The AM Forum

I got the 950SD in a couple of days ago. Went through it and it needed and alignment for sure. I have a bench with a HP8921A and several other pieces of test equipment including frequency counters, RF generators, audio generators, DMMs, Bird meter and dummy load, GPSDO, etc. It has full output(150 watts) with no fold back on all modes, USB, LSB, FM, FSK, CW. AM only had 3 watts maximum dead carrier, no modulation input. According to the specs it should be producing a maximum dead carrier of around 40 watts. After going through the alignment twice the output is now at about 10 watts maximum. When keyed it starts out at a higher output(12 to 13 watts) then folds back to about 10 watts. I have the service, technical and users manuals. I figured I'd start at the ring modulator 455 KHz (D41) and work from there. Everything mentioned for AM is also tied to SSB. What I"m trying to figure out what is it exactly that only affects AM and not SSB. I thought the balanced modulator might be the culprit but both sidebands produce full output and sound clean. I'm not well versed with Kenwood's design techniques and tracing out their circuitry is not for the faint of heart. Any help would be appreciated.
Problem solved. After doing some poking around I pushed against the AM filter and got full 150 watts output. Took the IF board out and resolder the pins and associated components to the AM filter pins then, resolder the joints on the AM filter itself. It's now stable and working. I'm going to get the better transistor for the 48 volt line and a set of spare finals. Although it's not new and the SDX is a better rig, I'll stick with this one for now. Thanks to all who offered the help. That's what's great about these sites.

I'm not familiar with the 950SD, but I've looked at the 940S and it's also a nightmare to trace-out the circuit — it jumps between several boards. That alone, makes me suspect a poor connector. You might try gently wiggling each one with the unit key-down and looking for a change in output (into a dummy load).

By intentionally mis-adjusting the carrier balance resistor/capacitor combo you should be able to get full carrier output, just as a check. If that works, the problem is probably along the path that unbalances the balanced modulator in the AM mode. They probably bias a switching diode "on" to do that, and have a carrier control pot to adjust the imbalancing voltage.

Happy hunting, and welcome to the forum.


Don

 I have a TS950sdx and TS990s which I often use as AM exciters into amplifiers.  The TS950sd,
put in cw mode, power output wide open, bring carrier control up to full, see if you have 150 watt output.  For am, am mode, power output wide open, carrier control at zero, bring carrier control up to the am power output you want.   For my L4b and 4-1000 home brew, about 10w carrier, and 40w or better audio peaks. On the 950, I run the audio gain wide open as some have recommended, on the 990 , leave it as is in low ALC  range. On the 950, did go into menu and turned off transmit filters, and left them off on am and ssb. Sounds much nicer and a little wider. On the ts990s, 4 kc wide transmit .

Bob Ohio W8LXJ