RCI-2970/2950 RF signal levels help needed.

[BayouRadioAmplifier]

any techs out there that are real familiar, or have a radio on their bench now.
I have been trying to get another radio in town here to compare with,
to save time and effort.

I have been working on this, my first 2970N2 main board EPT695015BR
that another tech was working on and gave up. not sure what he all did to it,
but he made things worse. When I got it, and keyed up the mic, it would
draw 18 amps from the power supply! (WTF!?!) got hot all over, with
only 2 watts RF output from the external 200 watt PA unit.

I replaced some IRF520's, and have the RF bias section repaired.
I have a post on that. not sure if RCI corrected this.

Also the receiver is working after bad soldering found (typical SMT stuff)
and tuning up some coils.

So to save time and work, in AM mode, I need to know the RF signal voltage levels
around the circuit on the way to Q62, the first IRF520. I have obviously
low levels, even after tuning up the coils.
What are the typical voltage levels at the collectors of
Q63 and Q64 ?? what level at TP3 ? the voltage at the gate of Q62 ?
RF P-P ? ?

all the DC levels on these transistors are correct, and the proper output
frequency as selected. I can put in SSB or FM mode and get a properly
modulated signal to Q64. And down the signal path from
here, the amplitude modulator is working, with this low RF signal.

and one more thing. sometimes when I touch my scope probe around
Q64 this radio transmit section starts to oscillate and I get over 120
watts of drifty RF out the PA. (at least I know the RF PA is good!)
so I need to remove the TP7/8/9 shorting board to probe around.

as everyone says, "thanks in advance"

I quit my job, at BearCom (Motorola Tech) a few months ago,
to kind of retire. So started repairing CB/HAM transceivers, old stereos,
guitar amplifiers again. always been doing that for many years.
drinkin' beer, fix'n radios, having fun! located in SE Louisiana on the bayou.

.....

 

[sp5it]

Take a look there: http://www.cbtricks.com/radios/rci/rci_2970/index.htm

 

[brandon7861]

Is there an error in that schematic? It looks like the inputs of the TR relay are backwards. It looks like the output of the finals will go into the radio, not the ant jack, when the relay is keyed. Bottom left.

http://www.cbtricks.com/radios/rci/rci_2970/graphics/rci_2970_main_sch.gif

 

[sp5it]

All OK on the schematic.

 

[nomadradio]

That schematic is for a RCI "2950"-series model.

Yours is the late "DX"-style circuit board, mostly surface-mount components. We just call it the "6950", even though I have never seen a radio sold with that number on the outside.

Here is that diagram http://www.cbtricks.com/radios/rci/rci_2950dx/graphics/rci_2950dx_70dx_sch.pdf

Good luck with documenting signal levels. Motorola's diagrams were always a cut above most other manufacturers. Always included everything you needed to know, pretty much.

Working backwards from Q62, you should see at least 6 or 8 Volts P-P on Q63 collector. Q64 collector will be more like a Volt or so P-P.

Touching a probe tip to the circuits upstream of that will throw the circuit out of resonance, making any useful measurement unlikely.

I would watch the signal level on a 'scope at Q63 collector and see that the behavior of the tuning slugs in L34,35,37 and L38 looks correct, with a reasonably-sharp peak. We see way too many of those coils with the tuning slug cracked or seized in place.

73