help with oscilloscope model 34

[Rob Strand]

between green wires 6.6 v
v7 pin 1 395 v   pin6  395v

pulled v8
v7    pin1 395v           pin6 395v

That's a relief.  Those look good.

We need to work out why the voltage on pin 7 of V7 is low.
Some possibilities:
- V7 has a problem.
- some fault is pulling down the voltage
- one of the parts of cap C33 has a problem

Put V8 back in.  And measure the voltage across R54 and R55.
That will tell how much current is flowing down each of them.
If the current is small and the voltage on pin 7 of V7 is low
then maybe V7 is stuffed or C33A has a "short".

[whomeno]

just rechecked v7 pin 7 953v

[Rob Strand]

just rechecked v7 pin 7 953v

V7?  953V is really high.  Are you sure it's not 453V?

What about the voltages across R54 and R55.

[whomeno]

r54 right by v7 on strip end to end 108

r55 by v2 on strip end to end 79

rechecked v6a pin 1   658    v6b  pin 6 330
v2a  pin 1  724  v2b  pin 6  524

but they do not move when switch is moved

yes pin 7 is 953 on v7

[Rob Strand]

yes pin 7 is 953 on v7

I can't believe it is that high.  I think you damaged your multimeter connecting it to V8.

If you have another meter try using that.

If you have some voltage sources or equipment with known the voltages,
check your meter can measure those voltages correctly.   The problem might only
show up on the highest voltage range.

[whomeno]

meter seems ok checked input v at 120 v
i did not have a good ground when i hit v8
have good ground now
only meter i have

[whomeno]

when you wanted me to read voltage across r54 r55 did you want it from ground to one end then the other?
or a lead on each end of resistor?
if i measure from ground to each end
R54
1 end 833   2nd end  947

R55
1 end  700  2nd end  782

[Rob Strand]

meter seems ok checked input v at 120 v
i did not have a good ground when i hit v8
have good ground now
only meter i have

Do you still get    V7 pin 1 395 VAC  and V7 pin6  395 VAC?

I'm having trouble trying to work out how V7 pin gets to 953V.   It should be about 450V.

If the meter is OK then I suspect the one of the power rails from V8 (V8 pin 1, V8 pin 9, C30, C31, R63, R61, R62) has shorted to V7 pin 7.

[Rob Strand]

when you wanted me to read voltage across r54 r55 did you want it from ground to one end then the other?
or a lead on each end of resistor?

Across each resistor.   The idea is it lets you work out the current through each resistor V/R.

[whomeno]

yes v7 pins are the same 1   395  6   395

[Rob Strand]

yes v7 pins are the same 1   395  6   395

OK.  So that says the transformer is OK.

Try pulling out V8 and remeasuring  V7 pin 7.

I'm worried about running the unit with 953V on the 450V caps they will blow up.

[whomeno]

dumb question how can a meter set to 600v measure 900 volts
meter is a utl 33

[whomeno]

with v8 out still at 955

[Rob Strand]

dumb question how can a meter set to 600v measure 900 volts

Well it depends on the meter. It could measure correctly but is stressing out the internal components.   Or it could be measuring wrong and the overload somehow isn't working.

with v8 out still at 955

Pulling V8 removes any chance of the 1200V rail shorting to the 430V rail and producing crazy high voltages.

I'm having trouble understanding how a transformer is producing 395VAC is getting to 955V DC.  The DC cannot (should not) get above 395V * 1.41 = 559V.   The 6X4 has voltage drops so the DC should be somewhat less than that.

I'm not sure how keen you are but at this point I would make a voltage divider probe:

  [Probe input +]  ----  10M  ---- 10 M  -----+-----  1M  --- o----  [Probe input -]

  [Probe input +] connects to the point to be measured in the circuit.
  [Probe input -]  connects to the circuit ground.
  The point + goes to the meter + and the point o goes to the meter -.
  This will divide the measured voltage down to about 1/23.
  So 955V should drop to 41.5V

[Rob Strand]

When I compare the voltages across the resistors with the difference of the voltage on each side of the resistor  the results are fairly consistent.

r54 right by v7 on strip end to end 108
r55 by v2 on strip end to end 79

R54  1 end 833   2nd end  947
R55 1 end  700  2nd end  782

947 - 833 = 114; only 6% higher than across resistor
782 - 700 = 82;   only 4% higher than across resistor

*IF* the meter is measuring OK on the low range then the measurements on the high range look OK.

I still can't explain the disagreement between the AC voltage and DC voltages.

[whomeno]

I made probe just as you said retested v7 pin 7 it droped it to 14.5v

[Rob Strand]

More trouble, I tried to find the input impedance of your meter (UTL33T) and the manual doesn't state it - ahhh!

Is there any chance you could replace 1M in the probe circuit with 100k? and repeat the test.   That helps remove the uncertainty of the meter impedance.   The multiplying factor will then be between x203 to x221;  ie. 10% uncertainty.

Anyway,

If your meter is 10 M ohm input impedance the real voltage is 23 times what you measured,

    V(V7 pin 7)  = 23 * 14.5 = 334V

If your meter was 1M ohm input impedance the real voltage is 41 times what you measured,

    V(V7 pin 7) = 41 * 14.5 = 596V

So despite the uncertainty of the measurement of 334V to 596V we definitely aren't getting 950V.  So something weird might be happening with the meter.

Based on that, it's looking like your meter is doing something weird on the 600V *DC* range.
But not I can't explain why the difference are only 10% out.

[whomeno]

will do it now

[Rob Strand]

If you have a 9V supply or a battery good idea to measure it *without* the voltage probe.
We need to measure something we really know.
I'm pretty confused about what's going on ATM.

[whomeno]

new battery is 9.56
redid probe with 100k retested at 3.2v

don't you mean ac 600