Authored by VE3EFJ
4.0 R4B AND R4C MODS AND TECH
I've had a few folks ask me which is the most desirable Drake receiver? Well, it depends. First, its a personal item and therefore open for debate. I've had folks from various backgrounds provide personal reasons for the R4A, B or C.
For reasons that are contained within this document, I prefer the B or the early model C - the one with the MOSFET mixer.
The front cover depicts the operation of the noise blanker.
Pay some attention to the quality of the female RCA jacks on the back of the receiver and transmitter. Quite often these connectors no longer make firm contact with the cables due to wear or abuse. Make sure that the center pin of the interconnect pin is pinched by the socket. In some other cases, you may find that the outside ring of the cable is similarly loose.
Cable quality may also be a factor, especially if you are using the original cables. Remember - in most cases you are using cables that are about 20 years old.
Adjustment is not at all difficult, but initially you can walk right by the T Notch null point if the trimmer is way off. Rather than try and see it at this point, listen for the null by a change in 'character' of the background noise or the calibrator. Start with the trim pot at about center.
If you get no null at all, more than likely someones been mucking with the slug in the notch can and the coil is not resonating at 50 kHz.
This change has been done by a number of people and is said to improve the audio considerably, especially when combined with the C100 change mentioned else where.
In mid May, word was received that International Fox Tango had been bought out. The above detail has been deleted.
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Crystal Filters - Tech
Based upon the behavior of the filter and the age of the unit, I suspect that the filter used in the TR3 is composed of discrete crystals.
Replace R33, a 470 ohm 1/4 watt with 680 ohm 1/4 watt.
In extreme cases, check wiring harness layout and add a 47K 1/8 or 1/4 watt resistor from the base of Q5 to ground.
Lead dress for this mod can be critical and is noted by an increase in audio hash and hum.
This change is applicable ONLY to R4C receivers that feed B+ to the plate of the 3rd mixer through the notch filter. My own experience is that this change seems to do little, but it does no harm either. Its a very popular 'secret mod' that you may want to try, just for the hell of it.
Its *supposed* to act like a tuned IF transformer for the plate of the 3rd mixer to minimize mixing products.
You don't need this change if there is no CW filter installed in the radio. Your receiver is eligible for this change IF you have a T7C (NOT a T7) IF transformer and if C49 is currently 390 pf in your radio. In this case, the coax in question will have a slightly larger diameter than the replacement RG/174 and will have a white colored center conductor.
An LM380 could be mounted on one of the support brackets for the noise blanker or, if you build a circuit board for it and use ground lugs ala Drake, you could use the 2 audio output transistor mounting screw holes.
Some folks have used the LM383 and this chip will provide a bit more audio output. I have no personal experience with this chip, but I've heard that it can be tricky to deploy without having it oscillate. Commercial users of this chip should not have this problem, but home constructors using the LM383 should be aware that the LM383 is a high gain, high current and high output linear audio power amplifier.
You cannot properly fix the power supply unless you make the audio changes because the class A audio output stage draws 1/2 amp (!!) and hauls the power supply down. Once you replace the audio stage, the low voltage supply will climb and you can use the EP487 as a pass transistor or install an electronic regulator.
Once you have made the audio amp AND regulator change, eliminate some of the heat generated from the PTO dropping resistor by power- ing the PTO from the low voltage 12/14 line instead of the 150 volt line. The PTO already has a series 100 ohm 1/2 watt dropping resis- tor so .... no problem to run with the 7812 regulator.
There is another mod circulating that uses the filament supply as a voltage boost for the low voltage line. Do not do this mod and if your receiver has had this change I strongly recommend you remove it and revert the supply to original. This mod cures nothing and actu- ally generates as much, if not more, heat. What it was supposed to have done was raise the input voltage above the 7812 input threshold so the regulator can work with the 1/2 amp load of the stock audio amp. While this does work and does reduce hum and noise considerably, it also creates a lot of additional heat from the regulator. This mod is on the right track, but the 'cure' is as bad as the disease.
These folks do small - read single - quantities and have historically dealt with amateurs since Day 1. They offer a high quality product at a reasonable price (abt $17 Cdn).
Crystal specs are series, 20 pf and HC6/U for band tuning. You can also use the crystal positions for fixed frequency operation, but this would most likely be used for MARS etc and I won't bother with detail. The crystal specs are different between these two applications.
