A question on checking PLLs/Oscillators for activity

[Bobcat]

Let's say that I did a recap of a 40 channel mobile (about 25ish caps total) and I get nothing at all. I mean... the power indicator and the lamp in the meter come on... but....

1) no sound comes out of the radio (with a temporary antenna plugged in!),
2) when you key, it doesn't seem to kick into transmit, and
3) I forgot to check if the PA works but will do so.....

I have physically been back over every one of those caps and they are "in the right polarity" AND there are no shorts around the pads.

But let's say that I am using this as an "opportunity to learn"....

One of the ideas that I have gotten is, if the radio is dead..... the first thing you want to do is check the reference and VFO oscillators and see if it looks like the PLL is working.

I DO have a 100 MHz Tektronics scope,
I don't have a frequency counter.....

With that in mind,
I probed and BELIEVE that I see the 1024 MHz reference oscillator....
When I go to look at the VFO
I see something that looks like a much higher frequency signal, but
It looks like it is a MUCH lower level than the reference oscillator....

If anyone knows.... is there such a thing as a "normal level" for that VFO signal? Or.... does it really just depend on the radio?

[MDYoungblood]

After you went through the caps did you check the diodes, make sure that power is going in the right direction and not shorted, I have but not recently have a couple of caps that the polarity was marked wrong, so much for quality control. Depending on where you probed 10.240.0 is the mixing crystal reference.

3's

Greg

[Bobcat]

Thank you for the info. What I'll probably do is walk through all of the power supply nodes I can find making sure they are all alive and correct. While I don't have the SAMS Photofact on it, I do have the correct Cobra schematic (it has a particularly unique PLL whose number I can't remember at the moment. At one time there were (it seems) a dozen Cobra 21 models with different PLLs and features!!!). But I can trace around the schematic, find the nodes in the power section and check them out.