RCI 2970 N4 Suddenly Lost Around 1/2 Power or more

[Wire Weasel]

Hi guys, Could use some pointers. Rig uses 4 pills, likely 2290 equivalents. Examined RF Deck. Heated rig up with 50W keydown. All 4 pills heating up about the same checked with infrared thermometer.
Normal: AM will key 150W swing to 400. SSB/400 (did not run wide open ever - usually key at 80 or 100 to talk)
Now: AM to 110W swing to 175. SSB 110W
.
What might be the problem? I am not a tech but do have some electronics skills. Can solder well and have an excellent VOM. Please throw me lots of details and pointers that might assist me in the diagnosis and repair if you would. A knowledgeable local has suggested the AM Regulator may have partially failed?
Thanks for any help !!

 

[Eldorado828]

If it were me, I'd start by turning off the 4 pill amplifier section to troubleshoot the radio alone first. The plug near the finals by where the mirror board is located is how you can disable the amplifier section. There's a thread around here somewhere that shows that part.

Once you can rule out the radio itself then move on to the amp section

 

[Wire Weasel]

Yes. If say the AM Regulator is bad - that's in the radio section, which is a 2950. However, it comes power-tuned to run the amp rather than as a stand alone 2950. So I also don't know what the factory settings are on it as is - to tell whether or not I'm seeing correct readings. I've had one of these before several years ago and the amp section went bad so I disconnected it and retuned the radio to operate as a 2950. I remember that the AM Low & Hi settings were turned down much lower - so I adjusted both to be 1W Low and 10W High.

 

[nomadradio]

The combiner that each transistor pair feeds into should have a resistor inside or alongside it. If this resistor is burned, one of the two pairs has dropped power. If I had to guess I'd look at the capacitor connected collector-to-collector on each pair. If one of those takes a hike you'll get crippled power.

73

 

[Wire Weasel]

The combiner that each transistor pair feeds into should have a resistor inside or alongside it. If this resistor is burned, one of the two pairs has dropped power. If I had to guess I'd look at the capacitor connected collector-to-collector on each pair. If one of those takes a hike you'll get crippled power.

73

Thanks for those tips Nomad ! Will definitely check. On first look didn't see anything burned or amiss.

 

[Wire Weasel]

The combiner that each transistor pair feeds into should have a resistor inside or alongside it. If this resistor is burned, one of the two pairs has dropped power. If I had to guess I'd look at the capacitor connected collector-to-collector on each pair. If one of those takes a hike you'll get crippled power.

73

Hey Nomad, took another closer look and damn if one of the 5W resistors next to the pill sets isn't blown. Doesn't test in-circuit but middle band is faded out, bulged in the middle a bit and can see a crack. Should be same as opposite resistor which looks perfect - and is brown-green-brown which is ..... 1,500ohm right? I chance to have a 5W 1,500 Block resistor in my stock that I can easily extend the leads on and this should work fine as a replacement you think?

 

[Gman]

Brown green brown is 150 ohms.

 

[Wire Weasel]

Thanks !! Was researching and the info I looked at seemed confusing.
No 5W 150's Metal Oxide on hand. Have a 5 pack ordered

 

[nomadradio]

The resistor is a casualty, not a cause. Just where is it on the circuit board?

73