BK Precision 2120 Oscilloscope - trace line not displaying on CRT - Page 1
Forum section: electronics / repair

author: AmpBuilder
author_rank: Contributor
timestamp: March 07, 2021, 11:50:33 pm
content: I have a BK Precision 2120 oscilloscope that the trace line is not displaying on the CRT. I can push the XY button and get the green dot on the CRT, but that is the only thing that is displaying. I replaced 3 resistors that had gone up in value and 1 open in the horizontal final amp R832,R834,R835 and R836. Is there a possibility that the CRT could be defective or a problem elsewhere? Any help would be appreciated.Marty

author: abrarbaig
author_rank: Contributor
timestamp: March 08, 2021, 07:30:14 am
content: Check the zener diodes.

author: AmpBuilder
author_rank: Contributor
timestamp: March 08, 2021, 07:47:49 pm
content: Which section do I need to check the zener diodes?

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2021, 07:51:35 pm
content: In the XY mode, can you get the green dot to move by applying a signal to the CH1 and CH2 inputs?

author: AmpBuilder
author_rank: Contributor
timestamp: March 08, 2021, 08:24:39 pm
content: I injected a signal in channel 1 and 2 in XY mode and was able to get the green dot to move.How do I need to proceed from here?

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2021, 10:40:57 pm
content: OK, did it move both up and down and side to side?Can you post a photo of the unit from the front? There were several versions of the BK 2120. How familiar are you with oscilloscopes?

author: AmpBuilder
author_rank: Contributor
timestamp: March 08, 2021, 11:26:38 pm
content: The line did go up and down and from side to side in the XY mode. I am an electronic technician,2way radio tech by trade.Also i deal in a lot of tube circuit amps,i know some about oscilloscopes and can repair if I could narrow down the circuit that is defective.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2021, 11:43:21 pm
content: OK. So to sort it out, it seems like it probably isn't triggering. If you set the trigger to LINE or AUTO with the intensity up high enough and CH1 and CH2 both on and inputs set to GND, you should be able to bring a trace onto the screen with the vertical and horizontal position controls. If you can't the problem is probably related to triggering or possibly blanking. Have you downloaded a manual and schematic?

author: AmpBuilder
author_rank: Contributor
timestamp: March 09, 2021, 12:24:42 am
content: I did what you said and still not getting my trace line,just the green dot in XY mode. So your saying the problem could be either in the triggering circuit or possibly blanking circuit? I have the servicing information printed out which shows the parts and schematics. I don't know if there is a service manual out there that goes more into detail on the different circuits.

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 12:31:34 am
content: Could you post a clear picture of the front panel and controls? You want to do as much sorting out as possible before you crack it open and I need to see exactly which version you have.And how bright is that dot in the XY mode with the intensity all the way up?

author: AmpBuilder
author_rank: Contributor
timestamp: March 09, 2021, 02:41:22 am
content: The dot is very bright with the intensity turned all the way up in the XY mode. I have already opened up the scope and done some minor troubleshooting.See attachment of the front panel of scope.

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 03:20:20 am
content: OK can you use the following settings:TIM/DIV to 2msVERTICAL MODE buttons--first two buttons out, second two buttons inCH1 and CH2 input selectors to GND, not AC or DC.TRIG LEVEL centered and pushed inXPOS centered and pushed in. Move it around each time you change any of the settings.Both POS controls centered. Move them both around each time you change something.COUPLING and SOURCE set to LINE and EXT. Try mixing these selectors up and try all the positions and combinations once everything else is set. Put an AC signal into the EXT trigger connection and try all 4 of the COUPLING positions with the SOURCE set to EXT.If you still have absolutely nothing, the next place to look is the schematic I have attached. What do you have for other test equipment?

author: AmpBuilder
author_rank: Contributor
timestamp: March 09, 2021, 06:39:27 pm
content: I tried the settings that you advised me to do and still have no trace line. I have checked the transistors,diodes and resistors in the circuit you sent and they seem to check good,also I have the correct voltages on the test points. Is there a possibility that U603 could be defective?Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 07:40:29 pm
content: Quote from: AmpBuilder on March 09, 2021, 06:39:27 pmAnything is possible, but the way to proceed now would be to track the trigger signal from the source switch to U601. Do you have any other equipment to do that with? Another scope would be the typical way. It becomes very difficult without one.

author: AmpBuilder
author_rank: Contributor
timestamp: March 09, 2021, 08:14:24 pm
content: I have a TENMA 72-300 as a backup. I don't like it but will use it to troubleshoot the circuit anyway. That is why I want to get the BK Precision scope back up and running.

