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Old 1st Feb 2023, 6:47 pm   #1
electroanorak
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Default Gould 1421 oscilloscope

Hi all.

Quite a while back there was a post about the OS300 scope and the EHT resistor network.

Now I have a 1421 scope here and suspect that there may be a problem with the network in it. From what I have found, it is the same one as used in the OS300.

A member was kind enough to post a schematic with the values of the network. However, the 2M2 resistor in mine is at 11M3 and the 560K resistor at 270K. All others are spot on.

Now, was he right or is mine correct. Any answers please.

Regards.

Dan.
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Old 2nd Feb 2023, 4:07 pm   #2
WME_bill
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Default Re: Gould 1421 oscilloscope

Advance Gould 1421 or OS300.
The thick film resistor RN720 is used in both these scopes. I suspect you have found my posting of 19 June 2020 giving the values of the resistors (under heading OS300 Focus Problems).
The values for the series resistor chain RN720(a) is 22M (not 2.2M as you suggest) and RN720(c) is 560k. As the values you quote are 11M and 270k, presumably for the same chain, I suspect they represent alternative manufacturing values. The ratio is the same, which is the important thing with these high voltage resistor chains.
I fear therefore that you are seriously in error.
If you really don't like the resistor values which I posted, then you can change them as much as you like.
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Old 3rd Feb 2023, 2:55 pm   #3
electroanorak
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Default Re: Gould 1421 oscilloscope

Hi.

Yes indeed, it was your post.You are right about the ratios. Sound l like the problem may be elsewhere. I have an intermittent trace with no intensity control. The focus control is workong fine.

I will battle on.

All the best.

Dan.
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Old 3rd Feb 2023, 4:57 pm   #4
WME_bill
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Default Re: Gould 1421 oscilloscope

Advance / Gould 1421.
That sounds more like Bright-up problems. Check the two high voltage transistors in the bright-up amplifier TR701 & 702. I have found that generally with modern scopes, transistor failures with the bright-up are the second most common fault, after the EHT supply itself. Incidentally, what is the actual transistor type used for TR701. The manual says TJ630, which sounds like a specially selection, and not a usual commercial type. Could you have a look and see please.
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