RCI 2950 Tx on multiple channels

[QuickdrawMcgraw]

This radio transmits perfect on sideband, but on AM it has garbled audio and transmits on 6 channels. I have checked the alignment of the PLL System and found everything perfect. The am detect circuits check good also D34 and D35 also Q24 all test perfect. I saw something on this before but can't find it now. Any help greatly appreciated. THIS IS MY LEAST FAVORITE RADIO TO WORK ON!

 

[999]

Quick... Is this the old one or the DX? (for clarity)

 

[brandon7861]

Turn mic gain down and try again. I'm thinking the limiter was clipped and you have boxcars coming out.

 

[nomadradio]

boxcars coming out.

Sometimes a 'scope is your best friend.

73

 

[QuickdrawMcgraw]

The audio is low and muffled. Transmits the same with mic gain all the way down. I have a full bench. Spectrum analyzer, scope, signal generator, frequency counter and audio analyzer. This unit is literally transmitting on 6 channels. I'll post images of the spectrum when I get a chance. Also, this is the orange face old unit...

 

[nomadradio]

Is it filling up the spectrum between channels? Or does the spec-an show a carrier on each 'extra' channel center frequency with sidebands on each channel above and below?

That would be different defect from overmodulated "bleedover".

RCI built a batch of 2990/Turbo radios in the 90s that had an assembly defect in the frequency synthesizer. Would hear on a few channels above and below, as well as transmitting on them. If the receiver also behaves this way it could be as simple as removing a bogus SMT resistor soldered across adjacent pins on one of the single-inline chips in the PLL.

Can't remember which chip. This was way before we had digital photography here, so no visual record.

Worth a look. There should not be any SMT resistors on the solder side of the pc board under PLL chips.

73

 

[QuickdrawMcgraw]

This is audio related. Thanks Brandon! I was mistaken. I thought I had tested this rig with the mic gain down. The problem goes away when the modulation is all the way down. I installed a new limiter and replaced the clipped resistor Q32 and R291 respectively. I also removed the resistor on the solder side across IC20. I haven't had time to test each of the audio amps. I did test Q51 it's good. The audio sounds good in PA mode. I will test the rest of the audio and AMC Circuits and check back in. Thanks for the input. I'm really sorry for the inaccurate post earlier. I'm coming off 10 days in bed sick.

 

[QuickdrawMcgraw]

Here is the Spectrum and Scope readings. I'm just getting back into this one. Thanks for your input. This is the result of even a slight amount of mic gain.

 

[nomadradio]

If I'm reading it right, the two spurious peaks are about 43 kHz above and below the channel center.

The PLL design in that radio is kinda wacky. Makes me wonder if someone twiddled one or more of the tuned circuits in one of the band pass coils. The alignment procedure has an element of by gum by gosh.

73

 

[NZ8N]

Those spurs do not have any modulation components like the fundamental. Only carriers.

 

[QuickdrawMcgraw]

if i showed a video you would see that there is modulation components in the spurs.

 

[QuickdrawMcgraw]

With no modulation there is a hum in the carrier. I can hear this on a radio without an antenna. Also I can see about 500 mv of that same hum on the scope with no modulation. I think if I can find the source of that hum, I will solve the problem. Also note that side band is perfect. No spurs and beautiful audio on the scope.

 

[QuickdrawMcgraw]

One more note. The PLL Alignment is spot on. Yes the PLL Alignment is different than most I've done. It is complex because of the 100 Hz resolution.

 

[nomadradio]

Don't know why I didn't think of this before. A X10 'scope probe onto one or the other exposed lead of L65 will let the spec-an eavesdrop on the output of the PLL synthesizer. If you see multiple peaks above and below the 16 MHz PLL output that would point you upstream into the PLL for the culprit.

L65 is a ferrite bead just to the rear of tuneable coil L19.

73