Author Topic: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?  (Read 14348 times)

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Offline richfiles

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Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« on: April 19, 2016, 08:23:35 pm »

So, I was given a blown 50 volt 20 amp single output bench power supply. It's a "Mastech" HY5020E.

opening it up, I can see that multiple parts have violently blown. The main board appears to be single sided, and I've not delved further into it to see how bad or how far charing reaches. The fuse on the rear is NOT blown, and I don't know if it was replaced or not after the initial magic smoke release. I have not attempted to power it up yet either.

If I can find service information, my question is, is this economical to repair? Obviously, if it's just a selection of transistors, diodes, resistors and capacitors blown, then maybe cleaning it up, rebuilding any vaporized traces, and replacing the bad parts might be worth while. 50 volts at 20 Amps isn't anything to scoff at, especially for a hobbyist on a budget. It's far overkill for anything I can think of doing, but if I could get it up and running again, it'd definitely be a nice supply... Also, the excuse of anything I can think of doing does not eliminate further proofing my shop.

If this level of damage is going to take a large investment though, I'd be better off putting it toward a dual tracking supply, which I'd find far more useful. I have a pair of Tektronix dual tracking supplies in a 500 series enclosure, but those are current limited to 400 mA, and I've already been held back by that no less than 4 times just this month! I need more power;D

So, seeing the multiple violently blown parts... Salvage parts or repair? What's more economical?

I must also say I was surprised at how small the toroidal transformer was, and theres'a a rectangular one as well. Am I possibly looking at a switching supply, vs a linear? Again, I've literally only popped the 4 cover screws, and not gone any further yet, so this is all pretty preliminary.
« Last Edit: April 19, 2016, 08:25:42 pm by richfiles »
 

Online SeanB

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #1 on: April 19, 2016, 08:46:40 pm »
Switch mode power supplt, and with the whole input side and power devices very well crisped. Repairable so long as you replace everything, resistors, transistors, diodes and capacitors, and get the charred board clean. Especially those Samwha input capacitors, looks like they dumped themselves on the board and ripple did the rest. Probably best to replace with better ones, and the output side as well.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #2 on: April 19, 2016, 09:02:36 pm »
Replacing capacitors? No such thing as buying cheapie crapola caps! :-DD
If I buy a replacement, I buy the rated value, and then limit search terms by manufacturer and highest available temp rating!  ;D

Yeah, I figured it'd be a full rebuild. I'm curious if anyone's done a rebuild like this before, can mayby throw me a guestimate at what I'd be in for.

Fortunately, Mastech's website specifically says that if you want a schematic, just email them and they'll send you a PDF of your model. Nice! More manufacturers need to do this! :-+
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #3 on: April 20, 2016, 04:06:19 am »
Well, got the schematic. We'll see if I can figure it out then.
 

Online Shock

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #4 on: April 20, 2016, 04:09:29 am »
I'd say max about $15-$30 in components and about 3 hours work for a proper job removing the pcb, components, scrubbing and rinsing, testing for shorts and other components, identifying the parts and ordering them, soldering them in, testing and assembly. Any consumables you don't have on top of that. Even the dirtiest unethical repair with components on hand would be about 30mins.

Of course if you don't know what your doing it could take longer and get you electrocuted.
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Offline edpalmer42

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #5 on: April 20, 2016, 05:13:21 am »
When it comes time to power it up for the first time, I'd strongly recommend that you use the series light bulb trick to limit the current and hopefully prevent more smoke release!

Ed
 

Offline MrSlack

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #6 on: April 20, 2016, 06:00:50 am »
Pah wire and fire!

More seriously, determining the actual cause rather than replacing the components and hoping for the best is something that should be done. The cause might still be hiding at which point you're going to light another fire.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #7 on: April 20, 2016, 08:02:10 am »
You know...I really ought to build a DBT (Dim Bulb Tester). I like working with vintage calculators, so it'd be nice to current limit those when doing power on tests. I have a variac, but that just varies voltage. Shorts or over currents depend on the fuse blowing. Good advice!  :-+

Last year I rebuilt a power feed for an old Bridgeport mill. The outlet is a 4 prong split single phase 240 volt outlet in a H1--N--H2 + Earth configuration. The building it had been installed at had the outlet wired wrong, and instead of fixing the outlet, they "fixed" the cord providing the 240 volt wiring. When my boss bought it, he discovered catastrophically that the power feed used 120 volts and pulled that from what SHOULD have been one of the hots and the neutral. Instead, it got hit by both hots (ouch).  :palm:

