One of the “must have” accessories for your TS-850 is certainly the DSP-100 Digital Processor, I was lucky to find one second hand years later it was discontinued.
But, what really makes a big difference are the set of optional crystal IF filters and the more recently added Roofing Filter.
Some years ago I heard about the filter manufacturer INRAD International Radio and I purchased some IF crystal filters for my TS-850. The reception perfomance improved dramaticaly. Those were filters easy to install, just plug ‘n play units.
Later, INRAD launched the roofing filter kit for TS-850, among other veteran equiment, that did not have the possibility to have roofing filters from the original manufacturer.
The kit is a 4kHz wide -6dB @20kHz, six pole crystal filter at 73.0500MHz, that comes installed in a small and conveniently shaped PCB together with a preamp that compensates for the filter insertion loss itself.
The IMD dynamic range will be improved up to 15 dB for signal spacings from 2 to 20 kHz. Also, the blocking dynamic range will be improved for close in signals. If you want to use this radio for AM, audio quality will be slightly degraded, but now it is an excellent SSB radio to compete side by side with modern equipment.
You can find the manual with step by step instructions on how to perform this easy modification to your TS-850 right here.