Ham radio manufacturers are funny (funny-strange, I mean).
They make 100 W radios needing lots of cooling, yet the PA's work through.
On the other hand, making QRP radios, PA transistors go KAPUT, drivers go KAPUT (FT-817 and IC-703 respectively)
SInce I have a FT-817 with a PA that died, I know it is just a matter of time before it happens again, unless the correct mods are made.
This week end ( with temperatures up around 30 C) I got to do the necessary mods for the IC-703
1) Driver modification. This one is not too bad, a matter of cutting a single PCB track and rerouting a piece of wire on the accessible side of the PCB, no need to remove the PCB. When at it, I adjusted the bias to the driver down to the recommended .5V, so the driver should be safe even with high temperatures now. In the original IC-703 the driver is connected directly to the full supply voltage, now it is connected to the 8V supply line.
2) CW Keyer input. For some obscure reason the keyer jack is grounded through a choke, giving rise to RF making it impossible to release the TX key. The solution, connecting the keyer jack ground directly to RF ground can be made provisionally outside the radio, but the more elegant solution requires the main board PCB to be lifted from the housing. Not too tricky, but beware of the flat cables (connectors).
3) 500 kHz TX option. This one required moving a *tiny* SMD diode. I managed to do it with a "normal" small solder tip, but the result is not that pretty.
The mod does open for TX on 500 kHz, but max output is about 40mW, and it looks like something is heating up, because the output reduces a bit after key-up, then stabilizes. I would probably not try to use the max output, but reduce it a bit, them make a power amp.
All in all not too bad for a hot summer's week end.
"The Art of Electronics" #8 -- Why Not a Simple Emitter Follower as The AF Output Circuit? - Click on the image for an easier read *Back when Dean KK4DAS and I were trying to find a suitable AF amplifier circuit for our High School Direct Conversi...
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