Mastech power supply

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If the Mastech switcher broke because the ouput was shorted at high voltage and the power supply now puts out around 80 to 90 volts and un-adjustable, the problem is most likely a shorted output current limit FET which in our Mastech's is a TO-220 150V N channel FET.

We've replaced many of these in many Mastech power supplies. If that's it, just find one of those low Ohm 100V or 150V FETs.

The problem as I see it is that the current limit drive to the FET is not fast enough because it comes from an op-amp with too high of gate drive R and an op-amp which can't supply very much drive current anyway.

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Great thanks Spehro. All through hole part on the linears's too. (and all resistors are 1% metal film.)

I got a quote today, savings in the 10-20% range when purchased at ~50 piece quantitites. Texio in Japan still hasn't gotten back to us. More business for China.

George H.

"The Journey is the reward"

eff.com- Hide quoted text -

Good to know. In this case, it's just dead (no output).

At 50A and 30V that would be a pretty hefty MOSFET to have enough SOA to shut it down safely.

You don't happen to have a schematic do you?

The cap was just after the rectifier on the mains transformer. It has

60Hz on it. The cap you see is probably the one that blew on our supply. I replaced it with a 150V 3900uF United Chemicon from Digikey. Try look up that capacitor in Nippon Chemicon's catalog. I couldn't find a match. If you have the 50 or 60VDC max output supply, it would be a good idea to change this capacitor.

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Ahh, got it thanks. I'll give it a look see. Grumble... they don't even list the KMG series in anything near 100V/ 4700 uF... it looks to have snap in leads, but that's a different series name (KMH) and it's a different physical size.... double grumble.

I'll look at the ripple on this baby. Did your's blow up in a short time?

I've really got to load it down.. max current and voltage to get the maximum ripple? I may need some bigger resistors.

George H.

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OK at 51 V and 1.5 amps there's 75 Vdc across this cap and ~ 2 volts of ripple. Which seems pretty good.

I couldn't find it in the Nippon Chemi-conn catalog. But here it is on the web

Maybe it's made in Brazil?

George H.

We have the GWInstek's, about twice the price. They are reliable.

Cheers

I've purchased a number of items from Marlin P. Jones. They even = purchased=20 some surplus displays I had some 20 years ago. I had a somewhat bad=20 experience with an MPJA brand labeled multimeter, and other items are=20 hit-or-miss with quality and value. I am tempted to buy one of their = mini=20 power supplies that are on sale for $50:=20

I also have a Mastech HY3006D, which I've used intermittently for = several=20 years, and it seems to be well made and reliable, and is essentially the =

same as this:=20

603+PS/=20 for $110. Or a mini version for $70:=20

I also have a Mastech MS8222H LCR multimeter, and I've been mostly = pleased=20 with it, although there are occasional problems with continuity of the = leads=20 or the range switch, and the audible beep is too high a frequency and = too=20 faint for me to hear. But it was a good deal for what I paid for it = (about=20 $50 IIRC). Still available, and I was not the only one to have problems. = See=20 the review:=20

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Thanks, Except for the handle and hi/low amps switch that looks identical to the Mastech.

George H.

We mainly used our 60V supply around 36V output and have been using it for, maybe, 3 hrs per week for the past 2 years. At that output voltage, the cap sees something under 100V, probably around 75V. When I was doing some pulse amplifier tests, I had the output voltage set to 60V which puts 105V across the cap in question. This lasted for 3 days of continuous use. Fortunately, I was walking by the room where the test was running and heard a bang and saw a ball of smoke (actually, vapor from the electrolyte) above the table. Lots of cursing thinking my amplifier croaked until I saw exactly where the offending spew was coming from. A whole bunch of cursing when I saw the extent of electrolyte spew inside the case! Those supplies now have official United Chemicon (one of my favorite cap mfgrs) 3900uF, 150V caps.

Seems odd that you could only find the capacitor in Brazil. They could be dealing knockoff products. Definately worth 10 minutes to replace the offending cap.

Mark

Maybe they all come from the same place in China :-)

--
Regards, Joerg

http://www.analogconsultants.com/

I think they were making bench supplies for B&K.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

And then a "Chinese multiplication" happened :-)

--
Regards, Joerg

http://www.analogconsultants.com/

I bought that one a while ago. So far, it works fine. I changed the original 1 turn pots for 10 turn pots. A little tricky mechanically - you have to enlarge the panel holes and the pot mounting nuts are recessed making them hard to get at - but the effort was well worth it. My guess is that it is equivalent to the Lascar $90 supply that JL uses.

Ed