BK Precision 2120 Oscilloscope - trace line not displaying on CRT

[bdunham7]

Were Q508 and Q509 confirmed bad by testing?  You should test D507 and D514 and maybe D502 as well to make sure they aren't blown open.

Do you have 24 volts at the R520/R522/Q505 junction?  And (carefully!) measure the voltage at pin 1 of U505.  You may need to adjust VR502.

I doubt the CRT would be damaged, but you might try removing Q508 and make sure you have a beam then. (do this last)

[AmpBuilder]

Yes Q508 and 509 were bad when i tested them.

[AmpBuilder]

I replaced Q508 and 509 with correct replacements,also I replaced shorted ZD501. I still don't have my green dot with the XY button pushed in,what else do i need to check? This was in reference to U505 being accidentally shorted out.

[bdunham7]

Did you check for 24 volts at the emitter of Q505 (and elsewhere) as I had asked above?

If you remove Q508 entirely, I think you should get a beam even if there are problems elsewhere in the circuit.  If not, then you have to start looking at the -1867, -1900 and -2000 volt points.  As far as measuring, unfortunately there is 2000 volts potential in that area and you really need the proper equipment before you go poking there.

[AmpBuilder]

I have lost my high voltage completely off the secondary winding of the power transformer,but I am still getting the 31 and 6.3 volts volts that are tapped off the high voltage winding,looking at the schematic on the power supply. So it looks like i can't go any further with this scope unless I can find another power transformer,any suggestions?

[bdunham7]

The floating voltages that you still have are not related to the actual HV generator circuit--look at your schematic and see if you have an input to T502 from Q511 and U506.  Unless T502 measures open on the secondary (or primary, I suppose) I doubt you've blown it up.

[AmpBuilder]

The transformer i was concerned about is T501. Looking at the schematic for the power supply it is the 1st winding at the top of the page,on one leg of the winding it shows 1,867 volts out,on the other leg it shows 1.9k out. Also I checked the input voltages in the high voltage generator and they are all there,I did find R546 820k way out of value,I replaced it but didn't fix the issue with the 2k volts out missing.

[bdunham7]

Those voltages from the power transformer are just 6.3V for the filament and the 33VDC supply for the beam current, etc.  T501 doesn't supply the HV, it just floats up with it.  The HV generator supplies the -2kV to the CRT control circuit, ZD502 drops it down (or up?) to -1.9kV and ZD501 to -1867kV.

If you have the all of the DC supplies to the HV generator but no output, then something has gone wrong there.  If you don't have another scope, testing components seems your only option.  I'd look at Q511 first.

[AmpBuilder]

I checked Q511 and it is ok. I have -11.5 volts off of pin 6 U506 and still not getting the -2kv out of high voltage generator circuit. I checked all caps,resistors and diodes in the circuit and they all checked good. I have continuity in the primary and secondary winding  in T502. Any suggestions where I need to go from here?

[bdunham7]

I guess start poking with your other scope and see what you have in the U506/Q511 area for voltages.  See what you have at pin 6 U506 and if that changes when you adjust VR508.  How did you test Q511?

[AmpBuilder]

I tested Q511 with my Simpson 260.

[bdunham7]

I tested Q511 with my Simpson 260.

I'm not sure how you do that--my Simpson doesn't have the hFE function nor the transistor test jack.   

I missed your statement about pin 6 on U506.  That looks like it is pretty much at the rail and trying to command the oscillator circuit to increase output, but I'm still not sure exactly how that oscillator circuit behaves.  Still, it is only a few parts plus Q511, so not much else to go wrong.  I would guess that the design depends on Q511 to have a pretty high gain.  It might be instructive to measure all the voltages around Q511 and see if you think it is on, off, or something else.

[AmpBuilder]

I retested Q511 with another piece of test equipment and reconfirmed it is good. I am still getting -11.5 volts out of U506,but still no -2K out of the H.V. generator. I am wondering if T502 could be bad. I have just about exhausted everything in this circuit that I know to troubleshoot.

[slbender]

Earlier today, I posted on another thread about cold solder joints throughout a B&K 2190, I suggest you closely inspect the boards and connectors on your 2120, and do some solder sucking and resoldering prior to replacing more parts…

Steven