2X BK Precision 2120C Oscilloscopes with Display Issues - Page 1
Forum section: electronics / repair

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: March 15, 2018, 09:44:48 pm
content: I recently rescued two oscilloscopes from being scrapped at work. They are BK Precision 2120Cs, both of which have display problems. They were likely bought around the same time and have been used by the calibration group, which really doesn't change the settings very often, if ever. For the most part I'm willing to bet the only control they ever touched was the power button.These scopes are probably left on all day every day and only turned off when the work day ends. The scopes have the following problems:Scope #1:The first scope I picked up reportedly had no trace. However, when I brought it home and powered it up a trace did eventually appear, though it took a while and was extremely dim:I opened up the case and performed the sniff test and a visual inspection, but I cannot seem to find anything out of place. I did notice that the coil wound around the tube was loose and I re-secured it with epoxy, but that did not seem to make any difference. I fiddled with the intensity knob and trimmer pot but could not seem to make the trace any brighter - only dimmer. The other controls appear to work as expected - just the trace is dim, and that's the case for both channels.Scope #2:The second scope I picked up had a nice bright trace, but its behavior was a bit funky. When I first powered it up it seemed to work just fine, with the exception of a little burning on the left hand side of the screen (the image makes it look worse than it really is). However, after letting it sit for a few seconds eventually the trace became very wide (1/4-3/8 inch or so?) and would not respond to the intensity control. It was also difficult to tell whether or not the focus control was making any difference.After initial power-up:After a few seconds:The trace would come in and out - sometimes it would be clean and focused, sometimes it would get thick and blotchy again.Does anyone have any ideas of what might be up with these scopes? Again, being that they were probably purchased around the same time and used for roughly the same purpose for the same number of years, I wouldn't be surprised if the problems were related and possibly a known issue with these scopes....?Thanks

author: DC1MC
author_rank: Super Contributor
timestamp: March 15, 2018, 10:17:37 pm
content: I can only help you with a split schematic and and a hint: capacitors, capacitors, capacitors, as Lenin said.Fortunately the schematic seem simple enough and you need just a multimeter, but be very carefully, the trace intensity and focus control has deadly voltages !!!Good luck,DC1MC

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: March 15, 2018, 10:46:37 pm
content: Quote from: DC1MC on March 15, 2018, 10:17:37 pmThank you for the schematic. Unfortunately that does not seem to be the C type, so the circuitry is actually quite different.

author: tautech
author_rank: Super Contributor
timestamp: March 15, 2018, 10:59:50 pm
content: Quote from: The_PCB_Guy on March 15, 2018, 10:46:37 pmEven so, I'd be focusing (excuse pun) on the EHT (HV) circuitry where the problem is most likely to be.Zeners, resistors, caps and pots are all under some stress so check them all.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: March 15, 2018, 11:10:12 pm
content: Quote from: tautech on March 15, 2018, 10:59:50 pmThis was my instinct as well (this is not the first time I've ever worked on failing scopes - I've done multiple repairs). I was just wondering if there's a specific section to focus (excuse the pun) on first, especially if anyone here had experience with this exact problem.

author: tautech
author_rank: Super Contributor
timestamp: March 15, 2018, 11:48:36 pm
content: Quote from: The_PCB_Guy on March 15, 2018, 11:10:12 pmVoltages first then the voltage differences between the HV sections eg. cathode vs grid.All high value resistors to check they haven't drifted as a result of the stress they're under.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: March 15, 2018, 11:54:34 pm
content: Quote from: tautech on March 15, 2018, 11:48:36 pmGot it. Wish I could find a schematic for the 2120C....

author: tautech
author_rank: Super Contributor
timestamp: March 16, 2018, 12:05:17 am
content: Quote from: The_PCB_Guy on March 15, 2018, 11:54:34 pmThe Mohm resistor dividers in the EHT section are normally pretty easy to spot likewise the HV caps.In circuit they might measure less that marked values, any that don't lift a leg and measure again.Generally all greater that 500k need be checked in the EHT area.

author: texaspyro
author_rank: Super Contributor
timestamp: March 16, 2018, 03:30:12 am
content: The dim unit could be a worn out CRT cathode. Swap the tubes... there's a good chance you will have a good unit and a double bad unit.

author: cvanc
author_rank: Frequent Contributor
timestamp: March 16, 2018, 02:00:47 pm
content: Quote from: texaspyro on March 16, 2018, 03:30:12 am^^^This.^^^

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: March 16, 2018, 04:32:37 pm
content: Quote from: texaspyro on March 16, 2018, 03:30:12 amUltimately this was what I planned to do but I was hoping I might be able to salvage both scopes.

author: DC1MC
author_rank: Super Contributor
timestamp: March 16, 2018, 08:58:03 pm
content: Quote from: The_PCB_Guy on March 16, 2018, 04:32:37 pmBarring a fully defective CRT or hi-voltage trafo, there is NOTHING preventing you doing this, I've seen in another thread that someone contacted BK Precision and got a full service manual, because this is the only thing that holds you back.Cheers,DC1MC

author: tautech
author_rank: Super Contributor
timestamp: March 17, 2018, 01:33:10 am
content: Quote from: DC1MC on March 16, 2018, 08:58:03 pmHear, hear.If these scopes were 40 years or older, yes the CRT's could be stuffed but I very much doubt these ones are.In a CRO the EHT and CRT circuitry is under the greatest stress and it's not uncommon to need to rework some areas.