I did succeed in cleaning the charing off that board, and rebuilding it. given the DOUBLE over voltage, I chose a very thorough rebuild. All semiconductors, all caps, and any obviously bad components. That job cost about $45 in parts, mainly cause of really spendy bolt ended diodes and drivers. No TO packages there! I took no chances though, and it was a rather simple op-amp based circuit. I could afford to spam! Glorious SPAM! It wasn't my money! >:D

It looks daunting, but I have a feeling that once I remove the PC board and unscrew the heatsinks, I'll see that it's probably manageable. I actually just snagged a nice 30 volt 2 amp dual tracing supply with an auxiliary 5 volt supply. It's more inline with the type of work I tend to do. Still, It'd be nice to fix this behemoth (to me anyway) of a power source!
« Last Edit: April 20, 2016, 08:22:33 am by richfiles »
 

Offline grumpydoc

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #8 on: April 20, 2016, 08:54:05 am »
You know...I really ought to build a DBT (Dim Bulb Tester). I like working with vintage calculators, so it'd be nice to current limit those when doing power on tests. I have a variac, but that just varies voltage. Shorts or over currents depend on the fuse blowing. Good advice!  :-+

I suspect a lot of designs would run off a 60V current limited DC supply, so if you have a 2-channel 30V supply you could use that to power the thing. Much safer than a "dim bulb" tester, if it works

I have one of a similar design in the repair pile, if I ever truely get to the end of the PM33[89]X repairs I have on the bench I'll go back to the PSU and try running it off bench supply to see how well the idea works (if at all).

 

Offline CJay

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #9 on: April 20, 2016, 12:48:16 pm »
When it comes time to power it up for the first time, I'd strongly recommend that you use the series light bulb trick to limit the current and hopefully prevent more smoke release!

Ed

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.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #10 on: April 20, 2016, 01:32:26 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.

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
 

Offline CJay

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #11 on: April 20, 2016, 02:01:29 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.

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

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).
 

Offline station240

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #12 on: April 20, 2016, 03:25:27 pm »
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.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #13 on: April 20, 2016, 07:20:56 pm »
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:
 

Offline albert22

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #14 on: April 20, 2016, 09:19:02 pm »
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.
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #15 on: April 20, 2016, 09:51:40 pm »
When you've got the supply torn apart and clean, it'd be great to snap some high-res photos of the PCB for reference and fixing the schematics.
I TEA.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #16 on: April 20, 2016, 10:11:14 pm »
I can do that. You should see the photos I've taken of my PC case build and my custom mechanical keyboard!  ;D
When I start, I'll keep this thread updated.
 

Offline grumpydoc

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #17 on: April 20, 2016, 10:58:42 pm »
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.
I have wondered about that in this design of PSU as well - but surely the thing would just blow up at high current/low voltage output settings?
 

Online Shock

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #18 on: April 20, 2016, 11:18:49 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.

I've not read much about this occurring, have you got any instances or references where this has happened and how a good SMPS failed from it? I can only seem to find it mentioned on the repairfaq.org site. Would the same occur using a variac?
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Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #19 on: April 20, 2016, 11:49:06 pm »
I'm 120v AC here (US), and that's what my input setting is set to. I do keep seeing a few references to 220v/240v here, so I thought I'd just mention that.
Man... I've never built a switching supply. Only ever worked with Linear ones. This will be my first switching supply repair, so be gentle!  ;)

The only thing I know for certain about switching power supplies, is how not to build one and throw a fake CE logo on them! :-DD Lookin' at you China...  ::)
 I know the basics of operation, the basics of how they work, etc. I just have not ever actually serviced one yet. I don't know what's typical of the controllers, and tend to follow schematics, instructions or guides to get through unknowns. I know well enough about upgrading to good high temp caps like Nichicons or Panasonics, and I can match a transistor or diode just fine. I used to do a lot of repairs as a profession, and got really good at tracking down parts lists and generating BoMs for them. These days, fewer professional repairs, but I still do this on occasion. More importantly, I will CERTAINLY take the best advice offered on more detail oriented suggestions, etc. I'll take into consideration the mention of C003 needing to be an X class cap, and I'll gladly take suggestions on Q401 This is the kind of gold I know I can find here, from people far more experienced than I am at this stuff. The whys behind everything... That's awesome too! Share why something should be different, why a specific change would be an upgrade, etc. I'll be listening. I wanna get this right. :-/O just know, it's all appreciated!  :-+
« Last Edit: April 21, 2016, 01:32:03 am by richfiles »
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #20 on: April 21, 2016, 05:32:21 am »
Just a dump of pictures. If you need any details enhanced, let me know.

