148 gtl rx/tx led issue

[nomadradio]

Yeah, I'm old school.

First I check to see if the circuit has power.

Then I look for circuit faults, like a PLL that's out of lock.

Just a habit.

73

 

[loosecannon]

Well yeah, logical approaches will work, but i'm pretty sure that only applies for electronics that aren't CB radios.

you know, like antenna theory.

I am just so used to seeing the "my RX/TX light doesn't change" type of posts, and the answer is "you misadjusted the VCO coil when you attempted an alignment" about 90% of the time LOL.

Glad to see you making the shift over here Nomad, even if we both hope the main hangout comes back online.
LC

 

[nomadradio]

Yeah, I'm still holding out hope.

But the "Tx/Rx light won't work" is a lot like "My car won't start". That's a symptom, but opens up a whole list of possible causes.

73

 

[Robb]

while all of the suggestions are good ones, i'm wondering if his VCO coil is just out of adjustment.
this would cause the RX/TX light to stay green when the mic is keyed, and the radio would not have any receive.

kaos, is this what is happening, or is there no illumination on the RX/TX light at all?
LC

I was wondering why the mode switch had supply voltage going in; but nothing coming out . . .

 

[kaos513]

Pin facing front of radio reads 8.97vdc middle pin reads nothing pin facing towards back of the radio reads 13.72 vdc.power supply is rated for 13.8 volt

 

[loosecannon]

Robb, i have to admit that i didn't have 100% faith that the test was done on the right switch terminals (no offence meant at all Kaos), and decided to post another idea in case this one hit a dead end.
LC

 

[nomadradio]

That's kinda non-specific.

If this 148 has the three-position rotary mode selector, it has four sections if memory serves. Some sections have no power feeding into or out of them. Only one section has (the schemo says) 8.4 VDC feeding into it.

Like saying "the spark plug has no fire" on a motor with eight plugs.

73

 

[nomadradio]

Does make it sound as if the 9-Volt regulator is working.

Pin 6 on the 8719 is the out-of-lock detector. Should be 8 Volts if the PLL is locked, lower voltage if it is out of lock.

If you can find a 1N60 diode marked "D31", that may be an easier spot to probe for voltage since D31 is connected to pin 6 on the 8719 PLL.

This is where a 'scope pays for itself. Pin 6 can still show voltage, with pulses going to ground when it's ALMOST locked on frequency.

Never did learn how to troubleshoot a PLL without a 'scope.

73

 

[Robb]

Ahh; but isn't the front mic 148 use a slide switch for mode - and not a rotary?

I haven't had good luck with the switches on these radios. Had a previous run in with the mode switch in particular. Cleaned the CRAP out of it SEVERAL times.

 

[loosecannon]

Kaos, if the VCO hasn't been messed with, and this radio just up and stopped working, try checking TR28.

you also might try confirming that D4 is not open. that one is just a wild guess.
LC

 

[sonoma]

Robb already gave him this info to check and not sure if he has or not. loosecannon he just has not came back with that info as of yet for some reason

 

[kaos513]

i get voltage coming in but not out of the am/lsb/usb switch.

 

[kaos513]

Tp9 i get my 3.2 vdc

 

[kaos513]

d4 reads 07.86 vdc

 

[kaos513]

d31 checks good