PSU's minimum output current. I think a few mA is the best it will perform.
W6A sets the PSU's maximum output voltage.

[Russ314]

For fine adjustment of voltage , I replaced front pot with a 10 turn Pot

Yeah I did that first, but wasn't happy with the results. That reminds me though - for anyone having strange intermittent problems, try adding conductive nickel ink to the rivets on the potentiometers. My NEW replacement pots ended up giving me problems on cold days until I coated them with nickel (using an MG pen). I can't believe that style of pot uses rivets to mash the pins directly against the carbon traces... what a joke. If I had to do it all again, I'd definitely use rivet-free pots.

I have a un balanced current situation in my PS (parallel mode) that I am looking into Also.
Good luck to you.

Thanks for reminding me! That's another symptom; very uneven current in parallel mode. If I set it to 5A in parallel, I'll get 1.5A from the weak channel and 2.5A from the strong one.