Authored by VE3EFJ
3.2 GENERAL MECHANICS
This book will provide you with an excellent background in receiver AGC, performance measurement, power supplies (R4C reference), noise figure, product detectors - an almost endless, PRACTICAL application of electronics to amateur radio. My copy is so ragged and worn, I need to replace it, just for that reason.
The 'aether' knows not if you are using transistors, glassFETS or even a coherer. Physics is physics. Far better to learn for $15 why .1 uV on 80 meters is a useless achievement rather than spend hours 'improving' your R4B for nothing.
Considerable detail is presented about receivers and multiple conversion designs. This can be applied to the R4B and R4C in that there need not be any difference in sensitivity and noise figure of a double conversion compared to triple or even quad. It all depnds on design goals, cost, engineering preference and other factors.
A highly recommended publication, even if you don't build. Do not let the title intimidate you.
Sometimes you may find a particular receiver will not peak properly when the preselector is set to the indicated value. Generally, don't worry about it. There is no law that says you cannot move the pre- selector a bit to accommodate the compression trimmers. Ideally, you want the compression trimmers set about 1 to 1 1/2 turns from 'snug'. If the trimmers are backed off too far to accomodate the (arbitrary) preselector setting, they won't hold long term alignment. In this case, rotate the preselector a bit to raise the slugs and try again.
Another potential source is:
Mouser has them (502-S-260, $7.08 ea.), so does Digi-Key You can usually find them at swap meets ....
(I have referred a few folks to Danny for the S/230 and there have been no reports of any problems whatsoever <wm>)
During reception, Drake wants you to treat an AM signal as 'Single sideband *Reduced* Carrier'. The sideband filter chops a sideband off and reduces the carrier. You detect it by zero beating the carrier thats left.
The transceivers and transmitters were designed to be operated with a linear. A true AM signal through a linear amplifier is not only very inefficient, its really rough on the linear, especially when operated improperly. Instead of simply unbalancing the balanced modulator to get sorta AM, Drake made a terrific compromise and went 'controlled carrier AM'. This mode has a small carrier at idle and the amplitude of *both* the carrier and sidebands goes up as you modulate. Its easy on the PA tubes and it works well. It is superior to unbalancing the SSB balanced modulator to generate AM and reason- able in terms of trade off to 'real' plate modulated AM.
This is similar in principle to the AM mode in the Heath DX60, Knight T60 and Knight T150.
These canned capacitors are dated and largely unavailable as re- placements. Due to their age, even if replacements were available, I would advise some caution. Whether in use or not, they've been sitting on the shelves for too long. Modern capacitors are much better. My recommendation is to replace the whole can with discrete electrolytics. You can either leave the old cap in there for appearances or pull it and put in a hole plug.
In the receivers, weak power supply electolytics are characterized by hum, low B+ and 'funny' audio on SSB. The funny audio is caused by harmonic mixing of the 60/120 cycle hum and the detected audio. In this case, sometimes there is not enough hum out the speaker to be too objectionable, but the speaker audio sounds 'funny' for some reason or another. You can verify this with the calibrator on SSB. As you sweep across the calibrator slowly, you'll hear some spots where the audio goes muddy. Once you detect it, and have your ears trained, its very obvious.
Additional care should be used when soldering to Drake hook up wire. The insulation melts very, very easily and will crawl up the wire and peel off while doing so. When replacing items like those canned filter caps, sometimes it causes less damage and is more expedient to simply cut the wires. One can easily and quickly loose 3/4" of insulation just in desoldering.
Some knobs have no set screw and instead are held in place by a piece of bent flat spring steel. Sometimes it is these knobs that always fall off, because the spring steel gets flattened just before the shaft rotates. In this case, try that particular knob in a location with less torque such as a volume control. Just swap the knobs.
The "B" knobs are of two types. One is a set-screw type (as in number 2 and 3 above,) and the other is a slip-on type with the little metal spring like Collins uses. The set-screw type will work to replace either of the originals.
On all the Drake C line and before, as in all audio power stages that have an output transformer, never crank up the audio gain with- out a speaker attached. Never connect an A/C volt meter across the primary. Transients generated in the output transformer, especially without a load, will create very high voltage spikes through the collapsing magnetic field. This is how output tubes arc, output transformers short and volt meter rectifiers get punctured.
It would be wise to verify the meter calibration against the idling current. Using the T4C for example, it has its PA bias set for 70 ma and has a meter resistor of 3.3 ohms. Set to 70 ma, you should measure E=I*R, or .07*3.3 or .231 volts across this resistor. Measure across the resistor and NOT at the meter terminals.
Similar problems with setting the proper idling current will be observed if the PA current meter needle is not resting at zero.
A TR3 suffered the above problems and was cured by the above approach. Most of this equipment is 20 years old or older. In the case of the TR3, it was 33 years. The obvious suspect is corrosion.
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