Author Topic: BK Precision 2120 trace issues  (Read 13574 times)

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Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #125 on: August 04, 2020, 04:05:06 pm »
The voltmeter modules draw their power from the circuit and the 10K pot can't supply much current.  They aren't a good idea for this application.

So on those tests, what ranges of voltages did you see on the input side and on the trigger level side?  You should do this again and make a chart showing what the input voltage side measure at with the pot all the way up, 75%, 50% and so on.  Then do the same for the trigger level pot.  You will need to be able to set certain voltages without too much measuring because the next step will be to send those voltages to the comparator U601 and observe its output.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #126 on: August 04, 2020, 10:33:48 pm »
The changes were very brief unfortunately, generally ranging between 1.25 V and ~3 V. I don't think I saw anything higher than 5.5 V. Of course, it might have just been noise since it also settled down to 0 V. This definitely seems like the sort of situation where another scope would be useful, is it time to try using a soundcard scope?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #127 on: August 05, 2020, 12:24:32 am »
The one on the trigger level side should be steady.  The other one, I think, should also be steady if the coupling is set to TV-V/DC, but let me review the schematic. Are you saying they both settle down to 0 volts no matter where you put them?

Yes, we are getting to the point where another scope would be extremely useful.  I don't want you to damage your computer, but I suppose as long as you used high value resistors for the input maybe it works.  Do you have soundcard scope software and an adapter cable?  Your sig-gen might be useful too, but the output is low.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #128 on: August 05, 2020, 12:46:48 am »
I'm not actually sure which side is which, but both sides settle at 0 V when changing the pot on the battery pack, one side settles at 0 V with minimum trigger level and 0.031 V at maximum trigger level, the other side settles at 0 V with minimum trigger and 0.015 V with maximum trigger.

Do you have soundcard scope software and an adapter cable?

I've got both of those things, it's somewhat janky because I bought the wrong connectors but seems better than nothing.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #129 on: August 05, 2020, 03:57:35 pm »
OK, why don't you see if you can get your soundcard scope to work with your signal generator and show some waveforms.  Then you can try feeding them in to see if  you can trip the comparator.  I don't actually think the problem is in this area, but it needs to be confirmed before moving on to the trigger logic circuit.  Use a resistor on the input of the soundcard scope, I'd recommend at least 10K, to minimize the possibility of computer damage. 

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #130 on: August 06, 2020, 03:06:00 am »
OK, why don't you see if you can get your soundcard scope to work with your signal generator and show some waveforms.

Done, waveform screenshot is attached.

Then you can try feeding them in to see if  you can trip the comparator.

What do you mean by that? Use the signal generator as the EXT input and run the previously mentioned tests?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #131 on: August 06, 2020, 04:33:05 am »
Yes, do the same tests to prove out the system.  Make sure you have the ground side of the signal generator and the soundcard scope both together at the same point (probably the black lead of your BNC clip cables).  You may want to pretest each new test point with your DMM in AC mode to make sure there isn't more than a few volts present before hooking up the computer.

Then, one additional step.  If you have a signal at S603, connect the scopecard to the #9 pin of U601 (you may want to either look up IC pinout rules--or post a photo) and see if you can get any signal by adjusting the trigger or even pushing it in and using auto.
You can see on the schematic that if finding and connecting to pin 9 is too hard, then the correct sides of R470 or R632 will do.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #132 on: August 06, 2020, 06:12:32 am »
So, BNC 1: red lead to signal gen out, black lead to signal gen gnd, connector to EXT. BNC 2: red lead to test point, black lead to signal gen gnd, connector to soundcard scope. Is this a correct understanding?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #133 on: August 06, 2020, 01:19:40 pm »
Yes, that's it. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #134 on: August 07, 2020, 05:37:35 am »
The ends of my bnc probes are bent out of shape and refuse to stay clipped to the gnd/out pins of the signal generator or the test points of the switch.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #135 on: August 07, 2020, 02:09:17 pm »
Crappy test leads are worse than crappy instruments.

These are not high-end, but they should be acceptable.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=401929974978&_sacat=0

EDIT:  The 10X feature of the probe will also protect your computer soundcard in case you make a mistake.
« Last Edit: August 07, 2020, 03:48:19 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #136 on: August 07, 2020, 07:54:32 pm »
Alright, guess this is on hold until I can get the money in one place.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #137 on: March 09, 2021, 04:42:54 am »
Well that took a bit longer than expected. New probes have been ordered and I should be able to resume testing this next week.