Thing was covered in flux like some cheap Chinese junk yet had Nippon Chemicon caps. There were actually a few solder balls stuck down in flux. Really should have been cleaned, especially if they are gonna splurge for good caps! ??? There's some smears cause I took a little isopropyl alcohol to a spot. Decided to deal with it later though. I didn't have time for more than the pictures today. There are two smaller module PC boards that plug in, but both are double sided boards using SMD parts. I can get more pics later.

A few pads are lifted, near the parts that blew, and in one spot where they soldered current handling wire onto the traces to beef them up. There is little to no charring on the solder side. I should have no trouble cleaning up the top side to prevent shorts, the bottom looks like it barely needs work!
« Last Edit: April 21, 2016, 05:39:13 am by richfiles »
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #21 on: April 21, 2016, 05:35:38 am »
And a detail shot underneath some wires, as well as the larger of the three modules. I got a kick out of the fact that they used that big 2 row header for only 2 conductors!
 

Offline MrSlack

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #22 on: April 21, 2016, 06:17:38 am »
That's actually considerably less terrible than I thought it was going to be. Thanks for posting the details.
 

Offline Richard Head

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #23 on: April 21, 2016, 06:35:41 am »
I had a look at the schematic. They use a full bridge topology on the primary side and a single ended (forward topology) on the output. Un-F#cking-believable. :palm:
 

Offline grumpydoc

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #24 on: April 21, 2016, 11:09:24 am »
I had a look at the schematic. They use a full bridge topology on the primary side and a single ended (forward topology) on the output. Un-F#cking-believable. :palm:
Could I pick your brains a bit for an explanation of which bits go with your comment above (well, I can see where the H-Bridge is) and why what they did was less than optimal.

As above I've always been puzzled why Q401 just looked to be a linear reg with no pre-regulation. Voltage feedback looks to go to the TL494 via R302. THat doesn't feel right because if you try to get 5V @ 20A out of this PSU (which should be well within its capability) the FET is going to try to disspiate 900W which doesn't add up.

Anything to help increase my understanding of the design would be great!
 

Online Andy Watson

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #25 on: April 21, 2016, 11:43:29 am »
As above I've always been puzzled why Q401 just looked to be a linear reg with no pre-regulation.
Would it make more sense if the label at the bottom of HY012 read "U+". I can't see where S4 connects to, but all the other sections appear to be referenced to U+.
 

Offline albert22

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #26 on: April 21, 2016, 01:47:57 pm »
I dont know where S4 is. In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
So I think that referencing it to U+ will not work with those values of voltage FB.
As grumpydoc said the power at the MOSFET will be huge (1100W). No way this can work, a pre regulator is required.  I suspect that the provided schematic is purposely wrong.
I also would like to know why a full bridge/forward converter is a bad idea and please what is the function of L201 ?
 
 
 

Offline oldway

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #27 on: April 21, 2016, 02:10:09 pm »
I dont know where S4 is. In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
So I think that referencing it to U+ will not work with those values of voltage FB.
As grumpydoc said the power at the MOSFET will be huge (1100W). No way this can work, a pre regulator is required.  I suspect that the provided schematic is purposely wrong.
I also would like to know why a full bridge/forward converter is a bad idea and please what is the function of L201 ?
I agree, it seems there are serious mistakes in this schematic.
I have never seen an electrolytic capacitor directly connected to the secondary of a switching power supply as C106 in H2 of auxiliary power supply.
It obviously lacks as a connection point between the rectified + (+ of the capacitor C007A / B) and Q203B and T101 connection.
 

Offline Theobald

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #28 on: April 21, 2016, 02:20:26 pm »
Same for H1, C107 is wrong and the H-bridge supply (VCC) should be connected to the D105 cathode.
Funny.

Theo
Edit: In fact they're twins chemicals: C106 & C106' and C107 & C107'. In the real world they're not across the windings. I hope.
« Last Edit: April 21, 2016, 02:35:59 pm by Theobald »
 

Online Andy Watson

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #29 on: April 21, 2016, 02:34:21 pm »
I dont know where S4 is.
S4 is to the left of U301. It shows where the "GND" for U301 is connected, but the other end of S4 only appears  to connect through to "HY022 on PCB", which I can't see anywhere!
I was suggesting that "U-" was a typo and it should say "U+" under U301.

Quote
In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
I make it 3.9V.
 

