THE ART OF CONVERSION

Since the MARS Chief has stuck his neck out and asked for contributions, here is mine. I have been horrified for some time at the butchery that has been perpetrated on some of the surplus gear in the name of "conversion."

Having converted some two dozen or so of the SCR 274 components myself, I would like to pass along the easy way to those who may not yet have converted theirs. My conversion takes about 20 minutes, with another 20 minutes or so being used up if the frequency range has to be changed. This applies to the BC-457, 458, 459, 696, and their ARC-5 counterparts.

Since these units were designed to use a fairly short, end-fed antenna, there is no reason to remove the antenna tuning coil as some of the published "conversions" have advocated. If portable work is contemplated, particularly this should be left "as is." When set to zero the antenna loading coil is completely out of the cireuit. The relays, however, should be either removed or rendered inoperative as their use will produce a thump or chirp in keying. The simple conversion using a 24 volt filament supply, follows:

1. Remove the resistor (either 1,500 or 200 ohms) from filament to cathode on the 1629.
2. Lift the arm of the antenna relay up over the antenna contact and solder in place; remove the relay if desired.
3. Short the two red wires on the relay under the chassis; the relay may be removed if desired.
4. Cut the white wires to the two relays and either tape the ends or trace the wires back and remove them.
5. Add a 0.006 MFD bypass condenser from cathode to ground on either 1625.
6. Add a 100-ohm filter resistor from cathode of the other 1625 to key. A key jack may be added to the front panel or the black wire that went to the relays may be used.

The transmitter is now converted! If it is a BC-459 or 696, it, is ready to go. A BC-457 can be realined to 80 meters and a BC-458 to 40 meters without touching the coils. Alinement procedure follows:

1. Remove the can which covers the oscillator coil and drill a 3/8" hole in line with the padding condenser shaft; loosen the set-screws in the shaft lock on the padding condenser; replace the can, replacing all the screws.
2. Loosen the set-screws in the shaft lock on the PA.padding condenser and remove the snap button (the small one) in the side of the chassis.
3. Insert a milliameter in the keying lead; apply 200 volts to the oscillator and PA plate and 100 volts to the screens of the PA tubes.
4. With the dial at the high-frequency end of its range, tune the oscillator padder for the desired frequency and then tune the PA padder for minimum plate current.
5. With the dial at the low-freqency end of its range, tune the slug in the PA coil for minimum plate current. If it was not possible to get the desired frequency range with the oscillator trimmer, the oscillator slug may also be moved before the PA slug is tuned.
6. Recheck at the high-frequency end, tuning the PA trim- mer condenser for minimum plate current, This minimum should now hold over the entire tuning range. If not, recheck, remembering that you tune condensers at the high-frequency end and slugs at the low-frequency end of the range.

With this procedure it has been found that the BC-457, 458, 459, and 696 can be used at full input. Rated input for the tubes used is 150 watts. They have been operated here quite satisfactorily with a considerable over-voltage on the PA plates; at present the voltages are 1100 on the PA plates, 225 screen, and 240 on the oscillator, the latter regulated with a VR-150 and a VR-105 in series. Rated voltages are: PA plate 750, PA screen 250, oscillator 250. In either case the rated PA plate current is 200 MA, and the PA grid bias measured from pin 2 on the cable socket to ground (with a 20,000 ohm per volt meter) should be at least -50 volts. The more bias the better the unit will work!