Saw tooth wave form on transmit???

[Lkaskel]

Hi Everyone,
It's been a few weeks.....

I have a mid 1970's Realistic mobile that someone asked for a recap and alignment on if the radio is working otherwise. I did a quick transmit, receive and frequency check on it and it displayed a saw tooth wave form on transmit. I did a few quick alignment adjustments on it and the wave form never cleaned up. In your experience, when you see this is it generally a bad driver/final or could it be something as simple as a bad cap? Or, could it be something else? The radio is a 452 with the 858 PLL so it's a decent Uniden product.

Thanks as always!!!

 

[loosecannon]

are you saying the deadkey showed as a sawtooth waveforem?

if so, and if it's not a problem with the scope, then i would first suspect a voltage regulation issue.
LC

 

[Handy Andy]

Sawtooth?

Eeewww.

Usually a mixer product problem.

Meaning, one signal combining with another - should result in a pretty clean match.

IF it's sawtooth - may be a combination of "minor glitches" making a mountain out of a Molehill...

In the effort, the Bennie schematic I got shows a TP203 and TR214 a twin Gate FET used as a combiner/Mixer amp using the main PCB's 9~10Mhz Xtal out and the PLL's own self-generated mess - to make a signal.

But to see sawtooth, can be 1 - a window for the mixer shuttering too soon, making a spike versus a pulse appear so the system attempts to "round" it into simple smooth sine wave, or the Bandpass Filter used to find and smooth out the signal this mixer makes - is off - considerably.

​

To me? TR213 - the "doubler" and TR214 - are doing their job.

Tr215 is simply taking in what that BPF is finished working on - so this tells me one side is "stronger" than the other - the only way to know "which output" the Mixer TR214 is seeing - that is causing the sawtooth - you'll have to look at individually.

Start at TP203 - see what its' P-2-P is...
TP202 is so you can scope the output without loading the BPF.
But TR13...

Has TP201...​

I'd look at output ranges of value (power levels) and see if any of the cap- resistor combos are open especially in the feedback between Base to Emitter - generating a full-on high gain loop instead of controlled feedback.


And it's also not uncommon to see resistors like R256 R235 in their own setting - crack - being a dipped porcelain (think FLUSHING, NY) enameled body - that carbon composition body can easily crack as age weakens and eventually this "pencil lead of graphite" turns into another broken pencil...

Excuse me Mrs. Johnson,
Can I have another sheet of paper to draw my family on?
​

Fortunately, or not - My Family Tree is the stick I use to draw with

 

[wavrider]

Maybe it is a bilingual transmitter?? Sawtooth an TR?