142 GTL Pll

[kaos513]

Im having some odd readings at the pll below are my readings.

Pin
1-1.925v
2-5.05v
3-8.22v
4-49.6 mv
5-51.5mv
6-2.289v
7-4.45v
8-4.28v
9-8.27v
10-000.2mv
11-8.23v
12-001.7mv
13-001.7mv
14-8.22v
15-17mv
16-8.20v
17-3.97v
18-000.3mv

 

[nomadradio]

Never have figured out how to troubleshoot a PLL with a voltmeter alone.

Always had a 'scope on hand since PLL radios got real. It's what I consider the Swiss-Army knife for a PLL problem.

Just what symptom prompted you to take the meter readings?

73

 

[kaos513]

Pin 7 & 8 is off frequency thats why i took the readings.

 

[nomadradio]

Pins 7 and 8 are for the 10.24 MHz oscillator crystal. Pin 7 has a 18pf capacitor to ground on it. Pin 8 has a 10pf cap.

The test lead you touch to either of these pins probably places an additional 20 to 50 pf or more into that crystal-oscillator circuit.

This is nearly guaranteed to throw the crystal off frequency.

Until you remove the probe.

If that crystal is off frequency, you will find that the spacing between channels is what it affects.

If you look at the counter and change from channel 1 to channel 2, the difference should be exactly 10 kHz. In real life, it will be off by just a little.

Now check how close the difference in frequency between channel 39 and channel 40 is to 10 kHz.

Should be the exact, same tiny error you saw switching between channels 1 and 2.

This is what that crystal controls in this model, the spacing between channels. And nothing else.

73

 

[kaos513]

I have another same chasis radio has no issues on pins 7 &8

 

[kaos513]

Never have figured out how to troubleshoot a PLL with a voltmeter alone.

Always had a 'scope on hand since PLL radios got real. It's what I consider the Swiss-Army knife for a PLL problem.

Just what symptom prompted you to take the meter readings?

73

I have sams photofact of the 142gtl pll readings dont match my readings.

 

[sonoma]

kaos has the radio been recapped. I have 2 of them and both were not able to be aligned and had bad readings every where. about 5 years ago I did a recap to both of them and every thing straightened out.
just a thought

 

[kaos513]

radio has been recapped

 

[Handy Andy]

Tried swapping in another 10.240 crystal?

And so others know, this comes up when the PLL got hit with RF/DC spikes. They can affect the PLL's "loading" of the Xtal - starts to short to substrate. Sucks out the signal from the rest of the PLL. So the Xtal "off frequency" reference may be this condition - may need to be swapped to see if PLL was bad if swapping the Xtal doesn't change anything.

Nomad, your reference to the scope - you would see it as noise on the Channel Pins (BCD stuff).

:+> Andy <+:

 

[kaos513]

I will look into the crystal

 

[nomadradio]

So, uh....

How much frequency error is there in the 10 kHz spacing between channels?

73

 

[kaos513]

So, uh....

How much frequency error is there in the 10 kHz spacing between channels?

73

Wierd thing is that there all right spot on frequency through antenna

 

[nomadradio]

This tells you that the 10.24 crystal was right on frequency before you touched the tip of the counter's probe to it.

And after, as well.

73

 

[kaos513]

Replaced crystal i now have my 10.240 frequency back.

 

[Handy Andy]

Tah Da!

Nice work!

Do the PLL voltages appear normal?

:+> Andy <+: