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Old 10-17-2020, 09:10 AM   #1
tester272001
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Default SMPS Resistor confirmation

I have not done too much SMPS repair. The resistor in this pic looks like a 0.68 Ohm 5% value correct? It is OPEN. The zener across it is a 36V that was shorted. When did they start making SILVER multipliers ? (if that is truly what that value is). if this is a 0.68Ohm Is a 1Ohm a reasonable sub? thanks folks!
Looks like a 2 watt size ?
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Old 10-17-2020, 09:33 AM   #2
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Default Re: SMPS Resistor confirmation

gold & silver multipliers are common, the value is correct, but I suspect there is more wrong. Where is the resistor located? is it in the source lead of a mosfet? If it is the source resistor, a 1Ω would not be a good choice.
That to92 transistor looks like it might be damaged, cracked.
Post a couple pictures of both sides of the complete smps, What is it from? that might be helpful.

Last edited by R_J; 10-17-2020 at 09:37 AM..
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Old 10-17-2020, 09:49 AM   #3
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Default Re: SMPS Resistor confirmation

I did not notice that transistor! thanks - here are both sides... the rectifier on the AC mains I replaced but it blew immediately. so those two are still missing. it is a full wave bridge there... Anything that is raised off the board was shorted and I raised the new part until I finish diag. the resistor I asked about was OPEN. All the other components appears OK from a multimeter perspective. The MAINS cap is .5 Ohm on ESR so thats good. This was a victim of lightning
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File Type: jpg smps_front.jpg (614.9 KB, 16 views)
File Type: jpg smps_back.jpg (1.07 MB, 11 views)

Last edited by tester272001; 10-17-2020 at 09:53 AM..
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Old 10-17-2020, 09:54 AM   #4
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Default Re: SMPS Resistor confirmation

My next step after fixing the resistor was going to be pull the transformer (on the output side of the MOSFET) and see if does not blow the fuse.
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Old 10-17-2020, 10:36 AM   #5
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Default Re: SMPS Resistor confirmation

The transformer is not the problem, removing it will prove nothing. if the mosfet is not shorted, then I suspect there is a short on the secondary. I see someone replaced D650 and I suspect they used a regular diode and not a fast recovery type.
If Q602 is damaged, then I suspect the Q601 mosfet is also damaged, R611(0.68Ω) is in the source of the mosfet and if the mosfet turns on hard or is shorted, R611 will blow.
What are you using for diodes D601~D604? I suspect they are also wrong.
Is that electrical tape as an insulator for the mosfet? that is wrong in so many ways.

I suspect Q601 shorted, causing R611 to open, this supplied +160vdc via a drain to gate short that damaged Q602 and likely other parts in that circuit

Last edited by R_J; 10-17-2020 at 10:53 AM..
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Old 10-17-2020, 12:04 PM   #6
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Default Re: SMPS Resistor confirmation

For the primary rectifier diodes I used what I had on hand. 1N4007 the originals are 1N539BL. The zener was 1ZB36BL. What looks like electrical tape is thermal padding I had for a big SCR since the MOSFET is not a plastic package it will short. So I put that non-conductive thermal pad in there temporary to see if it all would work. I was not planning to RUN that way only to test.

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Old 10-17-2020, 01:06 PM   #7
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Default Re: SMPS Resistor confirmation

On the mosfet with the tape, did you insulate the mounting screw, or just not use one? The zener is not that critical as it is just used to clamp any high frequency spikes across R611 when the mosfet is operating.
Q602 must be replaced if damaged as it controls the switching of the mosfet

Last edited by R_J; 10-17-2020 at 01:10 PM..
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Old 10-17-2020, 01:13 PM   #8
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Default Re: SMPS Resistor confirmation

I did not put a screw since it would likely short I will check Q602 - do you think i can get by with R611 as 1 Ohm? I def. do not have any SUB-ohm on hand.
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Old 10-17-2020, 06:20 PM   #9
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Default Re: SMPS Resistor confirmation

Using a 1Ω for R611 will change how the circuit operates, the voltage across the resistor will be almost double and that might upset the base voltage on Q602. Q602 sure looks bad from the picture.
You may want to check Q501 mosfet and D501, D502, D605, & D607 also.
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Last edited by R_J; 10-17-2020 at 06:27 PM..
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Old 10-17-2020, 09:12 PM   #10
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Default Re: SMPS Resistor confirmation

I appreciate the feedback RJ! i will check q602 and see about getting a low ohm kit.
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Old 10-17-2020, 09:38 PM   #11
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Default Re: SMPS Resistor confirmation

There are repair kits for these BA21N0F0102 Emerson/Philips/Magnavox power supply boards, it might make repair easier. Or at least point to the parts to test.
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Old 10-18-2020, 11:05 AM   #12
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Default Re: SMPS Resistor confirmation

I would suggest getting the repair kit as recommended by redwire, it may be cheaper and you will have all the correct replacement parts. Then it might be wise to use a 60~100 watt incandescent lamp in series with the a/c line to check the power supply, the lamp will limit the current if there is still a problem.
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Old 10-18-2020, 01:13 PM   #13
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Default Re: SMPS Resistor confirmation

Great idea fellas! The bulb trick I have heard of before, I will give that a whirl. I see that kit on ebay and its reasonable. Thanks for all the help and suggestions! Red and rj.
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