Offline oldway

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #30 on: April 21, 2016, 02:37:33 pm »
GROUND (GND) of U301 must be referenced to U+ and not to U-

So SMPS would act as a preregulator maintaining a fixed voltage drop on Q401 of 3.91V
 

Offline albert22

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #31 on: April 21, 2016, 05:34:19 pm »
So that was S4  |O. Sorry. Certainly connecting u301 gnd to U+ will work as a pre regulator but I dont understand how do you get 3.9V with the values for R302 and R304 shown in the schematic.  Another error in the schematic or I am not seeing something obvious (again)?

Please someone comment on L201. never saw that on a forward converter. I guess that it should be in series between C209 and C205 like L202 but on the + side.
 

Online Andy Watson

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #32 on: April 21, 2016, 06:31:02 pm »
how do you get 3.9V with the values for R302 and R304 shown in the schematic.
U301 produces Vref of 5V on pin 14 (according to datasheet). This is the source of potential divider R309 and R305 (100k, 3k3) producing 160mV. This drives the negative input of error amplifier, pin 2, which compares it against the feedback voltage on pin 1. 159mV x 49k / 2k = 3.9V.
 
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Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #33 on: April 21, 2016, 06:37:24 pm »
Desoldered all the large semiconductors and blown/burned parts, cleaned up the board, and took some more photos. That fan header... shorted with a solder bridge, and the fan is soldered directly to the PC board elsewhere. Q403 was unpopulated. R204 has 4 parallel shunt wires, while the schematic says 3. Many of the transformer's labels show different part numbers than what the schematic shows as well. Schematic also Labels U+ and U- as 30v. This model is supped to do 50 volts. I'm wondering if they took a general schematic and altered it for different models... and maybe missed some details.  :-\  :palm:

I also gave that thing a bath! Isopropyl alcohol and a brush, top and bottom, then let it sit on the oven rack for a bit at the warm setting, to bake out anything that remained.
(No wife... I'm allowed to do that with the oven!)  :-DD
« Last Edit: April 21, 2016, 07:07:05 pm by richfiles »
 
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Offline EPTech

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #34 on: April 21, 2016, 07:30:07 pm »
Hi There,

This is a very interesting design. The U- is a floating reference. Notice that the TL494 is also referenced to U-. The TL494 will actually keep the voltage before Q401 slightly above U+ to maintain a current flow that results in the set voltage voltage across the load. This supply actually works in CC mode. The supply actually requires a minimal load, in this case R203.
So the SMPS part regulates the voltage so that Q401 can maintain and regulate a current trough the load with minimal power dissipation.

If you plan on repairing this supply, I would recommend you put some 15V transient suppressors or zener diodes across the primary side fets to protect their gates. Because the pulse transformer probably killed them. The 15 ohm resistors are not sufficient for dampening that.

Good luck.
Kind greetings,

Pascal.
 

Offline albert22

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #35 on: April 21, 2016, 07:31:11 pm »
how do you get 3.9V with the values for R302 and R304 shown in the schematic.
U301 produces Vref of 5V on pin 14 (according to datasheet). This is the source of potential divider R309 and R305 (100k, 3k3) producing 160mV. This drives the negative input of error amplifier, pin 2, which compares it against the feedback voltage on pin 1. 159mV x 49k / 2k = 3.9V.

 |O |O |O.  Fool mistake. And I cannot blame, coffee, beer or wine for it. I took for granted 2.5v on p1 because that was used in a similar circuit. Shame.
Thanks
 

Offline edpalmer42

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #36 on: April 21, 2016, 07:51:47 pm »
Which components actually blew up - as in let the smoke out?  There are probably more than failed silently, but you can get useful info by tracing the smoke.

Ed
 

Offline Seekonk

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #37 on: April 21, 2016, 10:21:40 pm »
Just do it!   It is a simple H bridge transformer coupled.  Replace all the FET and gate resistors.  Before replacements installed power up to see if other supply is still working and that driver transistors were not damaged.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #38 on: April 22, 2016, 12:09:50 am »
 :(R206A, R206B, R208A, R208B, and Q202A, Q202B, Q203A, Q203B all blew up. Violently. Only one FET managed to not physically detonate it's die. The others all had an out of body experience. :o  :-BROKE

R207A, R207B, R209A, R209B all test okay, at 27 ohms, but have some significant charing, that goes into their housings. I'm replacing them regardless of them passing.

R002, R003, R004, and R202 Passed and cleaned up just fine (R202 was clean, the others just had a light covering of soot. R002 will need a new piece of heatshrink around it).

