PeteS in CA
Badcaps Legend
Last Activity: 02-25-2026, 06:37 PM
Joined: 08-10-2005
Location: San Jose, CA
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I recommend doing the re-forming process described here, https://www.badcaps.net/forum/genera...citors-and-why . -
The four series I listed are all older or mature low impedance series, NOT general purpose series by design. The PM and PS series are RoHS-compliant versions of the 1980s-1990s PL and PR series. The PJ series is the ancestor of the late-90s (or early-2000s) PW series. The older parts will be less easy to find, because they are older, but the reason I recommended these is that the P/S control loops are compensated to be stable with output capacitors of the impedance ranges of those series. It is possible for output capacitors to be too good and thereby cause instability (oscillation)....Leave a comment:
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Looking at the condition on the solder on the mounting pads, it may be that capacitors were intentionally not installed. Sometimes space and locations are designed into a board, knowing that the capacitors might not be needed.Leave a comment:
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When you do the re-cap, be aware that using the latest and greatest low impedance electrolytics could cause stability problems. I would suggest Nichicon PJ or PS series if either is available, and as second choices, PM or PW series.Leave a comment:
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One of those bad caps in the first and second pic is in a sort of pocket formed by a memory DIMM, an inductor, and a connector and wires. If the inductor was hot the cap would get cooked. The bad cap in the third pic is in a corner formed by two heatsinks, i.e. two heat sources, with the hot air from one blowing on the cap.
With enough time, an electrolytic cap used well within its parameters will eventually fail. It may take 10 or 20 or 30 years, but they do dry out. In the same way, an X- or Y-cap exposed to many years of surges will eventually fail, and may fail ugly (crispy with...Leave a comment:
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Nichicon's current 105C general purpose series seem to be the UVZ and UVY series.Leave a comment:
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If it's a transistor, that tab and center pin are probably the collector. The pin-out might be, left to right, Base-Collector-Emitter. There would be a P-N junction from base to emitter and from base to collector.
If it's a MOSFET, the tab and center pin would be the Drain. The pin-out might be, left to right, Gate-Drain-Source. Using an ohm meter, there would be very high resistance or an "open" between any two pins.Leave a comment:
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They may be used for sensing and scaling parameters such as voltages and currents. Do the outputs go to comparators (e.g. LM339 or LM393)? Op amps can be used as comparators, but actual comparators are optimized for that function.Leave a comment:
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Based on the silkscreen reference designator and it having three leads, I think it might have been a TVS diode.Leave a comment:
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1. DC fan current is not necessarily a smooth DC current. I recommend looking at the current using an oscilloscope and a DC current probe. This is not a suggestion for a change, but it will give you a better understanding of the actual load current.
2. Your MOSFET is a 600V, 11A part. Because of the high maximum voltage, the R(DS On) is pretty high, .55 ohm on the datasheet. Since the input electrolytic capacitor is rated for 50V, I suggest trying a 100V part.
3. The MOSFET you used is in an isolated ("FullPAK") TO-220 package. Unless you actually need the heatsink...Leave a comment:
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BTW, the "(K)" may mean 10% tolerance.Leave a comment:
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It's rated for 85C, so it's a general purpose capacitor.Leave a comment:
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One of the transistor pins, the emitter, may be connected to Return, so it should have 0V. So the problem is "upstream" from the base, possibly in the LM723 circuit as CapLeaker suggested. I suggest tracing the circuit back to that IC, possibly starting with whether the IC has Vcc....Leave a comment:
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Adding to what stj posted, MOVs are damaged by every surge they absorb. With every surge, the firing voltage decreases a little. Eventually, with many surges, the firing voltage drops to or near the nominal line voltage, at which tome conduction becomes continuous and the MOVs short and/or explode. Depending on the surges' intensity and frequency of occurrence, it may take years to reach the point of self-destruction, but if there are surges it is a question of "when", not "if".Leave a comment:
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Looks like it's available through Mouser and Digi-Key, but in Croatia? It's made by On Semi. IIRC (from ~45 years ago) that part is higher than the common maximum voltage for a PNP BJT....Leave a comment:
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Concur. That thing is on the order of 25 tears old, and cleaning it will help see what is what. The PCB looks singed under 1 or more of those 2W resistors. Have you verified the 2N5038 is not shorted or open?
Judging by the model number and the BJT switch device (2N5038), that looks to be a 3T designed by Boschert Inc., ca 1980, when I worked there (Robert "Bob" Boschert was, then, VP Engineering), though not with that project. You may have to create your own schematic. Boschert was bought by Computer Products, which merged with (IIRC) Zytec to form Artesyn, which was bought...Leave a comment:
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The part marked "1R5" is a 1.5uH inductor. The part marked "BABCNA" is a DC-DC converter IC; another member may be able to let you know what part number it is.Leave a comment:
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Curtis Instruments motor controllers are designed in their division in Livermore, California. https://www.curtisinstruments.com/pr...or-controllersLeave a comment:
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It looks like it might be a discontinuous flyback design. If my guess is correct, VR1 (TO-92 package), located a bit above T2 in your picture, might be a TL-431. While this is nominally a programmable reference IC, it can also be used as a comparator with a reference on one input, and through an opto-isolator or transformer (T2?) to turn of the switch device. Thus a TL-431 (or TL-430) can be the controller.
I don't have a schematic, but if the switch is not turned off, it might remain on long enough for the main transformer core to saturate, blowing the IRF840 and input fuse.Leave a comment:
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This may seem odd, but since the P/S is probably more than 20 years old, there is the possibility that:
* Output caps you replaced now have capacitors that are "too good", their impedance is too low, causing instability;
* Output caps you did not replace are high impedance, causing instability.Leave a comment:
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