If your final or driver transistors have suffered catastrophic failure, before installing replacements and after removal of the transistors, measure the base voltage on transmit. There is no bias adjustment for any of the stages in this amplifier chain. These changes look like they were done to improve stability, and the differences are minor. This one change for this specific board type is highly recommended, especially if you're thaving problems with MRF476 longevity.Ĭo incident with the different driver boards, Drake changed the PA stage around the ferrite transformers.
The degenerative feedback also makes the MRF476 easier to drive, so the net result is a wash. Yes, raising the emitter will decrease the gain. Removing the stub can be interesting for its soldered on both sides of the board. Remove the stub from the circuit board and put this resistor network between the emitter and where the the stub went into the circuit board. Make a tight bundle of 3 - 1.8 ohm 1/8 watt resistors in parallel.Ĭut the emitter lead of the MRF476 about where the lead changes width. The cure is to lift the emitter off ground with a resistor. You find its bad - usually leaky and low gain - replace it, and soon the new one dies an inglorious death also.
It may also be possible that the predriver board 'eats' MRF476s. If you were to feel the heatsink on the MRF476, it will be very, very hot. You let up on the mic for a few minutes and all is well. You'll notice this if all of a sudden the transmitter output drops or, on the lower bands, the ALC is gone and more mic gain is required. The 300 ohm resistor and 1N4005 diode is an acceptable method of providing bias, but with the grounded emitter, there is no way to guarantee thermal stability around the transistor. The 270 ohm resistor from base to ground is not enough to prevent this. Its bias level is such that the transistor will go into thermal runaway or may latch up by itself. The problem with the later model board is the bias network on the MRF476. Too much is just as bad as none - its a metal filler only. The purpose of this compound is to ensure a good thermal contact between the transistor and the heat sink by filling in the (natural) pits in the metal faces. Most folks use far too much of this stuff. If it is necessary to change any of the transistors in this area, you must use heat sink compound on the mounting bases. The emitter is the left hand lead, NOT on the right where you would expect a TO5 to be. If you orient the transistor so the base (center lead is furthest away from you) is in the centre and the transistor is held by the leads underneath, These transistors are somewhat unique - the case is the emitter and the collector and emitter pins are interposed. The MRF237 may also be used as replacements for the SRF2331. In my mind this is a little heavy for a TO92 transistor. I've used the MRF237 as a replacement because the transistor is biased for about 20 ma - about. This in itself is not a fault, but the way it was executed presents some problems that will be dealt with in a separate area. The board with the MRF476 predriver most likely was changed because it was much cheaper to make. The final amplifier board seems to have remained much the same, but the components around the PA input and output transformers were different. In this board, the last transistor is an MRF476. Early TR7s used 3 transistors the late model board used 2 transistors.
#How to use the drake tr7 as a vlf receiver full
Restoring Drake TR-7 to Full Output Power (PA repair)Īt least 2 different sets of boards were used in the driver stage next to the power amplifier. WHICH production version of the Drake TR-7 do you have?
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