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

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Offline vexatagTopic starter

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BK Precision 2120 trace issues
« on: July 11, 2020, 10:13:14 am »
In quite possibly the most foolish decision I'll make all year, I bought a broken BK 2120. There are some dents on the chassis that the seller didn't do a good job of pointing out, one of the feet on the back is cracked off, and several screws are missing from the bottom of the chassis.

The main issue, however, is the trace. Both channels behave identically. When hooked up to the calibration point, they show two shimmering ~7mm lines ~3mm apart from each other. The x position knob turned all the way left is off screen, but turned all the way right is dead center. Shifting this rapidly back and forth does reveal some amount of waveform.

Here is a link to the pictures I took this afternoon after unpacking it. If there are any other diagnostic steps I should take before poking with a multimeter please let me know.
 

Online med6753

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Re: BK Precision 2120 trace issues
« Reply #1 on: July 11, 2020, 03:07:12 pm »
In quite possibly the most foolish decision I'll make all year, I bought a broken BK 2120. There are some dents on the chassis that the seller didn't do a good job of pointing out, one of the feet on the back is cracked off, and several screws are missing from the bottom of the chassis.

The main issue, however, is the trace. Both channels behave identically. When hooked up to the calibration point, they show two shimmering ~7mm lines ~3mm apart from each other. The x position knob turned all the way left is off screen, but turned all the way right is dead center. Shifting this rapidly back and forth does reveal some amount of waveform.

Here is a link to the pictures I took this afternoon after unpacking it. If there are any other diagnostic steps I should take before poking with a multimeter please let me know.

I have a 2120 in my collection of scopes which obviously is in much better condition than that one. That's really been beat up.

Anyway, if you don't have the service manual download one. Make sure it's the service manual and not the user manual. There are plenty of on line sources. 

Start with the basics. All power supply voltages must be present. Then take it from that point.
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Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #2 on: July 11, 2020, 05:50:58 pm »
Has anyone pushed that XY button yet?  Did you ask the seller to do that and if so, what happened?  If pushing the button and setting the trigger to AUTO doesn't give you a trace, you know the horizonatal sweep isn't working--and that's a bad thing.

Also, do you look at eBay auctions on your phone or on a computer.  I saw that damage in the eBay photos, at least the cracked back part.

You're going to have a tough time.  the BK2120 was made in several iterations (not including the explicitly different A, B, versions, etc) and it is likely that what you have in front of you will not exactly match the manuals and schematics that are available. 

The first thing would be to take the covers off and examine it for physical damage.  Beware that although it is not an accelerated CRT, there is still a good -1500VDC at the gun and many hundreds of volts elsewhere, and then there's mains voltages in certain places. 
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 #3 on: July 11, 2020, 11:03:23 pm »
I have a 2120 in my collection of scopes which obviously is in much better condition than that one. That's really been beat up.

Anyway, if you don't have the service manual download one. Make sure it's the service manual and not the user manual. There are plenty of on line sources. 

Start with the basics. All power supply voltages must be present. Then take it from that point.

The first thing I did after buying this was download the service manual from one of your posts here. I'll check the voltages this evening (Pacific time) and report back.

Has anyone pushed that XY button yet?  Did you ask the seller to do that and if so, what happened?  If pushing the button and setting the trigger to AUTO doesn't give you a trace, you know the horizonatal sweep isn't working--and that's a bad thing.

Pressing the XY button moves what's displayed over a centimeter or two, no actual change in the trace generation.

Also, do you look at eBay auctions on your phone or on a computer.  I saw that damage in the eBay photos, at least the cracked back part.

I look at auctions on my computer, there's no excuse for my lack of diligence here  :-[

You're going to have a tough time.  the BK2120 was made in several iterations (not including the explicitly different A, B, versions, etc) and it is likely that what you have in front of you will not exactly match the manuals and schematics that are available. 

Is there any easy way you know of to check which version it is?

The first thing would be to take the covers off and examine it for physical damage.  Beware that although it is not an accelerated CRT, there is still a good -1500VDC at the gun and many hundreds of volts elsewhere, and then there's mains voltages in certain places. 

I understand and appreciate the advice.
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #4 on: July 12, 2020, 04:04:20 am »
Update: Took off the casing, no damage to the PCB amazingly enough. 140v test point is reading 141 on my DMM, 260v test point is only reading 79-83v. I can't test the 2kv test point unfortunately. I also went ahead and tapped the bottom of the case more flat, I want to sand it to bare metal and repaint it, question is what color is closest.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #5 on: July 12, 2020, 05:20:19 am »
Well, that could be somewhat good.  The -2kV supply is not an issue for now because you have a beam, so don't kill yourself trying to measure it. The 260V supply is sort of 'on top' of the 140V, so the fact that is measures less indicates it is not working at all.  The horizontal amp section runs off of this supply, so that might be the whole problem.  You can hope.

