help with oscilloscope model 34

[whomeno]

I got it to zero but still no dot

[whomeno]

c33A  =  425v
C33B  =  373v
C33C  =  348v
C33D  =  160v

[Rob Strand]

c33A  =  425v
C33B  =  373v
C33C  =  348v
C33D  =  160v

Those voltage look better.

We need to measure the resistors around the tube.
One could be open circuit.
However you will need to discharge some caps first.

*** Do not touch any of the parts or wires around
      this whole area of the circuit.
      The voltages in this part of the circuit are 1000V to 1500V.
      They will zap you and your meter!

- Power off the unit
- Discharge C21, C30, C31, C32
- Measure the resistances R60, R62, R63, R67.
  You might have to wait a 30 secs or more for some of the measurements to settle
  (because we are measuring in circuit).

[whomeno]

ok,  R59  1M = 888
R60 1M =   1.025
R62   560K = 606
R63   220K = 348
Is R64 the switch ?
I can not find a R67

[amptramp]

With equipment of this age, the high voltage capacitors, C30, C31 and C32 often leak to the extent that the voltage goes below the amount necessary to provide enough voltage for anything to be seen.  Discharge them and pull them out of circuit and measure their resistance.  It should be above 30 megohms and even that is suspect.

[Slowpoke101]

Is R64 the switch ?
I can not find a R67

I think Rob meant R57 which should be 3M3
R64 (spot shape) appears to be (referring to the schematic) a 2M pot as is R58 (focus).
R64 may have been replaced or substituted by a switch (odd but possible).

[Rob Strand]

ok,  R59  1M = 888
R60 1M =   1.025
R62   560K = 606
R63   220K = 348
Is R64 the switch ?
I can not find a R67

Yes, R64 is the pot.

I think the resistor numbers stop at R64 so no R67.   That's my typo I meant R57.

Obviously some of the resistors have drifted, especially R63.

*** Remember to be careful around this area of the circuit and don't touch anything
     while you are measuring.

The next step is to check if there's any CRT beam current.
- Turn on the unit and wait about 30 secs to 1 minute
- Turn the Intensity Pot to maximum
- Measure the voltages across R60, R62, R63
- Turn the Intensity Pot to minimum
- Measure the voltages across R60, R62, R63

After that you probably need to make a High Voltage Probe.
In order to determine the meter input impedance do the following:
- Set the multimeter to DC voltage
- Measure the voltage on say a 9V battery.
- Put a 10M resistorsin series with the +ve lead of the meter.
- Remeasure the battery voltage using the free end of the 10M resistor.
  You should expect somewhere between 1V and 4.5V depending on your meter impedance.

I think Rob meant R57 which should be 3M3

Yes!

[whomeno]

what voltage should I see across the resistors?
My meter only go up to 600v

[whomeno]

battery 9.56
with 10m =.87

[Rob Strand]

what voltage should I see across the resistors?
My meter only go up to 600v

Hold off on doing that measurement.
We need to setup a good/safe way to do it.

[whomeno]

OK I will wait till you have a good plan

[Rob Strand]

battery 9.56 with 10m =.87

FYI, your UTL33T multimeter input impedance is 1M ohm.
Most meters are 10M but the cheaper ones are often 1M.

OK I will wait till you have a good plan

Here's the problem:

I'm fairly sure there is a problem with the high voltage circuits around the CRT.  Exactly what I don't know.   In order to zoom-in on the problem we need to make measurements.

One way is to make voltage measurements but that means measuring high voltages.  We also have to be careful the meter doesn't load the circuit and give is a false reading.

Another way is to remove enough parts so all the parts around the high-voltage circuits can be measured individually.   That includes measure the pots and making sure the pot wipers are actually connecting properly to the resistive track of the pot.  Any part that is faulty you replace.    After replacing the parts you power it up and hope it works.    If it doesn't you have little option but to fall back on the measuring the high voltages.

I've put the Bell&Howell 34 and Heathkit IO-12 HV circuits side-by-side in order to compare the voltages.



The beam current measurement I mentioned before usually only requires the measurement of a < 100V voltage across a resistors.  However, that assumes it is working.  If there is a fault then the measurement might get up to 1500V.

Also there's one thing that looks weird to me about that circuit.  The cathode (pin 3 of the CRT) connects to the HV via a 1M resistor (R60).  I'd expect the beam current to be around 100 to 500uA however a 1M resistor will cause 100 to 500V drop.  Normally that resistor is smaller and the voltage drop is smaller.  I can't see how the beam current passes current through R60.  But then I look at the grid G1 (pin2 of the CRT).  On the Heathkit circuit there's (1200 - 1180) = 20V drop across a 100k so I = 200uA which is in the order of the beam current I expect.  However, I don't know why such a high current is passing through a grid.     Just to top it off, on Bell & Howell, the equivalent grid resistor is 560k.  At 200uA that's going to have about 112V drop across it - which is more than normal.  I obviously don't understand something about that particular CRT tube.

[whomeno]

I do have the devry 1s14 which dose go to 1500 volt. but there  is a pot on the back to calibrate it. How would I do that?

[Rob Strand]

I do have the devry 1s14 which dose go to 1500 volt. but there  is a pot on the back to calibrate it. How would I do that?

I already sent a reply to your PM.

[whomeno]

I got pm.
calibrated meter
5Up1 voltages
pin1 = 0
pin2 = 0
pin 3 =0
pin4 = 0
pin5 = n/c
pin6 =250
pin7 = 240
pin8 = 240
pin 9 = 190
pin 10 = 180
pin 11 = n/C
pin12 = 0
how do I read neg volts?
can not get a reading across resistors

[whomeno]

ok vm has dc- but still reads o for neg readings

[Rob Strand]

ok vm has dc- but still reads o for neg readings

***** Hold-off on using that meter any further *****

I found a schematic on the web.  It looks like the negative terminal is grounded!

When you connected it to another circuit the -ve lead on the meter might short the supply through mains earth.

Also, and more of a problem, when you connect the -ve lead to a dangerous voltage it might make the case of the meter the same dangerous voltage.


***** Also don't go using this meter to measure any mains voltages *****

[whomeno]

ok

[Rob Strand]

***** Also don't go using this meter to measure any mains voltages *****

[whomeno]

ok no more meter.