BK Precision model 1472C Oscilloscope Intensity problem ??

[guizmo1967]

Hello,

I have an old BK Precision model 1472C that is working but not the intensity ?

At connector P309:
Pin 1 +5V
Pin 2 when I turn VR2, goes from +.648 to +5.07V
Pin 3 GND

R356
Pin 1 when I turn VR2, goes from +.648 to +5.07V
Pin 2 no matter how I turn VR2, voltage stays at +5.7V

All the resistors around are good, except maybe R355 that reads 37K instead of 48K.

I also would like to know what TC302 (capacitor trimmer) is used for, because when I turn it, nothing happened ???

See the attach file, you can read the voltage at each transistor.

Thank you !

[stj]

Re: Oscilloscope Intensity problem ??

your missing a lot of the circuit, post the whole schematic/manual

[guizmo1967]

Re: Oscilloscope Intensity problem ??

Here it is !

[redwire]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

"Intensity Problem" what do you mean, it's dead, it's dim?
Why do you randomly turn trimpots? Hint: that will not fix the scope and it's not smart at all, it makes a mess.

Start by checking the power supply rails:
+195V
+145V
+18V
+17V
+13V
+10V (master rail)
-10V
+5V
etc.

If there is HV -1.9kV you will hear the crackle of static electricity when the scope is turned on or off. Do you see the filaments lit at the end?

I would not blame the blanking circuit yet, the voltages look ok. D315 WZ-050 zener is 5.0V 500mW.

[guizmo1967]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

Like I said the scope is working, there is a line, it's just the intensity knob that don't do nothing when I turn it. I mean by intensity, the brightness of the line.

If I turn the knob all the way to the left, the line should disappear and if turn all the way to the right, should reappear slowly to full brightness (intensity).

All the voltages you mentioned are good.

Why on one side of R356, I have 0.648V to 5.07V when I turn VR2 but Base of Q320 stays fixed at +5.7V ??????

VR2 is the intensity knob !!!

[redwire]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

Check the power supply voltages first before saying a stage is not working.
Again it could be the +145V or +195V rail is low or too much ripple.

IC204 TD3400AP is quad NAND gate but with open-collector outputs. Q319 is cascode. The whole thing is pulsed.

I think CRT grid bias comes from the D311, D312 switch. C337 1uF 250V is ok?
Don't mess with VR307 lol it's not the problem.

[guizmo1967]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

The board is out, I need to change R355, supposed to be 48K but reading 37K !

I will post when I reassemble everything ....

[guizmo1967]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

The two neon bulbs (NE-2) N301 and N302 are there for what exactly ??

I'm asking because they barely light up …

Some voltage measurements:

On one side of R349 +195V, the other side with anode of D313, cathode of D314 and collector of Q318 I get +63V.

VR307, C337 and cathode of D313, I get +112V.

In the service manual, collector of Q318 should be +80.6V ???

All the other voltages on that Power Supply are good !!

[stj]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

neons are used as spike/overvoltage devices,
the interesting thing about neons is they have infinate resistance between the pins till they reach strike voltage - usually 70-80v, then they look like a short

[redwire]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

The neon lamps protect the CRT grid-drive circuit from static electric discharges during power up/down, and protect the CRT from anything that could overdrive the grid positive into being a plate.
The neon lamps are normally never on. I would check D311, D312, R346, C343 for leakage as well.
The -ve grid bias for CRT comes off that -ve voltage multiplier which appears to be low.
How is the focus? To see if the high voltage is in the ballpark for the CRT.

[guizmo1967]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

D311, D312, R346, C343 are good !

Focus working as it should.

I'm going to replace C337 (1uF 250V axial). The only one I have near those specs is a 10uF 250V radial, so I think it's gonna be fine !

[guizmo1967]

Re: BK Precision model 1472C Oscilloscope Intensity problem ??

I replace C337, problem not solved. I'm running out of idea !!

One leg of VR2 (Brightness knob) goes to pin 3 of IC204, I'm beginning to suspect that IC, what do you think ??

Maybe a transistor after VR2 is not good but I don't like replacing components randomly !!!

[guizmo1967]

Well, I'm about to give up on that scope !! After replacing many components, that were probably good, the intensity knob still don't do nothing at all, the line on the screen is at full brightness !!

The intensity modulation (Z-axis) jack at the back is for as I understand it, modulating the brightness of the spot at specific time, correct me if I'm wrong !!

What I did is connect a 100k pot to this jack with a range from -24V to +24V, but nothing happened on the screen no matter how I turned that pot …

The service manual spec says 20 p-p minimum.

Am I testing the intensity modulation jack not the correct way ??

Please give me some help before I think of throwing that scope in the garbage ...

Thank you !!