METRIX OSCILLOSCOPE DEAD - Page 1
Forum section: electronics / repair

author: daverave56
author_rank: Contributor
timestamp: March 31, 2015, 02:28:31 pm
content: 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??

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2015, 07:39:32 pm
content: 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

author: nctnico
author_rank: Super Contributor
timestamp: March 31, 2015, 07:48:28 pm
content: 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.

author: daverave56
author_rank: Contributor
timestamp: March 31, 2015, 09:56:02 pm
content: what you said about supplying the power supply with external DC is a very good idea

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2015, 11:21:21 pm
content: 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 ?

author: daverave56
author_rank: Contributor
timestamp: March 31, 2015, 11:53:24 pm
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: April 01, 2015, 01:25:03 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 01, 2015, 04:44:35 am
content: 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.

author: daverave56
author_rank: Contributor
timestamp: April 02, 2015, 03:07:26 pm
content: 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.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 02, 2015, 06:30:22 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 06:52:02 pm
content: Quote from: daverave56 on April 02, 2015, 03:07:26 pmSeems like you have other feelers out there for a fix:http://www.badcaps.net/forum/showthread.php?p=546205Those 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.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 07:11:48 pm
content: Quote from: SeanB on April 01, 2015, 04:44:35 am+1I see on badcaps you say you have fitted those tant caps.Replace themAnd please show us the courtesy of disclosing your location as you have in badcaps

author: daverave56
author_rank: Contributor
timestamp: April 02, 2015, 08:08:56 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2015, 08:22:42 pm
content: Quote from: daverave56 on April 02, 2015, 08:08:56 pmIt'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.

author: daverave56
author_rank: Contributor
timestamp: April 02, 2015, 09:35:29 pm
content: 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.

author: daverave56
author_rank: Contributor
timestamp: April 03, 2015, 01:08:36 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2015, 04:32:42 am
content: Quote from: daverave56 on April 03, 2015, 01:08:36 amIt 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 a50Vcap 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.pdfHave you confirmed the chip is operative when powered with an external supply?

author: daverave56
author_rank: Contributor
timestamp: April 03, 2015, 09:34:42 am
content: 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.

author: daverave56
author_rank: Contributor
timestamp: April 03, 2015, 01:44:07 pm
content: done some tests today in the power supply wit the UC 3842 chip.Pin 1 - 0vPin 2 - 0vPin 3 - 0vPin 4 - 0.7vPin 5 - groundPin 6 - 0vPin 7 - 13vPin 8 - 2vi changed also the two tant capacitors with electrolytics i noticed a opto coupler nearby with the following voltages.Pin 1 - 114vPin 2 - 114vPin 3 - 0vPin 4 - 0vPin 5 - 13vPin 6 - minus 2v

author: nctnico
author_rank: Super Contributor
timestamp: April 03, 2015, 06:07:40 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2015, 07:06:11 pm
content: Quote from: daverave56 on April 03, 2015, 01:44:07 pmPin 7 13V UVLO will apply. See Datasheet

author: daverave56
author_rank: Contributor
timestamp: April 04, 2015, 09:55:06 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 04, 2015, 10:12:41 am
content: Quote from: daverave56 on April 04, 2015, 09:55:06 amWhat did you expect ?UVLO working, IC needs 16V to start, HV DC section will stay charged in this condition.Quote from: nctnico on April 03, 2015, 06:07:40 pmCan 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.

author: nctnico
author_rank: Super Contributor
timestamp: April 04, 2015, 11:42:00 am
content: Quote from: daverave56 on April 04, 2015, 09:55:06 amThat capacitor should have discharged itself! So there is definitely an open resistor somewhere. IMHO the answer is staring you right in the face

author: daverave56
author_rank: Contributor
timestamp: April 04, 2015, 02:11:05 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: April 04, 2015, 08:54:24 pm
content: Slightly off topic, be aware SMPS has been around for a good while and many of the last CRO's used it exclusively. A combination of linear for LV and switching for CRT HV has been used in CRO's since the 70's, maybe even earlier. Those that derive HV from a secondary winding on the mains transformer are very few, only some Hameg's come to mind.The required CRT HV has always been problematic, only very good design and quality componentry ensures reliable service.Even though one might think DSO's don't have these HV problems, be aware LCD backlight circuits commonly require 1 kV+ and this is always supplied with switching circuitry, albeit more modern and these days very very reliable.

