Author Topic: BK Precision 2120 trace issues  (Read 13575 times)

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Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #50 on: July 25, 2020, 06:28:53 pm »
R834 measured 68.4 ohms.

Unplugging PG106 leaves no dot on the screen. As for other equipment, I have some 100k pots, a 10k pot, a custom 16k pot, assorted capacitors, a Velleman MK 105 1khz signal generator, the ability to use my computer's soundcard or a usb soundcard as a somewhat limited scope (the wiring setup is admittedly janky and I don't want to risk super high voltage through it), a small breadboard, wire, solder, a soldering iron, and two multimeters (the DM 383 and a recently arrived AN860B+).
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #51 on: July 25, 2020, 06:46:00 pm »
OK, let's not burn up your computer just yet, there's enough useful stuff that looks helpful.  I'll try to think of a way to make it all work.

In the meantime, no dot with the connector unplugged seriously worries me, although I'm very unsure what to think at this point--it's bit of uncharted territory for me to.  What I was expecting was that the beam would just continue right down the center without being deflected.  You said there were other wires on PG106, can you tell where they go to?
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 #52 on: July 25, 2020, 06:55:55 pm »
All four wires on PG106 go to the base (I believe that's what it's called) on the back of the CRT. Re-testing, I saw a square faintly illuminate and quickly drift to the lower left corner while growing in size.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #53 on: July 25, 2020, 07:19:11 pm »
OK, I see that the other two are the FOCUS (orange) and ASTIGMATISM (black).  Be careful, theres about -2000 volts on the focus wire (but not much current).  That's why you need a few minutes of power off before handling anything.

Is there any practical way to disconnect just the two tan wires?  I don't know what the connector looks like.
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 #54 on: July 25, 2020, 07:27:32 pm »
I believe it's a JST connector?
 

Offline vexatagTopic starter

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Re: BK Precision 2120 trace issues
« Reply #55 on: July 25, 2020, 07:44:23 pm »
Oh, forgot to mention that I have some MPSA13 transistors from an impulse addition to an ebay purchase.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #56 on: July 26, 2020, 01:40:32 am »
So here's a simple test you can try since you have two multimeters now.  Put the first DMM back on the measurement between R834 and chassis ground.  Now go into XY mode and hook up that BNC-to-clips adapter I saw on the CH1/Y input.  Use the diode check or ohms function--it will generate a small voltage--and input it into the scope via the clips.  You will have to turn the V/DIV setting down on that channel to 1V/DIV or less--do that until the dot moves about halfway (up and down) across the screen.  You may have to reverse polarity to get it to go the right way.  Once that works, move the BNC connection to the CH2/X input and set the V/DIV to the same setting as you just arrive at for CH1/Y.  Now repeatedly connect and disconnect the signal-producing DMM (presumably then new one) and observe if there is any change on the other meter that is hooked up to R834.  Turn the range down one step at a time until you see some change--it should be clearly evident. 
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 #57 on: July 26, 2020, 02:09:05 am »
On CH1, I read ~1.558 V when the test load was connected and ~1.553 V when the test load was disconnected. On CH2 I read -4.34 V when the test load was connected and 1.45 V when the test load was disconnected.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #58 on: July 26, 2020, 02:21:18 am »
OK, does the dot move when you are connected to CH1/Y?
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 #59 on: July 26, 2020, 02:27:41 am »
I can get the dot to form a vertical line, yeah.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #60 on: July 26, 2020, 02:34:06 am »
OK, very good.  Now we have a way to trace the signal through the amplifier.  We might actually be close, I've been suspecting that Q808 may be bad, but haven't had a way to prove it.  So now repeat the test on CH2/X but hook the measuring DMM to the other side of R834 (the base of Q808) and write down the voltages each way, then at the emitter of Q808 (the side with the outward pointing arrow on the schematic.

EDIT:  If you have any difficulty getting at the base or emitter of Q808, you can use the something else connected to it like R835 or R837.
« Last Edit: July 26, 2020, 02:38:22 am 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 #61 on: July 26, 2020, 02:55:30 am »
At the base, voltage when the test load was connected was -4.37 V, voltage when the test load disconnected was 0.92 V. At the emitter, voltage when the test load was connected was -5.01 V, voltage when the test load disconnected was 0.28 V.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #62 on: July 26, 2020, 03:06:22 am »
That actually looks good--the emitter follows the base with about a 0.6 volt consistent difference.  Can you do the same tests on the base and emitter of Q807 and Q809?

EDIT:  And while we're at it, can you run the same tests measuring at the TO D1 and TO D2 terminals of the plug?

