Analog Oscilloscope Voltage Calibration Confusion

[Langdon]

I have an old (c. 1983) analog Metermaster 65601 oscilloscope (a rebadged Aron BS-601).

The manual (https://www3.langdonstaab.ca/Metermaster%2065601.pdf) says in the calibration instructions to turn a potentiometer until a pin on the P4 connector of the power supply board is 175±.5VDC.

However, the same manual also twice labled that exact pin as "200V"!!!!

Before I calibrated it to 175V, the voltage was 204VDC (which suggests 200V is actually correct).

Changing the voltage does not appear to affect the operation of the oscilloscope at all


Isn't it funny that 65601 stands for BS-601 (both contain "601"!)?

[Kean]

The documentation including the calibration info was almost certainly written after the PCB layout.

So I would guess they worked out that 175V was more suitable for production, compared to the original design spec of 200V.  The setting of 175V may put less stress on the components, but someone at the factory set it that way if an early production model, or later a user adjusted it to 200V based on the PCB labelling rather than reading the docs.

If you see no obvious effect on trace intensity, then I'd say the lower voltage should be fine.  With CRT aging it might have needed a tweak, but maybe it didn't see a lot of operating hours.

Not at all surprising that they have some commonality in part numbering.  Metermaster might have used 65 as a prefix for other rebadged models from Aron.

[Langdon]

Just strange they would say 175±.5VDC when it actually makes no difference and the original voltage was 200V!

[Kean]

Agreed.  It does seems like a tighter tolerance than would be required, but I've not analyzed the circuit nor are CRT grid drive circuits something I know much about.

It is possible that they changed vendors for the CRT at some point in production, and this could have changed the spec.  Maybe they just quoted the vendors recommended tolerance.

[Langdon]

Agreed.  It does seems like a tighter tolerance than would be required, but I've not analyzed the circuit nor are CRT grid drive circuits something I know much about.

It is possible that they changed vendors for the CRT at some point in production, and this could have changed the spec.  Maybe they just quoted the vendors recommended tolerance.

Good points.

If 204VDC is tolerated without issue, the tolerance is obviously more like ±30V, not ±.5V
That's a factor of 60!!!!