Old Radio Question

Snow,
That model # doesn't sound familiar to me; there's nothing in my service library about it, either. Is there any chance that it's "PC 77" rather than "PC 7x" ?

At any rate, on many radios a green light (instead of red) when hitting the PTT means that the synthesizer is out of lock. Then internal circuitry keeps the transmitter from coming on if the synthesizer can't confirm that either the frequency is within the limits programmed into the radio or the voltage coming from the VCO ("voltage controlled oscillator") is out of the "lock range" for the synthesizer to work with.

If that radio has a PA/CB switch, or a position on the channel selector for PA operation, plug a speaker (doesn't have to be fancy) into the PA jack and see if that function works. If it is equipped with a PA and that doesn't work either, you might well have a couple of mics that are wired for, say, a Midland, even though it fits. If there *is* a PA switch.....that switch hasn't sorta nudged off the CB position a little and slid over to the PA position, has it?

Also, switches can fail from lots of use, or no use, or just the accumulated number of years & cycles back and forth. The power switch on the volume control on lots of inexpensive radios is designed much like a tiny knife cutting a dowel at right angles, and the mating surface of the two intersecting parts is about the size of a pencil dot. It doesn't take much grunge or pitting to raise the resistance at that switch. The more current the radio draws, the more the voltage drops from the supply side (the big red wire from the power jack) to the load side. And, as luck would have it, the radio draws considerably more current on transmit than it does on receive, allowing the radio to receive but not transmit. It may even be severe enough that the receiver sounds horribly distorted and gets progressively worse as the volume is turned up (with signals to hear on the band.)

But I'd wager (not much; I'm retired) that your green RX light is telling you that the synthesizer is out of lock on transmit. Most synthesizer VCOs are so sensitive to small changes in their adjustment that it's easy to miss the right position by 1/8-turn. Some radios use just one VCO adjustment for both TX & RX. Some use separate controls, and they can be either variable caps or slug tuned coils.

The good news is that your receiver is working, which confirms that lots of the radio *is* set correctly, or at least close. But it might be worth a half-hour at a "real" radio shop to have the radio aligned to factory specs. That should be plenty of time to make sure the receiver is hearing as well as it should, your modulation is set correctly, etc.

That's just my two cents worth. Hope it helps some.

73

 

OK I am guessing that it is a pc-76xl from what you described.

OK let's look at this for a second, you said the transmit light comes on, but the meter goes to the bottom, and the meter switch is set for s/rf - right?

If so, I would guess you have blown finals or drivers.

I would take it to a cb tech and have them put it on a dummy load with an external meter to see if you get anything out of it.

 

Snow,
Thank you for the update to the OP. Given that data, I concur with my learned colleague Ridgeline. Since the TX light is red (I'd misunderstood originally) and it's the right mic, something has probably popped in the TX RF chain.

It also has a single adjustment point for the VCO, but as I understand it now the TX light is coming on red, rather than green. If you had a chance to get near another known working CB, and could find a quiet channel (like after the skip has dropped out), you *should* be able to be heard 10 or 20 feet away, even with a popped final, as long as the driver transistor is working, and the radio is actually producing an on-frequency signal.

But even then, you'll need a tech who has a service monitor (or at least a dummy load, calibrated wattmeter, & oscilloscope) and who knows how to use them to diagnose the fault, replace the part(s) and align the radio.

Neat older radio, and worth the time to fix. The parts cost will be negligible.

73

 

How far east?

 

If you get out to PA I hear great things about Bob's Cb.

http://www.bobscb.com/bobs_cb.htm

 

Next time you run to PA, I'm in agreement with Picklechips.

Bobs CB is a first rate CB Shop.

HWY 322 and I-80, Exit 70 across from the TS.

They'll bench check your radio for free. Plus, he's not the type to sell you something you don't need. The man is a rarity in this business.