PLL troubleshooting on a Motorola MC Micro

[gorplop]

Hello everyone,

I've recently decided to look at an MC Micro radio that was collecting dust in the basement. To be able to use it for anything sensible I've replaced the firmware with DG1YFE's version (original firmware is trunking, useless)
After powering it up, the red LED stays on, which, according to the firmware manuals means the PLL can't lock. (on both the custom and factory FW).

I've got some experience in general electronics, but this is the first time I have to troubleshoot a radio. The schematics for the unit are easily available on the net (can upload as well for you). My model no. is MAU34EVA5.

My question is what should I check to diagnose the problem and fix it? What precautions (Dummy loads?) should I use?

Cheers.

[gorplop]

Ive done some measurements around the PLL section and found two things:

1) the PLL is not synchronising (see scope image). Yellow trace period changes when tuning the radio (U101 pin 3). Blue trace is stuck at 80us. (U101 pin 13)


2) There's a discrepancy between the measurements and schematic:

There is 0.2VDC and a waveform as drawn between Q104 collector and Q105 collector. Schematic says there should be 5-6 VDC. (page 72 in service man.)
(note: huge image, right click and choose view image or scroll around... sorry.)

Service manual is here: http://www.repeater-builder.com/motorola/manuals/mc-micro-uhf-conv-svc-man.pdf
The PLL IC U101 is an MC145158, U102 says 77M45, also a motorola but I could not find any datasheet for that. Voltages on the pins of U102 are as labeled on the schem.

[PA0PBZ]

First step is to put the original firmware back in it and see if the PLL locks.

After powering it up, the red LED stays on, which, according to the firmware manuals means the PLL can't lock. (on both the custom and factory FW).

[PA0PBZ]

Also, measure the VFO frequency if you can, It is well possible that it got detuned in time and it could need a little tuning to get it running again.

[gorplop]

Thank you both for your replies, It's fixed now. It was just a matter of tuning the variable capacitor in the VCO circuit (not on the schem photo).

But, as the fault is 'hard', check voltages, once they're verified, check the input to the prescaler (if the test gear is available) from the VCO, then the output of the prescaler to the PLL, see if the VCO is at least running and somewhere in the expected 'range'.

So far I can get about 15MHz of "bandwidth", ie. the range that the PLL can lock at (which is around 1.2V - 7.5V on the control voltage, at the SL test point). Ive tuned the RX VCO at around the PMR446 range.

The input/output signals from the ICs are fine (phew! I couldn't even find a datahseet for the prescaler IC).

With further repairs Ill try to stick to the service manual which means that
Ill have to tune the TX VCO as well (no idea how to do that without a proper RF dummy load - I only have the scope, no counter, no other RF/ham equipment), then the actual filter array.

Overall the good news is the radio works - I can detect a PMR446 walkie (the squelch activates, but I hear no sound...).


Edit: Now that I think about it, I would just need to find a proper dummy load - the model I have (MAU34EVA5) is rated at 25W (which I will probably adjust down as well). The reference oscillator is spot on (the 2ppm model has an OCXO module in a can, measured 14.400MHz with the scope counter.), and I can adjust the TX VCO by keying the radio and tuning the capacitor until the SL voltage stabilizes and the red LED turns off.

The question is, though: How do I make sure I'm not blasting (up to) 25W in an uknown frequency while doing so?