I've also got the funds to get a cheap but somewhat proper oscilloscope like a Miniware DS212 and a slightly more proper function generator (some ~$80 Aliexpress special), would those be good purchases towards getting this thing running?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #138 on: March 09, 2021, 08:35:01 pm »
There's another recent BK 2120 repair thread in this forum with a problem sort of similar to yours.  Have a look.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #139 on: March 09, 2021, 09:55:54 pm »
I saw that, I'm keeping an eye on the thread to see where it goes.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #140 on: March 14, 2021, 12:32:13 am »
Probes arrived, how should I use them? Set them to 10x mode and set them up like in this post?
So, BNC 1: red lead to signal gen out, black lead to signal gen gnd, connector to EXT. BNC 2: red lead to test point, black lead to signal gen gnd, connector to soundcard scope. Is this a correct understanding?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #141 on: March 14, 2021, 07:39:05 pm »
It's been a while, remind me where you are at with your scope.  I seem to recall that you have a dot on the screen but no horizontal sweep, right?

As for your new probes and sig gen, can you just post a photo of everything so I can see it?  Far easier then explaining.  The first thing to accomplish would be to make sure you can generate a signal and view that signal on your computer/soundcard scope setup without connecting anything to the BK scope. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #142 on: March 14, 2021, 07:47:31 pm »
Yes, a dot but no horizontal sweep. I'll set things up and take a picture in a little while, need to clear things out.

Edit: I also checked the probes with just the signal generator last night, got a good waveform with 1x but not 10x.

Edit 2: Sorry if I misunderstood things, is this what you wanted to see?
1194020-0
« Last Edit: March 14, 2021, 10:48:45 pm by vexatag »
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #143 on: March 14, 2021, 11:38:14 pm »
A double post, for which I'm sorry, but I'm trying to figure out what I was trying to test back in August. Was it the waveforms on a few of the pins in S603 followed by one of the pins on U601?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #144 on: March 15, 2021, 03:13:14 am »
OK, what does the input to the soundcard scope look like?

And what is wrong with your signal at 10X?  We really want to use the 10X setting if possible.

You can use the signal generator or we can just use the line trigger if you can get a good picture of it on your scope setup.  Switch S603, then pin 9 of U601 and pins 3, 5 and 5 of U603.  Screenshots would help.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #145 on: March 15, 2021, 03:39:51 am »
OK, what does the input to the soundcard scope look like?

And what is wrong with your signal at 10X?  We really want to use the 10X setting if possible.

Answering these questions with two images, first is the scope with the lead at the 1x setting, second with the lead at the 10x setting. I zoomed in and compared it to the unplugged second channel, it's just background noise. I can most likely test the other components on Monday, the school quarter is almost done and I have a few more assignments to take care of.

One more thing, when checking the board over I noticed that all the variable resistors on the board have some adhesive goop on them, but the application on the sweep length resistor is nearly non-existent, almost like someone has adjusted it. I'm not checking it out just yet but I've attached a pic of what I mean for reference.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #146 on: March 15, 2021, 04:05:29 am »
OK, soundcard 'oscilloscope' apparently has a low input impedance, so no 10X won't work.   :palm:

How do the probes connect to the soundcard?  There needs to be some additional resistance put in there somewhere.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #147 on: March 15, 2021, 04:11:39 am »
Guess I should delay the testing process until it can be done safely, eh?

The signal chain is probe > BNC to RCA adapter > RCA Switcher > RCA to 3.5mm jack > 3.5mm cable > USB soundcard. Would it be a better idea to bite the bullet and get a different oscilloscope for testing? In my first 2021 update I mentioned that I was thinking of getting a DS212, so it would be within my budget.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #148 on: March 15, 2021, 06:51:03 pm »
Update, I bought a DS212 on Ebay, it should be here by next week.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #149 on: March 18, 2021, 10:49:27 pm »
DS212 arrived, it has built-in signal generation (square, sine, saw, from 10hz to 1mhz) and proper 10x attenuation support. I'm guessing I should hook the output channel of the 212 to the external trigger of the 2120 (dangerous potential for typos here) using a 10khz square wave, then connect the ground of channel A to the output channel ground and probe the points? Additionally, should testing be done with the 2120 on or off? It's been long enough that I don't remember how I was testing it.
 


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