Re: ViewSonic VP171b takes a long time to turn on
I have acquired a dozen or so of these "slow start-up" VP171b monitors.
I concur that the problem is in the 12V power supply. Some of them use the common 3842 IC used for years in smps designs. Pin 7 is power in. Starts up when voltage reaches about 16v, depending on the version (some low voltage versions lower startup volts).
Commonly the smps uses a "trickle" resistor from primary B+ (160+V) for startup, and a "run" winding from the smps secondary to keep running.
No "trickle" circuit is in this supply.
This power supply appears to have some degradation in a "marginal" component(s). I have found that the supply voltage to the smps IC is the problem. Some boards take a minute or two after the main switch is on to get the supply voltage to 16V to startup. This voltage is present on the 470Uf cap which is charged from the "run" winding and diode.
To get this big cap charged up to the start voltage, they use U03, an NPN transistor (mpsa44) as a "quick charge" circuit. This transistor is shut off after startup with Q06 which pulls the base of U03 low as the reference voltage (pin 8, 5v) is achieved. U03 collector goes to 160V B+ through 100K resistor. Base is forward biased with 2 3.3M series resistors to 160V B+. Emitter goes to the 470Uf cap and pin 7 of the chip.
For some reason the U03 is too slow at charging the 470 cap to get the power supply running in a timely manner.
My fix:
By shorting one of the base bias 3.3 Meg resistors thereby turning on U03 quicker, My monitor which took 35 seconds to start now takes 5-6 seconds to start up.
One interesting voltage reading: Q06, (NPN, emitter ground and collector to base of U03) collector voltage is around 3 V even with .67 v on the base. Seems like the base voltage should fully forward bias Q06 and bring the collector to zero volts.
Regards,
Mike
I have acquired a dozen or so of these "slow start-up" VP171b monitors.
I concur that the problem is in the 12V power supply. Some of them use the common 3842 IC used for years in smps designs. Pin 7 is power in. Starts up when voltage reaches about 16v, depending on the version (some low voltage versions lower startup volts).
Commonly the smps uses a "trickle" resistor from primary B+ (160+V) for startup, and a "run" winding from the smps secondary to keep running.
No "trickle" circuit is in this supply.
This power supply appears to have some degradation in a "marginal" component(s). I have found that the supply voltage to the smps IC is the problem. Some boards take a minute or two after the main switch is on to get the supply voltage to 16V to startup. This voltage is present on the 470Uf cap which is charged from the "run" winding and diode.
To get this big cap charged up to the start voltage, they use U03, an NPN transistor (mpsa44) as a "quick charge" circuit. This transistor is shut off after startup with Q06 which pulls the base of U03 low as the reference voltage (pin 8, 5v) is achieved. U03 collector goes to 160V B+ through 100K resistor. Base is forward biased with 2 3.3M series resistors to 160V B+. Emitter goes to the 470Uf cap and pin 7 of the chip.
For some reason the U03 is too slow at charging the 470 cap to get the power supply running in a timely manner.
My fix:
By shorting one of the base bias 3.3 Meg resistors thereby turning on U03 quicker, My monitor which took 35 seconds to start now takes 5-6 seconds to start up.
One interesting voltage reading: Q06, (NPN, emitter ground and collector to base of U03) collector voltage is around 3 V even with .67 v on the base. Seems like the base voltage should fully forward bias Q06 and bring the collector to zero volts.
Regards,
Mike
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