[Milsurplus] Alignment Info BC-221-AK?

Fri Feb 9 19:53:36 EST 2018

Yes-

The trick is to get the data to print as close to the orginal book as 
possible.
Anyone have luck with that?

Later
Hutch

On 2/8/2018 10:35 AM, Bill Cromwell wrote:
> Hi,
>
> I have kept two LMs here (passed some along to other hams) and mine 
> are still true to their 'cal' books. But with modern (and in fact even 
> outdated) computers we can easily generate new 'cal' charts with a few 
> (not hundreds) "checkpoints". The dials on the LMs (and BC-221) are 
> essentially "logging" scales. I have built tuning charts for my RAK 
> and SW-3 and then verified (signal generators, digital frequency 
> counters, etc) a number of points shown on the generated charts. I ran 
> the checks different days and confirmed it is repeatable. So enough 
> about the original cal books, please. They are NOT needed.
>
> 73,
>
> Bill  KU8H
>
> On 02/08/2018 02:05 AM, Robert Downs via Milsurplus wrote:
>> 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
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>>
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