C206A, C206B, C207A, C207B measure 2.1 nf consistently. They all appear to pass, but were pretty toasted in soot. I'll likely replace them to play it safe.

C003 and C004 are the caps across the mains. C001 and C002 are between mains and earth. I can replace these with your recommendations. If any other caps are sitting in critical places, and ought to be upgraded, like anything across the primary and secondary, I'm all ears.

D1-4 (actually 8 diodes, as they are in parallel pairs on my unit... something the schematic does not show) all tested okay.

C006A, C006B, C007A, C007B measure 765-767 µf consistently, well within the tolerance of these caps, but the two closest to the heatsink had ejected material from the resistors and MOSFETs melt the vinyl label. The two furthest from the heatsink had soot that could be wiped off easily. These big caps are all Nippon Chemicon caps. You can see on the PC board where one of the resistors that blew ejected material at the two closest caps. I usually order from Digikey, since I live in the same state as their location. Even cheap shipping shows up in a day or two. It'd be about $16 to replace these with equivalent Panasonic or $28 to go with Nichicon. Both options are uprated (2000 and 5000 hours @ 105°C, respectively). Honestly, I'll probably just go with the Panasonic caps. I've never had trouble with them, and it's already double the rating of the old ones. I figure I might as well hit all four caps.

C203A and C203B loved very clean. I left them on the PC board and tested them in parallel. they were 4.34 µf in circuit. since they are 2.2 µf caps, that's about dead on, well within tolerance.

I tested Q201, which is a half bridge diode pack. It appears to pass.

I did not do any significant testing of Q401 or SCR001. Q401 was a 5N2008, and SCR001 was an S6055M. S6055MTP is the replacement for S6055M (which is obsolete). Newark and Digikey both have no stock and a 250 part minimum. Mouser has no stock and a 10 week lead time, but will sell singles for $4.98... I have no idea how to test an SCR. I'll have to look that up, and see if I have what's necessary. Might have to order from Mouser if I need it. The 5N2008 is a different story. The 75NF75 listed on the thematic is a much smaller package component than the one in my unit. There isn't much showing up for 5N2008 either. Looks like a China special... Alibaba has s some, and Ebay seems to have ONE seller, who thinks it's an SCR. :palm:

I hope those last two are okay, but I'm not certain how to properly test either part. They aren't shorted, at least.
« Last Edit: April 22, 2016, 12:54:36 am by richfiles »
 

Offline Richard Head

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #39 on: April 22, 2016, 06:57:58 am »
This circuit is an utter joke.
I'll list the errors I can see.

1. There is a full bridge switching arrangement on the primary side of the HF transformer but only a single ended forward converter output topology. This means that the transformer is being used uni-directionally but with all the complexity of a double ended topology. Why not just use a forward converter? NOT TRUE. Actually a current doubler topology. That's why there's L201.
2. Auxiliary PSU output H1 connects directly to C107 with no rectifier at all! Can't work.
3. The voltage feedback resistors (R302/R304) of the pre-regulator produce a voltage at the error amp input of 160mV as reported earlier! Why so low? (I believe the pre-regulator is meant to referenced to U-)
4. Purpose of L201 is unknown to me. IGNORE
5. How does U301 get its Vcc? Missing connection.
6. What's the purpose of D? Is it a panel meter? If so it'll indicate the duty cycle of the pre-regulator. Why?

I bet there are plenty of other serious issues that I haven't noticed also.

Feel free to add to the list.
« Last Edit: April 22, 2016, 09:27:14 am by Richard Head »
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #40 on: April 22, 2016, 07:53:34 am »
(D) took me forever to figure out too, but I think it's the fan. The fan is powered by it's own transformer winding, and has a diode, cap, resistor and inductor isolated from the entire rest of the circuit. It has absolutely nothing to do with the header marked "fan"... That header is shorted on the bottom of the board...  ???

I don't understand the fine workings of switching supplies well enough to follow this circuit. I had no reservations that this wasn't gonna be some crap Chinese thing. The two questions are obviously first, can it be fixed. I think that's not going to be hard to do. The other question is can it be fixed? I mean, can dumb stuff be corrected, so this will be reliable in the long term? Who knows? I got it for free, so if I can fix it cheap, that's awesome. It's SO OVERKILL though for anything I actually have plans to do.

