Articles have been published covering conversion of these fine little units to permit their operation with AC on the filaments and with a separate plate supply
(Converting 274N Transmitter for VFO operation on FM or AM -- CQ Magazine, May 1946, pages 18-22
Conversion of SCR-274N Transmitters for Amateur Band VFO -- McCoy, W8CBI).
A subject on which little has been written is the frequency conversion of the BC-457 (4.0 to 5.3 mc) and the BC-458 (5.3 to 7.0 mc) to operate in the amateur bands -- and still give decent tracking of circuits. It is possible to modify these two transmitters to give comparable performance to the BC-459 and BC-696, and equal or better bandspread coverage of the ham bands.
It was desired, in the conversions made at this station, to retain the original dial calibration values of 10 kc per smallest dial division, so that a circular chart showing revised values at 100 kc intervals could be pasted on the dial, or a fixed value of a few hundred kilocycles could be added to, or subtracted from the dial reading to arrive at the correct frequency.
The first step in any conversion of this type is to remove the aluminum cover from the oscillator coil and condenser assembly. This assembly is just forward of the 1626 and 1629 tubes and held to the chassis by twelve machine screws.
On a line with the hole at the end opposite the tuning slug, and 7/16-inch below this hole, drill a 3/8-inch hole, smoothing off all burrs. This is to give access to the oscillator padding condenser shaft with an insulated screw driver.
Next put plastic solvent on the set screws holding the adjustment arm hub to the shaft and loosen both set screws "B" with a Bristo wrench. Loosen adjusting arm set screw "A" --put arm in center of slot and retighten screw "A."
Remove bottom plate from chassis and repeat the same operation on the PA padding condenser. It may be necessary to dismount this condenser and unsolder the leads, so you can raise it clear of other parts and get enough leverage on your Bristo wrench to loosen the set screws. This step cannot be dispensed with, however. Replace condenser leads and remount, then replace bottom plate, using enough screws (4 or 6) to hold it firmly in place.
BC-457:Add five turns of solid insulated hook- up wire to the oscillator coil at the top, after unsoldering the leads from the top pin. A small "C" clamp is helpful here to avoid letting slack into the oscillator coil while soldering the extra wire on. Close wound, this wire will probably extend a turn above the soldering lug on the coil form. It is well to leave at least the last turn and a half bare, and spaced, so that a portion of the coil may be shorted out to adjust the inductance if necessary.
The figure of "five turns" is not hard-and-fast as from four and a half to five and a half turns may be required. Be sure and use a good piece of spaghetti to carry the end of the winding across the bare turns, for soldering to the lug. Put a drop of plastic cement or two on the spaghetti and wire to keep it in place, being sure that there is no contact between any turn and any other turn.
Now screw the oscillator tuning coil slug to the center of its range of movement, and replace the oscillator assembly shield. The end screws and a center screw on each side are enough. Now add five turns of bare or enamelled No. 18 wire to the P. A. coil. You are now ready for the tedious job of adjusting the tracking. First, the oscillator with the dial, next, the P. A., with the oscillator.
You will need a good stable receiver, and a frequency standard of good quality, which will give you 100 kc check points. Find the one at 4.0 mc, and adjust receiver to zero beat-after both have been turned on for at least a half hour and have "settled down" in frequency.
Turn on the transmitter plate supply (the filaments are assumed to have been on for some time) and tune the transmitter until you pick up its 4.0 mc signal in the receiver. Zero beat it, and note dial reading of transmitter. The P. A. must be loaded with a dummy antenna of some sort and the antenna resonated each time a reading of frequency is made. Loading of the P. A. changes its apparent input capacity and failure to do so may result in errors of calibration of the oscillator up to 50 or 60 kc.
Let us say that you want 3.5 mc at the old 4.0 mc dial setting, and want 4.0 mc to appear on the dial at the old 4.5 mc dial setting. If it falls at a higher value oscillator padding capacity must be re duced. Set dial to 4.5 mc and insert an insulated screw driver into slot in end of oscillator padder shaft, and rotate counterclockwise to get zero beat in receiver. If you are lucky, you will find 3.5 mc close to the old 4.0 mc dial setting. Set receiver to zero beat with standard at 3.5 mc, and then tune transmitter to zero beat with receiver and note dial reading.
If dial reading is now within 50 kc, or five small dial divisions, of the proper reading you are lucky.
Adjust the oscillator coil slug to bring the zero beat note to the other side of the proper reading an equal amount, go back to 4.0 mc and repeat steps above until 3.5 mc is found within two dial divisions of the old 4.0 -mc setting, say at a point one- eighth inch to the left. Put dial on 4.25- mc setting and resonate to zero beat by tuning the slug on the oscillator coil. Then reset dial to 4.0 mc and resonate to zero beat with oscillator padding condenser. Now retune receiver to standard 4.0 mc (actual) and tune transmitter to zero beat. It should now be within one dial division of the proper reading of 4.5 mc and may be adjusted to zero beat with the small single plate trimmer which may be reached through a tube on top of the oscillator shield at the right end, using a small screw driver. This trimmer should be approximately in midposition before starting your adjustment.
If you are unable to resonate both ends of the band in the dial with readings fairly close to their proper readings, either the number of coil turns or their spacing, must be adjusted.
The procedure described should put the oscillator frequency "on the nose" with the dial readings selected. Recheck both ends, then lock set screws and recheck again after replacing all shield screws. Small trimmer should take care of any change required.
Note. The P. A. padder should not be touched during the oscillator calibration unless the plate current is excessive. In this case, insert screw driver through smaller hole on right side of chassis, and adjust P. A. padder for minimum P. A. plate current near midscale.
You are now ready to calibrate the P. A. Remove the load , and adjust tominimum plate current at 4.0 mc (or at a higher dial reading, if desired) , tune to 3.5 mc and observe change in P. A. plate current. If more than 6 ma change occurs, adjust at 3.5 mc to minimum using P. A. tuning slug. Retune to 4.0 mc and adjust to minimum plate current with P. A. padding condenser. Repeat these steps until plate current does not vary more than 3 or 4 ma over useful scale from high to low end.
This procedure may be abridged by using the calibrated crystal of the transmitter, but unless the crystal frequency will fall in the middle third of the revised dial, this method is not advisable.
To use the crystal for calibration, tune to lowest frequency at which "eye" opens wide, note dial reading, and, using both the oscillator padder and the oscillator tuning slug, adjust until resonance occurs at the proper dial setting.
BC-458:The BC-458 may be adjusted to cover the band 5.8 to 7.5 mc using the same calibration procedure. However, changing to a higher frequency coverage, we reduce the coil inductances by shorting out turns, and partial turns.
On the oscillator coil, short the last turn at the top, by soldering a short piece of wire across its beginning and end. Then, counterclock wise a quarter turn, solder another short across the two top turns.
Short the top turn of the P. A. coil at the solder lug. Proceed with calibration as above. This unit will provide all frequencies needed for multiplying into the 11-, 10-, 6-, and 2- meter bands. You will find the effort of conversion will have been well worth while.