Start by checking for AC (bridge input) and DC (bridge ouput) for the +260V branch.



« Last Edit: July 12, 2020, 05:21:59 am by bdunham7 »
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Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #6 on: July 12, 2020, 06:54:33 am »
For DC voltages I measured -130v on R504 and 38v on the positive leg of C503. I'm not sure which part you're pointing at for the AC voltage measurements, could you clarify?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #7 on: July 12, 2020, 01:20:21 pm »
I'm pointing at the two 120V winding wires marked ORG on the diagram--they should be orange--or the AC inputs to bridge rectifier BD101.

What exactly are you using to measure the voltages?  I'm assuming that you have been leaving the negative end on ground.  You will need to start moving both leads, but I want to make sure you have a proper DMM, preferably with test hooks so you can do this hands off.

OK, so you have +140V at TP140V but -130V at the junction of R509, R504 and BD501(-).  Now (only if you have a proper DMM--let us know) measure with the negative probe at the R509/R504 junction and the positive at C503(+) and TP260V (check both to make sure they are the same).
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 #8 on: July 13, 2020, 12:09:14 am »
I've been measuring with negative clipped to the chassis. My multimeter is a UEI DM 383 with some aftermarket leads (they've got screw-on alligator clips). I've ordered an Aneng AN860B+ but it won't be here for a while unfortunately.

So, negative probe at R504/R509 junction and positive at both C503 positive leg and TP260V? I'm unable to find BD101, are you referring to BD501?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #9 on: July 13, 2020, 01:29:32 am »
I've been measuring with negative clipped to the chassis. My multimeter is a UEI DM 383 with some aftermarket leads (they've got screw-on alligator clips). I've ordered an Aneng AN860B+ but it won't be here for a while unfortunately.

So, negative probe at R504/R509 junction and positive at both C503 positive leg and TP260V? I'm unable to find BD101, are you referring to BD501?

Yes, BD501.  The meter you have with alligators is actually good, hopefully you can do this hands-off with a minute of power-off time in between for capacitor discharge.  And still be careful even with that!
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 #10 on: July 13, 2020, 01:46:51 am »
To be absolutely sure: hook up when powered off, power on, record the data, power off again, wait a few minutes, carefully relocate the probes, rinse repeat (metaphorically)?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #11 on: July 13, 2020, 02:25:27 am »
Yes.  No rinsing.
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 #12 on: July 13, 2020, 05:48:07 am »
Alright, measuring from R509 to both the test point and the positive leg of C503 got 170v±1, measuring the AC lines of BD501 got 129v.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #13 on: July 13, 2020, 02:56:36 pm »
Very good.  You just might fix this thing yet! 

If you look at the diagram, you'll see that the only place you can be dropping all that voltage is across either R504 or Q501.  This doesn't mean that the problem is one of the two components, but that's where to start.  So the last two places I need you to test voltages is between TP+140V and both sides of R504. 

After you do that, let it sit for a few minutes and then use the ohms mode to check each resistor in the 260V circuit--R503 through R509.  In-circuit they could read lower then their spec, but they can't read higher.  Also do a diode check both ways on D502.  If you are very lucky, you will find that R504 has gone open or high resistance.
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 #14 on: July 13, 2020, 08:00:38 pm »
Some very interesting developments have occurred. Going from either side of R504 to TP104 revealed no current, but that's not the most unusual thing. R503 and R506, R507, R508, and R509 completely failed a continuity check. On R504 I measured 92Ω and on R505 I measured about 500Ω. On D502 I got 0.560, not sure what that means.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #15 on: July 13, 2020, 08:07:52 pm »
What do you mean by 'no current'??  There should have been a voltage reading of some sort on each.

The D502 reading is as expected--about a half a volt forward drop.  R504 and R505 are close enough to not worry about yet.  The others you probably have selected the wrong resistance range on your meter.  100K means 100,000 ohms, 47K means 47,000 ohms, 4K7 means 4,700 ohms, etc.  100KF means 100,000 ohms with a 1% tolerance.
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 #16 on: July 13, 2020, 08:10:00 pm »
By "no current" I mean I measured 0 volts of current. As for the resistors, I selected the continuity tester after failing to get any resistance in any range. As far as my meter could tell, the resistors are open.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #17 on: July 13, 2020, 08:22:08 pm »
OK.  Just say 'volts' not 'volts of current' because the latter gives me visions of sparks flying due to putting the test leads in the wrong jacks on your meter. 

You need to check your meter.  Given what we have so far, I don't think you would have zero volts there.  Just to be clear, one lead on TP+140V test point, the other on one side or the other of R504?  Try putting that second lead to ground and verify you get 140 volts again.  And add in a third measurement from TP+140V to the lower side of D502 (or both sides if isn't clear which is which.

As for the resistors, it seems unlikely that they would all be open, user error or a malfunction your meter is more likely.  So try measuring some resistors in the 140V supply , lke R516, R517, etc.  Since we know that supply works, the resistors will probably be OK.

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 #18 on: July 14, 2020, 12:27:36 am »
TP+140v to the chassis gets 140v, TP+140v to either side of R504 gets a 1 on the far left of my multimeter's display no matter what range I'm on. All of this is measured in DC. In AC mode some sporadic measurements happen but nothing more substantial than random noise. I connected in continuity mode very briefly but I started hearing a rapid clicking so I shut it down. TP+104 to the negative side of D502 got nothing, the positive side got 102v. After checking a couple known-good resistors on another PCB I have (little 1khz signal generator kit I have), I admit that I'm probably measuring them wrong.
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #19 on: July 14, 2020, 12:51:26 am »
OK, there's still work to do.

First, please only use the DC 1000V or AC 750 volt ranges when doing power-on measurements on the oscilloscope--and definitely never ohms or continuity.  There's no need for any resolution better than 1 volt at this point.  The '1' on the left side may indicate overrange, meaning the voltage has exceeded the range.  If this happened on the 1000V range, there's something very wrong, hopefully with your meter because I have no idea where 1000V could come from.  Do you have a 9 volt battery or some other low voltage source handy?  Try setting your meter on 2V DC and giving it 9 volts (it won't hurt it) just to see if that '1' does indicate overrange.  And does your meter should indicate zero when there's no voltage, so when you measured from TP+140V to both sides of D502, you said it was 102V on one side and 'nothing' on the other.  If that 'nothing' was a zero reading that's  lot different than an overrange reading.

Do figure out how to get the ohms function working and verify it.  Getting your test instruments to work properly is all part of the struggle!

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 #20 on: July 14, 2020, 04:15:42 am »
Do you have a 9 volt battery or some other low voltage source handy?  Try setting your meter on 2V DC and giving it 9 volts (it won't hurt it) just to see if that '1' does indicate overrange. 

The 1 indicates overrange, I'm getting 274 volts from TP+140v to either side of R504.

And does your meter should indicate zero when there's no voltage, so when you measured from TP+140V to both sides of D502, you said it was 102V on one side and 'nothing' on the other.  If that 'nothing' was a zero reading that's  lot different than an overrange reading.

My meter indicates 0.0 when there's no voltage, TP+140V to D502 resulted in 102V on one side and 0.0V on the other. I will be more precise in the future, my apologies.

As for the ohms I'll get it figured out,
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #21 on: July 14, 2020, 05:00:21 am »
Update, I've figured out the ohms, none of the resistors were open after all.

R503: 4.68k Ohm
R504: 99 Ohm
R505: 500 Ohm
R506: 48k Ohm
R507: 180k Ohm
R508: 1.68 mega Ohm
97.5k Ohm
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #22 on: July 14, 2020, 05:17:03 am »
If those readings are accurate, R507 and R508 are both bad and that might be the whole problem. They are the voltage divider that sets the output of the supply. It looks like they are a different type than the others?  They seem to be specifying 1/4 watt metal film 1% tolerance, which are readily available in 100K and 115K values.  I wouldn't do any more testing until you have replaced those.
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 #23 on: July 14, 2020, 05:56:59 am »
I'll round up a source for them (annoying that Mouser and Digikey shipping costs can easily exceed the cost of parts in situations like this), it'll be a bit before I'm able to replace them as something in my soldering setup is super janked, all my joints are cold.

1/2 watt metal film resistors are probably also fine, right?
 

Online bdunham7

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Re: BK Precision 2120 trace issues
« Reply #24 on: July 14, 2020, 03:50:22 pm »
!/2 watt are OK if they fit OK, but unnecessary.  Try eBay for very small orders, for something like resistors they'll be fine.  You might want to just get a big set of resistors if you plan on making stuff in the future.  You will have to buy the 115K separately, or else use a 113K or 120K, depending on the set you get.

https://www.ebay.com/itm/US-1460Pcs-1-4W-Metal-Film-Resistor-Kit-Assortment-Set-Labelled-1-Precision-Set/133464890909?hash=item1f13209e1d:g:B1MAAOSwDO9fDSDI

And yes, figure out your soldering before you start any repair work. 
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.
 


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