so this has been driving me nuts for a few weeks now. picked up a used 857D from a silent key estate sale, guy's family had no idea what anything was worth so i got it pretty cheap but obviously theres a reason for that. thing receives fine, sounds good on SSB, but when i try to transmit on any HF band it barely gets above 10 watts even with the power pot cranked all the way. VHF and UHF seem to put out close to normal, or at least closer to spec.

already checked the obvious stuff -- PA transistors look okay visually, no obvious burns or cold solder joints on the finals that i can see. SWR is fine, im using a dummy load to test so thats not the issue. ALC seems like its maybe clamping it down but ive messed with the menu settings and cant get it to budge much. someone on another forum mentioned it might be the driver stage transistors or the bandpass filter switching but i dont really know where to start probing without a service manual schematic in front of me.

anyone dealt with this exact symptom on a 857? the VHF working normal is what throws me off, makes me think its something HF specific in the signal chain before the finals.

yeah the VHF/HF split is a big clue there, on the 857 those share the PA finals but the driver chain is separate for HF vs VHF/UHF so if VHF is fine the finals are probably okay. id be looking at Q1026 or thereabouts in the HF driver section, Yaesu used some mitsubishi RF transistors in there that can degrade without failing completely -- like they still work but just dont have the gain anymore. you'll need the service manual, the full one not the trimmed down user one. its floating around on the usual sites.

also worth checking is whether all bands are equally bad on HF or if its worse on certain bands. if its worse on 10m vs 40m thats another data point. the LPF/BPF switching relays can also cause this if one isnt fully engaging but usually that causes more of a no-output situation than a low-output one.

had almost the exact same thing happen on my 897D which is basically the same guts. turned out to be a bad solder joint on one of the relay coils for the band switching, it was intermittent which made it really annoying to find. reflowed a bunch of joints in that area and it came back to full power. not saying thats your problem but worth a look before you start replacing transistors.