author: daverave56
author_rank: Contributor
timestamp: April 05, 2015, 09:03:31 am
content: i connected up a direct DC 18v supply to the UC 3842 chip and these are the results.Pin 1 - 0.09vPin 2 - 0.06vPin 3 - 0vPin 4 - 1.91vPin 5 - groundPin 6 - no oscPin 7 - 18vPin 8 - 5v

author: daverave56
author_rank: Contributor
timestamp: April 05, 2015, 12:00:01 pm
content: been looking at a schematic of the UC 3842 chip and it shows a resistor of 56k 1W from the 320v supply to pin 7 which i guess is the startup resistor but i cannot find any 1W resistor around the UC 3842 chip,all resistors are small surface mounted types

author: SeanB
author_rank: Super Contributor
timestamp: April 05, 2015, 12:28:45 pm
content: Quote from: daverave56 on April 05, 2015, 12:00:01 pmIt will be made up from a series connected string of smaller lower value resistors. Trace from pin 7 and you will find the set of typically 4 resistors, and one or more is either open circuit or high in value. Replace all 4 with new ones if that is so. You could also have a broken trace.

author: tautech
author_rank: Super Contributor
timestamp: April 05, 2015, 07:36:59 pm
content: Quote from: daverave56 on April 05, 2015, 09:03:31 amIn my Onsemi datasheet, the operating current is listed as less than 20 mA.Substantially more might will indicate a faulty IC.But when powered on with an external PSU, also check for an output waveform for confirmation of operation.

author: Ketturi
author_rank: Regular Contributor
timestamp: August 14, 2015, 02:03:33 pm
content: Was there ever any conclusions about fault? I have the same oscilloscope, with very same fault. It won't get on every time, and I have to pump power switch back and fourth until it starts (and even then sometimes CPU board locks up and won't work properly).I have changed C112 and C138 to corresponding 50V electrolytics, but that did not help anyway. If it goes to fault mode, US3842 vcc is only 10v and it takes only 1mA power, no output or any life in circuit. When it properly starts up, VCC is 16V with 15mA of current and everything seems to work as meant. Also, if running properly, if I touch error amplifier pin with multimeter PSU stops working and goes back to fault mode. Also, in fault mode, large 400V cap wont discharge, but in normal operation it's voltage drops quickly to around 60V after cutting power.I have checked every component in primary side, and all resistors and diodes seems to be just fine, no open or short circuits. I'm pretty bummed about this, just can't figure what the problem is.How can one power supply just be so frustrating. Should I just replace whole primary side with another switching mode controller...

author: tautech
author_rank: Super Contributor
timestamp: August 14, 2015, 08:49:49 pm
content: Quote from: Ketturi on August 14, 2015, 02:03:33 pmSMPS can do that to you. LOLSome can continue to operate with all the primary side perfect and very bad smoothing caps on the secondary, others will give incorrect FB and produce faults like yours.In a case such as yours I would check all secondary caps are close to spec, that is within 5% of labeled values and better than 100 milli Ohms ESR if you can test ESR.If you find low capacitance values, replace them all.

