[Collins] 75S3-B is receiving different bands

[Dr. Gerald N. Johnson]

On Sun, 2008-03-30 at 16:14 +0900, Osamu Hazawa wrote:
> Hello All,
> 
> My 75S3-B seems to receive different bands, other than normal bands.
> 
> 1. No modification but SB_2 has been made (by me).
> 2. It occurs on 7MHz band and above.
> 3. It occurs when the preselector indicates about half position it should.
> 4. e.g. on 21.4MHz position, the preselector is set at about 5, it receives
>     BBC and Radio Australia on the same band. They seem to be of 21.660MHz 
> and 21.725MHz.

Perfectly normal. At the higher bands the second harmonic of the crystal
is used for the first injection. Tuning the preselector to the lower
frequency uses the fundamental of the oscillator. For the 21.4 band that
crystal frequency is 12277.5 Khz, normally doubled to 24.555 Mhz, 3.155
above the selected lower band edge. When you use the fundamental of that
crystal, your band (from 0 to 200 on the dial) covers 9.1225 to 9.3225
MHz or possibly 15.4325 to 15.2325 MHz, depending on how the preselector
is tracking. 

> 5. CAL tone is heard at different position it should (30kHz away from the 
> tone).
> 
> Of course, it receives very good on normal bands with proper preselector 
> position.
> Yes, I'm very happy with that situation but very curious.
> Could this happen logically?

Yes. Covered at the end of section 2 of the operator's manual which
shows that band D tunes to about 15 MHz at a position of 5 on the
preselector scale, so it will be receiving 15 MHz SW BC band. If tuned
to 1, it may also receive the 9 MHz range I computed above.
> 
> 73's
> Osamu 
> 
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer