Faulty analogue scope - suggestions on where to look next

[timbos]

Hi,

Not sure whether this should be here or in the test-gear forum...

I've got an old Crotech 3132 20MHz scope that stopped working yesterday. After having it switched on for a couple of hours, it's released an acrid burning smell and isn't producing a trace on screen. I've taken the covers off and think that I've diagnosed that there's a fault on the EHT board (see page 4 of the attached diagram):

http://www.tjstephens.com/images/Crotech3132 _circuits(wm).pdf

Observations:

I've got the circuit diagram for the board after trawling the web, and I think that the board consists of an oscillator (a Wien oscillator?) and a two-stage (Cockroft-Walton ?) voltage multiplier. That's where my understanding kind-of ends.

The NPN transistor Q601 is on a heatsink and was very hot when I first checked it after unplugging the scope (and making sure that the various caps on this board were discharged).
The two resistors (R613, R614) on the output stage of the transformer have clearly been very hot. Their paint is peeling and the surfaces are charred, although they measure 5.8K and 6.1K, so they're still performing at about specification.



Can you suggest where I might look next? I'm not sure how to confirm whether these observations are a symptom or the actual fault (my guess is that something else may have failed upstream)? Is there anything else that I should be looking for?
 

[RiverTown]

I would first check all the capacitors on that board. That acrid burning smell could be electrolytic or tantalum capacitor failing. Even some block capacitors (mostly X2 rated) will produce bad smell when failing.

[valentinc]

      First, check for any obvious blown components... If it was a burning smell, certainly there is something obvious that needs to be adressed first... It could be a faulty capacitor, as RiverTown said, but check first... It could also be something else or, if it's a tantalum that shorted out, it could be blown out some active devices too...

[tautech]

Shift this thread to Test Equipment, where it will be seen by more members that can help.
The schematic has various test points for fault finding(circled).
Check all PSU voltages are in spec.
I could spot only 1 tant on the schematic, but if they are used at all, there are likely more.
Think about the components that are stressed(current, voltage, heat) and check them next.