author: Ketturi
author_rank: Regular Contributor
timestamp: August 15, 2015, 01:57:29 am
content: Quote from: tautech on August 14, 2015, 08:49:49 pmYup, I would do that, but my meter only goes to 22µF and without working oscilloscope I can't measure their ESR. So options are a) replace all capacitors at once b) get capacitance and ESR meters

author: daverave56
author_rank: Contributor
timestamp: March 01, 2016, 01:37:48 am
content: Hi everyone....my scope is still not working...i just decided to put it away in a box for another time...ive just decided to have another go at repairing it and noted many of your past advice posts...that guy with the same metrix scope as me....did you find the problemwhen i first bought the scope i also had to keep pumping the power button until the unit fired up but then one day nothing...100% dead

author: vk6zgo
author_rank: Super Contributor
timestamp: March 01, 2016, 02:14:03 am
content: Quote from: daverave56 on March 01, 2016, 01:37:48 amThis was a very common problem with domestic TV sets.A popular circuit used a 220k (or thereabouts) resistor from the incoming Mains to kick the SMPS into operation.The resistor would go high,& the TV would refuse to start.People found out that by "pumping" the switch,they could get it to go,so they would do this until they killed the switch,then would send the set in labelled "faulty switch".The "noobs" would replace the switch & be astounded when the TV set still "refused duty".

author: daverave56
author_rank: Contributor
timestamp: March 01, 2016, 02:27:01 am
content: i will check resistor again...i really want this time to get this beast of a scope working !!! i need a challenge !!! in my photo of the pcb it shows two tant capacitors and i forgot to solder them back in circuit before storing the scope...so not sure of the values or voltage of the capsany idea anyone

author: vk6zgo
author_rank: Super Contributor
timestamp: March 01, 2016, 04:47:08 am
content: Googling brought up this:https://www.google.com.au/?gws_rd=ssl#q=uc3842+power+supply+schematicClick on "more images" & you will be overwhelmed with 3842 circuits.You will note that the schematics are almost identical,apart for the few being used for applications other thanAC to DC SMPS.If you trace through your supply using these schematics,you should find one that is most like it.You can probably cheat with a few capacitor values.If you can't remember what they were,use one from these circuits.I also found:http://noel.feld.cvut.cz/hw/philips/acrobat/8014.pdfNote: the 14 pin DIL changes the Vcc pin from 7 to 12.P.S---I doubt if there would be an external "soft-start" circuit involved with pin 7,as the chip does that internally.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 01, 2016, 05:58:24 am
content: Test the optocoupler! If that marked as Z105 is an optocoupler it can be the culprit of your headpain!!! Replace it.

author: oldway
author_rank: Super Contributor
timestamp: March 01, 2016, 07:51:22 am
content: Quote from: daverave56 on April 05, 2015, 09:03:31 amIn this condition, UC3842 MUST oscillate.Replace it.

author: tautech
author_rank: Super Contributor
timestamp: March 01, 2016, 08:00:59 am
content: Quote from: daverave56 on March 01, 2016, 02:27:01 amWith study of the App Notevk6zgolinked and the Datasheet I linked in reply #16 AND your pics, the caps should be 100uF (C112) and 10uF (C118?) (but I'd use 22uF) and all at least 50V rated and only ELECTROLYTIC.BTW, meter out both these caps footprints and check against the overlay which is the GND pin. (to be sure, to be sure)You've still got more to work though IMO, pics are barely enough detail, there's parts of the circuit we can't see, the switching Mosfet/BJT for one and trying to identify the flyback VDD diode.Please confirm if the via next to pin 8 is GND. (3842)Where does pin 5 go?What component is the TO-92?As SAUL BRITTO says replace the opto too.Another pic from a bit further out and lit in the currently dark areas would be a great help.Try as I might to find a "nice" circuit example similar to what we can see in the OP's images WITH opto feedback, a flyback PSU winding, multi PSU caps and multi outputs, this is the closest I could:

author: Ketturi
author_rank: Regular Contributor
timestamp: March 01, 2016, 10:22:04 am
content: This scope is cursed, after using far too much time to repair it, the front panel board gave up, and now all buttons and leds are dead and do nothing, scope just shows channel 1 voltage and timing and trace slowly sweeps outside the image area creating large flare on screen. Probably the MCU in the front panel is shot, and it may be as well mask rom one, so fixing it could be nearly impossible. I gave up and retrieved my old analog kenwood cs-5140 from other side of country (400km and 5 hours at one direction just to get one working scope...).

