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Power Supply Board (Pre Amp Part 3)

HiFi, LoFi, Tubes, Transistors, ICs, Commercial, DIY

Postby muskyhuntr » Fri Jun 22, 2012 12:00 am

After I finished assembling the power supply board for the pre amp, I ran some voltage tests. The results I got, I think are a bit high. For the +17V line I got 20.0 volts and for the -17V line I got 20.3V. The 15V line is at 16.9V. These were all read under no load conditions. I am not too concerned with the 15V line being high as it will just activate the relays. But I am worried that the 17v lines are way too high. What do you think.

Some additional measurements are 32.9V on the positive rail before the LM317 and 33.1V on the negative rail before the LM337. AC voltage is 25.3. I am using the transformer in the parts list. I also measured the R1, R2, R3, and R4 resistors and they are on the money. Both LEDs light up.

Problems like this are usually just me, but 3V over spec. seems a bit much.

Jim
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Postby muskyhuntr » Sun Jun 24, 2012 12:00 am

I think I have an additional problem with the power supply (maybe).
Diodes D3, D4, D5, and D6 are extremely hot to the touch. They heat up within 10 to 20 seconds after power up. This is happening with no load attached. Is this normal for this device? I have no experience with Schottky rectifiers.

I am using a Vishay MBR1045PBF-ND.
Voltage - DC Reverse (Vr) (Max) = 45V
Current - Average Rectified (Io)= 10A
Voltage - Forward (Vf) (Max) @ If = 570mV @ 10A
Current - Reverse Leakage @ Vr = 100µA @ 45V
Speed = Fast Recovery =< 500ns, > 200mA (Io)

Thanks in advance.

Jim
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Postby thijsbeckers » Mon Jun 25, 2012 12:00 am

Hi Jim,

Seems like something went terribly wrong with your power supply. The voltages should be no different when the supply is unloaded.

D3-6 becoming hot can point to capacitors being soldered the wrong way around. Have you checked their polarity? Are they becoming hot as well?

And why is your transformer 2x25V and not 2x18V? Is it connected the right way to K5/K6?

I suggest you check components on their polarity (again)... It should be working without a hitch!
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Postby muskyhuntr » Mon Jun 25, 2012 12:00 am

Thank you thijsdeckers for your reply.

Part of my problem has been solved. My meter was bad! Found out by checking readings on known sources. Using a cheap model now, but readings are now what I consider good.(or maybe just better)

17V lines now read -17.85 and +17.55.
15V line now reads 14.84.
AC voltage is 21.5 on both legs.

The problem still remains on the heat issue with D3, D4, D5, and D6.
I double checked the polarity on all of the capacitors. Did this using both the pictures in the article and the printing on the PC board. Matched up. I then traced paths on the PC board according to the schematic. Again, matched up. It's possible C8 and C10 are bad, but according to the way the LEDs dim after I remove power, I think they are alright. (I know that is not a very good method of checking capacitors, but I have no other way)

The transformer is the same one recommended in the article. Markings on the side of the transformer show 18v per leg.

Jim
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Postby thijsbeckers » Tue Jun 26, 2012 12:00 am

Hi Jim,

D3-6 shouldn't get hot! Period. Perhaps only a bit warm to the touch, but with the currents flowing they should definitely NOT get hot.

Did you used our PCB? Did you check for shorts and solder blobs? Are the regulators ok? Etc.
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Postby muskyhuntr » Wed Jun 27, 2012 12:00 am

OK, I double checked the board with a 4x magnifying glass, everything checked out. The board is one of yours (nice quality). That leaves the parts. I removed some of them and still had the problem. I pulled the diodes and had one reading almost a volt when hooked in reverse to a 10V supply. It read 10V forward. So I went ahead and ordered new diodes and regulators. I'll let you know what happens with the new parts.

Thanks again for the reply.

Jim
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Postby thijsbeckers » Thu Jun 28, 2012 12:00 am

Ok Jim. I hope these were the culprit. It's not a real complicated circuit, so there's not that much that can go wrong.
Let us know the outcome.
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Postby muskyhuntr » Tue Jul 03, 2012 12:00 am

OK. Put the new diodes in, still have heat problem. Have this problem with down stream regulators removed and installed.
Voltage readings are essentially the same as in prior posts.

Thijsdeckers mentioned in a previous post that my AC voltage sounded high at 25.3. That was with the bad meter. With it's replacement it now shows 21.5 on the secondary side. I'm now thinking that the transformer may be bad and have ordered a second one. See How that works.

One thing that I tried was to run 19VDC to the AC terminals. Did this one leg at a time. The diodes did not get hot and I got +16.97 and -17.10 output from the regulators.

Jim
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Postby flange » Wed Jul 04, 2012 12:00 am

This really looks like a capacitor(C8, C10) problem. The PCB is not very helpful not marking + and - for these two, so it's easy to get wrong. I know that this has been said before but recheck them. If you have the regulators out then no current should be flowing. Try taking the capacitors out then check for overheating.
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Postby muskyhuntr » Mon Aug 13, 2012 12:00 am

Sorry to not post an update sooner. I found the problem soon after my last post. The problem was finally traced to a faulty capacitor. C12 to be exact. The capacitor also killed a couple of the MBR1045 rectifiers. Interestingly, when I removed C12, the diodes still became hot but at a much slower rate. That is when I found the bad diodes.

After finding the problem, the power supply works fine. I did play around with the values of R1, R2, R3, and R4 to get the voltages as close to each other as possible.

Flange: Thanks for the suggestion. I thought the problem might be reversed caps also.. The parts outline on the board is a bit hard to interpret as you mentioned. When you consider that it is the largest capacitor part outlined on the board, you would think that they would have room for a little plus sign. I did 3 things to triple check my parts orientation. One, I checked the schematic, two, I checked the photo of the build, and three, the part outline has a schematic of a capacitor within the outline, plus (positive) is the unfilled rectangle.

I learned a couple of things from this adventure. The first is that whenever you trouble shoot a circuit, it will always be the last thing you check. Second, good multimeters are expensive. Note that I blew mine at the beginning of this thread.

Thanks to all who offered help

Jim
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