Switching Power Supply - Can not get it to work - sigh - FIXED!!!!!

[daveyk]

Please look at the picture of the resistor, attached.  Tell me what its value is please. 

If I go by an on-line resistor value calculator at All About Circuits, it tells me 33K, 3%, 50ppm Temco. 

If I put the colors in the other way, I get: 212 ohms, 3%, 15ppm tempco.

The BOM shows an 18K Ohm in the location.  That can't be correct.  It reads 30 ohms.   Everything else in the circuit around it is burned up, so I can not trust that reading although the resistor looks okay (there are two 1.5 ohm metals film resistors that look perfect but are open.

I have uploaded a copy of the circuit.  Those numbers are NOT component values, but a reference number to the BOM.  The resistor in question is component 188.

200 shorted
160 & 162 burned open (1.5 ohmes 1% each)
194 open (SCR)
184 shorted (Fairchild shows to use a 1N914B now, was a 1N4446)

Input voltage to this circuit is 325-350volts DC.  This is the last stage PCB of a 5 board power supply.  I need to check 158 and 172.

I have ordered all the parts so I hope the transformer is okay.  That damn 188 resistor is a mystery to me.  Maybe it is a 33 ohm; I dunno.  Originally SCR 194 was supposed to be a 2N5061 (or equiv) by the BOM.  New it had a BRY39 installed with its anode gate lead cutoff.  Maybe 188 was changed for that part?  Maybe 188 is just burned up?  Usually a resistor doesn't short or go down in value when it burns up or fails.

Help and ideas appreciated.

Dave

[pigrew]

The BUZ80 (Q200) has a max Vgs of +/-20V. It needs 5 to 10 V to turn on according to the datasheet. The gate is pulled up by R164+R166 (820k+470k=1.3M).

Using voltage division from 325VDC, R188 would only yield 50 mV if it were 212ohm, which is far too little. 18k would yield 4.43 V, which is nearly enough to turn on the FET, but it wouldn't surprise me if they were having issues that the FET is too close to its threshold voltage, causing it to overheat. 33k would yield 8V, which is a good voltage for turning it on, but may suffer slightly from too much charge being needed to turn on and off the FET. I think that 33k is likely the true value.

I think that the resistor failed by going short. They can do that. You may want to check other resistors in the circuit, too, and probably replace R164 and R166 (the high voltage ones).

EDIT: The BUK456-800B has an even higher Vt, so would need a higher value resistor to fully turn on.

-Nathan

[daveyk]

Thank you for the response. I’m not at my computer in the shop and responding on my iPhone.

The +325 feeding this circuit is thru a 1.5amp pigtail fuse. So hopefully if the buzz80 shorted spontaneously the fuse popped (and it sure was flashed black) before the transformer could fry. Hopefully it’s okay and not the cause.

200 was actually a BUK456-800B, so that’s what I ordered, not the BUZ80.

I have orders in with EBAY, Mouser and Digi-Key. Mouser was the only one with 1.5 ohm 1% resistors. I prefer Digi-Key as their shipping charges are a lot less than Mousers, but I’m did what I could to source   the six+ parts, ~$55-$60.

I had this instrument last week and it went through the 13+ hour calibration. Shut it off and shipped it to them. They hooked it up, turned it on and it only came on for <minute and started flashing on the crt and shut down. They thought it took a bad hit in shipping (114 pounds shipping weight). I think it is a coincidence.

The 325bolt rail is not well regulated. If the line voltage goes up it goes up to about 350 volts. Hopefully their line voltage doesn’t have nasty harmonics on it. I’ve seen than before in factories when not running off of solo transformers.

Anywho, I should know around next Wednesday if it works again.

Dave


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[duak]

I was surprised that a metal film resistor could be damaged  and have its value lowered by 3 orders of magnitude, but apparently it does happen in 5% of cases: https://www.navsea.navy.mil/Portals/103/Documents/NSWC_Crane/SD-18/ResistorsFailure.pdf  I understand the mechanism is metal migration, probably from the end electrodes.  It would seem that just the right amount of energy in a certain period would be needed.  Too much and it would be more obviously damaged.

I've never seen a resistor with that many bands so I found this table: https://static4.arrow.com/-/media/arrow/images/miscellaneous/h/how-to-read-resistor-color-codes.jpg?la=en&hash=21018796E46BD04CD6E01630B12B645BB011FBA2

[xavier60]

Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?

[daveyk]

Hello Duak. Those bands confused me and went beyond my electronics schooling from 1980 -lol. What was nice is that we still had a vacuum tube course back then. Anywho, thanks to the innerwebs, I can find a six band resistor color code decoder. I found a free app for the iPhone but it only goes to five bands. It’s lazy but it does hurt to double check my color code memory after all these years.

Now in this case, I wonder how important it is to match that 33k exactly with its weird 3% tolerance and 3% temperature coefficient? Rather than pulling a 33K from the bins, I may try to order the exact one. There’s another $9 shipping from Mouser.

Dave


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[daveyk]

Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?

There are no rail zeners. That rail cap was perfectly fine,although I replaced it (4.7uf 400volts, 105 degree nichicon).

On the first supply board past the 325volt supply. There was another 4.7uf 400v cap completely dried out and open; replace it too.

The opto tests fine but I have been tempted to try and order replacements too. It’s the only regulation I see, but I’m no switcher expert. I don’t fully understand them and dislike working on them.


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[daveyk]

Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?

