METRIX OSCILLOSCOPE DEAD

[daverave56]

I have a problem with my Metrix ox 8620 oscilloscope,i bought this scope about two years ago second hand and from the start had a problem with it !!! ive always had to push the power button in hard and quick then the scope would power up,but now it will not power up at all,i checked the power switch which is ok,and checked the capacitors on the primary and secondary side of the switch mode power supply with esr meter but all seem ok,ive replaced the UC 3842 PWM/oscillator chip but scope still dead with no fire up noise whatsoever,somebody told me it maybe something to do with the soft start circuit involving the UC 3842 and i looked on the internet and it said there are three components that control the soft start which are a pnp transistor/resistor/capacitor but i have no schematic to work from as metrix will not supply a service manual,there are lots of unmarked surface mounted resistors and capacitors around the UC 3842 chip but i dont know which are the three components that i should be looking at !!! is anyone on this forum familiar with this UC 3842 chip and could tell me what pin is used for the soft start function and maybe the component values am looking for ??

[tautech]

Welcome to the firum.

Some pics of your scope might help, a member here might have schematics. 

In many datasheets there is a Typical application that may match your PCB soft start.
Hunt out a few of the 3842, there is more than 1 manufacturer that makes them

[nctnico]

I agree about getting datasheets and application notes on the switcher chip. It can also be the power switch itself or an open resistor in the primary circuit. At some point the circuit needs to get running. Sometimes the current is supplied using a resistive divider & zener diode. If that resistor goes open or has an intermittant contact you can get funny results.

What often helps to find problems is to supply the low voltage supply of the primary supply from a DC power supply. This should bring the switcher chip up & running so you can check various signals without the mains plugged in. Check the datasheet which voltage range is acceptable for the switcher chip.

[daverave56]

what you said about supplying the power supply with external DC is a very good idea 

[tautech]

So is it a Metrix OX 8620 ?

Sorry I don't recognize the UI, but I'm guessing it's a rebrand.

Any clues to the manufacturer on the PCB ?

[daverave56]

ive looked on the pcb and there is no markings of manufacturer,the scope is labled metrix ox 8620,im really frustrated with this power supply problem...its such a nice scope and i dont want to scrap it if i can somehow get it working,i find this uc 3842 chip difficult to understand as in the data sheets there is no diagram of soft start shown.   

[nctnico]

It looks like someone already did a poor job at replacing those capacitors.
There is only one way out: retrace the diagram and look harder for examples with the UC3842. TI must have a bunch of appnotes. There repairs require a lot of creativity in order to succeed.

[SeanB]

Replace those tantalums with 100V electrolytics, and look for every resistor over 47k on the board, especially those in a series chain, and either measure to see if the value is correct in circuit ( they will be lower than the printed value, short is not good, higher and it is open, unsolder then check again) or just replace them. Those are the startup circuit, especially on the top left where there are chains of resistors between the big blue electrolytic and the blue tantalum capacitor.

[daverave56]

i did replace those capacitors that you said replace several months ago but still scope is dead,would electrolytics make any difference ? i will check those resistors that you said were between the blue capacitor and the tant capacitors,i find the surface mounted resistors hard to read   as i have no schematic to show values of resistors and capacitors.

[SaabFAN]

Maybe silly question: Did you check the power-button? My first PC failed when the Power-Switch just wouldn't connect anymore. I had to really punch the several times from Off to ON until it worked.
If you're lucky, it could be something that simple.

[tautech]

i did replace those capacitors that you said replace several months ago but still scope is dead,would electrolytics make any difference ? i will check those resistors that you said were between the blue capacitor and the tant capacitors,i find the surface mounted resistors hard to read   as i have no schematic to show values of resistors and capacitors.

Seems like you have other feelers out there for a fix: http://www.badcaps.net/forum/showthread.php?p=546205

Those colour codes on the Melf SMD resistors will be the same as TH types, 3 or 4 band.
If you can't read them, get some magnification.
Indentify those that are related to the UC 3842 power up, probably a high value one from the main rectified mains cap, it might be open or gone high value. There is probably a ~22 uF cap on the VCC of the chip, these are often faulty. Remove and test. Check all component values in the FB circuit also.

[tautech]

Replace those tantalums with 100V electrolytics, and look for every resistor over 47k on the board, especially those in a series chain, and either measure to see if the value is correct in circuit ( they will be lower than the printed value, short is not good, higher and it is open, unsolder then check again) or just replace them. Those are the startup circuit, especially on the top left where there are chains of resistors between the big blue electrolytic and the blue tantalum capacitor.

+1
I see on badcaps you say you have fitted those tant caps. 
Replace them

And please show us the courtesy of disclosing your location as you have in badcaps

[daverave56]

i will check the resistors and try and replace the tant capacitors if i have the same values in electrolytic ones,can i test the power supply without reconnecting all the plug in pcb's and crt tube ?? i did test the mechanical mains switch and its working correctly as 320v appears across the main power supply electrolytic capacitor.

