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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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[LONG] Another switching PSU question...
Hi !
I gave up on the PCServer 325 PSU, since at 99% was damaged a custom board. It was a standard ATX with different connector, so it was enough to adapt the failing PSU connectors to a "standard" (I chose 285W instead of 230W, just to be sure) and it worked. Now, there's a another small PSU which I can't substitute, belonging to a DECServer 700. I followed your suggestions and hints. The only ICs are UC3844N PWM SFH601-3 Opto IRFPE40 AVS10 kit (AVS10CB, AVS1AC) Automatic voltage switch I checked voltages (after the usual short-circuit hunt), and around PWM and Opto there's nothing or some mV. Voltage stops around the AVS10 kit. The Triac and the surrounding high valued resistances are really hot. I checked the datasheet of the AVS10CB controller against measured values, finding something odd. http://www.st.com/internet/com/TECHN...CD00000665.pdf I haven't figured well, yet, AVS10 kit behaviour. By the way, according to the datasheet typical app, Vm (main) is connected through a partitor to the AC main. Following the circuit is not easy, due to plane changes So, Vm should be an AC value, around 4 V. I found 2.8V. DC values are around 150 V. I checked R1 (exactly 1MOhm) and I changed, just to be sure again. Can't identify still R2, the scheme differs. Removing R2, I should have no Vm: I found 4V, so the PSU circuit differs from the datasheet typical one. Since on the two big caps after the rectifier bridge I have 150V, I should assume the AVS kit is working, anyway, even if the associated Triac is really *very* hot. On the IRFPE40 pins I found the whole rectified voltage, over 300V. Then I lose track of the signals. Next step would be to follow 3844 supply line, in order to see where it comes from. Any hint is welcome ! |
#2
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[LONG] Another switching PSU question...
On Sat, 22 Sep 2012 17:32:32 +0000, John wrote:
Hi ! I gave up on the PCServer 325 PSU, since at 99% was damaged a custom Removing R2, I should have no Vm: Removing R1, 1MOhm, of course |
#3
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[LONG] Another switching PSU question...
On Sat, 22 Sep 2012 17:32:32 +0000 (UTC), John
wrote: Hi ! I gave up on the PCServer 325 PSU, since at 99% was damaged a custom board. It was a standard ATX with different connector, so it was enough to adapt the failing PSU connectors to a "standard" (I chose 285W instead of 230W, just to be sure) and it worked. Now, there's a another small PSU which I can't substitute, belonging to a DECServer 700. I followed your suggestions and hints. The only ICs are UC3844N PWM SFH601-3 Opto IRFPE40 AVS10 kit (AVS10CB, AVS1AC) Automatic voltage switch I checked voltages (after the usual short-circuit hunt), and around PWM and Opto there's nothing or some mV. Voltage stops around the AVS10 kit. The Triac and the surrounding high valued resistances are really hot. I checked the datasheet of the AVS10CB controller against measured values, finding something odd. http://www.st.com/internet/com/TECHN...CD00000665.pdf I haven't figured well, yet, AVS10 kit behaviour. By the way, according to the datasheet typical app, Vm (main) is connected through a partitor to the AC main. Following the circuit is not easy, due to plane changes So, Vm should be an AC value, around 4 V. I found 2.8V. DC values are around 150 V. I checked R1 (exactly 1MOhm) and I changed, just to be sure again. Can't identify still R2, the scheme differs. Removing R2, I should have no Vm: I found 4V, so the PSU circuit differs from the datasheet typical one. Since on the two big caps after the rectifier bridge I have 150V, I should assume the AVS kit is working, anyway, even if the associated Triac is really *very* hot. On the IRFPE40 pins I found the whole rectified voltage, over 300V. Then I lose track of the signals. Next step would be to follow 3844 supply line, in order to see where it comes from. Any hint is welcome ! A little history is in order. The original PC - XT - AT power supplies had a 110/220 switch which should be in the appropriate position according to the local AC supply. In the simplest terms the switch changed the rectifier configuration from a bridge (for 220 VAC in) to a voltage doubler for 110 VAC in. Note the in either case the electrolytic caps are in series. The original design using a mechanical switch became a problem - if set to 220V the power supply would not work with a 110 VAC input. If set to 110 volts the power supply would be damaged when connected to 220VAC mains. The AVS10 kit automates that switching. Of course, in most modern power supplies the SMPS components have been selected to operate over a 90 - 250 VAC input range. I tend to troubleshoot by looking at critical points. You SHOULD have 1.4 x your AC input voltage across the series electrolytics if the AC input is in the range of 200 - 240 volts, 2.8 x the AC input voltage if the input is in tha range of 100 - 120 volts. For a nominal AC input of 120 VAC I would expect about 165 VDC across each cap, 330 VDC across the pair. PlainBill |
#4
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[LONG] Another switching PSU question...
I tend to troubleshoot by looking at critical points. You SHOULD have
1.4 x your AC input voltage across the series electrolytics if the AC input is in the range of 200 - 240 volts, 2.8 x the AC input voltage if the input is in tha range of 100 - 120 volts. For a nominal AC input of 120 VAC I would expect about 165 VDC across each cap, 330 VDC across the pair. PlainBill On one capacitor there is 156V, 165V on the other, and it should be correct ... -- http://www.supervinx.com/Retrocomputer |
#5
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[LONG] Another switching PSU question...
Well, I made some more measurements.