The TR4any uses overtone crystals.
Band range crystals are interchangeable between the entire R4any and T4any and even between the receiver and transmitter.
Another source is Jan Crystals in Fort Meyers, FL.
The National LM380 is near perfect for this application and will, together with the voltage regulator change, clean the audio and reduce current consumption.
LM380 Pin outs
Pin Use Pin Use === === === === 1 bypass 8 aud out 2 +ve input 9 N/C 3 grd 10 grd 4 grd 11 grd 5 grd 12 grd 6 -ve input 13 N/C 7 grd 14 Vcc
I have included a schematic drawing program - SKEM. This program is evaluation software only - you can load it up under DOS and view and print enclosed schematics. This version of SKEM demonstrates a lot of potential. Its most frustrating area of operation is in the 'undo', for it backs out the schematic in reverse order to input. A lot of input gets wiped out to fix an earlier mistake.
All electrolytics have -ve terminal on the ground unless stated otherwise. Observe good wiring practices, as this is a high gain, high current and high output IC. None of the parts are especially critical except perhaps the 2.2 and 220 ohm reststors. If all you have is a 4.7 K resistor instead of a 5.6K, then by all means use it. Instead of a 2.2 ohm, if all you have is a 2.7, then use it. Value changes such as this will not stop the circuit from working. Much the same applies to the voltage regulator changes.
Remove the audio output transistor and socket from the R4C. You could mount it on brackets right where the original transistor was. If you do this, simply connect the volume control center pin to the 4.7 uF input coupling capacitor. Connect B+ to the LM380 from pin 14 to the 12 volt line on the original BFO/audio board. Output from the LM380 (through that 220 uF coupling cap) goes to the headphone jack where the black wire is.
I suggest you build the circuit on Radio Shack perf board and run it stand alone from a seperate 12 volt supply. If more gain is required, you can increase the value of the 2.2 ohm resistor slightly. If the ratio of the 220 and 2.2 ohm resistors exceed the gain of the LM380, it will oscillate. And loudly! Parts layout is not critical so long as you keep in mind that this is a high gain, broad band power amplifier.
Next, on to that 'voltage regulator'. You may as well use the EP487 as it was intended - as a regulator. Remove R116 from the circuit board. Remove CR18 and CR19. Remove C201. Connect a 12.6 volt 1 W zener from one of resistor pin holes that went to the base of Q12 and ground. Cathode going to base, anode to ground. Please don't use the wrong R116 hole. The 'wrong' side has 160 volts on it! Zener diodes make very smelly firecrackers. Last, install a 180 ohm resistor from base to collector of the EP487 'regulator'.
The above changes make a useful difference in the R4C audio and will clean it up nicely. You need to make both of these changes at once. The LM380 needs clean DC to operate from. The regulator change for the pass transistor will not work with the stock audio amp because it draws too much current.
Some have noted that an LM380 produces 'cross over distortion'. This is simply not true. The distortion of an LM380 at 13 volts nominal is around 1%. The TR7 uses this IC for its audio output and I can testify that the audio is *clean*. What an LM380 is prone to do, though, is oscillate at super audio frequencies. You may not hear the oscillation, but you will hear its effect on the normal audio frequencies. That is why there is considerable bypassing in the above design.
Included SKEM files:
All of the above are in SKM.ZIP, plus the original sample files.
The schematic drawing program is in SKEM.ZIP.
Some R4any that allow for 160 operation may have either the 12.6 or the 12.9 installed - I've seen both. If your R4 and T4 both included 160 but use different 160 meter crystals, it can cause some con- fusion until you get used to it.
".... designs with shape factors between 1.4:1 and 1.2:1 have two unpleasant side effects:
Continuing, Rohde says ".... SSB reception should be between 1.9 and 2.4 kHz to limit operator fatigue .... (The) bandwidth on the famous KWM-2 was restricted to 2.1 kHz for this reason."
Ignore it. You can't fix this without hurting the otherwise wonderful AGC.
Many theory books show 'ideal' filter passband as an oblong box on its edge. This is not inaccurate when confined to desirable IF bandpass characteristics. 'Practical' filters have skirts. Some filters with sharp skirts will not cause severe AGC pumping but they may have quite severe ripple, depending on the response type. In general, the 90's approach is for large stopband attenuation and filter shape factors of around 2:1. IF DSP can clean up the skirt problem. This is overkill for Amateur applications, but does illustrate the move away from 1980-think of severe skirt roll off being desirable.
