[Drake] Drake R4C - Unstable VFO

[Garey Barrell]

Bob -

There are several possible causes for this.

Most common is the grounding of the PTO lead screw via the ball bearings 
at the front.  This typically presents as small 'jumps' in frequency as 
you tune, usually disappearing when you stop tuning.  Drake added a 
small, braided ground wire from the side wall of the gearbox to the 
follower fork, soldered at each end.

Another likely cause is the Zener diode inside the PTO housing.  It's a 
1N714, 10V, 500 mW Zener, replaceable by a 1N4740A or similar 1W unit.  
You can often see this voltage instability by measuring at the Red or 
Wht/Red wire entering the PTO can from under the chassis.

Yet another cause is the FET, 2N5950.

Lastly, there have been cases where the Buffer transistor, 2N3563 or 
2N3858, has caused small frequency jumps.

ALL of these can be changed without damaging the linearity of the PTO, 
AS LONG AS you don't disturb ANY of the other internal components OR 
leads inside the PTO can.  Some have managed this without removing the 
PTO from the radio, but it is a little more difficult.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>


Robert Wood wrote:
> Hello Drake Techies,
>
> I have a possible PTO issue with my R4C.  When listening to CW on any band, the
> signal is not stable (tone jumps around) intermittently.  I do not encounter the
> issue when switching to my TX4C PTO.  When using the T4XC PTO I can work other
> stations with no problems :).
>
> At this juncture, here are the steps I have taken within the R4C:
>
> 1. Cleaned all of the vacuum tube sockets and pins (deoxit)
> 2. Cleaned all of the switch contacts (deoxit)
> 3. Swapped 6HS6s (Premix Tube with the 1st Mix tube) no change
> 4. Replaced the two electrolytic capacitors (C163, and C163)in the 150V power
> supply section. (I thought I noticed the voltage fluctuating so went ahead and
> upgraded the caps)
> 5.  Tightened the VXO mounting screws.
>
> I am thinking about swapping the PTOs between the units to see what happens, but
> before I do I wanted to get other suggestions?  I really appreciate any help.
>
> Thank you very much,
>
> Bob
> K5MBA
>
>
>
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