| Electronics > Repair |
| Mastech HY5020E 50v 20a Bench Supply: Economical to repair? |
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| albert22:
--- Quote from: Andy Watson on April 21, 2016, 06:31:02 pm --- --- Quote from: albert22 on April 21, 2016, 05:34:19 pm ---how do you get 3.9V with the values for R302 and R304 shown in the schematic. --- End quote --- 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. --- End quote --- |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 |
| edpalmer42:
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 |
| Seekonk:
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. |
| richfiles:
:(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. |
| Richard Head:
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. |
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