[tautech]

i will check the resistors and try and replace the tant capacitors if i have the same values in electrolytic ones,can i test the power supply without reconnecting all the plug in pcb's and crt tube ??

It's generally not a good idea to power SMPS without a load on the rail that the FB is derived from.
Identify that rail and solder a suitable light bulb in. Current draw is not too important.

If the CRT is powered from this PCB, then no, the HV developed without a load may break down componentry.

[daverave56]

the power supply and the main pcb are all one big mother board,there is a pcb that plugs into the mother board and of course the front control panel,up until now the only thing i have not connected is the crt tube,and i put the eht cap safely into a plastic bottle,then tried to power up !!! but of course the scope still being dead.

[daverave56]

i checked all the surface mounted resistors around the uc 3842 and all are correct...i noticed the supply pin 7 is decoupled with a 16v tant capacitor so will check later if that 16 v is derived from the 320v supply via a 150k resistor.

[tautech]

i checked all the surface mounted resistors around the uc 3842 and all are correct...i noticed the supply pin 7 is decoupled with a 16v tant capacitor so will check later if that 16 v is derived from the 320v supply via a 150k resistor.

It is not a decoupling cap, it is for bulk capacitance & smoothing on the VCC rail.
I have never seen a tant used for this. Replace it with an electroylitic, commonly ~20 uF but a datasheet I'm looking at shows 10 uF for 110 VAC supply.
It is likely the tant is also blown and there should be a 50V cap there.
Via the 150K this cap is charged until the IC min operating voltage is surpassed, the IC then kicks off and this cap must maintain operating voltage until the primary side FB supply takes over.
Unless it happens in this manner IT WILL NOT START.
Get this part right and the IC is good there is only a gate or base resistor and the switching BJT/Mosfet and a possibly open tranformer winding that will prevent it from starting.
On P15 of this datasheet there is a typical circuit in which you can see the points I have raised.
http://web.arrownac.com/sites/default/files/pdfs/UC3842.pdf

Have you confirmed the chip is operative when powered with an external supply?

[daverave56]

i checked all the resistors around the chip and all read ok,the only thing i found a little strange was a zener diode in para with a resistor,on my meter i was getting a diode reading of about 177 in both directions,if i unsolder the surface mounted diode and damage it if its ok then i have no idea of the zener voltage rating so i will be in bigger problem.   

[daverave56]

done some tests today in the power supply wit 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

i changed also the two tant capacitors with electrolytics i noticed a opto coupler nearby with the following voltages.

Pin 1 - 114v
Pin 2 - 114v
Pin 3 - 0v
Pin 4 - 0v
Pin 5 - 13v
Pin 6 - minus 2v

 

[nctnico]

I'd use an external power supply and put power on the UC3842 directly. That way you can measure using an oscilloscope. You should at least see something on the gate drive output.

[tautech]

done some tests today in the power supply wit 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

i changed also the two tant capacitors with electrolytics i noticed a opto coupler nearby with the following voltages.

Pin 1 - 114v
Pin 2 - 114v
Pin 3 - 0v
Pin 4 - 0v
Pin 5 - 13v
Pin 6 - minus 2v

 

Pin 7 13V UVLO will apply. See Datasheet

[daverave56]

i was really supprised this morning when i checked the main 400v electrolytic capacitor even though the scope had been switched off for over 15 hours the capacitor still had 320v across it,im really getting frustrated with this scope now and i feel like binning it !!! the fault is just too hard for me or anyone to know what is causing this shut down.   

[tautech]

i was really supprised this morning when i checked the main 400v electrolytic capacitor even though the scope had been switched off for over 15 hours the capacitor still had 320v across it,im really getting frustrated with this scope now and i feel like binning it !!!

What did you expect ?

UVLO working, IC needs 16V to start, HV DC section will stay charged in this condition.

I'd use an external power supply and put power on the UC3842 directly. That way you can measure using an oscilloscope. You should at least see something on the gate drive output.

Can you confirm operation of the IC with this simple and standard SMPS test?

This is the THIRD time you have been asked to do this.
We are loosing patience with you too.

[nctnico]

i was really supprised this morning when i checked the main 400v electrolytic capacitor even though the scope had been switched off for over 15 hours the capacitor still had 320v across it,im really getting frustrated with this scope now and i feel like binning it !!! the fault is just too hard for me or anyone to know what is causing this shut down.   

That capacitor should have discharged itself! So there is definitely an open resistor somewhere. IMHO the answer is staring you right in the face 

[daverave56]

i did try a direct DC from my bench power supply but i did it wrong as i connected the DC power supply directly across the large 400v electrolytic capacitor and there were no voltages present on any pin of the UC 3842,  i should have connected power supply direct to the chip which i will try today ad see what happens,im very grateful to you guys for your help but im finding this circuit with the UC 3842 hard to learn and understand, my electronics knowledge is more about making hobby metal detectors and thats why i use an oscilloscope and i prefer these old CRT type osci  lloscopes compared to the modern digital ones.