B. P. Precision 2121 Oscilloscope Smoking Capacitors- need help repairing

[jkbk1969]

Hi, I work in a Technical college and this week alone I had 5 bk precision Oscilloscope pop and blow smoke and 2 power supplies do the same. I have been able to repair the PS but the scopes have been more challenging. The students said they turned them on and they popped then started to smoke.
 I found that one or two of the capacitors in the Power supply section. So I changed the two capacitors (C902-C903) I get power, and the freq. display lights up, but it doesn't read and the CRT does not display anything.
I was wondering if anyone else has had this problem.
Thanks Jackie

[Chris56000]

Hi!

Can you provide a link to the rest of the schematic/manual please?

I can help you go thro' this and fix it if I have the rest of the bookwork – from the snippet you provided it's possible C902/C903 failed off their own bat but there's not enough on that extract to help fault find the rest of your scope!

Chris Williams

[SilverSolder]

Wow, so many things breaking at the same time?  Do you have a problem with the mains voltage being too high or spiking?

Or are the students plugging 110V equipment into 220V?    lol

[jkbk1969]

This class has been special, we just have a lot more students, but I'm thinking they are just old.  I tried to upload the manual but it it not working right now. I can not find the bk precision manual. But the Elenco has all the same boards in it. I looked. The elenco only needed some adjustment.

https://www.manualslib.com/manual/235958/Elenco-Electronics-S-1325.html

[Doctorandus_P]

How old are these CRT scopes, and how long have they not been used?
It seems that you dusted of some long unused equipment for a big class...

A known failure of (high voltage) electrolytics is if they are stored for a long time the oxide gets damaged (Disolves in electrolyte? Converted back to aluminimum?) As a result the capacitors start leaking a lot of current when voltage is reapplied.

Old Electrolytics can often be regenerated by slowly increasing the voltage, limiting the input current, and checking the do not get to hot in the process. When they are able to withstand the full rectified mains voltage again you should keep them charged for a few hours or days to strengthen the oxide layer.

There is plenty of literature about gegenerating old Elco's.