Authored by VE3EFJ
Tom Taylor, N7TM
9.2 TR-4 MODS AND TECH
The changes or mods for this equipment are few. The tube line up changed a bit - different 100 kHz oscillator tubes and such, but for all practical purposes the radios performed about the same.
 | TR-4
Manual Trivia The front cover of the manual depicts the 2 crystal filters in the radio showing the skirt selectivity and bandwidth. | ||||||
| Increase IF Gain The TR-4Cw had 1 pf capacitors across the IF transformer hot side (T11 and T12) to increase the IF gain. Since bandwidth is determined by the crystal filter, this had no effect on the receiver. | ||||||
| Different TX and RX Preselector Peaking Especially noticeable on 10 meters, its 'normal'. There is not much you can do about it. | ||||||
| Sick Receiver In instances of a weak receiver or where the transceiver shows a very low sensitivity receiver after transmit, check the AC4 negative bias supply. This bias supply is used principally for final bias and receive AGC. In some cases, 'funny' receive AGC problems can be sourced to the 12AX7 AGC amplifier, but check the bias supply first. | ||||||
| 9 MHz BFO Imagine the passband curves of the two sideband filters together as the capital letter 'M'. The BFO is set dead center in that middle valley between them. Proper setting of the BFO is to listen to the receiver with no antenna and switch the sideband selection, adjusting C130 for the same pitch. Sometimes you'll adjust it and 5 minutes later, the adjustment has drifted. In almost every case, this is caused by C130 losing its temperature characteristics. Two things will cause this - the ceramic has a hairline crack or there is crud in the trimmer. The following is not for the heavy handed .... All of these Centralab trimmers are held together from the bottom by a tripod clip that fits into a ring machined on the rotor shaft. Grab the long pin firmly with some needle nose (bottom chassis) and GENTLY pull and push down at the same time. At the same time, push very gently on the tripod clip with a small screwdriver just behind the center of the clip where you see the rotor pin . If you get this just right, the little ceramic disk on the top will fall out as the clip extracts. Don't apply so much force that the trimmer is smashed or, when the clip lets go the pliers run amuck. Now, inspect both inside surfaces for cracks. If its cracked, replace the trimmer. If it looks OK, clean both surfaces with alcohol and a fresh J-Cloth. Don't touch the surfaces! Oil from your fingers will ruin the repair and you'll be punished by having to do this over again. Now put it back together (heh heh). Allow the radio to heat soak for 15 minutes with the top cover on and then adjust C130. I've done this a number of times over the years with these trimmers on various radios (NCX-5, most Heath). Oh yes - NEVER put a pencil mark on the side of these trimmers to indicate calibration. Guess where the graphite goes in about 3 months? | ||||||
| C130 TR-4Cw and TR-4Cw/RIT The adjustment of the above trimmer is somewhat critical for proper CW reception, for the CW filter frequency is specifically designed for the 9 MHz BFO to be precisely on frequency. The sideband balance adjustment of C130 will affect the CW reception of the transceivers - the place where the note peaks to a *very* large degree. Be careful setting this BFO trimmer, for there is a filter match procedure to follow also. Without the filters properly loaded, the BFO adjustment using the 'hiss pitch' will be colored by a poor filter match setup. | ||||||
| TR-4any VOX Delay The TR-4any has a fixed VOX delay. There is no adjustment for this delay; it has been set at the factory. The delay is about a second. The manual outlines a simple procedure for setting this delay to other than factory default. In most cases, the delay is about right. | ||||||
| Antenna Fuse Bulb This is located inside the final cage and is a #12 bulb. A #12 is 6 volts at 150 ma - exactly the same as a #47, but with a different base. This bulb is a bit silly, for it will take well over a watt of RF to open it. By then, the receivers ruined anyway, most likely. If you really want this protection (its good Stupid Insurance), pull the bulb and put a Radio Shack peanut bulb (6 V at 50 ma or so) across the terminals. The cold resistance of this bulb will not affect the receiver adversely. | ||||||
| TR-4
Improved RX Audio On the TR-4, C212, a .0015 uF on G1 of V17, a 6AQ5, should be paralleled with a .01 uF 300 volt cap. This will remove a lot of the brassiness and distortion. Following the TR-4, Drake made some changes around the audio output stage, but they employed negative feedback to recover the frequency response of the sharp roll off of the coupling cap and grid resistor of V17. | ||||||
| External Antenna Switch The switch on the side of the TR-4 allows for an external antenna to be connected. Whenever you move the transceiver, the switch moves to external by mysterious cosmic forces. You connect the antenna and wonder why the receiver is dead. To prevent this, you can lock the switch by placing a 4-40 nut in the exposed slot where the tab slides back and forth. Cover the nut with some tape to prevent it | ||||||
