Oscilloscope B&K Precision 1472C Intensity problem !

[guizmo1967]

I have an old B&K Precision 1472C that is working except when I turn the intensity knob nothing happened, the line is always at full brightness. The service manual says the line should disappears when the knob is at the 9 o'clock position but it does not !!

This is what I have done so far:

1: Replaced all the electrolytic capacitors.
2: Replaced all the infamous 2SC458C transistors with 2SC536NF.
3: Replaced some other transistors.
4: Replaced IC201, 202, 203 and 204.
5: Replaced diodes D311, D312 and D313 V06E with UF4005.

Check every resistors, diodes on the power supply board. I even check with my multimeter on the solder side, EVERY traces to make sure none were broken and on the component side EVERY connections between them to be certain there was not a cold solder joint somewhere. Very time-consuming !!!!


Voltages measurement:

Connector P303
1:   +86V
2:   +83V
3:   NC
4:   NC
5:   -1900V
6:   -1895V
7:   -1895V
8:   -1895V
9:   -1560V

CRT socket (removed or not from the CRT, I get the same readings)
1:   -1895V    Heater
2:   +86V      Astig
3:   -1900V    Grid
4:   +102V    Horizontal plate
5:   NC
6:   +116V    Horizontal plate
7:   -1895V   Cathode
8:   -1560V   Focus
9:   NC
10:  +92V     Vertical plate
11:  +83V     Astig
12:  +78V     Vertical plate
13:  NC
14:  -1895V   Heater

I know it's a lot of numbers but I would really appreciate if someone could have a look at the schematic and try to tell me what maybe the problem because I'm running out of solutions !!!

[bdunham7]

Are you adjusting VR307?  If so, and you can't get other than full brightness, then the next thing to do would be to check the voltages at VR2 and VR307, seeing what they are and if they change at all when you adjust the control or trimmer.  Which version do you have?

[guizmo1967]

Thank you very much for your help bdunham7 !!!

No matter how I turn VR307, all I get is full brightness.

I check VR2 (500 Ohms) itself, not for voltage and it is ok, goes from 2 Ohms to 500 Ohms.

Connector P309
1:   +5.05V
2:   +5.06V  (no matter how I turn the knob)
3:   ground

Directly at the VR2 pot
Leg 1:          +5.03V
Middle leg:   +5.04V (no matter how I turn the knob)
Leg 3:          fully CW +5.05V, fully CCW +4.96V  (From pin 2 of connector P202, I think it's the blinking pulse ??)

VR307
Fully CCW +146V, fully CW +85V

I have the later version with mini board X77, I replaced almost every components on that little board !!!

[bdunham7]

That schematic makes my eyes water.  I haven't figured out what everything does yet, but I would disconnect the side of VR2 that goes to J202 pin 2 and tie it to ground through a 50R or so resistor, then try it to see if the intensity control responds and is adjustable.  Once you know that, you know which direction to concentrate your efforts.

[guizmo1967]

I disconnected pin2 from P202 and tied it to ground through a 50 Ohms resistor and now pin 2 of P309 goes from +0.4V to +5.04V, but no matter how I turn the knob, line is always at full brightness

The service manual shows what voltage should be present on each CRT pins. It says pin 3 and 7 should be at -1400V but they are at -1900V and -1895V respectively !!

I really hope that together we are going to find the problem, because all those hours troubleshooting it, all the money for the parts, I could have bought a better working oscilloscope. Well mine is working but not the intensity LOL

Please don't let me down ......

[bdunham7]

I really hope that together we are going to find the problem, because all those hours troubleshooting it, all the money for the parts, I could have bought a better working oscilloscope. Well mine is working but not the intensity LOL

Sure you can buy a scope, but where's the fun in that?

I merged the 3-piece schematic in case anyone else wants to jump in.  Can you tell me whether your unit actually has a Z-axis input on the back?

[guizmo1967]

Yes it has a Z-axis input on the back and I did try something with that.

I used four 12 volts battery to make a -24Vdc to +24Vdc power supply, connected that to a 100KOhms pot and the middle leg to the Z-axis input and still nothing happened !!

I told you, I spent many hours on that scope .....

[bdunham7]

Do you know where the Z-axis input is on the schematic?  I don't see it.

Can you verify whether P302 has a shorting plug installed?  And if removing it changes anything?

Try this experiment:  Set the scope up so that it is triggering and showing a trace, then adjust the horizontal position so that the trace is starting on the left side but well onto the screen.  Then remove the signal or stop it from triggering and sweeping--is there a bright dot remaining on the screen?

I haven't really figured out exactly how the high voltage section works on this.  Do you have another scope for testing?