The IRFPE40 shows the following voltages: VDS=312V (max. 800V) VGS=0 (?????) What's the IRFPE40 function ? Looking at the PCB it seems that the 3844 supply voltage comes from it... |
#6
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[LONG] Another switching PSU question...
On 9/22/2012 4:52 PM, supervinx wrote:
Well, I made some more measurements. The IRFPE40 shows the following voltages: VDS=312V (max. 800V) VGS=0 (?????) What's the IRFPE40 function ? Looking at the PCB it seems that the 3844 supply voltage comes from it... Are you looking at this with a scope? I've seen many instances, especially in monitors where a cap has gone high resistance. You measure the average volts with a voltmeter, cause that's what's being regulated. But look at it with a scope and you see high peak voltages that make stuff HOT...just before it explodes. Probing around in the line side of a switcher with a scope is a good way to make a dead psu and a dead scope and a dead technician. I use fully isolated probes. |
#7
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[LONG] Another switching PSU question...
On Sat, 22 Sep 2012 17:32:32 +0000 (UTC), John
put finger to keyboard and composed: I haven't figured well, yet, AVS10 kit behaviour. DC values are around 150 V. I checked R1 (exactly 1MOhm) and I changed, just to be sure again. Can't identify still R2, the scheme differs. Removing R2, I should have no Vm: I found 4V, so the PSU circuit differs from the datasheet typical one. IMO you should not be experimenting with R1 or R2. This potential divider senses the line voltage. If you make the AVS10 believe that the line voltage is 110VAC when it is fact 220VAC, then you will have 660VDC across the two capacitors rather than 330VDC. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#8
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[LONG] Another switching PSU question...
On Sat, 22 Sep 2012 23:52:57 +0000 (UTC), supervinx
put finger to keyboard and composed: Well, I made some more measurements. The IRFPE40 shows the following voltages: VDS=312V (max. 800V) VGS=0 (?????) What's the IRFPE40 function ? Looking at the PCB it seems that the 3844 supply voltage comes from it... I would expect that the MOSFET is the chopper. Its gate would be driven by the UC3844 PWM controller. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#9
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[LONG] Another switching PSU question...
On Sun, 23 Sep 2012 21:48:11 +1000, Franc Zabkar
put finger to keyboard and composed: IMO you should not be experimenting with R1 or R2. This potential divider senses the line voltage. If you make the AVS10 believe that the line voltage is 110VAC when it is fact 220VAC, then you will have 660VDC across the two capacitors rather than 330VDC. If your application is 220VAC only, then you could disconnect the AVS10 section completely. Just remove the triac, or cut the link to the junction of the bulk capacitors. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#10
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[LONG] Another switching PSU question...
John wrote:
Hi ! I gave up on the PCServer 325 PSU, since at 99% was damaged a custom board. It was a standard ATX with different connector, so it was enough to adapt the failing PSU connectors to a "standard" (I chose 285W instead of 230W, just to be sure) and it worked. Now, there's a another small PSU which I can't substitute, belonging to a DECServer 700. I followed your suggestions and hints. The only ICs are UC3844N PWM SFH601-3 Opto IRFPE40 AVS10 kit (AVS10CB, AVS1AC) Automatic voltage switch I checked voltages (after the usual short-circuit hunt), and around PWM and Opto there's nothing or some mV. Voltage stops around the AVS10 kit. The Triac and the surrounding high valued resistances are really hot. OK, the classic circuit here is a rectifier feeding the main input cap with a resistor. When the cap is charged to some value, a circuit tickles the power transistor and starts the (usually) flyback converter. A winding on the transformer turns on a triac to shunt the charging resistor. Usually, if the converter starts without turning on the triac, the resistor burns up in a couple seconds. If the converter starts to power up, then the problem is likely the rectifier, etc. that triggers the triac. If the converter doesn't start, then the problem could be the PWM chip, the power transistor, or maybe the rectifiers, etc. on the output side. Jon |
#11
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[LONG] Another switching PSU question...
On Wed, 26 Sep 2012 14:59:08 -0500, Jon Elson wrote:
John wrote: Hi ! I gave up on the PCServer 325 PSU, since at 99% was damaged a custom board. It was a standard ATX with different connector, so it was enough to adapt the failing PSU connectors to a "standard" (I chose 285W instead of 230W, just to be sure) and it worked. Now, there's a another small PSU which I can't substitute, belonging to a DECServer 700. I followed your suggestions and hints. The only ICs are UC3844N PWM SFH601-3 Opto IRFPE40 AVS10 kit (AVS10CB, AVS1AC) Automatic voltage switch I checked voltages (after the usual short-circuit hunt), and around PWM and Opto there's nothing or some mV. Voltage stops around the AVS10 kit. The Triac and the surrounding high valued resistances are really hot. OK, the classic circuit here is a rectifier feeding the main input cap with a resistor. When the cap is charged to some value, a circuit tickles the power transistor and starts the (usually) flyback converter. A winding on the transformer turns on a triac to shunt the charging resistor. Usually, if the converter starts without turning on the triac, the resistor burns up in a couple seconds. If the converter starts to power up, then the problem is likely the rectifier, etc. that triggers the triac. If the converter doesn't start, then the problem could be the PWM chip, the power transistor, or maybe the rectifiers, etc. on the output side. Jon The PSU was repaired, with the aid of a friend technician. The PWM was faulty, together with a couple of resistors. Then it started and went down again. All caps were desoldered, finding that many of them had leaked (the PCB under them was wet). Checked with the ESR meters, they looked good. Changed all caps resolved the remaining troubles. Thanks to you all ! |
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