RCI 2950DX 6 - One issue at a time - this is a story with a question or two.....

[Lkaskel]

You know how discovery works? We have all had a radio with what we thought was one issue and then the discovery happens. So I had this radio come in for 2 things. The 1st was that it was over 100Hz off on transmit. The second was a complaint that the radio's TX audio tone was not amazing and wondered if there was something wrong or could be done. Now this radio was modded from a shop that adds a built-in amp to the factory setup so there are some minor differences including the fact that the finals had the part number removed for some reason. The drivers are RT1's. The frequency alignment was tricky because I could not get any adjustment out of VR8. I did find that the clarifier mod was at some point returned to stock but the diode (D63) was installed backwards. No big deal, I reversed the diode and VR8 was back in service. I was able to get the radio on frequency but I will admit that the audio did sound pinched or just off. My 1st thought was that the finals were some cheap E-**y parts so I started with replacing them with two ST IRF-520's I get from Digikey and then do a TX alignment (1st mistake). I should have just tried the TX alignment 1st. The issue (that is still current) is the voltage to TP8 is low on SSB at 9.5VDC with the front panel power knob at max and VR18 at max. Also, on AM it was at 2.7 VDC but I could adjust it to 6VDC per the alignment guide. This is where I should have just gone to bed as it was already late but I was locked in. Did I do something? Was the reversed diode a greater problem than I realized? I decided to remove the shield of the amp and saw that the FET's all had thier part numbers removed as well. AH HA!!! The finals had the part numbers removed from the amp guy as well. I don't know why he did that but no big deal. That was enough for me to call it a night.

I do some Saturday AM routines (chores) and just sat at the bench. At this point the IRF 520's are still in the radio and I can deal with that later. I went through the TX alignment again and sure enough the voltage at TP8 is low on SSB at 9.5VDC. The radio has 13.97VDC in. Q66 has a little above 13 volts on the outer pins and the center pin does change when you adjust the power knob.

2 quesions at this point.
- What would you guess the part number of the finals are? I was thinking maybe RT6's.
- What would cause low voltage through Q66? Could it or Q65 be bad even though they adjust the voltage as designed?

Thanks for reading my story!!! I'll be sure to finish it when I know more.

 

[BayouRadioAmplifier]

Q67 can be bad which drives the modulator Q66.
a known problem in these. I did this repair on a 2995.

I have a 2970N4 here with a blown external amp
to play with. "015" main board is now working.

yes, "pinched audio" in transmit I was told,
bad in SSB and AM.
I tested the stock mic and others on my computer.
recorded in Audacity. sounded pinched.

Maybe I can get some voltage readings around Q67
for you, to save some effort.

but gotta make a "beer run" now, get some Lotto tickets.

 

[BayouRadioAmplifier]

OK, got Lotto tickets. a little bottle of Jack Daniels.
(gonna be cold down here again tonight!)

on this 2970N4 here,
in AM mode, mic all the way down, key up, and turn the
power control down for 3 volts at the shorting board T.P. 7/8/9.
and looking from the front of the radio at transistor Q67
you got B C E left to right.
at B 11.7 V at C 3 V at E 12.3 V

the V to look at is the V at E should be .6 to .7 V greater than V at B

if not, then a problem.

and while you are at it. I had trouble with the bias circuit to
RF driver Q62 on some radios. a missing resistor.
(I only have worked on a few so far)
so test it. put in LSB mode, MIC down off,
pull out shorting board, key up and note V at the banded end
of D93. note the voltage, then turn VR13 back and fourth
and note the voltage change. then turn back to the origional
voltage reading if you can.