TS-940S Audio Mods

[Steve - WB3HUZ]

Don - KE1IZ has shared some nice audio mods for the Kenwood TS-940S. Check them out in the Technical Section of The AM Window Website.

http://www.amwindow.org/tech/htm/ts940s.htm


Enjoy!

[KM1H]

That looks real nice and well thought out. The PS board is a weak link on the 940 and Ive blown several over the years just on SSB in contests with max power out 100W key down. Rebuilt much heavier duty plus a couple of spares that havent been needed......yet. Transformer takes it and never lost anything on the PA/driver module. A small quiet external fan is used on the heat sink.

At last count I have 6 TS-940's they seem to breed. I only use 2 of them.

Carl
KM1H

[WW4Q]

Hi All,

All I did was disconnect the ALC side of the RF Power Pot(front panel) and tied a 47K Ohm resistor to ALC connection. ALC no longer tracks with power adjustment.
I have pure forward modulation.
I have NO ALC up to 100W
So now if I set RF power to 80W, then I drop carrier down to 20W, I get loud and clear audio with no backward modulation. On sideband, I set the processor out adjustment to 100w with same results. I get a some ALC at 150W, but AL-80B uses about 80W so there's no need to run radio at 150W. ALC is the enemy! TS-940SAT is awesome.
Well, there my 2 cents worth....
WW4Q

[WW4Q]

The Mod works great. I just did it over the weekend.

One issue is that I can no longer drive my AL-80B, since the radio peaks at 225 watts. Is there at fix for this?

Modulation is really strong now.

Thanks,

WW4Q

[Steve - WB3HUZ]

Cannot drive the AL-80 in which mode? AM? Does the mod remove the ability to reduce the power output level? What is the unmodulated carrier level output of your 940 in the AM mode?

[K5UJ]

I only vaguely recall the mod but if the 940 is like most s.s. ssb/cw transceivers there should be a front panel power output pot that adjust the PA collector current.  You should be able to turn down the output RF carrier to 10 watts, which is where I'd start to drive a single 3-500.    You probably only want to drive the AL80B to a peak power of not more than 700 w., maybe even 650 i.e. < = around 150 w. carrier output with 100 to 130% pos. modulation and I'd do that with a lot of cooling on the tube, i.e. more than the typical stock ssb level of cooling.   Also look at how much continuous plate current the h.v. supply can give. and adjust your carrier output to a power that demands around 50 to 75 ma. below the continuous current limit. Then you have some excess current for modulation power and with a lot of air moving through the p.s. and tube you can not sweat transmitting in AM for more than a minute without nervously eyeing the tube.   the 940 can probably put out 10 w. 100% modulated all day.   If you can't get the current spec from Ameritron find out the plate transformer part no. and mfr. and go to them (for example Dahl or Magcap) to get the CCS current limit.


Rob

[WW4Q]

AM Mode.

Carrier is about 25W, Peak is 225W, I tried RF Power Output, but all it seems to control is the carrier.

The modulation is incredible. Its really loud. The radio vibrates when I modulate.

Maybe I could put a outboard compressor/limiter in line to control the peaks?

Thanks,

WW4Q

[WW4Q]

The RF Power adjustment controls TGC/ALC. I disconnected the ALC side of this pot and tied a 100K resistor to it a while back. But since I adjusted the ALC loop to 225W, it shouldn't matter, right?


WW4Q

[Steve - WB3HUZ]

Reduce the audio level. Whether you have a compressor or not, just reduce the audio level until the peaks are not flat-topping your amp as shown on a scope.

AM Mode.

Carrier is about 25W, Peak is 225W, I tried RF Power Output, but all it seems to control is the carrier.

The modulation is incredible. Its really loud. The radio vibrates when I modulate.

Maybe I could put a outboard compressor/limiter in line to control the peaks?

Thanks,

WW4Q

[WW4Q]

I used the built-in Kenwood compressor and limit the peaks. Audio is still loud and clear.

