Since the beginning of the week I have experienced what appears to be a temperature related problem, whilst receiving. The problem appears to be initiated by temperature rise during transmit. Once the transmission stops (and the fan which is only active on transmit, if the temp exceeds a certain value, stops), the receive function fails, shortly after stopping to transmit. The latter failure can be prevented by keeping the PTT pressed, but not modulating, i.e. the fan is kept running, but no extra heat is produced. This appears to sufficiently cool the circuits, to prevent the failure. The failure manifests as a click on he audio, followed by the S-meter gradually falling to zero and no further audio. If the PTT is pressed under these circumstances and modulation applied, there is no RF output. Switching off and waiting for a while (30 seconds or 1 minute)(circuitry cools down?), normal functionality resumes and the set stays functional for hours, provided that it is not heated by transmission. (The fan appears to be working normally)
Change the way your fan is driven.
Use an temperature controled driver wich will also let the fan work in recieve mode.
For my FT 847 and FT100 and Alinco DR 605 in the van i use an circuit consisting out of an lm317 regulator, an 2.2 k Ohm variable potentiometer and an NTC of 1 k Ohm.
The NTC is mounted on the heatsink, the lm317 output is now controlled by the resistance of the NTC and adjustable by the variable resistor.
That way the transciever will never overheat anymore.
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