Moseley PCL-505 STL Xmtr / AFC Drift

[RememberWHEN]

Does this unit have a history of AFC problems with normal room temperature variations? Power supply voltages appear normal. After several hours of operation the unit's AFC drifts to the point where it unlocks. Unit is in temperature conditioned environment. If the unit is on standby, drift issue is at a minimum. Any  help appreciated.

[littlejohn]

Given the age of the radio, it wouldn't be uncommon to find that some of the caps in the AFC section are drifting with temperature.  If it's in an airconditioned environment, it will take longer after turnon for the modules to warm up.  You >might< be able to get the can top off the AFC and use some freeze mist on the components in there when it starts to drift.  At the age, and given the marginal performance of the box by today's standards, you might want to consider replacement.

[littlejohn]

I assume you've toughed up the AFC loop gain?

[Dr. Bob]

I'd check for a stuck crystal oven and/or an old crystal.  The oven should cycle on & off for a constant temp.  I have seen them stick ON and "cook" the crystal leading to premature failure. 

Also, in the 505,606 & 6010 xmitters after a number of years, the reference frequency crystal will age and drift from the thermal cycling.  Eventually, the cap to pull it back will not work.  International Crystals in OKC can fix you up with a new crystal.

Dr. Bob

 

[whitfm]

Doesn't Moseley still refurbish these units? Seems like they did that on our 6010 a couple of years ago.

[OKCRadioGuy]

Nope.  But J-squared sure does. 

[David Reaves]

I'd check for a stuck crystal oven and/or an old crystal.  The oven should cycle on & off for a constant temp.  I have seen them stick ON and "cook" the crystal leading to premature failure. 

Also, in the 505,606 & 6010 xmitters after a number of years, the reference frequency crystal will age and drift from the thermal cycling.  Eventually, the cap to pull it back will not work.  International Crystals in OKC can fix you up with a new crystal.

Dr. Bob

Makes you wonder: How can a piece of rock go bad?
 

Kind Regards,
David

[littlejohn]

They go bad in several ways.  The most common is, the etchant isn't completely removed from  the galena in the etching process, and continues to slowly etch it.  The symptom is, the frequency walks up the band over time.  The ones from The City had this problem so badly in the late 70s we used to call them 'Wanda's Rubber Rocks'.  Their process is now much improved.

A stuck oven can cause fracturing and total failure, as well as driving the thing down the band just from the heat.

Poor connection or a lack of good cleaning at manufacture can also cause corrosion on the wires and contacts within the mounting structure.   This can cause a myraid of symptoms.

Remember, this gent is using a third overtone crystal, and that is then mutiplied about twelve times if memory serves.  A little bit of change goes a very long way at the frequencies employed.

[TomT]

While you have the box open--check for corroded IC's. These boxes used TI IC's from an era when they had problem with the silver plating on the leads.  I've found a number of 505's with the leads rusted away to nothing. Lots of these Texas Instruments chips used in the AFC sections of the transmitter.  May not be the source of your AFC drift, but could be a problem in the near future.

[Pete Partenio]

I had one that had the TI chip problem Too! I would shotgun the supply caps, clean up the pins, check the oven.
All the caps in the little boxes are tant. not a problem.

If you are still having problems,contact me OL and I will fix it for you....need work!! I am also tight with Dave Chancy and can get parts.