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Old 10-18-2019, 11:09 AM   #21
ajshoe
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Default Re: eax55357701 repair

should I bench test it with load on 12V & 20V outputs ?
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Old 10-18-2019, 11:12 AM   #22
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Default Re: eax55357701 repair

Compare all the diodes and resistors from both boards, as in check D501 on one board and the other, see if any are different, sometimes not all, you get fluctuating voltages with bad voltage regulating diodes or bad solder joints on transformers or elsewhere.

Last edited by nomoresonys; 10-18-2019 at 11:15 AM..
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Old 10-18-2019, 11:29 AM   #23
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Default Re: eax55357701 repair

It is either the PS-ON is fluctuating or the VCC that is generated by the standby power supply power transformer AUX winding is fluctuating that is why the PFC Voltage on the main caps and the 12/20V are fluctuating.
Verify that the PS-ON is constant, if it is then check the VCC Voltage of the PFC IC and the 12/20V SMPS IC. May be the filter caps for the VCC from AUX winding are bad.
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Old 10-18-2019, 03:18 PM   #24
ajshoe
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Default Re: eax55357701 repair

ps_on does cycle every 3 secs after you push on/off button.
+5V is steady
+12V cycles every 3 seconds 0 - 12.2V
+20V cycles every 3 secs 0 - 21.33V

I've checked both boards - ohming out resistors. checked caps. checked diodes. I'll check again tomorrow

The LM431 to-92 did fix the as431 that were bad. This shunted regulator should have been a higher rated part. The SMD is only 0.28 watt where the to-92 pkg is 0.8 watt
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Old 10-18-2019, 03:57 PM   #25
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Default Re: eax55357701 repair

Right now you have PS-ON issue. You need to run the power supply board by itself and connect 1K resistor between PS-ON pin and the standby power supply output to force on the power supply manually and see if the PRC and the switched power supplies are steady or not.
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Old 10-18-2019, 07:24 PM   #26
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Default Re: eax55357701 repair

thats exactly what I was thinking. i try that tomorrow morning. since I replaced the 2 as431 - it is acting differently.
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Old 10-20-2019, 10:55 AM   #27
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Default Re: eax55357701 repair

bench testing PSB: PS_on jumpered with 1K to +5V gives
+12V is 12.3V
+20v is 19.64V
PRC is 0V

PSB stays on - does not cycle

I replaced the AS431 with new LM431 TO-92 pkg on the working board and it still works perfectly

Last edited by ajshoe; 10-20-2019 at 10:59 AM..
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Old 10-21-2019, 08:06 AM   #28
ajshoe
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Default Re: eax55357701 repair

was using the wrong ground for PFC - it holds steady at 386V along with the other voltages.
so bench testing both PSB shows voltages:

lbl Good board non-working board

+5V +5.14V +5.1V
+12V +12.28V +12.30V
+20V +19.6V +19.63V
PFC +380.2V +385V

I've going to give up on this - seems like its either a PSB current problem or some sort of over protection circuit.

Last edited by ajshoe; 10-21-2019 at 08:45 AM..
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