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 08:30:50 pm
content: Quote from: AmpBuilder on March 09, 2021, 08:14:24 pmOK, something is way, way better than nothing. Switch the trigger source to LINE and look for a 60Hz signal at the COM pins of S602. Then look for the signal at pin 9 of U601 and pin 3 of U603.

author: AmpBuilder
author_rank: Contributor
timestamp: March 09, 2021, 10:05:11 pm
content: I have signal on pin 9 U601 and signal on pin 3 U603. The signal looks different pin 3 U603. Should I see a signal on U603 other than pin 3?

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2021, 10:27:31 pm
content: The signal will be different, it's gone through filters and the op amp. U603 pins 5 and 6 should be alternating high and low in normal operation. You'll have to look at the 7474 diagram and the BK 2120 schematic and sort of figure out how it works. One pointer I'll give you is that the unblank pulse should be allowing the CRT beam to turn on. Even if there were no horizontal sweep, you would see a dot on the screen when you turned the horizontal position knob to the right--that would be the normal resting spot for the retraced beam. So either you don't have an unblank pulse or the CRT drive circuit is not responding to it. I'll be travelling the next few days, but I'll check in and see how you're doing from time to time.https://console5.com/wiki/7474

author: AmpBuilder
author_rank: Contributor
timestamp: March 10, 2021, 10:57:20 pm
content: I did some checking in the blanking circuit. I have +24 volts going to R520 2.2k and .8 volts on the other side of that resistor going to Q505,shouldn't the voltage be higher than that?

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2021, 12:36:09 am
content: Quote from: AmpBuilder on March 10, 2021, 10:57:20 pmYou should scope it and see if it varies. Try going back to XY mode and see what you get at Q505. BE CAREFUL--you are in close proximity to and only one component away from -2kV.

author: AmpBuilder
author_rank: Contributor
timestamp: March 14, 2021, 03:31:24 am
content: I was checking around that area with my scope and accidentally shorted the base on U505 and now the base reads open. I now have nothing on my display even with the XY button pushed in. I have another U505 on order,i hope I didn't damage the CRT when i shorted it. I will update once i replace U505.

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2021, 04:12:02 am
content: Quote from: AmpBuilder on March 14, 2021, 03:31:24 amWelcome to the club! (Unless you were already a member...) If you fix stuff long enough you'll learn to appreciate shielded test probes and sharp tips.OK, so try this:First, remove Q505 entirely, see what you have both on the display and for a voltage at the junction of R520 and R522. Then put a 47R resistor across where the C and E of Q505 goes and try everything again. I doubt you damaged the CRT, but hopefully that short wasn't to the high voltage or you might have some additional damage.

author: AmpBuilder
author_rank: Contributor
timestamp: March 14, 2021, 05:43:49 pm
content: Will this work even with pin 6 of U505 opticoupler open?

author: bdunham7
author_rank: Super Contributor
timestamp: March 14, 2021, 06:32:19 pm
content: Ooops! We were discussing Q505 and I didn't read carefully. So no, don't do any of what I said.Why would you probe anywhere on the high voltage side? You can't do that safely without special equipment and you may have damaged something else in the process. Be glad you weren't zapped.In the meantime, what you need to be looking for is for Q505 to be turning on and off when an the scope has an appropriate trigger input. You don't need a CRT beam to do that, so you can try it before your parts arrive.

author: AmpBuilder
author_rank: Contributor
timestamp: March 17, 2021, 08:25:12 pm
content: Replaced defective U505,Q508 and Q509 still no green dot displaying when the XY button is pushed in. I replaced Q508 and Q509 with some generics that i had in my parts bin. Didn't know if i need to replace with original replacement's for this circuit to work properly. Also i was wondering if i blew the CRT when U505 was accidentally shorted with Q508 and Q509 being damaged in the process. Is there a way i can check to see if the CRT is still good or not?

author: bdunham7
author_rank: Super Contributor
timestamp: March 17, 2021, 08:38:20 pm
content: Were Q508 and Q509 confirmed bad by testing? You should test D507 and D514 and maybe D502 as well to make sure they aren't blown open.Do you have 24 volts at the R520/R522/Q505 junction? And (carefully!) measure the voltage at pin 1 of U505. You may need to adjust VR502.I doubt the CRT would be damaged, but you might try removing Q508 and make sure you have a beam then. (do this last)

author: AmpBuilder
author_rank: Contributor
timestamp: March 17, 2021, 09:20:38 pm
content: Yes Q508 and 509 were bad when i tested them.

