i've noticed some interesting drift characteristics on my r4c. i live in
florida.. shack in the garage.. during the summer months , with 80-90 deg temps
and high humidity, the r4c drifts approx 50 hz per hour. during the winter with
50-60 deg temps and reduced humidity the drift is reduced to 10 hz per hour .
in both cases these measurements were observed after a lengthy warm-up period
using a digital display counter. i have no internal fan or aux cooling in the
radio. only significant change in the vacinity of the pto is the sherwood audio
amp mod . temp and humidity obviously a significant factor in dealing with
drift. i know this is no big revelation.. thought the differences were
interesting though.
Larry WA0QHF
Sent from my iPhone
On Jan 5, 2012, at 12:44 PM, Garey Barrell wrote:
> Curt -
>
> Yes, that covers it. I believe the core is precise, and determines the
> linearity between the end points. The df/ds 'centers' the span in the
> 'right' portion of the coil. There is a t/c cap in most of the PTOs, but not
> related to the coil cap combo. The only other 'failure' would be a variation
> in the ferrite core, which could crack or even lose permeability from heat or
> excessive dc current. This would probably be fatal. I suspect the core and
> coil are built together and are not interchangeable. I know this is the case
> for the coil / slug assemblies in the Passband Tuner.
>
> 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
>
>
>
> Curt wrote:
>> Hi Gary:
>>
>> Let me do a brain verification on this linearity issue. My thought is that
>> the linearity is nearly 100% dependant upon the coil, core consistency, and
>> screw thread linearity.. THe only contribution the caps can make to
>> non-linearity is if they are capacity dependant upon frequency over the span
>> of the VFO. While this is a real effect, my thinking is it is miniscule--as
>> is the screw lead. However, because the coil/core combo does vary its df/ds
>> (delta freq per delta distance), the capacitor setup can "place" the
>> useable span + or - somewhat to utilize the best linearity section of the
>> coil/core.
>>
>> Obviously there is the temp compensation issue to consider as well, where
>> the cap Tc can help to temp stabilize the LC and stray temp stability but
>> that is different issue.
>>
>> Is this about the way you look at the linearity issue?
>>
>> Thanks
>>
>> Curt
>> KU8L
>>
>>
>>
>> On 1/5/2012 11:14 AM, Garey Barrell wrote:
>>> Steve -
>>>
>>> I don't recall ever seeing one of those pots changing that much, but I
>>> guess it's possible. It may have been fried and the 600 ohm is what he
>>> could find. Not a major problem as the two resistors can be changed to
>>> center up the pot.
>>>
>>> I don't know about the linearity with the shield can off, but I suspect it
>>> would be affected.
>>>
>>> Those glass caps are the 'trimmers' for C194. I have seen two, one, or
>>> none on various PTOs. NP0 ceramics are probably the best available today
>>> for replacement. Dur-Micas woud probably work fine, although they DO have
>>> a slight T/C. If you have a small air variable trimcap it might be
>>> interesting to see what happens as you vary the capacitance.
>>>
>>> The 'sweet spot' seems to be about 92.5 pF. The nominal value of C194
>>> could be anywhere from 85.5 - to 94.5 pF. I think they may have selected
>>> 90 pF caps that were 92.5 or less, and then used small trim caps to bring
>>> it up to 92.5 pf. Most that I have seen have had one or two small value (2
>>> - 7 pF) caps. Remember that end-to-end specification is +/- 3 kHz.
>>> LInearity between those two points is determined by the precision of the
>>> coil, which is apparently pretty good.
>>>
>>> 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
>>>
>>>
>>>
>>> Steve Wedge wrote:
An interesting thing I've found in the S-meter circuit is that the
sensitivity adjustment (the one on the board) is listed as being 1k and
I'm measuring about 600 ohms. A wee bit out of tolerance, methinks, or -
the PO changed it. I can't see any markings on it to ascertain its
intended value.
After just putting the front panel back on, it breaks my heart to have to
remove it again, but I'm getting darned good at it. since I now have a
counter and power supply, hooking it up on the "bench" should be easy.
Garey, in your experience, does the linearity change when the cover is
removed? I know that the frequency overall sure does.
Most PTO's I've seen have two glass compensating caps on the solder side
of the board and this one only had one. It could well be that it only had
one to begin with, but I know for a fact that the original owner was in
there, so everything is suspec