[Zenith]

I don't find the circuit diagram easy to follow. I think pin 3 of the CRT (pin 5 on P303) is the first grid of the CRT and should be several volts negative to the cathode for normal operation, just like with a vacuum tube. Possibly at -10V it totally cuts off the beam and at -5 it's as bright as it should be. The brightness control and flyback blanking affect the voltage on that grid.

As a hunch I'd change R346, a 22M resistor, for a suitable high voltage replacement.

If your luck's out, you have an internal disconnection between pin 3 on the CRT and the grid. You should still see the voltage on pin 3 change, with respect to the cathode, when the brightness is adjusted.

You could measure the voltages, and with a codge force pin 3 to be say, 18V negative to the cathode. That should at least dim the display.

I'm sure I don't need to tell you that oscilloscopes can bite, so be extra careful.

I told you, I spent many hours on that scope .....

Consider it an investment of time rather than an expenditure, and all part of the joy of learning.

[guizmo1967]

Do you know where the Z-axis input is on the schematic?  I don't see it.

Just look to the left of the CRT

Can you verify whether P302 has a shorting plug installed?  And if removing it changes anything?

Yes it has shorting plug but ounce removed, still no change !

Try this experiment:  Set the scope up so that it is triggering and showing a trace, then adjust the horizontal position so that the trace is starting on the left side but well onto the screen.  Then remove the signal or stop it from triggering and sweeping--is there a bright dot remaining on the screen?

If I turn the SWEEP TIME / CM knob to CHB, I get a really bright (over-bright) spot in the center of the screen.

I haven't really figured out exactly how the high voltage section works on this.  Do you have another scope for testing?

No I don't !

[bdunham7]

I can't see the Z-input anywhere.  Is it on the schematic that I attached?  What is the closest component to it?

Let's try the sweep thing another way.  Set the scope up for NORMAL and LINE trigger, only Channel A selected and 1µs/div sweep rate (this should dim the trace a bit, but not for helpful reasons), then move the trace to the right so that it starts on the screen.  Is there a bright dot at the point where the trace starts?

Not having another scope often makes these things tough, but this just might be solveable. 

[guizmo1967]

I don't find the circuit diagram easy to follow. I think pin 3 of the CRT (pin 5 on P303) is the first grid of the CRT and should be several volts negative to the cathode for normal operation, just like with a vacuum tube. Possibly at -10V it totally cuts off the beam and at -5 it's as bright as it should be. The brightness control and flyback blanking affect the voltage on that grid.

Pin 3 of CRT (pin 5 of P303) is grid
Pin 7 of CRT (pin 8 of P303) is cathode
Pin 8 of CRT (pin 9 of P303) is focus
Pin 1 of CRT (pin 6 of P303) is heater
Pin 14 of CRT (pin 7 of P303) is heater

If you look at my voltage measurements in my first post, CRT pin 3 is at -1900V and CRT pin 7 is at -1895V. That's the reason for the full brightness, but why !!!

As a hunch I'd change R346, a 22M resistor, for a suitable high voltage replacement.

I already changed that resistor !

If your luck's out, you have an internal disconnection between pin 3 on the CRT and the grid. You should still see the voltage on pin 3 change, with respect to the cathode, when the brightness is adjusted.

I don't think the CRT itself is the problem because if I removed the adapter from the CRT and measure each pin of that adapter, I get the same voltage readings as in my first post.

You could measure the voltages, and with a codge force pin 3 to be say, 18V negative to the cathode. That should at least dim the display.

How do I do that

I'm sure I don't need to tell you that oscilloscopes can bite, so be extra careful.

[guizmo1967]

I can't see the Z-input anywhere.  Is it on the schematic that I attached?  What is the closest component to it?

It's connector P302, R363

Let's try the sweep thing another way.  Set the scope up for NORMAL and LINE trigger, only Channel A selected and 1µs/div sweep rate (this should dim the trace a bit, but not for helpful reasons), then move the trace to the right so that it starts on the screen.  Is there a bright dot at the point where the trace starts?

OK I turned SWEEP TIME knob to 1us/div, TRIGGERING LEVEL knob to normal, not AUTO and if I turn the HORIZONTAL (POSITION) knob I can see a really bright dot moving across the screen.

[bdunham7]

It's connector P302, R363

Do you have a different schematic?  I still don't see it....

OK, you don't have LINE trigger, but what you did confirms that you have no blanking as well as no intensity control.  I'll think about what to do next if you don't have a scope.  If you can, check the voltages at the emitter and collector of Q321, both with the intensity turned up and turned down.  Leave it set up with the wire at VR2 grounded through the resistor for this so you have a varying voltage at the center of VR2.

[BlownUpCapacitor]

Check the voltage of the anode of D312.