author: AmpBuilder
author_rank: Contributor
timestamp: April 10, 2021, 09:50:55 pm
content: I replaced Q508 and 509 with correct replacements,also I replaced shorted ZD501. I still don't have my green dot with the XY button pushed in,what else do i need to check? This was in reference to U505 being accidentally shorted out.

author: bdunham7
author_rank: Super Contributor
timestamp: April 14, 2021, 03:45:12 am
content: Did you check for 24 volts at the emitter of Q505 (and elsewhere) as I had asked above?If you remove Q508 entirely, I think you should get a beam even if there are problems elsewhere in the circuit. If not, then you have to start looking at the -1867, -1900 and -2000 volt points. As far as measuring, unfortunately there is 2000 volts potential in that area and you really need the proper equipment before you go poking there.

author: AmpBuilder
author_rank: Contributor
timestamp: April 15, 2021, 06:22:56 pm
content: I have lost my high voltage completely off the secondary winding of the power transformer,but I am still getting the 31 and 6.3 volts volts that are tapped off the high voltage winding,looking at the schematic on the power supply. So it looks like i can't go any further with this scope unless I can find another power transformer,any suggestions?

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2021, 12:08:56 am
content: The floating voltages that you still have are not related to the actual HV generator circuit--look at your schematic and see if you have an input to T502 from Q511 and U506. Unless T502 measures open on the secondary (or primary, I suppose) I doubt you've blown it up.

author: AmpBuilder
author_rank: Contributor
timestamp: April 16, 2021, 03:38:22 am
content: The transformer i was concerned about is T501. Looking at the schematic for the power supply it is the 1st winding at the top of the page,on one leg of the winding it shows 1,867 volts out,on the other leg it shows 1.9k out. Also I checked the input voltages in the high voltage generator and they are all there,I did find R546 820k way out of value,I replaced it but didn't fix the issue with the 2k volts out missing.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2021, 04:08:51 am
content: Those voltages from the power transformer are just 6.3V for the filament and the 33VDC supply for the beam current, etc. T501 doesn't supply the HV, it just floats up with it. The HV generator supplies the -2kV to the CRT control circuit, ZD502 drops it down (or up?) to -1.9kV and ZD501 to -1867kV.If you have the all of the DC supplies to the HV generator but no output, then something has gone wrong there. If you don't have another scope, testing components seems your only option. I'd look at Q511 first.

author: AmpBuilder
author_rank: Contributor
timestamp: April 17, 2021, 05:33:38 pm
content: I checked Q511 and it is ok. I have -11.5 volts off of pin 6 U506 and still not getting the -2kv out of high voltage generator circuit. I checked all caps,resistors and diodes in the circuit and they all checked good. I have continuity in the primary and secondary winding in T502. Any suggestions where I need to go from here?

author: bdunham7
author_rank: Super Contributor
timestamp: April 17, 2021, 11:16:40 pm
content: I guess start poking with your other scope and see what you have in the U506/Q511 area for voltages. See what you have at pin 6 U506 and if that changes when you adjust VR508. How did you test Q511?

author: AmpBuilder
author_rank: Contributor
timestamp: April 18, 2021, 03:32:25 am
content: I tested Q511 with my Simpson 260.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2021, 04:44:09 am
content: Quote from: AmpBuilder on April 18, 2021, 03:32:25 amI'm not sure how you do that--my Simpson doesn't have the hFE function nor the transistor test jack.I missed your statement about pin 6 on U506. That looks like it is pretty much at the rail and trying to command the oscillator circuit to increase output, but I'm still not sure exactly how that oscillator circuit behaves. Still, it is only a few parts plus Q511, so not much else to go wrong. I would guess that the design depends on Q511 to have a pretty high gain. It might be instructive to measure all the voltages around Q511 and see if you think it is on, off, or something else.

author: AmpBuilder
author_rank: Contributor
timestamp: April 21, 2021, 12:13:31 am
content: I retested Q511 with another piece of test equipment and reconfirmed it is good. I am still getting -11.5 volts out of U506,but still no -2K out of the H.V. generator. I am wondering if T502 could be bad. I have just about exhausted everything in this circuit that I know to troubleshoot.

author: slbender
author_rank: Regular Contributor
timestamp: July 03, 2021, 03:35:38 pm
content: Earlier today, I posted on another thread about cold solder joints throughout a B&K 2190, I suggest you closely inspect the boards and connectors on your 2120, and do some solder sucking and resoldering prior to replacing more parts…Steven