Help fix benchtop power supply MP-3086)

Man this must be a common problem with this PSU.

r17 , 2n3055 , 741 op-amp , L7812CV had all died.

Power supply all of a sudden stopped producing current displayed showed 50v dc followed buy a buzzing and a puff of smoke.

Tested and replaced the following all happy and well again.
Thanks for the mention of R17

Don't charge bapperies without a series diode.

Hey hello , I'm a total fool when it comes to electronics, but as a tattooist, I have been using an MP-3086 DC power supply without a drama, to power my tattoo machines, for a good 2 + years. Anyway, the Current pot seems to have spat the dummy to a degree. That being, it just turns without a stop point , ( excuse my limited knowledge ) but , yeah, the thing just turns and turns, so something in the pot has no doubt broken. I have two of these supplies, and both seem to have done the same thing . I'm a busy artist, and am confident enough to pull stuff apart, and replace bits and Pieces/ soldering etc. anyway the question is , where do I find replacement pots to suit . I have one removed now, sitting in front of me rear of the pot reads , SK8+ with the negative below the + marking followed by 10 %
I've included a JPEG of the little sucker, and would be truly great-full if anyone can assist me in tracking down a replacement, or even better a less flimsy pot ?
Cheers Bobby

Maybe SK8 = 8K8, which would mean 8,800 ohm resistance. On second look, it could also be 6K8, which would mean 6,800 ohms. That is the maximum resistance of the potentiometer and can be measured between the two outer pins. The +/- is a tolerance of plus or minus 10% of that value. The pot's total resistance is an unusual value. It may be available, though, as I have not checked. The other value you need is the power capability of the pot, which is usually specified in watts. Based on the relative size in the picture, you might get by with using a similar sized replacement.

Whether the full scale value make a difference is hard to tell without knowing the circuit. However 8,800 + 10% = 9680, so a 10K pot may work. It's a little unnerving to speculate on something used on humans. Be sure to fully validate any repair you make.

John

jpanhalt said:

Maybe SK8 = 8K8, which would mean 8,800 ohm resistance. On second look, it could also be 6K8, which would mean 6,800 ohms. That is the maximum resistance of the potentiometer and can be measured between the two outer pins. The +/- is a tolerance of plus or minus 10% of that value. The pot's total resistance is an unusual value. It may be available, though, as I have not checked. The other value you need is the power capability of the pot, which is usually specified in watts. Based on the relative size in the picture, you might get by with using a similar sized replacement.

Whether the full scale value make a difference is hard to tell without knowing the circuit. However 8,800 + 10% = 9680, so a 10K pot may work. It's a little unnerving to speculate on something used on humans. Be sure to fully validate any repair you make.

John

Click to expand...

In other words ... try it on yourself first !

There appears to be a small cap blown on the circuit board. it is connected across the two ac rails that come from the transformer, then through 2 relays, then into the bridge rectifier. the cap is across the input to the bridge recifier. I have no idea why. because normally any signal smoothing is done with a cap on the output of the bridge rectifier. and its a tiny cap, so i dont know how it would fullfill that purpose even though its in parallel with the transformer, so it might be a resonant/filter of sorts. i know nothing about those guys.

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These are RF supression caps and are known as safety caps. or X and Y caps. I forget the dsignations, but one is designed for across the line and one is designed for line to ground. These MUST be replaced with safety caps.

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If you can, it's always wise to put a reversed biased diode across the outputs and you can also place one in reverse across the 2n3055. These will help prevent problems. When using the supply to charge a battery, insert a series diode.

These parts are expensive and are usually left out.

Only 4-quadrant power supplies can be directly connected to a battery.