You want to see another interesting schematic of self oscillation with NO feedback as far as I can tell; I don't think it's regulated against load.  Here is the schematic of the 325 volt supply attached. Oh, and part 42 is a TLC555 timer! Simple switcher.

[xavier60]

Looks like it's supposed to be some sort of PFC but I can't make much sense of it. Might make a bit more sense knowing what else is connected on the left and also the component values.
But the MOSFET doesn't appear to be doing the usual high frequency switching thing. Its Gate appears to be fed with rectified and filtered signal from the 555 driven transformer.    The on/off switch short circuits one of the transformer's windings.
It all looks like an elaborate DC power switch.

[duak]

I wouldn't think that the value or tempco are particularly important, unless the resistor is used to compensate for something like temperature.  Are there other resistors on this board that have six bands?  Could this resistor have been replaced after a repair or upgrade?  The specs aren't particularly exacting yet Mouser wants $9.  Maybe there's some other attribute like higher than expected voltage breakdown.  One would need more design info or have one of these boards on the bench to probe.

[daveyk]

Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?

You want to see another interesting schematic of self oscillation with NO feedback as far as I can tell; I don't think it's regulated against load.  Here is the schematic of the 325 volt supply attached. Oh, and part 42 is a TLC555 timer! Simple switcher.

I've replaced all the film capacitors, SCR, FET, Resistors, etc, but I can not get it to fire up.  I did pull the 100 ohm pot and made sure it was good.  I put $250-$300 (with shipping, and getting the exact parts) in to this power supply and it doesn't oscillate.

The initial problem was a shorted FET and SCR and those 1.5 ohm resistors.  I tripple and more checked the correct insertion of the SCR.  I can not get this bitch to oscillate.  There is no shorts on the secondary supplies as far as I can tell.

I'm stumped.   I guess it's time to call it as un-repairable.  The customer has a bunch of parts units that I can get power supplies out of.  He is going to have to send me one now.

Dave

[daveyk]

Okay, dummy me, I got it working.  I had 1N914B, 184, in backwards.  If the very tight space, it was hard to tell.  I kept buzzing out the circuit and FINALLY found that.

Does anyone have theories on how pot 186 is adjusted?

Anywho, as is it's oscillating and the power supply outputs are correct!!

Dave

Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?

You want to see another interesting schematic of self oscillation with NO feedback as far as I can tell; I don't think it's regulated against load.  Here is the schematic of the 325 volt supply attached. Oh, and part 42 is a TLC555 timer! Simple switcher.

I've replaced all the film capacitors, SCR, FET, Resistors, etc, but I can not get it to fire up.  I did pull the 100 ohm pot and made sure it was good.  I put $250-$300 (with shipping, and getting the exact parts) in to this power supply and it doesn't oscillate.

The initial problem was a shorted FET and SCR and those 1.5 ohm resistors.  I tripple and more checked the correct insertion of the SCR.  I can not get this bitch to oscillate.  There is no shorts on the secondary supplies as far as I can tell.

I'm stumped.   I guess it's time to call it as un-repairable.  The customer has a bunch of parts units that I can get power supplies out of.  He is going to have to send me one now.

Dave

[daveyk]

I wouldn't think that the value or tempco are particularly important, unless the resistor is used to compensate for something like temperature.

The board didn't appear to have an other re-work.  That 6 band 33K, 1%, 50PPM tempco resistor has been replaced with one of the exact value.  You may be right that the value doesn't mean much.  The BOM calls for an 18K.  Go figure.  Probably an update since they weren't using the BUZ80 anymore and the SCR was a different type.  Everything else was exactly as called for in the BOM.   From what I can tell, any SCR/Thyristor may have worked as long as the voltage spec was high enough.  I dunno.

Any idea how that pot (100 ohms) 186 was supposed to be adjusted?  I left it as it was.  It is paralleled by two 1.5 ohm resistors and then the tap has a 470 ohm resistor going to ground.  The adjustment was probably fairly minor.

I'm mentally burned right now.  I think I am shutting the shop down early today and go out and pressure wash - lol.  I had a very hard time getting it to work until I realized, I installed 1N914B diode 184 in backwards.  I think it would have just mimicked the SCR turned on all the time. 

When I try to figure out how this power supply is supposed to work, with the secondary feedback through the opto-isolator, I get a migraine.  When I went to Electronics school in the late 1970's, switching supplies were not taught, and if they would have, they would have just skimmed over the basics.   I have been watching switching power supply repairs on YouTube.  Some actually try to teach the theory, some just show the repair.  None come close to matching this monstrosity.  Keep in mind this is just one board out of five that could have problems.  The other power supply boards have the same kind of excellent schematics (sarcasm).

I don't have all the caps needed to recap all five boards.  I only found one swelled capacitor and leaking capacitor and it's been replaced.  For the future, I need to get a supply of capacitors in-stock.  That isn't easy.  I haven't found a good 105 degree electrolitic "kit" for purchase that covers most of what someone would need.  I am going to go through the BOM and order 10 of every value I see and stock them.

Dave

[xavier60]

The Pot adjusts the power supply's current limit.
While the MOSFET is on, primary current ramps up. This causes a corresponding ramp up of the voltage drop across the 1.5Ω shunt resistors. The Pot divides this voltage and applies it to the SCR's gate which turns off the MOSFET.
When regulation voltage is reached, the opto-coupler adds current to the SCR's gate circuit causing it to fire earlier causing the MOSFET to be turned off at a lower primary current.
If you decide to adjust the Pot, make certain that is has no dead spots and stay away from the bottom end. This will totally disable the current limiting.