Author Topic: BK Precision 2120 Oscilloscope - trace line not displaying on CRT (Read 4465 times)

AmpBuilder · « **on:** March 07, 2021, 11:50:33 pm »

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

abrarbaig · « **Reply #1 on:** March 08, 2021, 07:30:14 am »

Check the zener diodes.

AmpBuilder · « **Reply #2 on:** March 08, 2021, 07:47:49 pm »

Which section do I need to check the zener diodes?

bdunham7 · « **Reply #3 on:** March 08, 2021, 07:51:35 pm »

In the XY mode, can you get the green dot to move by applying a signal to the CH1 and CH2 inputs?

AmpBuilder · « **Reply #4 on:** March 08, 2021, 08:24:39 pm »

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?

bdunham7 · « **Reply #5 on:** March 08, 2021, 10:40:57 pm »

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?

AmpBuilder · « **Reply #6 on:** March 08, 2021, 11:26:38 pm »

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.

bdunham7 · « **Reply #7 on:** March 08, 2021, 11:43:21 pm »

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?

AmpBuilder · « **Reply #8 on:** March 09, 2021, 12:24:42 am »

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.

bdunham7 · « **Reply #9 on:** March 09, 2021, 12:31:34 am »

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?

AmpBuilder · « **Reply #10 on:** March 09, 2021, 02:41:22 am »

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.

bdunham7 · « **Reply #11 on:** March 09, 2021, 03:20:20 am »

OK can you use the following settings:

TIM/DIV to 2ms
VERTICAL MODE buttons--first two buttons out, second two buttons in
CH1 and CH2 input selectors to GND, not AC or DC.
TRIG LEVEL centered and pushed in
XPOS 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?

AmpBuilder · « **Reply #12 on:** March 09, 2021, 06:39:27 pm »

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

bdunham7 · « **Reply #13 on:** March 09, 2021, 07:40:29 pm »

Quote from: AmpBuilder on March 09, 2021, 06:39:27 pm

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?

Anything 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.

AmpBuilder · « **Reply #14 on:** March 09, 2021, 08:14:24 pm »

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.

bdunham7 · « **Reply #15 on:** March 09, 2021, 08:30:50 pm »

Quote from: AmpBuilder on March 09, 2021, 08:14:24 pm

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.

OK, 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.

AmpBuilder · « **Reply #16 on:** March 09, 2021, 10:05:11 pm »

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?

bdunham7 · « **Reply #17 on:** March 09, 2021, 10:27:31 pm »

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

AmpBuilder · « **Reply #18 on:** March 10, 2021, 10:57:20 pm »

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?

bdunham7 · « **Reply #19 on:** March 14, 2021, 12:36:09 am »

Quote from: AmpBuilder on March 10, 2021, 10:57:20 pm

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?

You 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.

AmpBuilder · « **Reply #20 on:** March 14, 2021, 03:31:24 am »

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.

bdunham7 · « **Reply #21 on:** March 14, 2021, 04:12:02 am »

Quote from: AmpBuilder on March 14, 2021, 03:31:24 am

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.

Welcome 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.

AmpBuilder · « **Reply #22 on:** March 14, 2021, 05:43:49 pm »

Will this work even with pin 6 of U505 opticoupler open?

bdunham7 · « **Reply #23 on:** March 14, 2021, 06:32:19 pm »

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.

AmpBuilder · « **Reply #24 on:** March 17, 2021, 08:25:12 pm »

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?


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Author Topic: BK Precision 2120 Oscilloscope - trace line not displaying on CRT (Read 4465 times)

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