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 01, 2016, 12:42:18 pm
content: Stay calm!

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 01, 2016, 09:32:54 pm
content: Ketturi>> Has this scope a 3V battery type coin? If yes, measure it.

author: daverave56
author_rank: Contributor
timestamp: March 02, 2016, 03:33:38 am
content: been doing some testing on the metrix scope tonight....tested capacitors again with esr meter but all seem ok...the mosfet is a buk 456....i did a resistance check between the positive of the 400v capacitor and VCC of the UC 3842 and i got a reading of about 600k....so i connected a 16v supply to the UC 3842 and the 5v reference was now present...and a saw tooth waveform on pin 4...but alas no oscillator output from pin 6..i did replace the UC 3842 some time ago when the scope first stopped working so it should not be the chip faulty but not sure now...i will post photos soon as requested.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 02, 2016, 03:49:40 am
content: And the optocoupler?

author: tautech
author_rank: Super Contributor
timestamp: March 02, 2016, 05:20:29 am
content: Quote from: daverave56 on March 02, 2016, 03:33:38 amSeems much too high, I'd expect 150K max. It seems in the pic there is a chain of same value resistors from HV DC+ to UC 3842 VDD (pin 7), have you checked they all are the same and none have gone high?Good progress, keep it up.

author: daverave56
author_rank: Contributor
timestamp: March 02, 2016, 07:26:15 pm
content: still cannot find the fault...this power supply is complecated compared to other schematics ive seem using the same UC 3842 chip...i made a mistake today by not putting the insulator back around mosfet and tab touched ground while i was working on it and we had some smoke which took out three low value resistorsi nearly thought about binning it but im still persisting although i will need now to get another mosfet and small resistors.

author: tautech
author_rank: Super Contributor
timestamp: March 02, 2016, 11:13:40 pm
content: Quote from: daverave56 on March 02, 2016, 07:26:15 pmFrom your pic (without not being able to meter it out), there'll be a dropper resistor chain from HV DC+ and an electrolytic for this (IC startup supply), AND then a series diode from the transformer primary flyback winding with a further electrolytic (IC run supply) AND a ceramic SMD cap at pin 7 for local IC decoupling.Hope that makes sense.That's how I interpret your pic.Having 2 electrolytics for the IC VDD is a little confusing but I'd also like to know what the TO-92 is close to the opto-isolator, is it a BJT or a V-ref?I'd suggest you make a dim bulb tester, it uses an incandescent bulb in series with the mains to limit the current available in case ofmoments.Google dim bulb tester.

author: daverave56
author_rank: Contributor
timestamp: March 04, 2016, 11:35:32 am
content: had a little break from trying to repair oscilloscope...been doing a blue tooth audio project for my metal detector....hopefully will start looking at scope tonight...i would really love to see this scope light up again.

author: Agent24
author_rank: Regular Contributor
timestamp: March 05, 2016, 11:02:19 pm
content: You can contact Metrix throughhttp://www.chauvin-arnoux.com/en/contact-us/internationaland they may just help you out with a schematic.. (they did for me, anyway)

author: Ketturi
author_rank: Regular Contributor
timestamp: June 14, 2016, 12:35:05 am
content: I gave one last chance for this scope. Front panel was not working because one smd 74HCT08 was fried on the front panel PCB. It had visible burnt hole at middle of it. I also changed all electrolytic capacitor from the power supply section and also UC3842 IC.I also had to repair some bodges that were broken, this scope is so full of them, every section has some smd components soldered together and there was some drilled holes on traces and diodes&transistor soldered over them. Power is still bit intermittent, and it does not want to power on every time.I found small metal contact inside the scope from time base encoder that also was not working, glued the contact back. It is possible that it had broken of and then shorted something else inside scope. I do not know if I want continue troubleshooting this scope because it is terrible mess. I have never seen such badly designed and bodged scope, and repairing it is pain because everything must be removed to get access to main board. CRT can't be disconnected easily as it has soldered ground connections everywhere (even inside metal shielding)