[R-390] More R-390A Questions...

[Tom M.]

Mike,

There are two main modes of PTO error.

1) non-linear error
B) endpoint error

If a PTO is non-linear (it can happen) a graph of the output vs dial reading
would not be a straight line (within 200 hz at ea 100 kcs checkpoint).

If the PTO endpoint is off (very common to be "long") it has an output of other
than the proper 1000 kcs in ten turns of the pto shaft.

The PTO should have an output of 3.455 kcs at 000 on the dial and 2.455 kcs
tens turns higher than 000.  Yours is requiring more than ten turns to achieve
the 1000 kcs of full output.  While not a major problem, it can harm
sensitivity on the low bands as this can be a significant percentage out of
tune when it is supposed to be joined at the hip with the RF deck and the
variable IF's (less of a problem on the higher bands).

It can also be a tuning hassle, and after all if you fiddle with 390's at all
this has to be right!

The endpoint adjustment is under a screw behind the transformer on the PTO. 
Turn it to the right to get more output is ten turns.

It may also be nonlinear, but fix the endpoint first and then regroup.

You should have a manual handy when attempting this.

Cheers!

Tom N5OFF
--- Mike Melland <w9wis@charter.net> wrote:
> I've been ringing my "new to me" R-390A out all weekend. 
>  I received it last Wednesday.... It's a Motorola, serial 
> number 714 from the 1956 contract.... parts inside have 
> 1955/56 date stamps on them.  I noticed that the PTO is a 
> Collins.  I have checked over most of the things all of 
> you told me to check and find that the radio works very 
> well indeed.  It is very clean inside and the front is 
> also in nice shape.... has all internal covers plus top 
> and bottom as well as tube shields (silver ones) and has 
> what appear to be the original meters.  It does not appear 
> to have any modifications at all.  Everything appears to 
> work pretty much as it should.
> 
> I do have a couple of questions that perhaps some of you 
> could answer for me.
> 
> How linear is a Collins PTO ?  If I calibrate against the 
> calibrator at for example 6.000 MHz and then turn up to 
> 6.900 MHz and re-check against the calibrator It's off 
> about 1.0 to 1.2 kHz high.  
> 
> If I listen to my local AM station at 1.490 MHz and have 
> the radio set up medium AGC, RF gain up all the way at 10 
> I read about 80 dB of carrier level and the audio sounds 
> fine.  However if I leave the radio on for several hours 
> and return I notice the audio is distorted.  If I then 
> reduce RF gain a bit I hit a point where the audio clears 
> up and is fine.  I can then turn up the gain again after a 
> bit and the audio remains fine.... but will eventually 
> distort again.  This seems to happen only on this very 
> strong very local signal and in the 4,8 and 16 kHz filter 
> positions.  MGC appears to work normally. Is this overload 
> when in AGC ?  Is it normal ?  What would be likely to fix 
> this if it's a problem ?
> 
> BTW.... I was checking Chuck's procedure for adjusting IF 
> gain.  What is meant by "terminate antenna" ?  If I put it 
> into a 50 ohm Bird load I hear nor see any receiver 
> noise.... perhaps it means tune to 15.2 MHz with the 
> antenna hooked up to hear background noise ?  At any rate, 
> the carrier needle on mine using the antenna hooked to a 
> dead spot around 15.2 pins to the right when using the 
> procedure to check the IF gain.... could that be part of 
> the cause of the distortion when listening to a very 
> strong signal in AGC if the gain is in fact set too high ?
> 
> 
> Thanks,
> 
> Mike, W9WIS
> 
> 
> --
> Michael Melland, W9WIS
> Winneconne, WI USA
> _______________________________________________
> R-390 mailing list
> R-390@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390


__________________________________________________
Do You Yahoo!?
Great stuff seeking new owners in Yahoo! Auctions! 
http://auctions.yahoo.com