Re: UC3842 basics
between pin 1 and ground there is a resistance of around 200k.
daverave56
Member
Last Activity: 03-02-2016, 07:08 AM
Joined: 03-30-2015
Location: kent
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Re: UC3842 basics
i noticed that there is a opto coupler 4N25 near to the UC 3842,i measured 13v going into pin 5 of the opto coupler but nothing coming out of pin 4,the 13v which should be present on pin 4 of the opto coupler then go's through several resistors and two small transistors and then to pin 1 of the UC 3842, i was tempted to power up the UC 3842 with the 18v external supply and short pins 4 & 5 of the opto coupler and see what effect this has on pin 1 of the UC 3842Leave a comment:
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Re: UC3842 basics
just been looking at a schematic of he UC 3842 and it shows a 56k 1W resistor from the 320v supply to pin 7 of the UC 3842,which i guess is the startup resistor,but i cannot see any 1W resistors in the power supply but only very small surface mounted typesLeave a comment:
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Re: UC3842 basics
i connected 18v DC supply to the UC 3842 and these are the results.
Pin 1 - 0.09v
Pin 2 - 0.06v
Pin 3 - 0v
Pin 4 - 1.91v
Pin 5 - ground
Pin 6 - no osc
Pin 7 - 18v
Pin 8 - 5vLeave a comment:
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Re: UC3842 basics
the large 400v electrolytic has 320v across it and after 15 hours of the power supply being disconnected the 320v was still present and i had to bleed it with a resistor,i think i will try connecting a separate power source direct to the UC 3842 and see what happens and measure pin 6 with a frequency counter,i just wish i could understand the workings of this UC 3842 chip as i find it very complecated.Leave a comment:
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Re: UC3842 basics
im coming now so close to binning this scope which will sadden me as i really wanted to see this scope light up again !!! i feel the fault is too difficult for me or maybe other people to work outLeave a comment:
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Re: UC3842 basics
both my scope and multimeter failed after being in the same room,we had some water come into the room from the above flat which went over my oscilloscope but i did take the lid off at the time and could not find any water inside scope maybe just a coinsidence,even after the water problem the scope worked for several months after,when i first bought the scope second hand i always had to push the mains switch in quick and firmly then the scope would fire up and was like this for about two years then one day i went to use the scope and it would just not fire up,i...Leave a comment:
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Re: UC3842 basics
i did check the secondary electrolytics some time ago with esr meter and found nothing wrong,i will have another look at the regulators and diodes,could any faulty chip on the secondary side cause this shut down ??? we did have some dampness in the room where i keep the scope,my digital multimeter also will not work anymore,maybe same problem.Leave a comment:
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Re: UC3842 basics
i replaced the two tant capacitors with electrolytic 63v 47uf and did some voltage tests on the UC 3842 chip.
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 0v
Pin 4 - 0.7v
Pin 5 - ground
Pin 6 - 0v
Pin 7 - 13v
Pin 8 - 2vLeave a comment:
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Re: Metrix 803B Oscilloscope power supply issue
i did some voltage tests today in the switch mode power supply with the UC 3842 chip.
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 0v
Pin 4 - 0.7v
Pin 5 - ground
Pin 6 - 0v
Pin 7 - 13v
Pin 8 - 2v
changed the two tant capacitors with electrolytics and made no difference scope still deadLeave a comment:
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Re: UC3842 basics
checked all the resistors around the chip and all are ok,i wonder if the opto coupler circuit may be causing this ??? its so frustrating trying to fix this switch mode power supplyLeave a comment:
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Re: Metrix 803B Oscilloscope power supply issue
i also have a metrix scope that is dead with switch mode power supply problem which has caused me much frustration cause metrix refuse to supply me with a service manual !!!!!!! my model is a metrix ox 8042 but uses a UC 3842 pwm/oscillator chip rather than the TDA 4605 as used in your scope.Leave a comment:
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Re: UC3842 basics
i replaced the uc 3842 but no difference,i find the surface mounted resistors and capacitors hard to read.Leave a comment:
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Re: UC3842 basics
the scope is a metrix ox 8620...the 12v supply is steady but the ref pin of the UC 3842 only measures about 2v...the tantalum capacitors in the photo are ones that i replaced some months ago but made no difference the scope being still dead.Leave a comment:
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Re: UC3842 basics
sorry im new to this site,i did try and put this in the right section but i was blocked from doing so,not sure why i cannot post in correct section,my capacitors ive checked with esr meter and no bowed tops,im very frustrated trying to get this power supply to work....Leave a comment:
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Re: UC3842 basics
i have a problem with a UC 3842 which is used in my oscilloscope power supply,the chip will not power up,the supply voltage is around 12 volts and the reference pin is about 2 volts,someone told me it maybe a soft start problem,if so what pin of the chip and components are involved in soft start,there is also opto coupler nearby,can anyone please advise....Leave a comment:
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