BK Precision 2120 Oscilloscope Repair - No Sweep
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- Опубліковано 1 бер 2021
- Another episode in vintage electronics. Troubleshooting loss of horizontal sweep in a BK Precision 2120 analog oscilloscope reveals a defect in the voltage regulator circuit of a linear DC power supply. What does it mean when a resistor reads spot-on the correct value in one direction and infinity in the opposite direction?.
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Nice catch with that sneaky resistor. Usually an open circuit resistor would look toasted!
Hi Lawrence, here we have a master piece of how to perform a trouble analysis and fault finding. Like a real teacher...Thanks a lot
Thanks so much. Glad you liked it.
Nice troubleshooting and circuit analysis.
Things usually don't go so smoothly! Thanks for commenting.
Thank you. Excellent troubleshooting and repair. Repair Tip = I do not fault you, at all, for soldering the replacement resistor from the topside of the board. Sometimes, it just makes better sense. However, as I am sure that you are aware, there are two possible negative outcomes... 1) The pin stubs may reflow the solder, on the bottom of the board, and the stubs drop out. 2) If they do drop out, they may fall down on a lower component and cause a short. So, it is a good idea to secure the pin stubs, before soldering to them. A little dab of super-glue between the stub and board seems an easy way to accomplish that.
Thanks so much. I did not expect so much interest in a 30?-year-old oscilloscope.
Love your work sir. Thank you!
Glad you liked it. Thanks for commenting,
Thank you, I really enjoyed the logical diagnostic steps taken to find the fault as well as the explanation of the effect that the open resistor had on the circuit. And as far as your SMD and "SMPS rubbish" comment I say, "'You are preaching to the choir with me!'"
Glad you found it helpful. I wish all my repair attempts were as successful. I don't do surface-mount, period. Thanks for commenting.
Love the video.
Thanks so much!
That resistor might be giving you a false reading on your multimeter if there's another component in parallel with it, that's why it's always a good idea to try measuring the resistor out-of-circuit, if you still get the same readings with the resistor out-of-circuit definitely replace it for sure.
Thanks for the video. I've recently gotten an old Hameg 203-6 where the sweep doesn't work and this gives me a good idea on how to figure out what's causing it.
Glad you found it helpful. Hopefully it will be something simple like a power supply problem. Thanks for commenting.
Well, that's definitely a new one on me, never seen a resistor go faulty like that 47k resistor before.
Nice video! :D
Thanks!
My first scope was the B&K 2120, still have it.
Now I move to digital scopes.
Best Regards.
This is my first oscilloscope too. I did a lot of electronics as a kid, but never had my own scope. 50 years later I finally bought one. Thanks for commenting.
@@quetzal4042 I start in electronics at early 8 years old.
I bought the B&K 2120 maybe 35 years ago.
Now I move to digital scopes only.
I love electronics too.
Best Regards.
I think I figured-out why you're getting those funny resistance readings with that 47k resistor (R506), and it all has to do with the Base-Emitter junction of Q 502, here's my theory when you measure the resistor in one direction and get a resistance reading of 47k you're effectively biasing the Base-Emitter junction of Q 502 off, remember that the Base-Emitter junction of a transistor forms a diode, when you measure the 47k (R506) resistor in the other direction, you're effectively biasing Q 502's Base-Emitter junction on, so you get a false reading because then the 100kF resistor (R 507) is effectively in parallel with the 47k resistor (R 506), that's why you should always try to measure resistors out-of-circuit if you suspect them as being faulty.
Wish you could help me with mine...goldstar 9020p....only works on x10 mag..
It could mean that the resistor that reads spot-on in one direction and open-circuit in the other direction, has somehow become a diode, usually the normal failure-mode of a resistor is to go open-circuit in both directions, one time I did encounter a resistor that had gone short-circuit, not exactly sure how that happened, are you sure it's a resistor and not a diode?
Hi Lawrence, where did you find the circuit diagram for this scope? I have the exact same model!
Paul,
Here is the link:
www.opweb.de/english/company/BK_Precision/2120
Thanks for commenting.
sir I have a cro with a short horizontal beam(sweep) around half division but the intensity, focus, vertical & horizontal controls are working fine, which section should I check to get a full sweep(line) on the screen, please help
If it has an X/Y mode (one channel vertical input one channel horizontal input) put it in that mode, then see how much horizontal deflection you get with a known AC voltage on the X channel, compare that with same voltage on the Y channel. If horizontal deflection is much less then must be a problem in the horizontal amplifier and output section. If horizontal deflection is ~ same as vertical, then must be in the horizontal sweep generator circuits.
@@quetzal4042 thanks sir for reply
hi lawrence, i also have as 2120 that i just got from a thrift shop and it also does not sweep. we might have a similar problem. may i know the value of the reisistor you replaced? ohm and watt. i can only do minor electronics. and i might be able to do the same. thanks!
It was R506, 47,000 ohms and 1/4 watt. But first, check the 260V test point with a multimeter to determine whether the problem is in in the sweep circuits or the power supply.
@@quetzal4042 ok got it. thanks!!
Hello Sr. Francis, I need one more time your help. The oscilloscope on channel 2 holds the 1Khz test signal well at 2 Volts peak to peak with no signal loss. While channel 1 reflects this same signal to me below the volts on a scale of 50 mVolts and using the attenuator it is possible to see the signal. The button that selects channel 1 only works by placing the reverse signal button. You can see that the 15 volt supply drops to only 5 and I have already separated the supply and isolated it and the drop is maintained, the 7812 regulator is in good condition. What opinion can you offer me to see if I can repair it, this was a gift that my uncle gave me, may he rest in peace.
Hello again. First let me say I am no expert on oscilloscopes and I don't fully understand how the vertical selection works but I will help if I can. Second, I am a little confused by your description. So you are saying that channel 2 works normally but channel 1 gives no response in normal mode and a very low signal in inverted mode? Is that right? Then you say that the 15 volt supply drops to 5. I don't see a 15 volt output on the power supply, the outputs are -12, +5 and +24. The 7812 is supposed to regulate at 12 volts. Also there are two 7812's in the instrument, so you mean U502 in the POWER SUPPLY, not U602 in the TRIG & SWEEP? And under what conditions does it drop to 5 volts? Even if it is disconnected from its load? And I don't understand how it can be a power supply problem if channel 1 works but channel 2 doesn't. Does the vertical amplifier even use the -12 volt power supply? Not that I can see. So at this point I am pretty confused what could be wrong and I need more information.
Hello Mr Francis, excellent video. Will you have the electrical diagram of the oscilloscope?Ihave faults on Channel 1.
Juan Carlos, So glad you liked the video. The service manual with complete circuit diagram is available for free download at:
www.opweb.de/english/company/BK_Precision/2120
Thanks for commenting,
@@quetzal4042 thank you mr. Francis. I already downloaded the manual.
God continue to bless you and your family.
Greetings from Panama.
I have the same model and the x is not parallel with the grid. Thoughts?
You mean the trace with no signal isn't exactly horizontal? Isn't that the Trace Rotation adjustment, between the Intensity and Focus knobs?
I should add after adjusting trace rotation it tilts again when turning the var sweep.
And after trace adjustment it tilts back on its own.
The sine wave does not l ook like a sine wave. It is distorted.