| Mixing Scheme - TR-4any The TR-4 uses the same PTO as the rest of the 4 line, but it has a 9 MHz IF. It covers 80 to 10 meters. Hetrodyne mixer crystals are not used on 80 or 20 meters. For these two bands, either the sum of the IF and PTO is used (20 meters) or the difference (80 meters). Thats why 20 meters has its unique dial markings that are backwards to the rest of the bands. All other bands have premix crystals and follow the formula of Fxtal = f + 9 + 5.5. The injection into the first mixer is 9 MHz ABOVE the lower band edge and is made up from the band crystal frequency MINUS 5. In the case of 80 meters, there is no crystal and the formula is simply f + 5.5. All crystals are HC/6U 3rd overtone. This is accurate for all bands but 80/20. In this case no crystal is used and the 5-ish MHz PTO is used directly. The 6EA8 PTO premix circuitry is diabolically ingenious in how it uses and does without a crystal oscillator depending upon the band switch. Having a TR-4 operate on different bands is more of an operation than simply changing crystals. The front end is tuned by a variable capacitor, not by slug racks as in the case of the R4any. Moving a TR-4 to the WARC bands, say 18 MHz in exchange for 20 cannot be done (no crystal, remember?). Generally, what you see is what you're going to have. | ||||||
| What Happened to 15 Meters? There is no 15 meter adjustments in the radio aside from the band crystal. Make sure you place the preselector where the manual tells you to during alignment of the various bands. If you inspect the band switch, you'll see some small air wound coils about 1/4" in diameter. These coils are used for the three 10 meter crystal oscillators and for 15 meters. Now that you know this, that does not give you an excuse to muck with them if you have trouble in these areas. Those coils have sat there for 20 years. If you have trouble in any of these areas in your radio, it will never, ever be with these coils. | ||||||
| Low Sensitivity 40, 15 or 10 Meters First, check for sensitivity on 20 meters. Is it OK? What you've just done is verify that the front end is just fine and that the problem is in the VFO premixer - the 6EA8. Quite often people will twiddle the transceiver - see "15 Meter Osc Inj" on the coil can and tune for max S meter. This is OK, but they forget that there is a similar slug on the bottom of the coil can too. Of course one slug affects the other. And, again, you needn't bother with the loading network. | ||||||
| Relay Cycling Especially on the transceivers, sometimes when you put the unit in TUNE, it will drop out or cycle as you advance the DRIVE control. Nothing is wrong - its caused by having the RX audio set too high in relation to the anti VOX. Its actually the sidetone signal thats doing it. Turn down the audio gain, pull the mic or adjust the antivox. Another cause of relay cycling can be the filter can as mentioned in the general comments section. | ||||||
| Relay Specifications The relay changed from year to year, from open frame to enclosed, depending upon the model of the transceiver, but the relay coil specifications did not. The relay is 120 volt and 15,000 ohm coil. What if I can only find a 120 VAC relay? Measure the resistance and if its 12K to 18K, use it. In most cases, AC relays are the same as the DC relays except for a shorting turn. In all likelihood you can use one and never notice the difference. | ||||||
| Ventilation All Drake vacuum tube equipment that transmits should be placed in such a way that adequate air flow is provided. This is especially true for the transceivers. If there is adequate airflow, you'll find Drake equipment to be quite gentle on components. Conversely, if you choke a TR-4 off from free air circulation, you'll eventually cook the components. The first to go usually are ceramic disk capacitors. If you have to replace more than one or two of these, it is a sure sign that someone cooked the radio. | ||||||
| Fan The PA cage area gets quite hot when in use and some forced air cooling is desirable. There is quite a lot of heat trapped in that final cage that is trying to escape by convection. The answer is a fan, not so much for cooling, but to help purge the hot air inside the final cage. The only place to mount a fan is on the back of the final cage. A small 12 volt 70 ma 2 1/4" fan just fits nicely. If you route the leads through one of the corner chassis holes, they will come out in the final compartment. You can pick off the 12 VAC from the junction of the feed through and the filament choke. Do not go to the final tube filament pins - they are RF isolated by the chokes. Power the fan from a half wave rectifier filtered with about 100 uF at 20 volt. This mod can be done without drilling any holes or destroying the units originality. Orient the fan to blow in. I use a larger fan on the TR-4 than a T4any simply because the TR-4 needs some good air movement. With the 2 1/4" fan on the TR-4, the unit can be used indefinitely and does not give any signs of doing a mini-Chernobyl. Please note that the TR-4 is not unique regarding heat. Almost all other radios of this era used convection cooled finals. They too need some forced air cooling or circulation. This is true of all Heathkits (inc HW12 series), Collins, Galaxy - the list is endless. This mod will greatly extend the service life of the 6JB6 tubes. For the most part Drake did a good job designing the chassis for ventilation. An inspection of the radio from this aspect will reveal thoughtful placement of power resistors and discrete chassis holes. | ||||||
| TR-4
Noise Blanker This blanker is very similar to the blanker on the R4C and is similarly very effective. Note that there is a different blanker model for TR-4 serial numbers before 31321. Quite a few transceivers were sold without noise blankers. As with other Drake accessories, the 34PNB is difficult to find by itself. The R4C noise blanker is unique from an TR-4 blanker and cannot be modified to operate in the transceiver for the following basic reasons:
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