Drake Mods - T4 Mods and Technical

R.L. Drake

Drake Mods - T4 Mods and Technical
Authored by VE3EFJ

6.1.2 T4any Mods and Tech

Can't Tune 10 Meter Driver Plate
This is characteristic of 'generic' 6JB6 tubes. Replace with Sylvania 6JB6. Problem caused by high input capacitance of tube(s).

VOX Gain - T4X and T4B
In cases where more VOX gain is required pick up the VOX input from the mic gain control instead of from the plate of V9b pin 6. When re-routing the audio pickup point, bypass pin 6 with a .05 uF cap.

T4C TX Lock up
Lock up on a T4C is usually caused by a gassy mixer or 12BY7 driver tube. Other causes include leakage either to ground or B+ of the TX keyed line. This is a high impedance line. In extreme cases the cause can be T6. Inside T6 is a rubber washer that can contact the coil pins internally. The fix is to remove T6 and the shield and install a fish paper washer between the bottom of the rubber washer and the bottom of the transformer base.

T4C Side Tone - Elimination
Standby CW sidetone may be eliminated by placing a 22 Meg ohm resistor between the pin in the centre of the circuit board in front of V7 (the one with the wire going to pin 1 of V7) and the lug at the top left of the board (the one with the 150K resistor).

Substitute VOX relay tube
A 6AQ8 will replace the 6EV7 if a 47 ohm 2 watt resistor is added across pins 4 and 5 of V10. The tube change is applicable across the entire 4 line (and TR3). The resistor IS NOT needed in any of the transceivers. Just swap the tube, in this case.

Fan
The PA cage area gets quite hot when in use and some forced air cooling is desirable. The easiest way of accomplishing this is to use a small 1 1/2" 12 volt DC fan. Mount the fan on the outside of the PA cage through the perf holes on the back of the PA cage. Power the fan from a half wave rectifier filtered with about 100 uF derived from the 12 volt filament supply. Route the fan wires through a chassis hole along back top of the chassis. The rectifier can be connected between the filament fuse and an insulated standoff. This mod can be done without drilling any holes or destroying the units originality. Orient the fan to blow in.

These types of fans are CPU coolers and can be bought for less than $10. They do not move a lot of air and move even less through the perf holes. What is necessary is not cooling, but circulation, and this simple expedient helps a lot.

This mod will greatly extend the service life of the 6JB6 tubes at the expense of faster accumulating radio dust bunnies.

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