Thanks,

WW4Q

[K5UJ]

If you don't have one, it is an extremely good idea to get a cheap scope and look at the modulation envelope coming out of the al80b.   you can get a good old Tektronix scope for under 100 dollars -- i just got an old 30 mhz tek scope at a hamfest for $73 last sunday.   you can tap rf off an unused position on a coax switch that you use to switch antenna feedlines etc. (just don't transmit into that position!) and put the 940 RF to the amp through an old cheap CB swr bridge.  drill a hole in the swr bridge box and mount a coax jack, bnc is good, and run the center to the meter pot that adjust voltage off the diode that rectifies the RF to drive the meter.   now you have adjustable audio voltage for the scope to trigger on.  you can google and find the way to connect this to the scope it is either the X or Y and the RF from the amp is the other.   I can't remember off hand; maybe there is instruction somewhere on this site.  Mess around with the scope trace speed, brightness, and v./division until you get a sinusoidal-ish pattern when you do ooooooo into the mic when keying the rig and amp.  this will dramatically improve your ability to see what is going on.  if you are still driving the single 3-500 with 200 w. pep you are dramatically over driving it.  If the processing in the 940 is RF clipping it is far better to disable that for AM and use audio frequency processing and audio peak limiting.  RF processors are designed for SSB usually with narrow freq. response and the process can add an element of distortion that may be okay for SSB but might degrade AM quality.

rob

I used the built-in Kenwood compressor and limit the peaks. Audio is still loud and clear.

Thanks,

WW4Q

[WW4Q]

Thanks for the info.

I don't have a scope, will try to get SM220 in near future.

Don't know if processor is a RF Clipper.

I could use the processor on my MC-85 mic. I'm having some trouble with the switches on that mic right now.
Switches cause squeeling if not tweeked just right. I guess I could try a switch cleaner....

WW4Q

[Ralph W3GL]

Yes, the processor on the TS-940S/at is an RF Clipper... Not great in AM mode!
When all else fails, READ THE MANUAL!

The squeal you have with the MC-85 (as well as from an MC-80) is because the goose neck is not grounded to the common side of the condenser mike element.

RF is getting in there. "GOOGLE is your friend"...

This is a known problem ,  you should find a fix in one of the many Kenwood mods lists...

Get a scope, even if it is a humble HO-10.   No AM operator should be without a basic scope.  

The SM220 is nice and matches the 940 but a bit on the pricey side.
Also, I don't remember but I don't think it will look at the transmitted AM signal.
I know my 940 does not monitor the AM signal in xmit like it does the SSB signal.

A good two channel 10mhz scope at a ham fest should not be over 50 bucks..

 

[WW4Q]

I have to disagree with you on the MC-85 mic.

I grounded the gooseneck, ensured that there is 0 ohms between the ground in the mic cable, mic case, gooseneck, etc...

When I press down on the low cut switch, the squeeling stops. then after a couple of days, the squeel starts up again, I press down on the switch, it stops....If I flick the switch back and forth then press down, squeel disapears for a long time.

I either have to replace the switch, or clean the heck out of it with switch cleaner.


WW4Q

[WW4Q]

Update....

The Peaks can be controlled by Carrier adjustment on top of radio.

With the power adjustment to max, I set dead key to 15 watts and it peaks to around 100 - 125 Watts, The audio maintains its loudness with clarity. With my AL-80B, it works out to be 150 watts deadkey to about 1100 watts peak.

I cleaned off my ground connections and MC-85 squeeling stopped.

WW4Q

[KA2PTE]

Can this be done to the TS-930, its similar in design....?

[Pete, WA2CWA]

You could take the TS-930 schematic and match it up with the TS-940 schematic in the audio area of interest. I suspect the wiring probably will be different, the component designations probably will be different, and the component values would be different.
The TS-930 was back in the day when using it on AM, you had to play with the carrier level and audio gain controls. Too much carrier and you generally had distortion. If you're going to use the TS-930 on AM, it should have an AM filter. Pushing AM audio through a SSB filter isn't worth the effort.

Some of the posters in this thread have left the building so you won't get an answer from them.

You might want to read this:
http://amfone.net/Amforum/index.php?topic=36889.0