If anything, I seem to have given the forum a nice challenge to rip a new one dig into!  ;)
« Last Edit: April 23, 2016, 07:03:38 am by richfiles »
 

Offline cs.dk

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #41 on: April 22, 2016, 09:06:50 am »
I suspect a lot of designs would run off a 60V current limited DC supply, so if you have a 2-channel 30V supply you could use that to power the thing. Much safer than a "dim bulb" tester, if it works

Sorry for offtopic;
But why on earth is it so difficult to find a "LAB PSU" that outputs 230VAC with current limiter and other features we know from all our DC PSU's?
 

Online TurboTom

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #42 on: April 22, 2016, 09:11:39 am »
It's not only the schematic that's questionable, the PCB layout is the same nightmare. Just look at the creepage distance of the negative rectified primary voltage to one pad of the secondary shunt array. Is it substantially more than a millimeter? On the control supply plug-in board, it's even worse. Adjacent terminals of the transformer are life and isolated side without any additional insulation, even with flux residues in between. An arc-over is only a quastion of time. If I'd been given that thing, I would have dumped it immediately. Maybe kept the enclosure with the displays for some project of my own but I definitely wouldn't use it in its original configuration as a lab power supply!

Btw, the power ooutput rectification is symmetrical and not single-ended as Richard mentioned. It's drawn a little awkward in the schematic but actually, a "virtual center tap" of the power transformer secondary is produced by L201 and L202 each of which carries half the DC current but basically blocks the AC from the power transformer's secondary. Quite an unusual design but probably nothing wrong with it.

Cheers,
Thomas
« Last Edit: April 22, 2016, 09:13:57 am by TurboTom »
 
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Offline Richard Head

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #43 on: April 22, 2016, 09:23:28 am »
Turbo Tom
You are quite correct about the output inductor. It's the so called current doubler configuration. I didn't recognize it the way it was drawn. It's actually very effective for high current outputs.
I'll have to update my comments.
 
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Online Andy Watson

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #44 on: April 22, 2016, 09:44:50 am »
This circuit is an utter joke.
Too right ;) It's hard to imagine how anything useful could be produced from such a circuit - I wonder if it's been deliberately obfuscated?

Quote
2. Auxiliary PSU output H1 connects directly to C107 with no rectifier at all! Can't work.
There is something very wrong with this part of the circuit. Perhaps there should be a note that says "arrange these components to form a rectifier and filter."  If you do that you will also answer this:
Quote
5. How does U301 get its Vcc? Missing connection.



 

Offline CJay

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #45 on: April 22, 2016, 09:47:03 am »
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.

I've not read much about this occurring, have you got any instances or references where this has happened and how a good SMPS failed from it? I can only seem to find it mentioned on the repairfaq.org site. Would the same occur using a variac?

Personal experience from a couple of years of my life when I worked in a PC repair company, some power supplies aren't 'wide input range' but have a switch to alter the smoothing capacitor configuration (turns it into a doubler configuration and yes, they used to be expensive enough to be worth repairing).

That switch can either be a physical panel mounted one or an electronic one. The electronic ones can get latched into 120V operation and winding the input voltage up higher can have dire consequences for the PSU.

Which is quite frustrating when you've spent an hour drilling out rivets, cleaning up soot and replacing a couple of dozen components.

Some PSUs were (may still be) too sensitive to dips and spikes on the mains supply and latched to 120V in normal operation, we used to call them one shot brownout detectors.

A supply that has a single smoothing capacitor and is rated for 100-264V input is a wide range supply, a switched supply will have series connected smoothing capacitors and the 'centre' connection between them is where the input voltage switch is connected.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #46 on: April 22, 2016, 12:30:00 pm »
Confirming, this supply has a manual 120/240 switch.

Hearing of the issues in the design instills me with such "Damn it, China!".
 

Offline MrSlack

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #47 on: April 22, 2016, 02:25:53 pm »
It's the Chinese design ethic. Steal someone else's well thought out design, go on a parts minimalism trip removing every bit of protection and reliability so it works long enough to sell it, slap times new Roman on it and whip an army of cheap labour to knock it out.

Like Sinclair but with less black and red and a different typeface.
 

Offline CJay

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #48 on: April 22, 2016, 03:22:50 pm »
Confirming, this supply has a manual 120/240 switch.

Hearing of the issues in the design instills me with such "Damn it, China!".

A proper switch, excellent, one thing less to worry about.

Looks to me that you should be able to test the various boards in there one at a time and build it up as you verify each one.

I think you can ignore Q401 for the moment, leave it out of circuit, it's being used as a linear pass regulator.

HY023 is an independent power supply, used for soft switching the main supply, powering the PWM controller, linear regulator drive and logic/display so that should power up if you can feed it rectified and smoothed line voltage or a suitable alternative.

