Linear for mobile use
This battle scarred old road warrior was bought on eBay.
The heat sink was scratched and dusty and it wouldn’t fire up on RF. A look at the ad said it would work on both DC and RF switching but other ads only mentioned RF switching. A hunt on Google didn’t throw up a circuit diagram but I did find one for the 100w model which I guessed would be similar.
Opening it up showed signs of modding by previous owners and I quickly located the problem – a small RF choke had been inserted to provide the DC switching and was blocking the RF sensing. A quick snip with some sidecutters restored the RF switching but at the expense of removing DC switching.
The linear then found a home under the driver’s seat and worked quite well producing 20w out from 0.5w in with the switch at 1w. Strangely enough it also produced 20w out with the switch in the 3w position and still sat stubbornly at 20w with about 4-5w in. Obviously some other tinkering had been done over the years but I didn’t investigate what as the difference between 20w and 30w is pretty trivial in practical terms.
I used this setup for a few months but eventually decided that the THD-7E was not really suitable for mobile APRS as replying to any call while driving just couldn’t be achieved safely. The linear was relegated to the spare bits pile and eventually eBay probably having picked up a couple more scratches and certainly more dust and the TMD-700 base unit took its place.