OSCILLOSCOPE PROBLEM

[daverave]

soft start power supplies are a pain when bust as they feedback to make work, and they oscillate or chop the dc in a ferrox cube xformer often. Sometimes the windings in the xformer are not secure and the singing windings can rub thru the thin enamel and short a bunch of turns. Better xformers are resin dipped and vacuumed to help this.

Sometimes there are flyback or freewheel diodes in the primary chopper side and can age as they can be spec'd close as cheap.

Get the pcb under a small microscope to really look at the solder joints, there is normally a couple of jokers on a board.

Try warming it with a hair dryer and seeing if it will come on.

Or try a light dust with freezer spray, dont get it wet like snow though.

Locally bash the pcb with the plastic handle of a screwdriver repetitively as you turn on off.


I used to use these techniques fixing avionic kit at marconi's in the 80's


Something could be loading the psu output, could look at ciruitry hanging off it.

What is make and model of unit

when you mentioned about feedback to make the circuit work do you mean the opto coupler that isolates the cold from the hot ???? also i use to leave the window open in my workshop and i wonder if any cold damp air maybe got onto the pcb and caused this problem cause my digital multi meter also failed for no reason !!!!

[CAS]

What he is saying is that if there is a fault on the secondary side that does not allow the correct voltage to come up at the instant of turn on, the p/s wont start. Did you end up replacing the electro caps? Even if they read ok on the meter, I would always change primary caps. I have replaced many caps before that read ok but are in fact faulty. When you turn it on does any of the the secondary voltage rails come up slightly? Does the supply make a little squeal on start up?

[golfnut]

When you used to stab at the power button, could this have been bending the pcb and making a joint connect, after repeated operations the tiny gap could be too big to span with a decent bend? OR if the pcb can take a bend from the button push, new issues like surface mount resistors or caps may have an end cap fractured, this would be almost impossible to see.


I find Ic,s are normally pretty robust. They often have s/c protection, thermal shutdown, esd clamps etc

If the pcb has been made with lead free solder Im sure there will be a duff joint on it somewhere. NASA wont use it at all as its generally crap.


Tantalum caps are **** poor longer term. They pass DC sometimes.
The electrolytics can dry out a little if used in hot kit for years, or they can bulge or split, they used to go off like detonator, I deafened myself once and filled the room with chaff and horrible smelly yellow oily stuff ... Castanet wet tantalums will smash their way thru three 2mm pcbs, smashing the lids off big leaded ICs.


The opto couplers are used to insure high voltage isolation is guaranteed

put a few good images up

[daverave]

before i dismantled the scope when i pushed the power button the led's on the front panel would light for about 1 second then nothing...there is no squeal when powered up !!!! i must admit in the area where the chopper transformer is soldered the fiberglass board is not level !!!! i have not replaced any electrolytics as they read ok on a esr meter and i cannot see any fractures...the tantalums i replaced !!! i did notice the ref voltage on the UC 3842 AN is only 1.2v and documents say it should be 5v so im not sure about this !!!!!!!!

[daverave]

[CAS]

Because you have said that the leds come on for a second before it turns off, I would probably be looking for a fault in the secondary side of the power supply. If, when the supply fires up, the voltages dont come up to spec, the primary side will switch off. Check for shorted diodes in the secondary stage or faulty electros. If this was a job I had on my bench for a customer, I would just change all the electros (in primary and secondary) and double check all diodes. Caps are cheap and easy to replace and this would eliminate all the easy things. If it was still faulty then I would start looking for more complicated faults. Dont always trust your esr meter. I have had plenty of caps that read ok on a meter but were still faulty.

[daverave]

what worries me is that about 2 months ago we had a water leak upstairs that came into my workshop and went over the scope and bench...later i took the top casing off the scope to check for any water inside scope but could not find any dampness inside or on the pcb !!! the scope has been working ok since this happened until now !!! maybe a coinsidence but not sure ?????????????? i will go through the electrolytics again...some of them are like 120v type which i havent got so would have to order them....do you think its a waste of time changing the oscillator chip cause my gut feeling is that the chip is not the cause ???

[golfnut]

The only thing that looks like an issue visually is the attached.

[daverave]

i checked the diodes with meter and seem to be ok !!! i will check the solder joints on the surface mounted components...not sure what else to do ???

[golfnut]

without a block diagram, its hard to split up the sections to get one bit running

#i went for an older scope that wasnt made in china ( not saying yours was ) but that has lasted well and is stored in a damp garage. A Hameg from the 80's £75 from the bay


Some libraries can get manuals for TVs etc have you tried them for a copy of the manual, try a main city library.

I noticed on other forums people having faults with these scopes and there was a schematic but I couldnt download it, seemed to be belgium or the netherlands

[daverave]

the metrix scope is from a french manufacturer...i did phone them but got nowhere as they do not sell their service manuals to the public but only offer to repair the scope themselves !!! if i cannot get anywhere with this scope i will just leave it boxed up and try and forget about it...i will never buy another metrix product again.

[F117]

There is a thread in this dutch forum http://www.circuitsonline.net/forum/view/71495 about a guy having trouble with the power supply of a second hand 0X8620.
They mention that Metrix is now part of Chauvin-Arnoux in France.
He sent a mail to [email protected] in French and they sent him the service manual in pdf.
You could give it a try - or maybe someone living in France could ask for the manual ?

[daverave]

im still battling to fix this oscilloscope....just swopped the oscillator chip but still dead...thanks for the info about chauvin arnoux in france....i did try their uk office but got nowhere so i will try their french office and hope they understand english

[elektro]

Hello everyone,
daverave check the resistors of high value power section could be that someone has altered value.
Sorry for my English.

[daverave]

most of the resistors are low wattage surface mounted type with small surface mounted brown capacitors...im really getting frustrated trying to repair this power supply....i thought maybe the chopper mosfet might be faulty but there are no shorts....what i cannot understand is that the oscillator chip only has a 2 volt reference on one of its pins when i read somewhere it should be 5 volts ???? but the supply voltage to the chip is correct at 12 volts.