SB-610 Oscillator Problem

[Garey Barrell]

Randy -

My experience with these oscillators (including those in the HO-10) is
that they have marginal gain.  Also, I would say it's highly unlikely
that your problem is in the PEC's as they are about as reliable as
ceramic capacitors, as in VERY reliable.

The changed cathode resistors ARE suspect however, since they set the
operating point of the oscillator stages.  Some have recommended
reducing the value of these resistors, but I have found that if the
components are "as designed", they will work.  A weak tube will also
cause this problem, since as I said before the gain is marginal at best.

There is no way to check the PEC's with an ohmmeter, since there are no
DC paths between any of the leads.  The PEC's in the HO-10 consist of
500 pF caps and 470k resistors.

I would begin by replacing the cathode resistors, and then swap the tube
with a known good one before worrying any more about the PEC's.

Let us know what you find!

73, Garey - K4OAH
Atlanta


Randall C. Iliff wrote:

>Hi All,
>
>I'm working on an SB-610 and for some reason neither oscillator will
>function.  Parts are the right ones, soldering is ok, switch cleaned, tube
>checked.  The cathode resistors have grown from 6,800 (heath value) to a
>current value of 10,150 and 7,800 ohms respectively.  I can certainly swap
>those out, and the 10 mfd caps that bypass them, with parts on hand but have
>not yet done so.
>
>Anybody else ever encounter a similar problem with these seemingly simple
>oscillator circuits?
>
>My concern is that the PEC phase shift networks seem to be open between pin
>1 and 2.  (It seems odd to me that they'd both read that way, so maybe I'm
>not measuring them properly.)
>
>Anybody have the component values inside that overgrown ceramic package?  At
>least then I could check these units or build up individual component
>substitutions if they are bad.
>
>Thanks, and best wishes for a wonderful new year!
>
>All the best,
>
>Randy (KE0BN)
>
>

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