Authored by VE3EFJ
Tom Taylor, N7TM
9.2 TR4 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.
In some cases, 'funny' receive AGC problems can be sourced to the 12AX7 AGC amplifier, but check the bias supply first.
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?
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.
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.
Following the TR4, 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.
Having a TR4 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 TR4 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.
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.
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.
Another cause of relay cycling can be the filter can as mentioned in the general comments section.
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.
If you have to replace more than one or two of these, it is a sure sign that someone cooked the radio.
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 TR4 than a T4any simply because the TR4 needs some good air movement. With the 2 1/4" fan on the TR4, the unit can be used indefinitely and does not give any signs of doing a mini-Chernobyl.
Please note that the TR4 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.
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 TR4 blanker and cannot be modified to operate in the transceiver for the following basic reasons:
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