[Milsurplus] Alignment Info BC-221-AK?

[Robert Downs]

No, that isn't correct.  Absent leaky or shorted paper capacitors that keep one circuit or another from working properly, the majority of unmolested BC-221's will still calibrate to their original calibration books.

Unfortunately, AFAIK no repair manual was ever written on the BC-221.  The only manual that I have found that covers calibration of any part of BC-221 is TM 11-4700, and it only covers re-calibration of the crystal oscillator.

I can't find any manual that covers re-calibration of the variable oscillator in the BC-221.  So can only assume that it would have been done in the same basic manner as that in which the variable oscillator of a transmitter or of a super-heterodyne receiver is done, which is usually by adjusting the inductance of the coil (or sometimes by adjusting a series capacitor) at the low end and the ca[acutance of the variable capacitor at the high end.  That much seems fairly logical and straight-forward.  

However, in the case of the BC-221, the issue is further complicated by the presence of two trimmer capacitors for the low range and two for the high range instead of only one for each range.  The only explanation for this that I can think of is that one trimmer in each case has a negative temperature coefficient and the other a zero one.  If this is the case, I know of no way to work out the temperatures used other than extensive trial and error.  If one doesn't plan to use the BC-221 on the ground in Tripoli in August and at 30,000 feet over northern Germany in January, this requirement can probably be safely ignored.

That being decided, the other unknown is what four frequencies to use.  My guess would be that the high range ones were either 2.0 and 4.0 MC or maybe the checkpoints nearest 2.2 and 3.8 MC.  With a mid point at 3.0 or thereabouts.  The test procedure would be to set the correction capacitor to 50% and then crank back and forth between the two end points adjusting one of the trimmer capacitors at the high end and the trimmer coil at the low end.  And then confirm that at the mid point, the correction capacitor had enough range to correct the frequency to match the book.

I think that this would probably work, but don't plan to try to prove it.

Robert Downs - Houston
WA5CAB


-----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Bob kb8tq
Sent: Tuesday, February 06, 2018 15:18 PM
To: Michael D. Harmon
Cc: Milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Alignment Info BC-221-AK?

Hi

I think that the simple answer is to re-zero it and move on. Back in the day, you probably sent it back for a new calibration book. Id bet that most of the calibration books are past their use by dates anyway. 

Bob

> On Feb 5, 2018, at 8:12 PM, Michael D. Harmon <mharmon at att.net> wrote:
> 
> Does anyone have information regarding the procedure for setting the "factory" trimmers in a BC-221?  I have a model AK which had a shorted 3-2 trimmer (HIGH range trimmer).  I have replaced the trimmer, but I think someone has been inside the unit with an alignment tool, probably not realizing that the trimmer was shorted.  I can't find any info regarding recalibration after replacement of the components in the oscillator section of the unit.  I'm sure at one time, there was something out there designed for depot techs who had to overhaul the units.
> 
> I have a paper copy of TM-11-300, but all it says is "don't touch the trimmers"!
> 
> Thanks!
> Mike Harmon, WB0LDJ
> mharmon at att dot net
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net Please help support this email 
> list: http://www.qsl.net/donate.html

______________________________________________________________
Milsurplus mailing list
Home: http://mailman.qth.net/mailman/listinfo/milsurplus
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Milsurplus at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html