Teardown B&K 2120 Scope

[Performa01]

I took a look at the scope running with my Flir E4 and, wow, it has some resistors (R838,R839) that run 100 deg C !  Would this unit benefit from forced air?  That seems pretty warm but perhaps that is just the way it is made.

While usually not really a problem for a resistor, 100°C isn't particular sexy either. But the same goes for forced air cooling, doesn't it?

Why not just replace these hot resistors with bigger ones (rated for 2 or 3 times the power)?

This will provide a lower thermal resistance, thus lowering the temperature on the surface.

If the footprint doesn't fit, then just leave the resistor leads a little longer and bend them in a nice curve so they can fit. This way you also increase the distance between resistor and PCB, which isn't a bad idea for hot components anyway.

[alank2]

The two resistors in question are already 3W !!  I think mouser did show some 5W ones were available...  They do have a hole in the pcb underneath them probably to provide for some air flow...

[Performa01]

The two resistors in question are already 3W !!  I think mouser did show some 5W ones were available...  They do have a hole in the pcb underneath them probably to provide for some air flow...

What resistors are R838, 839 exactly? Carbon film? Carbon composite? Wire wound? And what values?
Maybe you could also show the relevant part of the circuit diagram, so we could decide which kind of replacement would be appropriate...

...because if inductance isn't a problem and ww. is an option, you can easily get cemented resistors with very good properties, e.g. KH214 series from Vitrohm.

[alank2]

They are for "Horizontal Final Amp" from XY mode. and go to the CRT.  15K.  Page 18 of the 2120 schematic.

[Performa01]

Okay, had a look at the service manual and found the answer: R838,839 are metal oxide resistors, hence have a rather low inductance. But then, there are two inductors L801,802 in series ("peaking coils")...

If your adventurous and like experimenting, you could get two 15k Vitrohm series 214-8 resistors. These are 9W and will have some 250µH inductance (measured at 100kHz). When replacing R838,839, the total inductance will rise, but that probably won't hurt. But these are rather big: 9 x 9 x 38mm - and Mouser doesn't seem to have them.

The 5W metal oxide resistors Xicon 286-15K-RC (Mouser) would be the safer bet.

And then, you could not change anything and just ignore these two little toasters in your scope ...

[alank2]

What about these:

MOS5C153J

Mouser has them in stock...

[Performa01]

Yes, they would be fine too of course.

The Xicon would just have been a bit cheaper, sorry, didn't realize there is no stock...

[elex_enthusiast]

Great teardonwn. I tooo have the same scope given by a friend here on eevblog with the same peaking issue on both channels and definitely needs calibration.. Here's the story: http://everist.org/NobLog/20150731_simple_scope_funny_fault.htm
It has been repaired by him but he didn't had time to calibrate. Its my first scope and its was a great opportunity to own one to learn from it. Sadly I dont have any function gen to use for calibration.

[alank2]

Thanks everyone for the help given in this thread - my 2120 is working so great it is like a different piece of equipment!!!

elex - calibration was not too difficult - the service manual covers it in very good detail.  Just be careful with the high voltages.  I picked up (any excuse!) a high voltage probe so I could check the -2kV test point as well.

[elex_enthusiast]

I already had the service manual. Unfortunately acquiring a descent function gen is way far beyond my financial means,  another thing is, I haven't figured out yet if there's somebody near my place has a function gen that I could borrow or rent for calibrating my BK scope. I'm leaving in an urban place and there's no such place like a cal lab here..

[alank2]

Some of the calibration steps are visual, "adjust this until this is visible on the display", and some are voltage only, "adjust this until this test point is 95VDC +/-5V", so you might be able to calibrate some of them.  I didn't end up having to change any of the trimmers for the various vertical gain settings which was nice, but it certainly helped having a frequency generator.  I picked up a SDG2082X last year and it is now one of my favorite tools.

[med6753]

Thanks everyone for the help given in this thread - my 2120 is working so great it is like a different piece of equipment!!!

elex - calibration was not too difficult - the service manual covers it in very good detail.  Just be careful with the high voltages.  I picked up (any excuse!) a high voltage probe so I could check the -2kV test point as well.

You're very welcome. Always glad to help!