Troubleshooting Issue with Oscilloscope

[bjosephs]

Hi All,

I fired up my BK 2120 today and have an issue. The trace is completely unfocussed regardless of channel, probe or AC/GND/DC setting. Focus control has no affect. On low interval settings it's essentially a big dot. In contrast, in this post from last May (the last time I took a photo of my scope in operations) I had a nice, crisp trace. It's been very cold lately in my basement shop so I've allowed it an hour or so to warmup with no change.

It's the weekend and I don't even know if there's a repair/calibrator in the area (or that the price of a repair is worth it for a scope I paid $100 for 10 years ago) so I'm posting here to see if any members have ideas for what to poke at. Thanks in advance.

 

[moamps]

Most likely the problem is in the power supply because a lot of functions do not work (deflections, focusing). It is good that the cathode ray tube is probably working, and that the high voltage is working. Attached is a schematic. Try checking the voltages in the power supply, + 140V, +/- 12V, + 5V. Page 5, upper part. Be careful, at some points the voltage is very high.

 

[JohnRoberts]

+1 check PS, especially hi Voltage.

$100 ten years ago, means it cost you $10 a year... that's fair. May be time for a new scope.

JR

 

[bjosephs]

Most likely the problem is in the power supply because a lot of functions do not work (deflections, focusing). It is good that the cathode ray tube is probably working, and that the high voltage is working. Attached is a schematic. Try checking the voltages in the power supply, + 140V, +/- 12V, + 5V. Page 5, upper part. Be careful, at some points the voltage is very high.

Thanks. This is the kind of insight I needed. This version schematic seems bit different from what I find under the hood (Not surprising given the production time frame and probable revisions) but it gave me some help in poking around. I was able find test points for 260v, 140v, +12v and -12v. All present and within 5% of target. I couldn't find a test point for +5. It looks like the Intensity and focus controls (middle of page 10) derive from the 2kv rail and I don't dare try and measure that because of my multimeter range. Any other thoughts on what to look at? Maybe I should make or buy a 1/10 divider to try and get a high voltage measurement into range?

 

[JohnRoberts]

be careful messing with kVolts... but you can make a crude voltage divider with resistors, that's all a 10x scope probe is (10M in series form 1/10 divider with 1M input termination). Be careful about current and power if not using megohm resistor values for a divider (ohms law is your friend).

Just be careful... I kept my old Heathkit (kit) scope working years beyond its life expectancy but at some point you just need to punt. I don't follow the market for used scopes but there are probably lots of them out there.

JR

 

[moamps]

I was wrong, the 2120B is quite different from the 2120. This is the schematic of the 2120. The power supply schematic is on page 19. Don't check the high voltage, it's too dangerous, and even if there's a dot on the screen, it probably works ok.

 

[CJ]

you might try to re-solder any power resistors you see,

 

[bjosephs]

I was wrong, the 2120B is quite different from the 2120. This is the schematic of the 2120. The power supply schematic is on page 19. Don't check the high voltage, it's too dangerous, and even if there's a dot on the screen, it probably works ok.

Those find numbers look correct. Thanks. Not sure what to check beyond the rails being present though

 

[bjosephs]

Just be careful... I kept my old Heathkit (kit) scope working years beyond its life expectancy but at some point you just need to punt. I don't follow the market for used scopes but there are probably lots of them out there.

JR

I agree 100%. That said, I haven't been staying up on good models. I literally walked into a local electronics repair shop and asked if they had an extra scope lying around when I bought this one. The guy took me out back and we tried a few together. I really don't like shopping for test equipment with so much out there. I guess I'll start googling for a place that repairs/calibrates to make I buy something that's been looked over a bit.

 

[moamps]

Those find numbers look correct. Thanks. Not sure what to check beyond the rails being present though

+24V, +5V

 

[bjosephs]

+24V, +5V

Of course, thank you. I was able to pick up both of these at their regulators (U503 and u504). I can't find a TP further downstream like with the other rails. The only weird thing is with U502: Get -12v on pin 2 but pin 1 I see +6vDC... perhaps this is a characteristic of using a 3 terminal positive regulator in this configuration (never seen it done like this).

 

[moamps]

Unfortunately, I have no more ideas that would not involve the use of oscilloscope.

 

[bjosephs]

Unfortunately, I have no more ideas that would not involve the use of oscilloscope.

Well I definitely appreciate the help. Time to start shopping I guess.

 

[scott2000]

I had all kinds of weird issues when I bought my used Tek scope and, being limited in my knowing what to do, I just started checking caps and there were a load of bad ones. Bunch of those blue Philips that I guess are sought after for audio and some others. Most were the small voltage electros iirc.. Guess I was lucky because everything worked and still does.

 

[ruffrecords]

Does the intensity pot work at all?

Edit: What I am thinking is there is a string of resistors associated with the focus pot (R526, R527, R528 and R529) plus the focus pot itself. Any one of these going open would give a problem.

Cheers

Ian

 

[bjosephs]

Does the intensity pot work at all?

Edit: What I am thinking is there is a string of resistors associated with the focus pot (R526, R527, R528 and R529) plus the focus pot itself. Any one of these going open would give a problem.

Cheers

Ian

Neither intensity nor focus have any effect. All other controls seem to be normal, I just have either a 1/4" diameter circle the nets sweep speed is set low.

I also tried twiddling the the focus and intensity trim pots on the board and neither of those seem to do anything.

 

[ruffrecords]

From the schematic posted above, there is a string of resistors and a 100V zener from the -2KV supply to ground. If any one component in this string goes faulty then neither control will work. At least you can check these with no power on.

Cheers

Ian

 

[bjosephs]

I haven't had the guts or interest to keep poking around though I may at some point in the future. I decided it was time to get another scope anyway. So, after shopping around for fancy new digital scopes and being turned off by the after-purchase licensed features, then trying out a friend's Hitachi V1050-f that had more issues than my B&K, I ended up buying a refurbed Tektronix 2465b on eBay. Seems like a good scope and the seller had two decades of 100% positive feedback for repaired/refurbed/recapped scopes. Fingers crossed it gets here safe and lasts 5 years or so.

If/when I crack the B&K open again I'll report back on findings. Right now I want to get back to tinkering with the real projects.