RCI 2950DX - Internally adj. power?

[buickid]

Got a 2950DX driving a Texas Star 350HDV. With the variable power all the way down (box off), I'm still getting about 30W swing on sideband. AM gets me 2.5W swinging 8-9W, no problem there, although 1.5-2W would probably be a little better. I want to turn down the low-power on the 2950DX a little, so I'm not pounding the snot out of the poor box. The service manual shows VR19 controls AM low-power, and VR14 controls SSB ALC, which looks like SSB high-power, but I'm unsure which pot would affect SSB low-power. Would I need to realign anything after tweaking the power down a hair?

Also, slightly unrelated question, my radio is kind of quiet on FM modulation (receiving end needs to turn up the volume to hear compared to AM). Is there an adjustment that just affects FM? The service manual shows VR17, but is listed under AM and FM modulation. My AM modulation is reported as fine right now.

Aloha,
Trevor

 

[9C1Driver]

Based on what you have stated I would say your radio was mis-tuned at some point. That said you can back the AM carrier down to 1.5-2 watts, and then turn the SSB ALC down so maybe you get 20 watts PEP. If you had to you could run it as is and you would be ok, 30 watts peak on SSB will not hurt a TS350. You might also try backing the mic gain down when in SSB mode, might drop the peak wattage to 20-25.

Since you live in Hawaii for future radio purchases I would buy from a shop that does good tech work. This way your radio gets tuned & aligned right ONE TIME and you never have to mess with it for years. This way you don't spend a bunch of money on shipping twice.

 

[buickid]

Hey there 9C1Driver, thanks for the advice. I purchased the radio from DTB, he did whatever he does to it before shipping it out. I ordered the IF filter mod done too, but the RCI warranty sticker covering a top cover screw is still intact, so not too sure about that one... Maybe I'll give Dave a call and see what he advises too.

Does the AM carrier adjustment have any bearing on SSB output? I know the SSB ALC will affect SSB power, but does it affect the whole range on the variable?

The reason I want to turn the power down a bit is because the TS350 is taxing my power supply when it goes full tilt boogie on SSB. My Fluke inductive clamp registered 40A on SSB, which I'm sure my RS-35A Astron doesn't appreciate very much :redface: On AM, amp draw is fine, but I have to engage the AM button (equivalent to Dial-A-Watt) or it'll dead key a little over 150W, not so hot when it swings to 300W.

Thanks again,
Trevor

 

[buickid]

Dave suggested using the variable knob on the 350 to back down the power, but I've already tried that (and fried it, as expected). Any other suggestions? The radio does seem to get very hot when the variable is wide open, so perhaps it has been tuned a little too hot.

 

[9C1Driver]

Hmmmm, sounds like you bought it right then, DTB is top notch and a forum member here. Is your watt meter working as it should? Most 2950's I have owned I could use the variable power to control SSB power from say 5-30 watts. I would get with DTB after you confirm your meter is ok.

 

[buickid]

So it looks like Dave forgot to do the IF Filter modification I ordered. He send a refund, but I'd like to have the mod done since I use the radio primarily on SSB. Plan on sending him the 2950 as well as a 949 to have aligned. I suppose this offers the ideal opportunity for me to update the IF Filter mod thread I started. I'll make a video of before and after on SSB, and attempt to replicate the conditions as best I can.

 

[Shockwave]

Just a note on the FM issue. The capature coil in the FM detector may be out of alignment. The bad news is most of the "export" radios sold in the USA have a defect that prevents good FM reception.

Proper FM reception requires a receiver with a 10 KHz. bandwidth since narrow band FM can deviate plus or minus 5 KHz. from the carrier frequency. The ceramic IF filter used in just about all of these radios has a 6 KHz. bandwidth.

That's because they got cheap and run the FM signal through the AM filter which is far too narrow to allow loud distortion free audio to pass through it.

 

[buickid]

Just a note on the FM issue. The capature coil in the FM detector may be out of alignment. The bad news is most of the "export" radios sold in the USA have a defect that prevents good FM reception.

Proper FM reception requires a receiver with a 10 KHz. bandwidth since narrow band FM can deviate plus or minus 5 KHz. from the carrier frequency. The ceramic IF filter used in just about all of these radios has a 6 KHz. bandwidth.

That's because they got cheap and run the FM signal through the AM filter which is far too narrow to allow loud distortion free audio to pass through it.

Thanks Shockwave. Actually my radio picks up FM well, my buddy on a 2970 comes in on FM just as loud as AM. The concern is my FM transmit, which actually now that I think of it, actually tends to overmodulate unless I back the mic gain down, at which point it becomes quiet. Is there some kind of independent modulation limiter adjustment for the FM circuit?

 

[Shockwave]

That is part of the symptom. They tend to sound overmodulated on FM even though the deviation is set below 5 KHz. They don't sound clean in the receiver until you get the transmitter below +/- 3 KHz. deviation. The thing is they sound fine when you listen to them on a quality HF rig in the FM mode but not on another receiver using the AM filter. There is an independent control for the FM audio and it's called FM deviation or FM DEV on the board. The potentiometer for this is towards the middle front section of the board as I recall but you should look it up on cb tricks .com.

 

[buickid]

Gotcha. So its not necessarily my radio's transmit, but the the receiving radio's design. That makes sense, because if a receiver isn't passing the whole signal, its essentially clipping the audio off, hence the distortion. I'll have to check out the board on my radio when I have time. The FM Deviation adjustment affects TX deviation correct? Although its probably just easier to turn down the mic gain when I go FM...

Thanks again,
Trevor

 

[Shockwave]

If you turn the FM audio down at the mic gain, you just reduced the mic sensitivity which makes you sound softer. If you speak up or move closer to the mic, the over deviation would still cause distortion. By adjusting the internal FM deviation control, you can turn the mic gain up with much louder average speech and still not go over the maximum deviation level the internal control is set at. Setting the deviation to 3 KHz. is probably the best choice if you're talking to mostly people on export radios. The slightly lower level is not as noticeable as long as you're not turning the mic gain down alot to get there.

 

[buickid]

Gotcha, I'll take a look at that when I get a chance!
I suppose I would need more equipment than a Frequency Counter to accurately set that hm? (Its what I have on hand).
Aloha,
Trevor

 

[hotrod]

Thanks Shockwave. actually tends to overmodulate unless I back the mic gain down, at which point it becomes quiet. Is there some kind of independent modulation limiter adjustment for the FM circuit?

fm de
vation is set up too high.
set mic gain to max setting
go find fm devation adjustment inside radio turn it down till other say your audio is clean
unless you have proper equimebnt to set it right

 

[w9cll]

I use the radio primarily on SSB.

I would dump this radio and purchase a HF ham rig if your plan is to run ssb mostly. Proper vari-power, better receiver performance and 100 watts out of the box.

 

[buickid]

fm de
vation is set up too high.
set mic gain to max setting
go find fm devation adjustment inside radio turn it down till other say your audio is clean
unless you have proper equimebnt to set it right

Thanks hotrod, that sounds like what I pictured would be the way to do it without equipment.

I would dump this radio and purchase a HF ham rig if your plan is to run ssb mostly. Proper vari-power, better receiver performance and 100 watts out of the box.

Its something I'd really love to do eventually, but right now its what the budget allows for. Certainly a bit better than my DX979 on initial sideband stability though! :laugh: