[Collins] 70K - 2 Fault

[Dr. Gerald N. Johnson]

On Mon, 2006-03-27 at 20:44 +0000, Jim Smith wrote:
> Hi,
> 
> I have just experienced an unusual fault with a 70K-2 pto. The rig I had 
> purchased arrived with the dial carefully wrapped up, the pto had obviously 
> been got at - with only one of the chassis mounting crews in place. (other 
> screw safely wrapped up)
> 
> I quickly set every thing back in place and switched on and on 20 m around 
> 14100 (my favorite check area) found the Cal and realigned the dial to 100. 
> Tuning down I found the 00 cal quite a long way off. BUT on tuning up I 
> could not find the 200 cal signal.
> 
> I won't waste your time further but I put a counter on the pto and found the 
> pto was not going down to 2500 -hence no 200 cal.
> 
> The lowest frequency was something like 2547 and continuing the tuning to 
> the actual end stop of the coil slug (a small movement) - the freq folded 
> back on itself rising to about 2560. (I had of course loosened the shaft 
> stop.)
> 
> I have made the quickest of fixes by adding small silver mica across that 
> 'adjust on final test' C 301 (as it is the easiest fix to do) which is 
> across the pto assembly. The pto is back tuning 2470 and through 2700. Now I 
> have my three cal points and the pto is tuning over the 2500-2700 plus a few 
> kHz in the usual way.
> 
> Has anyone experienced this - and what is the recommended fix - should I 
> change all of the oscillator silver mica caps. Start from scratch to set it 
> up again?
> 
> I feel that this pto was very close to its spec originally and age has 
> pushed it outside - with no adjustment possible. The pto is pristine inside. 
> It must have been the most frustrating of faults - an endless loop!!
> 
> Sorry for the long post,Norfolk island is very lonely right now - HELP - 
> please reply to my email address.
> 
> Regards
> 
> 73
> 
> Jim, VK9NS
>  
C301 I think is an open capacitor, that is, has no coating. Chosen for
temperature compensation. The lack of a coating makes it vulnerable to
corrosion in the long run. Your tuning range symptoms are that there's
not enough C across the slug tuned coil, at least not until you added
the capacitor. I suspect that if you touch one of the leads of C301 that
you will find one lead or the other is no longer attached to the
capacitor. Then you wish to have a collection of 20 pf capacitors with
varying temperature coefficient. Failing that, you try what 20 pf you
can find, silver mica and any available temperature compensating
ceramics to find one that keeps the drift within reason. You may need to
give the PTO several hours of cooling after soldering the temperature
compensating capacitor to dissipate ALL of the heat from soldering
before you can see if its working for temperature compensation. You can
check frequency range earlier.

I suspect the technique at Collins was to build with a middle
temperature coefficient capacitor, then after sufficient time for the
soldering heat to have dissipated to set the oscillator to a standard
frequency, my guess would be mid range while at room temperature, then
with the shaft locked, place into a hot box, let the frequency
stabilize, and check the frequency change. From a chart, choose the best
available capacitor, replace C301 and repeat.

C305 as well as all the capacitors connected to the tuning coil are
suspects. C305 shows two different values and connections according to
age. The older schematics and parts lists show a 68 pf across the whole
coil. The later schematics and parts lists show a 100 pf with a couple
possibilities of temperature coefficient connected in parallel with
C302. ALL schematics and parts lists have a MAJOR discrepancy between
the schematic value of 2000 pf and the parts list staying 3000 pf for
C303. CPN 912-1748-00. C302 varies from the schematic and parts lists
too. Most show 1000 pf on the schematic and 1100 pf in the parts list.
Though my latest manual shows 1000 pf in both the schematic and parts
list. Somewhere along the line the part number changed fro 912-1747-00
to 912-1749-00. With the consecutive part numbers these were probably
special order dipped silver mica parts.

I still suspect the uncoated parts first.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer