help with oscilloscope model 34

[whomeno]

ok replaced the 1v2 tube. no line on scope tried all settings. what should I look for now.
the only tube i could not test was the 6af

Schmatic

[PRR]

Generally troubleshooting involves Voltage Checks.

BUT this beast has 2,000 Volts at potentially lethal charge.

Are you experienced enough to work in there and not be killed?

(It isn't always the electric shock. I picked up a 20KV CRT, it "stung me", I fell down and bumped my head on the bench.)

While I am fond of tube 'scopes, and free 'scopes, I an NOT fond of repairing them. Especially today when you can get a pocket-'scope kit for far under $100. Cheaper than a  concussion or a funeral.

[whomeno]

I have worked on older projection tvs and got them working, never worked on a scope. my dmm is only rated for 600. did not get test leads for the 1s14 I got with the scope. tring to find the right banana plug for leads, they are small.

[Rob Strand]

OK we don't know  if there's a problem with the HV (~1200V) or not.

Don't put your meter across the 1200V it will get zapped.   There's a trick to measure this but normally you don't have to measure it.  It is often checked by measuring current by measuring the voltage across R62.  Don't worry for now deal with that later.

There's many things to go wrong and it only takes one to stuff it up.
So it's best to go through each section one by one.

For reference:
http://steptorun.com/shop/icon/2/EX-3-0022.jpg

Power:
- Are the heaters on all the tubes lighting-up?  (Including the Display you should see a glow at the back of the tube.)    Can't remember if you can see the heater working on 1v2 or not.
- Check all DC power rails

Check Horizontal amps:
- Switch the sweep selector to Horizontal input  (EXT on the Sweep Selector *NOT* the Sync Control).
-  Measure the voltages on the plates of V6A and V6B. 
   Check the  the Horizontal Position makes the voltage go up and down on each.
   When one plate goes up the other should goes down and the two voltages should be
   more or less equal with the Horizontal Position roughly in the mid position.

Check Vertical amps:
- Set the Vertical Sensitivity to 50V
- Measure the voltages on the plates V2A and V2B
   Check the  the Vertical Position makes the voltage go up and down on each.
   When one plate goes up the other should goes down and the two voltages should be
   more or less equal with the Vertical  Position roughly in the mid position.
-  Wouldn't hurt to check plate voltages on V1A, V1B, V1C


[EDIT:
FYI:  With the Horizontal mode set as above you will no longer get a line.  You will get a dot which you can move up/down left/right with the Vertical and Horizontal position controls.

Don't let the dot stay in one spot it will damage the screen.   Keep the intensity down on this mode so you can just see it.   Only turn up the intensity while trying to find the dot.
]

[Rob Strand]

BUT this beast has 2,000 Volts at potentially lethal charge.

Based on the cap voltages I'm guessing 1.2kV.  1.2kV wasn't uncommon in simple CRO's of the day.   Better models were 2.5kV.  More modern models are 8kV or so.

[Rob Strand]

The manual in the other thread was for the transistor version (unless I've missed something).

Here's a link to a tube version which is similar but not the same (eg. V1 is split into V1 and V2).
Until I find an exact match it can be used as a guide.  The main point is it has some test voltages marked on it.  They should be a good ball park to get you started.
http://www.heathkit.nu/O-12_mod_schematic_2428.png

Non-mod version:
https://elektrotanya.com/heathkit_o12_oscilloscope_sch.pdf/download.html
https://elektrotanya.com/PREVIEWS/38933510/23432455/heathkit/heathkit_o12_oscilloscope_sch.pdf_1.png

[PRR]

> I'm guessing 1.2kV.

His 'scope plan shows 1600V AC.

Your similar plan shows 1040VAC making 1340VDC, as expected from *1.414 - losses.

I will not argue the exact value; it is not critical either for operation or shock. 500V would "work" at lower brightness and focus; and still be dangerous. As you say, he wants "some" current at the *bottom* of some handy divider where the KV is few-V.

[Rob Strand]

His 'scope plan shows 1600V AC.

I only see that as the 1600V cap voltage.
1.2kV is a number I've seen kicking around a lot (the new schem actually shows 1200V at the display pin).
Anyway the last couple of schematics I posted shows some more precise voltages, unfortunately for other models, but there's a good chance they kept the same transformer.

As you say, he wants "some" current at the *bottom* of some handy divider where the KV is few-V.

It's common to *set* the beam current as part of calibration. Which is usually done with a pot.

[thermionix]

Which is usually done with pot.

[Rob Strand]

Which is usually done with pot.

That's so you get twice the buzz if you slip  .

[whomeno]

ok here are pics.  which 12au7 should i be testing for the horiz  part and which one for the other? there are 3 12au7's   all tubes lighting up display also

[whomeno]

ok they are marked. meter set on 600
v6A  69 pin 1
v6b   32  pin 6
They do not move when switch is moved
v2 a   65   pin1
v2b    45   pin6
dose not move with switch

[Rob Strand]

ok they are marked. meter set on 600
v6A  69 pin 1
v6b   32  pin 6
They do not move when switch is moved
v2 a   65   pin1
v2b    45   pin6
dose not move with switch

They look low.  Should be 260V and 280V.

Have a look at the Heathkit schematics I posted a few posts ago (reply #5).
They have voltages marked on the schematic.
Start by measuring the voltages in the bottom right hand corner of the schematic.
They have  labels A, B, C etc.

These ones are probably the most important to check at the moment:
G_semicircle  +350V
G_triangle  +385V
G_square   approx +430V

Then,
A +105V
B +110V
C +120V
D +115V

Actually the Heathkit circuit is a little different.  Here's my best guess at the voltages:



Use the caps and the resistors on the supply to help you find them on the actual unit.  When you measure the voltages make sure you measure the correct side of the resistor.

[whomeno]

going by the schmatic v8 pin 1 & 6 should have power, but on scope not hooked up  pins 4- 5- 9 are used.


I have 4 red 2 white and 2 green coming from power supply.
the 2 white go to v8  4  & 5 both at 146v
v7 pin 7 is 68v
the red wires has nothing over 70v

[whomeno]

if you look at the schmatic i posted thats how v7 and v8 is wired. i am charging up my camera to take better pics for you. I am trying my best to follow along with you. i can understand the schmatic but it don't make sense the way the unit is wired.

[whomeno]

here are better pics

[Rob Strand]

I've annotated your previous pic with the voltage points and some part values.
To me it looks like C33C and C33D are swapped on the built version; assuming we go A, B, C, D
counter-clockwise on the caps. (I'm just mentioning that. It is how it is drawn in my pic and it's fine like that.)



Give me a chance to look over your new pics.

[Rob Strand]

I have 4 red 2 white and 2 green coming from power supply.
the 2 white go to v8  4  & 5 both at 146v
v7 pin 7 is 68v
the red wires has nothing over 70v

Those are really weird voltages.

I think you need to measure:
- The heater voltage, should be 6.3V AC between the green wires.
- The voltage on pins 1 of V7 to ground and pin 6 of V7 to ground, should be maybe 330V AC.
=================
Ahhhh

v8  4  & 5 both at 146v

Don't measure stuff around V8 with your multimeter.  V7 is OK not V8!

[Rob Strand]

After that, pull out V8 and remeasure the voltage on pin 1 and pin 6 of V7.

[whomeno]

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

pulled v8
v7    pin1 395v           pin6 395v