Kenwood TS-440S

[TheGman]

I just aquired a 440S. Had PLL unlock (dots on display) and no tx rf output. Resoldering cold joints on the 4 VCO's section on the RF board cured the PLL unlock problem. Replacing a fried bias driver transistor for the finals and replacing the finals (2SC2879's) pretty much got the rig up and running. The problem now is that I have to turn the ALC pot up for only 25 watts out on 10/11m. Less ALC gives more power but the rig acts likes the power supply has no regulation. Lights dim, display dims to drops out, and the signal is distorted. 40m and below, I get full 100 watts output and no problems. The 2SC2879 finals were used from a 10m power amp. Spurious ? Bad used finals ? Any ideas on what's happening ? Thanks.

[w3kq]

A few suggestions come to mind you may wish to look into...

1st, since you listed the unit as a 440S, I'll assume you do not have the ATU installed.  If you do, start by bypassing it completely at the RF level and make certain there are not strange things going on there.  Even if you have an ATU installed, odds are slim it's the source of problems, but I always check even the obvious.

Much more likely to be the problem area is the filter board assembly. You want to make certain that you don't have a switching problem on the filter unit board.  The output of the final amp feeds this board.  The bandpass filters are switched in and out by relays controlled from logic generated on the RF board.  The key here to the situation is the fact that all of your RF power, SWR and protection circuity is AFTER these bandpass filters.  If you have a bad relay, or a bandpass filter that is always staying switched in regardless of band, your RF amplifier is looking into a bad load which the protection and metering circuit does not see. 

Some key indicators that point to this area as a likely suspect...

1.  You're replacing finals.  Unless someone really messed around with the RF protection circuitry, this is enough of a red flag to get my attention.

2.  You're getting good power output on other bands.

3.  Your description of only 25 watts out, but dim lights, etc. point to a final amp working into a very poor load, but with no protection or fold-back circuits to back off on the power.  As I mentioned above, this could and would occur if you have a band switching board problem in the bandpass filters or switching circuitry, since all of the metering, fold-back protection, etc for the finals are monitored after the bandpass filter and the switching relays.

Without looking at the schematic, I'm shooting from the hip here, but I think you can hook up the output of the final directly to the SO239 jack on the back. I know I've done this many times on the TS 930 and 940's, but it's been awhile since I had a 440 in with an output problem.  You need to be very careful, and have a pretty good idea of what you're doing, otherwise you might have an "oops" that makes the situation worse..... just a thought.

Like I said, that's just a few thoughts that come to mind... I'm pressed for time so probably won't be able to check back in here for awhile....

Good Luck,

Rick

[TheGman]

Rick, thanks for the reply. I guess I wasn't that clear in explaining. I can get about 100 watts intermittently on 10/11m, but the display flickers badly and the panel light dims real bad, kinda like trying to draw 20 amps of current out of a 5 amp supply. Sometimes the display goes out completely, a relay clicks, and although you have the mic still keyed, I guess the fail-safe kicks in and kills tx, like your back to rx mode. Lowering the mic gain extremely low or turning the ALC pot on the filter board to lower SSB power out to roughly 25-30 watts out, is the only way to stabilize operation. My guess is mr. golden screwdriver was the prior owner, ran the radio in AM mode at full carrier, and pranged the output. Am wondering if more got pranged besides the 2 finals and the bias driver transistor. Maybe a bad capacitor or something. I pulled the board and had to replace the two 15 ohm resistors across the bases to ground on the finals. All else was clean. Was wondering if used 2SC2879 finals from a 10m amp, which is what is in there now, would cause this problem (being already used and stressed already). Stumped for the moment.........

[TheGman]

OK, FWIW, I replaced the capacitors for the 22-30 mhz filter section on the filter board. Didn't cure a thing. So I'm guessing the used finals I put in are just too stressed for further duty. They were cheap ($20 ea) and do fine down on 40m & 80m. Will just have to fork out more for a new pair of 2SC2879's. Sometimes you just can't take the short path home.................