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RCI 2980 - no low power control

Hawkeye351

Hawkeye351

Well-Known Member
Jun 27, 2021
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Hi all,
Got another 2980 on the desk. Everything works great on this radio with only 1 issue.

Rf power knob all the way up works great, it's when you roll it counter clockwise that's the issue. It will key 7 watts all the way up, but it will only drop to 5 watts all the way down.

Adjusting the low power VR (801) on the switchboard will only drop to 5 watts (goes up fine). The Hi power vr I have set to 5.45v (7 watts).

I removed TR801 off the switchboard and tested it in the tester. Shows fine with a good hfe. Removed VR801 and tested, it shows 2.93k ohms.

R238 had a 47k ohm (yellow, purple, orange) in its spot. I removed it and installed a 5.6k ohm.

There was also a 1n4148 diode shunted across the center pin of the 2sb754 passthrough regulator to another spot. I removed that diode.

Tr32 was removed. Replaced with another 945.

No other mods that I can see. Everything else looks stock.

What could be causing lo power to not go lower than 5 watts?

Thank you in advance.
 
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Really sounds like VR801 is bad. Turning the front-panel control to minimum and then bridging across two of VR801's lugs with a screwdriver tip would confirm this.

A quicker test than trying to juggle two meter leads onto it.

73
 
Thank you for responding.

I already tried the shorting trick for vr801. No change.

I did volt out the passthrough regulator.
Receive AM:
Emitter = 13.78
Collector = 12.75
Base = 13.28

Transmit AM:
Emitter = 13.34
Collector = 4.56
Base = 13.01

Does that seem right? Seems off to me.

I also noticed that when you turn the RF power all the way down (counter clockwise), it stops at 5w and there is no audio swing neither.

Vr801 on the schematic calls for a 3k.
The one in the radio at vr801 measures 2.93k.

Also, while listening on a nearby radio, I noticed that AM sounds tight at any RF Power level. But SSB sounds great.
 
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This is where a 'scope pays for itself. Just seeing how the DC voltage feeding from the control into the base of TR49 behaves as the control is turned up and down would be informative. If it won't turn down, a DC voltage is feeding into the base of TR49. Just don't know where it's coming from.

And if the voltage to TR49 base really is turning down properly, the problem is downstream, in TR50 and/or 51, maybe. Not a symptom we see. Then again, the voltage difference between base and emitter on TR51 should be close to 6/10 of a Volt. Your reading on receive is close, but only about half that on transmit. Not sure what that reading means, but it's not right.

Or something else is broken. A 'scope is the only way I learned to track that down.

73
 
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UPDATE 7/14/22:
1. Replaced the passthrough regulator (2SB754-Y) to a 2SB817-K. Because the voltages didn't seem right.

2. Replaced the AF Regulator (2SA473) to a 2SA1012-Y. This one showed some oddball readings also.

3. Double checked my work.

4. Let the radio warm up for about 15 minutes.

5. Everything is back to working normal and sounds great.

Thanks again to yours and Andy's helpful info on this site elsewhere. I scoured through some of y'alls info when helping others and got the info that helped me out.

Thank you nomadradio, you guys/gals are the best.
 
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Rock on! Congratulations for sticking with it. Any time TR51 gets replaced, we change TR50 at the same time. Even if it checks okay, a failed TR51 could put excessive stresses on it. Only had to have one or two radios come back in a week or two because TR50 was weakened and failed after TR51 got replaced. That was enough of a pattern for me.

Cheap insurance to just change both when the big one has to go.

73
 
Now to pull the pll's out of these 2 cobra 2000 and 148. That dadgum glue ate legs off the PLL's.

1 more down, 17 more to go.... I need an assistant, lol... I delivered 4 last weekend but brought back 7 more, lol...cant catch up...
 
Hawkeye351 said:
Now to pull the pll's out of these 2 cobra 2000 and 148. That dadgum glue ate legs off the PLL's.

1 more down, 17 more to go.... I need an assistant, lol... I delivered 4 last weekend but brought back 7 more, lol...cant catch up...
Click to expand...
I just had that on an old McKinley this week. Out of all the places they smeared that stuff, PLL pin 1 was the only thing that had corroded.
 
Question for the experts:

I'm getting into the OP Amp (njm4558d) scheme of things in these radios. You know, the typical resistor, cap changes. I looked up a better replacement for the NJM4558D and came up with 3 suggested replacements.

NE5532
NJM2114D
TLO72

From what I've read on the 3 above, the ne5532 is a much better replacement for the njm4558d. The claim is it produces a better sound in the LF and midrange, but is somewhat shy in the HF range (hf in sound, not frequency).

Question is,
Seeing as how the 5532 is better than the typical 4558, could they be used in these radios? Has anyone tried them in these radios?
 
TM86 said:
I just had that on an old McKinley this week. Out of all the places they smeared that stuff, PLL pin 1 was the only thing that had corroded.
Click to expand...

Check around the crystal and vco area (including the vco block) also.
 
Hawkeye351 said:
Replace all the 10v caps, the others you may can get away with.

Is it working good for you now?
Click to expand...
Recap is done, waiting on a voltage regulator, which is due in Monday. I think this was someone's parts queen before I got it,so there's probably more stuff that will need replacing. No biggie, I didn't pay a lot so I don't expect a lot.

The important thing is your 2980 is working again, and I've gained a few clues of what to look for should I see this problem someday.
 
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Nomadradio and handy Andy taught me a lot in a short time. Those two still teach me by their posts in helping others. The one thing I've learned from them on this stuff is, check input voltages, output voltages, gate voltages, collector voltages and current. The input voltages and output voltages on all your transistors, ICs and amps will help you in more ways than you can think.

Patience goes a long way in this line of work. Be patient, take your time.
 
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On the older type radios you're working on, if they have the 1419 regulator in it, then I'd toss that thang as far as I could and install an 1135 or 6487g.
 

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