EDIT2:  Also at the junction of 835 and 836, which is the collector of Q807.
« Last Edit: July 26, 2020, 03:18:40 am 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 #63 on: July 26, 2020, 05:30:11 am »
Here's the result of my testing

Load appliedNo load
Q807 base-8.63 V-8.63 V
Q807 emitter-9.36 V-9.36 V
Q809 base1.04 V1.65 V
Q809 emitter0.24 V0.88 V
Left tan pin0.28 V0.91 V
Right tan pin0.20 V0.65 V
835/836 junction-5.37 V-2.40 V
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #64 on: July 26, 2020, 06:02:53 am »
OK, we're missing something here.  Where's the power?  You had some voltage on TO D1 and TO D2 before.

Can you check the voltage at the junction of L801 and L802 (this is the+260V supply we fixed before) and then power down and measure the resistance from that junction to TO D1 and TO D2. 
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 #65 on: July 26, 2020, 07:48:23 am »
Note for clarification: The left tan pin is D2, the right tan pin is D1.

Voltage to D1, -116 V. To D2, -121 V. I tried measuring the resistance but got either overload or around 5 megaOhms on both my meters.

EDIT: I also remeasured the voltage between D2 and ground, got a result of around 3 volts. Maybe my testing methodology is borked somehow?
« Last Edit: July 26, 2020, 07:50:09 am by vexatag »
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #66 on: July 26, 2020, 04:21:17 pm »
Yes, there seems to be some issue measuring, so let's back up.  Is the scope partially disassembled so that any of the grounds are disconnected?

So first, let's go back and measure the +260V supply that you fixed earlier, make sure it still has power.  DMM neg lead to ground and pos to +260V.  Then move just the pos lead to the L801-L802 junction, the same 260 volts or so should be there as it is a direct connection.

Next, power down and look at L801, L802, R838 and R839.  You can see how they are connected, L801 and L802 are inductors so they should have a very low DC resistance.  If you measure from the L801-802 junction to either TO D1 or TO D2, you should get approximately 15K ohms--and if not, you need to measure each component separately. 

Also look at all the connections and wiring to make sure everything is firmly connected and you haven't missed anything.
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 #67 on: July 26, 2020, 10:41:05 pm »
Scope is fully assembled, just out of the case.

TP260V and the L801-L802 junction both have 265V. As for the megaohm resistance I was measuring on the wrong side of the inductor :palm: Both are around 15K to D1 and D2.

Ground to D1, 37.4 V. Ground to D2, -3.3 V.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #68 on: July 26, 2020, 11:40:35 pm »
Alright, just to be clear, you measure from the L801-802 junction to D1 and D2 and each time you get 15K?  I don't see why using the wrong side of the inductor would matter.  Can you just check L801 and L802 to see what their DC resistance is?

I'm trying to figure out how D2 can be at -3.3 volts.  Can you also measure again voltage from D1 to D2?
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 #69 on: July 26, 2020, 11:54:40 pm »
Alright, just to be clear, you measure from the L801-802 junction to D1 and D2 and each time you get 15K?

Measuring from what I believe the junction is, I get 15k ohms. Measuring from the other side, I get 5.3 mega ohms.

L801 measures 7.3 ohms, L802 measures 7.1 ohms. Each leg has continuity with every other leg as well, don't know if that's important to mention or not.

Checking D1 to D2 gives 73 V.
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #70 on: July 27, 2020, 12:31:16 am »
OK, let's try this.  On the picture I've attached, can you confirm that your board looks like this, more or less?

I've illustrated where the +260V point is, that is also the L801-802 junction.  The A side of L802 is connected to that, then the PCB trace on the B side of L802 goes under and around it back to the other B, which is the B side of R839.  Then the C side of R839 is connected to D1.  So when you measure from A to C, you should get about 15K, which is the combined resistance of L802 and R839.  Apply the same logic to the other side.  So where are you seeing the 5.3M ohms?

« Last Edit: July 27, 2020, 12:37:57 am 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 #71 on: July 27, 2020, 12:47:30 am »
That's about what my board looks like, yes.

I'm seeing 5.3M ohms from L801 A to R838 B and C. (L802 A to R389 C gives the normal value of ~15k, I was measuring from L801 because it's more convenient to clip to)
 

Offline bdunham7

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Re: BK Precision 2120 trace issues
« Reply #72 on: July 27, 2020, 12:56:33 am »
Well, there's your problem. Or at least 'a' problem--I can't promise you there won't be more!  :)

So if L801 measures 7-ish ohms and R838-B measures 15K all the way to D2, then measure the trace (from B to B) between L801 and R838.  There must be an open circuit there somewhere.  If you can't easily see the problem but you can confirm that there is not continuity between the two B points, you can run a jumper wire as I've drawn it.  Just tack on a wire to the component leads--it's called a bodge wire.

« Last Edit: July 27, 2020, 01:01:43 am 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 bdunham7

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Re: BK Precision 2120 trace issues
« Reply #73 on: July 27, 2020, 01:02:06 am »
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 #74 on: July 27, 2020, 01:44:10 am »
Hmm, this might be the problem  :-DD

I'll try cleaning these joints up a bit first, see if that helps.
 


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