Re: Very high input impedance high voltage meter???

From: Chuck Harris (cf-NO-SPAM-harris_at_erols.com)
Date: 09/26/04 

Date: Sat, 25 Sep 2004 23:29:24 -0400

Norm Dresner wrote:

> Related questions: The HP200CD service manual says, in part, "The DC
> voltage between the cathodes (pin 3) of the 6AU5 tubes should be 1 volt or
> less. If this voltage is excessive, it indicates a bad tube in the
> oscillator (V1-V4) which must be replaced to insure low distortion in the
> output waveform."
>
> 1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...

What they are trying to show is the balance between the different tubes.
Anything over 1V is excessive (in their opinion). If you let the balance
get too far off, the oscillator will be prone to distortion, and hum.
>
> 2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
> them is way off, it's pretty much the one that should be replaced. What if
> they're both close but the difference is a little over 1.0V? Should I just
> replace both?

It probably doesn't matter what the voltage is exactly, as long as it is
around 3.5V, and less than 1.0V difference.
>
> 3. Should I just replace all 4 tubes in the circuit? There's also a
> rectifier tube whose output is filtered by a couple of 10 uF capacitors
> [actually three sections of a single capacitor) and a 6H choke. How much
> 60/120 Hz ripple on the DC output of the power supply would suggest that the
> tube and/or the capacitors should be replaced too?

I would suggest that you pay more attention to how much ripple is on the
output of the oscillator. What I generally do is test the caps with a good
capacitor tester. If they seem a little leaky, I reform them for a while.
If they don't settle down to very minimal leakage current, or if the ESR is too high,
I replace them.
>
> 4. The really hard question: Am I still trying to polish a turd and should
> just replace all of the electronics with a good quality IC-based Wein Bridge
> using the dual variable capacitors as the basis for this?
>
> Norm
>

Keep the 200CD! They are virtually bullet proof. They will last virtually
forever. You can always build an IC Wein bridge oscillator if you really need
super low distortion.

-Chuck

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