Author Topic: help with oscilloscope model 34 (Read 16404 times)

Rob Strand · « **Reply #60 on:** October 12, 2018, 04:59:52 PM »

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sw on 0 2.0
sw on 10 it jumps around from -8.0 to -34

Not quite as symmetrical as I'd expect but at least you can get + and - so that's a good start.

The jumping around sounds like a bad contact in the pot.
Maybe the pot needs cleaning.
Sometimes moving it backward and forward a few times helps (for now) but later it will go bad again.

The main point is you can get + and -. You should be able to measure the voltage between V2 pin 1 and V2 pin 6 and adjust the pot to get zero output voltage.

Another thing you can check is the voltage on pin 4 of the pot to ground. It should stay at a fixed voltage when you adjust the pot. I expect about 40V there.

Rob Strand · « **Reply #61 on:** October 12, 2018, 07:22:20 PM »

Oh, another test is to measure the voltage difference between pin 3 and pin 4 of the pot for different pot settings.
If it's not symmetrical try pulling out V2 and re-checking.
If it's still not symmetrical there might be something weird going on with the pot, or perhaps leaky caps (but that would be low on the list).

whomeno · « **Reply #62 on:** October 12, 2018, 09:41:30 PM »

across v2 pins 1 & 6 I can take it too 0
which would be pin 4? here is a pic

Rob Strand · « **Reply #63 on:** October 12, 2018, 09:49:30 PM »

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across v2 pins 1 & 6 I can take it too 0

OK cool. That means V2 and surround is probably working.

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which would be pin 4? here is a pic

Pin 4 of the pot is single terminal with green wire on the left.

whomeno · « **Reply #64 on:** October 12, 2018, 09:54:09 PM »

pin 3 would be below 2 right? not Above

whomeno · « **Reply #65 on:** October 12, 2018, 10:02:14 PM »

ok pin 4 to pin above pin2 12v to 14v
pin 4 to pin below pin2 -12 to -14

whomeno · « **Reply #66 on:** October 12, 2018, 10:35:58 PM »

will be back on saturday.

Rob Strand · « **Reply #67 on:** October 12, 2018, 10:58:32 PM »

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ok pin 4 to pin above pin2 12v to 14v
pin 4 to pin below pin2 -12 to -14

Those look good.

I think you measure the right voltage despite my mistyping.
I meant measure between pins *2* and 4 on the pot.

Here's the marked up pic:

Anyway, is it possible to adjust the Vertical Position pot to get zero voltages between pin 6 and pin 1 of V2 ?

***** For some reason the pot is wired up with pin 1 in as the clockwise position. Don't worry about that.

Rob Strand · « **Reply #68 on:** October 12, 2018, 10:59:34 PM »

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will be back on saturday.

OK no problem.
I think we are getting close to checking/debugging getting a green dot.

whomeno · « **Reply #69 on:** October 13, 2018, 10:38:28 AM »

yes i can adjust the Vertical Position pot to get zero voltages between pin 6 and pin 1 of V2

Rob Strand · « **Reply #70 on:** October 13, 2018, 05:49:09 PM »

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yes i can adjust the Vertical Position pot to get zero voltages between pin 6 and pin 1 of V2

OK cool.

Do the following:
- Make sure all the tubes are in.
- Make sure the Sweep Selector (Red) is set to EXT.
- Set Vertical position so the voltage between pin 1 and pin 6 of V2 is zero
- Set Horizontal position so the voltage between pin 1 and pin 6 of V6 is zero
- Set the Intensity pot to full

Note down the positions of the Vertical position and Horizontal position pots so you
can set them back to that position quickly.

Do you see a dot or any form of green light from the screen?

Also, try wiggling the Focus control back and forth.

whomeno · « **Reply #71 on:** October 13, 2018, 06:30:41 PM »

I think there is something wrong with the Vertical switch. I can't get it to zero now.

Rob Strand · « **Reply #72 on:** October 13, 2018, 06:34:32 PM »

FYI, my only "concern" at this point is normally the voltages on V2 and the voltage on V6 are about equal.
The Heathkit schematic shows these all at 280V. From above, after adjusting the pots to get equal voltages, you were getting about 320V on V2 and 225V on V6. So V2 high and V2 low and they are not equal.

Need to check if the drifted resistors values are contributing to this.

Maybe measure the voltages:
- "B+b" marked on the schematic, where R32 (1k) and R26 (1K) join
- "B+c" marked on the schematic, where R52 (27k) and R53 (27k) join.
- While you are there, measure "B+a+, at any point on the wire connecting R56, R31, R45 etc.

Rob Strand · « **Reply #73 on:** October 13, 2018, 06:36:16 PM »

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I think there is something wrong with the Vertical switch. I can't get it to zero now.

It might be dirty. You can clean it with electronic contact cleaner.
Sometimes you can wiggle it backward and forward a bit to get it going however it often goes bad again later.

whomeno · « **Reply #74 on:** October 13, 2018, 07:14:48 PM »

where R23 (1k) and R26 (1K) join 359v
where R52 (27k) and R53 (27k) join 356v
for the life of me I can not find these resistors R56, R31, R45

Rob Strand · « **Reply #75 on:** October 13, 2018, 07:46:07 PM »

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for the life of me I can not find these resistors R56, R31, R45

Have a look at my pic from post #16.

One side of R56 (5k) is B+b = 359V = the voltage you measure between R23 (1k) and R26 (1K).
The other side of R56 is B+a ; should be about 135V.

whomeno · « **Reply #76 on:** October 13, 2018, 07:53:44 PM »

200 but it drops as time moves

Rob Strand · « **Reply #77 on:** October 13, 2018, 08:10:47 PM »

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200 but it drops as time moves

OK a but higher than I expected from the Heathkit circuit - not a crazy value.
Same as the other voltages, B+b and B+a, not a close to the Heathkit as I hoped - again, not crazy values.
They aren't exactly the *same* circuit so we might expect some differences.

Anyway, did you have any luck getting the Vertical Position pot to work?

whomeno · « **Reply #78 on:** October 13, 2018, 08:23:58 PM »

not yet I will let you know when I got it working

Rob Strand · « **Reply #79 on:** October 13, 2018, 08:29:54 PM »

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not yet I will let you know when I got it working

It's best if you can get it working.

If you are really desperate and want to continue there is a trick to force the balance of the tubes. You simply short the two anodes of V2 together and short the two anodes of V6 together. From that point you can debug the CRT part.

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Author Topic: help with oscilloscope model 34 (Read 16404 times)