OSCILLOSCOPE PROBLEM

[daverave]

To make my metrix crt oscilloscope switch on i had to push the power button in quickly and firmly and then the oscilloscope would fire up...but yesterday i went to switch on the scope and i cannot get the scope to switch on,i have the scope now in pieces and ive checked the mechanical power switch which seems to be ok and i cannot see any damage in the power supply section,could anyone give me an idea to what maybe the problem ???? (i used the scope for about 2 years by quickly and firmly pushing the power button in and now nothing).

[downhalo]

Probe the output of the power supply with your multimeter, also check continuity of the power switch in the on position..

If both are fine, you've got a bigger problem

[daverave]

its a bit difficult to test as ive separated the pcb from the crt tube...and the eht lead is disconected...i think its a power supply problem but not sure what to do ????

[Silver Dollar]

Some times there is a "soft" switch where the power button goes into a micro to wake it up.
And sometimes it goes to a 4013 flip flop that soft-starts a switching PS chip. That chip
monitors current and won't start if there's a problem. Can you post a clear picture of the
PCB with chips visible?

[daverave]

i decided to do some tests on the power supply of the oscilloscope and i first isolated the eht lead for safety....i found the voltage is getting to the primary side of the power supply but the primary chopper transformer transistor is not being driven by oscillator chip (UC 3842 AN) no oscillator output from chip !!! the supply voltage to the chip is 12v and the reference voltage about 1.5v....So i dont know whether to blame the chip or maybe something shutting the chip down as some sort of protection circuit ???

[CAS]

Try the easy things first. Replace any electros in the primary circuit. Also check for shorts on the secondary side. If at the moment of turn on there is too much current drawn, the ic wont start up. I would also check the cap off pin 8 (VRef). Should be a .1uf or something like that.

[daverave]

i bought this scope about two years ago second hand and it would only power up if i pushed in the power button quick and then would fire up...like a quick surge of power pressing button in quick made it work...but now nothing....i will check all the electrolytics but none have any bulged tops !!!! pin 8 (v ref) seems a bit low at 1.5v so will check the cap as you said...the cap is a tantalum type..is there any other things i can check concerning this (UC 3842 AN) ????

[CAS]

I have replaced many electrolytic caps that were faulty but we're not bulged. That issue you had with it not starting unless you pushed button quickly sounds like the startup cap in power supply is faulty. I always replace all electros in startup circuit if power supply won't start and there are no obvious faulty components. I am not a huge fan of tantalum caps as in my experience they don't seem to have the longevity of other high quality caps.

[daverave]

i will replace the tantalums with normal electrolytic and see if this solves the problem...when you say startup cap do you mean the tantalum connected to the ref pin of ic (UC 3842 AN) ???? or where would i look for this startup cap ???

[CAS]

Without seeing the circuit its hard to say exactly what cap it is but any smaller value electrolytic caps hanging off the ic I would replace. If that doesn't work, check the values of the resistors as some values may have gone high. If that still doesn't produce results, replace the ic. I don't think they aren't that expensive.

[daverave]

ive been working on the scope tonight without any success....i changed the two tantalums near to ic (UC 3842 AN) but still the same !!! i checked the electrolytics with a esr meter and all seem to be ok...i looked on the internet for a diagram of (UC 3842 AN) the resistor and cap in the ref network seem ok when i measured them within correct values..but the ref voltage is only 1.2 volts so im not sure if this is correct !!! and vcc is around 12 volts which i think is ok !!! most of the resistors and caps around the chip are surface mounted type and the caps unmarked to their value i also noticed an opto coupler nearby...not sure if that would play a part in the problem !!! as you say i may have to replace the ic and see what happens.....i really dont want to scrap the scope cause it was a top model several years ago and is a good spec...its very frustrating trying to fix this problem and im not having much luck lately with my test equipment.

[CAS]

Power supplies can be a pain in the a***, especially when its one you are not familiar with. I am repairing them every day and still have some I just want to throw in the bin in frustration. (Please note that when I say 'throw in the bin' I am being polite as I am pretty sure the forum rule wouldn't allow me to say exactly what I want to do lol.)

[daverave]

ive had really bad luck buying second hand test gear from ebay...this scope was faulty from the start...and i bought a hameg scope several months ago and even that has problems with encoder controls that need replacing...plus my digital multimeter packed up on me !!!!!!! i think the next time i buy a scope it will be the new lcd digital type as the old crt types are a pain in the a*** to dismantle and repair !!! i think i will have to get a service manual for this scope and will replace the (UC 3842 AN) and check these surface mounted resistors around the chip.

[golfnut]

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

[daverave]

the oscilloscope is a metrix OX 8620...i telephoned today the company that deals with metrix scopes and they will not even sell me a service manual...they said send the scope to us for repair...i said the scope is in pieces and anyway i cannot afford their repair cost...ive been going through the surface mounted resistors and they seem ok !!!!! but the small brown surface mounted caps i havent got a clue as to their correct value....i think i will replace the oscillator chip as the ic is very cheap....the print seems ok !!!! this scope always use to fire up if i pressed the power button in quickly and firmly...but now nothing !!! its very frustrating trying to fix this switch mode power supply....i prefer the old mains transformer technology at lest it was more reliable.