There will always be trade offs.
It is the 1st mixer that sets the sensitivity of the receiver. It is the third IF amp that provides a significant amount of the receiver gain. By the time the signal gets to the 3rd mixer it should be processed enough to easily overcome 3rd mixer noise. And it does. If your receiver works well, leave the 3rd mixer alone.
Drake employed considerable changes over the years to this area throughout the R4C series. Improvement in an early R4C can be rendered by installing a pair of back to back diodes from the junction of C53 and C52 to ground. Use 1N4148.
When Drake employed the 6EJ7s as mixers, the injection was moved from the cathode of the 3rd mixer to the control grid. It is this connection that some Drake enthusiasts assume to be 'noisier'. This connect point isn't 'noisier' (white noise). It *is* very capable of creating hash and is much less tolerant of sloppy lead dress. I have a mod for this further on with much greater detail.
There was also a Sartori mod that injected the LO signal into the third mixer from the bottom end of the secondary of the 3rd mixer grid input transformer. This mod follows good engineering practices and one of my R4C receivers has had this change. My other R4C, an early one, has the injection to the cathode of the 6HS6. I cannot tell much difference.
The file 'NEWR4C.SKM' contains a schematic of the changes to use the EP487 as a real voltage regulator.
The following will not cure white noise in the 3rd mixer, but if you have the above problems, it will kill this hash, buzz and assorted garbage once and for all. All mixers make white noise - consider it incurable. Basically, this mod changes the 3rd mixer to cathode injection and allows G1 to be DC grounded. The verbal text describes the end result of the circuit changes and is not a step by step procedure.
Change CR20 and CR21 to 1N4148. Change C52 to .005 uF. Remove C200 and C199. Replace C199 with a straight piece of wire. Where C52 connected to pin 1 of V6, connect it to pin 3. This essentially reverts the 3rd mixer of late model R4C with the 6EJ7 to the circuit used in the early models.
I cannot give you a before and after comparison, for I never have had a chance to play with a stock late model R4C. However, after this mod my '6EJ7' R4C is dead quiet with the stock audio amp and power supply. With no antenna and normal volume I'd swear the speaker was disconnected. Additionally, the problem with the S meter moving as the PBT control was rotated was all but eliminated. Signals literally jump out of the speaker from nowhere. This change will not make the receiver more sensitive, but it did clean up significantly the garbage in the audio (for me). The reason for this change revolves around the need for a DC path for G1; cathode injecting the LO is the easiest way of provide injection. Additionally, the concept is proven from the earlier R4C designs. Indications are that the 6EJ7 is a pretty 'hot' pentode mixer.
I cannot testify to what an original late model R4C was like. Before you try this change, I'd expect that you have some audio hash that gets almost unbearable in the AM position. When you place a screwdriver near C199, the hash and 'junk' increases in amplitude. If you do not suffer these symptoms than rule 1 of modifications takes precedence ....
"If it ain't broke, don't fix it."
Connect a series network of 4.7K 1/4 W and .0012 uF across R83.
Noticeable improvement can be made by returning the power supply secondary grounds to the canned filter caps. Some folks have put a copper strap under the circuit board ground lugs on the circuit boards, running a strip of copper under the whole length, grounding the lugs. I'm skeptical about the long term. Copper corrodes.
The low voltage supply/regulator/audio is marginal, at best. Measure the voltage on the audio output transistor emitter. If its above about .5 volt (assuming the proper emitter resistor), the transistor could be drawing too much current or be going into thermal runaway, hauling that marginal supply down.
The previously listed mod changing the 3rd mixer 6EJ7 to cathode injection helps considerably, for it grounds G1 to DC. This (original) floating grid can be responsible for an incredible amount of crud.
For a simple solution to the inherently lousy audio response in the stock audio amp, the C100 change makes it much more pleasant.
For the price this receiver sold for, it shouldn't have these problems in the first place. What makes it worthwhile is how good the receiver becomes once these marginal and frankly unacceptable characteristics are attended to.
If a new 12AX7 still does not cure the problem, it could be caused by the clamp diode. An acceptable substitute is a 1N270.
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