Electronics > Repair

Mastech HY5020E 50v 20a Bench Supply: Economical to repair?

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richfiles:

--- Quote from: CJay on April 20, 2016, 12:48:16 pm ---With the dire warning that if it's got auto input voltage sensing then using it on 220V with a series light bulb could just trigger the rapid, violent release of more smoke.

If however it's got a nice switch to select between 120/220 on the back then yes, use a light bulb or, even better, a variac.

--- End quote ---

The real trick comes in using both. A variac will limit voltage, but doe not limit current any more than just popping the fuse under and overload. The DBT in front of a variac will auto current limit, while still giving you voltage control. Also, I presume the suggestion of using the current limited bench meter to power it, refers to attaching it passed the rectifier... Never really thought of doing that before.

My old bench supply is a Tektronix 500 series power supply module installed next to a DMM and a function gen module. It's limited to 400 mA. I'll have to learn to think entirely differently once that dual 30v/2A bench supply arrives! Not used to having anything good on my bench! :-DD

CJay:

--- Quote from: richfiles on April 20, 2016, 01:32:26 pm ---
--- Quote from: CJay on April 20, 2016, 12:48:16 pm ---With the dire warning that if it's got auto input voltage sensing then using it on 220V with a series light bulb could just trigger the rapid, violent release of more smoke.

If however it's got a nice switch to select between 120/220 on the back then yes, use a light bulb or, even better, a variac.

--- End quote ---

The real trick comes in using both. A variac will limit voltage, but doe not limit current any more than just popping the fuse under and overload. The DBT in front of a variac will auto current limit, while still giving you voltage control. Also, I presume the suggestion of using the current limited bench meter to power it, refers to attaching it passed the rectifier... Never really thought of doing that before.

My old bench supply is a Tektronix 500 series power supply module installed next to a DMM and a function gen module. It's limited to 400 mA. I'll have to learn to think entirely differently once that dual 30v/2A bench supply arrives! Not used to having anything good on my bench! :-DD

--- End quote ---

The reason for mentioning a variac is that you can wind up the supply voltage and monitor the current as the supply starts up, but of course you're right, a variac won't limit the current in any sort of sensible way so yes, using both would be useful if you had no way to monitor current.

Doesn't matter if you bypass the rectifier or not, you'll just have to suffer the forward voltage drop of the diodes but at a hundred volts it's unlikely to be significant enough to affect the operation of the supply and will work (lots of switchers will happily run on DC applied to the input as long as the voltage is high enough).

station240:
Oh good, it has transformer isolation between the destroyed mosfets and the driver ICs.
Looks fairly straightfoward to repair, new caps, mosfets and a few resistors.
I'd also replace C003 with a proper X class capacitor, given it's directly across the mains.

richfiles:
All these part upgrades and repairs remind me of what it takes to get your average harbor Freight <Insert X Here> to be remotely usable!  :-DD

To the credit of this PSU though, it did do the job, for a time. I think it died when an entire bank of DC brush motors on a test rack got reversed while attached to it! Brutal! :wtf:

albert22:
Take care with the provided schematics. I spotted a small error and may be there are others.
The positive rail at R004 should connect with R207B and R105 (a dot is missing at the crossing of two lines)
Also I may be wrong but it seems that the switching supply is not working as a tracking pre-regulator so Q401 can get very hot and even fall outside its SOA. It is rated at 200W at 25degrees. And probably is not a MOSFET suited for linear operation.

Good luck with your repair.

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