HY012 will produce dual, complementary PWM modulated signals on V01 and V02 if you give it, I suspect, 12V or thereabouts on VCC, you should be able to vary the pulsewidth by varying the voltage on V1 (VI?).

Make sure the PWM drive is making it through the drive transformer to the gates of Q202, 203 A and B

Definitely add some 15V zeners to the gates of the primary MOSFETS to ground as someone else suggested, they won't save anything if it goes bang again but they'll help stop it going bang.

Once you get to this point you should be able to power up the main supply and get a non variable output on the drain of Q401. I'd guess somewhere around 60V.

You'll need to test HY013 and HY014 as one unit but again, you should be able to do that without the rest of the supply running. I'm guessing they're not going to be faulty but you don't need much more than 12V to do that.

Front panel meters are powered by HY023, they're real simple.


 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #49 on: April 23, 2016, 06:59:36 am »
Thanks CJay for that finely detailed troubleshooting information!

I won't be able to get to it immediately. I had the other day off, so I was unusually active with this, and I also just got an order for some cables that I build (the cable provides power and commutation for a brushless motor in a surgical tool. The cable is built to be able to withstand being autoclaved repeatedly to sterilize it). I'll have to do those first. Hopefully, by the time I finish those, my dual tracking power supply that I bought will have shown up, and I can use that for the testing...

Heck... maybe i should open THAT ONE up and have a look to see if it needs any safety updates. It's a used unit, from a liquidation sale. All I could afford at the moment. Just got SO TIRED of current limiting my Tektronix 500 series supplies. 400 mA is NOT reasonable for a bench supply to max out at. Fine for breadboarding a few logic chips or an op-amp on a breadboard, but not anything more!

I think I can follow most of what you said. I tend to work visually, hence why I do stuff like annotate pictures of the PC board and stuff. You broke this down into a nice bullet point list of tasks, and that's really awesome!  :-+

My boss had a Sorensen DCS 300-3.5 power supply on hand that he uses in place of this unit. He has another Sorensen supply that's 3 or 4 times thicker. I don't recall what that model was It's a Sorensen Elgar SGA400/12C-0AAA (Input 187-242V 3 phase, Output 0-400 Volts DC @ 0-12 Amps). It's a beast of a machine, probably similar to the size of four of these units. Both are rack mount units. As far as I know, he's not even using the big one yet (He's not, he wants to sell it). Needless to say, with those beastly power supplies, I don't think he has a need for this sad little Mastech unit, especially in a production setting, if it's gonna just up and pop again someday. I kinda wanna fix it, and I kinda don't even have a clue what to do with it if I succeed!  :-//

Can always do it for the challenge.  ;D

What are thoughts on the 4 big caps? Obviously, they were in circuit at the time that things blew, and the vinyl labels suffered some cosmetic damage... Given that all 4 of them measure within 2 µf of each other and are within the +/- 20% tolerance for their value, can I just leave them be for now? Replacing them will cost $16-28, depending on the grade I buy (and not downgrading to cheap craps... I mean caps). I do not have an ESR meter, but I should be able to do it with a function gen and scope, right? If they are fine, I'd rather leave them be to keep the repair cost down. If they are borderline though, I would replace them. It just seemed interesting to me that they'd all test so consistently.

Another question, regarding the creepage distances of some parts. Obviously, the design is... less than stellar. My question, is will a thorough isopropyl alcohol cleaning, a bake to dry out, and a conformal coating help with this, or is the issue of an arc over something that would be a potential hazard regardless? I only occasionally work with 120 volt stuff, and have never serviced a switching supply before. About all I've done have been linear supplies and voltage doublers (to run nixie tubes). I've always conformal coated my high voltage stuff... But is that actually good to do?
« Last Edit: May 13, 2016, 05:10:12 am by richfiles »
 

Offline Seekonk

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #50 on: April 23, 2016, 01:18:26 pm »
Send me your old caps! I'd be perfectly happy using them.  I'm pretty amazed at how expensive even crap ones are from China. Doesn't matter if some of the shrink is burnt off.  If you were happy before, they aren't an issue. 
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #51 on: April 23, 2016, 11:37:37 pm »
I would actually like to get one of those component/ESR testers. I know they're pretty much just a micro with and LCD running a testing program. I think it'd be useful to have around, and they are pretty dirt cheap too. Every gut instinct tells me the caps are just fine. I just have not tested the ESR yet. I feel like it's probably gonna be fine, but I just don't know yet. This puppy went bang pretty hard.

