BK 2120 scope problem

[pneumaticdril]

I bought a 2120 from Ebay and it has a problem(Of course). The display shows the trace in about 1/4 inch square of the screen. It seems like it's working but the display size is too small. I have checked the output of the transformer and it seems within spec. I do have schematic, but not sure where to start looking. Any help is appreciated. Thanks.

[pneumaticdril]

Re: BK 2120 scope problem

Haven't gotten too far, only discovering what is not causing the problem. I had thought I was onto something when I disconnected the leads to the coil surrounding the CRT and nothing changed. I now believe that coil is responsible for trace rotation. Still unknown what can cause the screen to shrink to it's current tiny size. Unfortunately, I have no experience with video circuits. Mostly worked with audio equipment, cell phones, and power supplies. Any guidance would be appreciated.

[newtothis]

Re: BK 2120 scope problem

Contact the seller?

[redwire]

Re: BK 2120 scope problem

It sounds like it is out of focus, and no horizontal deflection. Does the trace move or blink at really low sweep speeds, or is it in X-Y mode?

See this old post:https://www.badcaps.net/forum/showthread.php?t=15169

Schematic is here: http://elektrotanya.com/b_k_2120_osc.../download.html

[pneumaticdril]

Re: BK 2120 scope problem

The trace only moves about 1/4 of inch. The best I can get is a 1/4 inch line in center of screen. The focus and intensity adjust work. I need the trace to go completely across screen to see wave form. When I turn off scope the trace will dance to the left half of the screen, leading me to believe the CRT is good. It's like trying to view the world thru a pinhole.

[redwire]

Re: BK 2120 scope problem

I would guess the power supply voltage is low for the CRT deflection.
Take a multimeter and measure the PSU voltages. The +140VDC, +260VDC rails have test points, and +24VDC, +12V, +5V.
The high voltage -1.9kV you need a HV probe to measure.
It is set up for the right line voltage 115V/230V?

[pneumaticdril]

Re: BK 2120 scope problem

I did read correct voltages at 140 and 24v test points. I had a critical malfunction when attempting to read 2KV without HV probe and consequently I need to get a new DMM. I will post when I get a new meter and verify the voltages. Thanks.

[retiredcaps]

Re: BK 2120 scope problem

I had a critical malfunction when attempting to read 2KV without HV probe and consequently I need to get a new DMM.

Which meter paid the price for trying to measure 2000V?

[pneumaticdril]

Re: BK 2120 scope problem

it was an old Radio Shack. had it for about 20yrs. Gave me an excuse to get a new one. Ordered an Extech MN16A. Kinda cheap, don't have the money for a Fluke at this time.

[pneumaticdril]

Re: BK 2120 scope problem

Ok, I got new meter and tested all the voltage test points and the 240v is only 25v. I checked back at the transformer and only getting 130vac. I am thinking the transformer is the culprit. Now I just have to locate a new one. Any suggestions?

[budm]

Re: BK 2120 scope problem

Ok, I got new meter and tested all the voltage test points and the 240v is only 25v. I checked back at the transformer and only getting 130vac. I am thinking the transformer is the culprit. Now I just have to locate a new one. Any suggestions?

Not the transformer if you are getting 130VAC, if you look at the Diagram, the 260VDC is generated by stacking the two power supplies. You need to check the components in that section of the circuits.

[pneumaticdril]

Re: BK 2120 scope problem

My bad, I will go back and check the components. Thanks much!

[pneumaticdril]

Re: BK 2120 scope problem

I think I got it now. I am getting 25v out of BD501 and 140v out of BD502. I will order new rectifier. Will post with results.

[budm]

Re: BK 2120 scope problem

You need to measure the DCV between the two legs of the filter cap C503 for the top power supply, it should have (as you indicated 130VAC reading) 130VAC * 1.414 = about 183VDC.
So check the rectifier but I would suspect bad C503.

[pneumaticdril]

Re: BK 2120 scope problem

Yes, I ordered the cap also. No Eparts stores around here, Radio shack was the last to go about 6 months ago.