UC3842 basics

[daverave56]

Re: UC3842 basics

between pin 1 and ground there is a resistance of around 200k.

[rievax_60]

Re: UC3842 basics

The transistor inside of the opto coupler should not be passing any current until its LED on the isolated side gets some current passed through it when the output rails rise to their regulation voltages.
Don't force anything until the circuitry is better understood. Is there a transistor's Collector pin connected to pin 1 of the UC3842?
Its best if you draw out the circuitry involved with pin 1. You have not mentioned the result of checking for a short to ground on pin 1.

[daverave56]

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 3842

[rievax_60]

Re: UC3842 basics

I made a mistake. It's pin 1 that has the problem. With the voltage readings on all the other
pins, pin 1 will be wanting to go up to a few volts. Something external to the IC is holding pin 1 low. Look for a transistor. Check for a short to ground first.

[ben7]

Re: UC3842 basics

Something is holding pin 2 low. You will have to trace it.

I second this.

You should be looking for somewhere around 2.5V on the feedback pin.
Perhaps there indeed is some sort of protection circuit kicking in.

As to where the large resistor is, I can't see the whole power supply, so I can't say where it might be. But it might be hiding inside a heatshrink sleeve, or something like that.

It is also interesting that the oscillator isn't running. I don't know why that could be.

[rievax_60]

Re: UC3842 basics

Something is holding pin 2 low. You will have to trace it.

[daverave56]

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 types

[daverave56]

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 - 5v

[rievax_60]

Re: UC3842 basics

A while back, the power supplies in my HP 54645A DSOs failed. I found a protection circuit that was meant to disable the UC3842 while ever the mains voltage was too low. The UC3842 was being permanently disabled because of a high voltage resistor going high in the protection circuit.

[rievax_60]

Re: UC3842 basics

The start resistor might be connected back to one of the AC phases, this can be traced and confirmed. We need to know what is really happening on pin 6.

[stj]

Re: UC3842 basics

startup resistor(s) - they used to change value slowly in a lot of set-top boxes i had to fix.

[ben7]

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.

Yes you can try connecting a separate power source to the 3842. I don't know what sort of other circuitry could be connected to the power pins, so be sure to use a current limited PSU, maybe to say 100mA or so. Just in case! Also, I'd probably use about 18V or so.

Strange that the capacitor still has a lot of charge left after a while, because there should be a resistor which supplies initial start-up power to the 3842. I would think that resistor would drain the voltage on that capacitor much more rapidly.

[daverave56]

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.

[ben7]

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 out

Well, I'm sure one of us could figure out what is wrong with it, it's just that it is harder to do that through the computer screen ... lol

Don't bin it just yet! :S

https://www.badcaps.net/forum/showpo...9&postcount=89

Have you checked the voltage on that big blue capacitor, in that picture you posted (in the above post I linked to)?

Also, for example, start checking diodes, for example, like D109, and D110.
Could we possibly get a picture of the whole power supply too?

[rievax_60]

Re: UC3842 basics

Pin 2 measuring 0V is suspicious. There could be a short to ground or a protection circuit acting on it.
You need to confirm that pin 7 is getting to 16V. The best is with another oscilloscope and isolation transformer. The Min/max on a DMM might catch it.
Or measure pin 7 while slowly increasing the input ACV with a variac.

[daverave56]

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 out

[daverave56]

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 really dont want to scrap this scope cause i like it very much,but im running out of idea's on how to fix this as im not expert on switch mode power supplies.

[ben7]

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.

Sounds like some electrical gremlins! :P

Strange that your meter won't work anymore ... I'm assuming you tried replacing the battery. What brand is it? (Like, is it a cheap one or a good one?)

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 ???

Yes, a shorted diode or voltage regulator (or anything connected to the output of the power supply) would cause it to shut down.

[daverave56]

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.

[ben7]

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 - 2v

Definitely something going on here.

Did you check for shorts (and shorted diodes) on the output side of the PSU?