TS-820 Low output

[mschmitz57]

I picked-up a nice old TS-820. This rig looks like new inside and out. Receives great but has low output power. In CW mode to a dummy load it produced between 15-28 watts output depending on the band. I tried a new driver tube with the same results. Final RF stats appear to be good, HV is a little over 900 V. IP is good at 60 ma. Tubes appear to be the original S2001's. Cathode resistors test good at 5.2 ohms. Everything about this radio appears new / unused. I suspect this radio was stored back into it's box and never used after the low output power issue was discovered. The plate cap choke looks like new, nice and white, no signs of heating. Same with cathode resistors. No signs of overheating. The most obvious symptom with this rig is no ALC deflection on the meter during tune-up, none. There were 2 resistors added to the back of RF board near one of the MOSFETS. The modification consisted of 47 Ohm resistors from G of Q6 to G of Q7 & a second 47 across Q3 which actually went from a ground foil to a ground foil. This does not appear to be any known modification that I can find on any of the Kenwood mods sites. I'm hoping it was a mistake by one of the previous owners. My best guess is that one / all of the MOSFET's are bad. Replacements might be hard to find. I've been told that ALC problems are hard to find due to it being a closed loop system. So any thoughts would be appreciated. I would also like to find a good spare 820 RF board if anyone has one for sale.

Thanks,
Mark
KB9M

[WA9WVX]

Hello Mark,

I suspect the RF Finals (6146B) haven't been properly neutralized thus causing the unstable and Low RF Output covering 160 thru 10 meters Neutralization procedures can be found in the owners manual. You may also have a bad set of 6146B RF Finals as vacuum tubes don't last forever. Just looking at components will not tell if something is wrong inside especially vacuum tubes.

If you do not have the proper manuals for Kenwood TS-820, then go to the following mods DK website:

http://www.mods.dk/manual.php?brand=kenwood

73,

Dan
WA9WVX

[mschmitz57]

Update:
The problem turned out to be something in the CW circuit. I disabled the CW filter and it tunes to full output (100 watts) just fine.
I discovered this when I finally got around to connecting a mic to the rig and found that SSB output was normal.
I moved the jumper on the bottom of the IF board to SSB only. CW filter was replaced with another but no help.
I suspect something on the IF board but I don't work CW so I could care less.