There will be a little bit of a high voltage there so be aware of that so you don't blow up your meter. At least 500V, likely higher. Look at the schematic for more details.

When you adjust the intensity control, there should be a voltage change.

If there is none, check the cathode of D313. The voltage there is low so adjust your voltmeter range accordingly. The voltage there should change when the intensity control is adjusted.

[guizmo1967]

It's connector P302, R363

Do you have a different schematic?  I still don't see it....

OK, you don't have LINE trigger, but what you did confirms that you have no blanking as well as no intensity control.  I'll think about what to do next if you don't have a scope.  If you can, check the voltages at the emitter and collector of Q321, both with the intensity turned up and turned down.  Leave it set up with the wire at VR2 grounded through the resistor for this so you have a varying voltage at the center of VR2.

I have the same schematic as you. The image you posted, it's right there INT MOD, shorting plug !!!!

Voltage at Q321 when I turn VR2:

Emitter: from +40.5V to +86.5V
Collector: from +123.5V to +128.5V

[guizmo1967]

Check the voltage of the anode of D312.

There will be a little bit of a high voltage there so be aware of that so you don't blow up your meter. At least 500V, likely higher. Look at the schematic for more details.

When you adjust the intensity control, there should be a voltage change.

If there is none, check the cathode of D313. The voltage there is low so adjust your voltmeter range accordingly. The voltage there should change when the intensity control is adjusted.

Voltage at anode D312 is -1908V, and no change when I adjust VR2

Voltage at cathode D313 goes from +84.6V to +91.2V when I adjust VR2

[bdunham7]

I have the same schematic as you. The image you posted, it's right there INT MOD, shorting plug !!!!

Voltage at Q321 when I turn VR2:

Emitter: from +40.5V to +86.5V
Collector: from +123.5V to +128.5V

Oh!  I was expecting a BNC type connector and slightly different wiring.  I thought that was an internal connector!  So the Z-axis input is not DC coupled, which means using batteries won't have any effect.  Is there any way you can come up with an AC voltage of about 10V_RMS?  If so, set it up so that the signal is displayed on CH A with as much amplitude as possible without clipping, then simultaneously feed the same signal to the Z-axis input.  When you do, you should see the tops of the waveform dim and the bottoms brighten (or vice versa).

The voltage change at the emitter of Q321 shows that the circuit is working (somewhat, anyway) that far up.  Your problem is likely in the area of Q318, but I'm not sure how to test that without a scope.

[guizmo1967]

Is there any way you can come up with an AC voltage of about 10V_RMS?

Well if you are talking about a function generator, I don't have one yet !

Just to let you know, Q318, Q319, Q320 and Q321 were replaced.

Will get back tomorrow, here it is 11:00pm, time to go to bed .... I really hope that we will find the problem.

[bdunham7]

Just to let you know, Q318, Q319, Q320 and Q321 were replaced.

Were all those parts replaced trying to fix this specific issue? 

The 10VAC can come from a transformer, 60Hz should be fine.  Maybe a 9VAC wall wart?

[guizmo1967]

Yes I replaced all those parts trying to fix this issue, like I said I ran out of solutions ...

I have a transformer laying around, it's a 27Vac center tap. Black probe on center tap does not give -13.5Vac and +13.5Vac, they are both at +13.5Vac or +27Vac when I only measure both ends.

Service manual says:

INTENSITY MODULATION
Voltage: 20 p-p minimum
Input resistance:  470KOhms

[bdunham7]

I have a transformer laying around, it's a 27Vac center tap. Black probe on center tap does not give -13.5Vac and +13.5Vac, they are both at +13.5Vac or +27Vac when I only measure both ends.

There is no + or - with AC.  Go ahead and connect the center tap and one winding to Channel A, get a signal on the screen and then connect it to the Z input as well.  Make sure you connect the same wire (the center tap would be best) to the grounds.  You can alternate which non-center-tap wire goes to the Z-axis and the dimmed part should appear on the top or bottom depending on which one you select.

[carlitos]

One of the Neon, N301 and N302, may be leaking.
Capacitor C343 or C333 open, without capacitance.

[guizmo1967]

Ok this is what I did:

ground clip to center tap, probe to one winding connected to CHA, sine wave on the screen.

Z-axis BNC ground to center tap, Z-axis input to same winding as my oscilloscope probe.

Result = sine wave at full brightness !!!

[guizmo1967]

One of the Neon, N301 and N302, may be leaking.
Capacitor C343 or C333 open, without capacitance.

Those two caps has been replaced as well !

Since I never had this oscilloscope before, first time I turned it on without the casing, I noticed that the two Neon were just slightly glowing at the tip and I ask myself should they be completely off

Do you think that since the beginning, the problem was just the Neon Well the problem is not solved yet, but just asking ....