And you're right... those caps can get spendy! I normally spam part replacements, just to be on the safe side. If I think a part was even slightly stressed, I prefer to replace it. When you have 4 parts that cost $4-7 each... The cost adds up quick, though. As long as they test okay though, i'm gonna reuse them, I think. At least for testing anyway.
« Last Edit: April 23, 2016, 11:51:53 pm by richfiles »
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #52 on: April 24, 2016, 12:31:18 am »
An ESR meter is definitely a good thing to have. If you have an oscilloscope and function generator and don't mind a little math, you can still measure ESR.

I TEA.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #53 on: April 24, 2016, 08:01:42 am »
Haha! I already have THAT video up on another tab! I just haven't had the time to do anything. Probably won't have time for about 2 weeks, due to work. I'll have to clear my workbench for my current order...

But yeah, definitely looking at getting one of those cheap combo component testers. At the price, why not. Looking at http://www.ebay.com/itm/2016-EZM328-transistor-tester-ESR-table-frequency-meter-square-wave-genera-/172129915560 at the moment. Seems to be latest rev, with socketed micro (good for testing flashed rev updates).
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #54 on: April 24, 2016, 09:13:59 pm »
Yeah, real life has a way of taking over. I haven't seen that particular version of tester. It seems to have most of the right stuff (encoder, socketed uC, plug for leads). I couldn't tell if it has input protection. I wonder what the little trimmer pot is for. Screen brightness? Anyway, that'd be a discussion for https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/

Looking forward to seeing how the rest of this supply turns out when you're back from your income-generating project(s).
I TEA.
 

Offline MrSlack

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #55 on: April 25, 2016, 08:53:14 am »
Watch out for the cheap Chinese "transistor tester" units. They don't seem to consistently measure capacitor ESR. I've had readings that vary by as much as 200% in successive measurements even if I thoroughly discharge the caps between each reading.

I opted in the end to purchase an ancient LCR bridge. That is incredibly consistent, plus it does inductor Q which is nice. Cost about the same on ebay as a higher end transistor tester. Uses the same method w2aew uses to calculate the ESR as well wrapped in a bridge.

 

Online Shock

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #56 on: April 25, 2016, 04:24:21 pm »
Probably worth a punt for a $10 or so Chinese meter with free shipping though, mine seem to take reproducible measurements but I've hardly put them though a workout.

I agree sinking some cash into a decent LCR meter is worthwhile in the long run. Multiple frequency support and a wide capacitance range is important as well, as you can be comparing apples and oranges if you're not testing to the frequency on the datasheet. So the DER EE DE-5000 is another option.

Some decent caps can self heal or reform unless they are too far gone, like leaking electrolyte or bulging. After a time sitting idle, depending on when you are testing them in their life cycle this can throw out results.

I've seen SMPS caps or ones placed next to warm heatsinks (especially the cheap brands) their life expectancy seems to be under 3 years for partial or total failure. Heavy hours or really poor design I'd give them a year tops.
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Offline MrSlack

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #57 on: April 25, 2016, 04:34:57 pm »
I've got 55 years out of some Sprague ones so YMMV :)
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #58 on: June 13, 2016, 07:01:44 am »
@richfiles: Any news on your Mastech?
I TEA.
 

Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #59 on: June 13, 2016, 03:14:28 pm »
No new news... Hours have picked up at work, and I've just not sat down with it. What free time i have, has been applied to other projects, like my Kerbal Space Program controller.

Don't worry! I haven't forgotten about this thing.
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #60 on: June 13, 2016, 06:25:59 pm »
No worries. Just thought I'd check when I saw an online mention of one of these supplies. It sounded familiar and a quick search brought me back here.

I haven't played KSP (though I've spent a good number of hours in Orbiter a number of years ago). Your controller project sounds intriguing. I'll have to check it out.
I TEA.
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #61 on: June 15, 2016, 04:00:44 am »
I ran across this schematic of the HY5020E from an older thread around here that was actually about the HY3040E. This schematic and the PDF posted earlier in this thread are very similar to each other except on this one,

1. The output is actually labeled 50V instead of 30V despite both being titled "Diagram for HY5020E"
2. Changes to PCB HY023 (top of the schematic)
I TEA.
 
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Offline richfiles

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #62 on: June 15, 2016, 06:15:12 am »
Hey, thanks! Always good to seek moar information!
 

Offline bitseeker

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Re: Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
« Reply #63 on: June 15, 2016, 07:47:46 am »
I'll second that. :-+
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