Old President Grant same cobra 138XLR and Trc449 odd problem not sure where to start.

When a PLL won't lock properly, there is more than one possible fault. And if someone has tried to fix it already there may be more than just one.

Most-common fault is one (or more) required inputs to the PLL chip either missing or too weak. This chip really only has two input signals. The 10.24 MHz reference crystal and the difference frequency coming out of FET4 derived by mixing the VCO signal with the output of L24. The difference between those two frequencies is what feeds into the PLL's divider input at pin 11.

Until you have 33.855 (or so) at TP6, the PLL will never lock. L24 is peaked for max level at TP6.

73

Now there's a product waiting to be developed. A tiny pc board with four wire leads and a surface-mount dual-gate MOSFET on it.

73

Low_Boy said:

I could not screw up that way.

Click to expand...

I bet I could!

Low_Boy said:

all PLL voltages are correct

Click to expand...

You didn't mention if the tuning voltage increases smoothly from channel 1 to channel 40. The lock-detect output is all very nice, but there's no substitute for seeing it behave smoothly when changing channels. And if the tuning voltage isn't correct, the rest of them don't matter. That tuning voltage is what locks the frequency to those crystals.

I completely gave up that part of Sams alignment procedure. Once the two carrier crystals are set properly I just use the receiver frequency to set those trimmers. After all, the radio's operating frequency is the objective. Don't see why measuring 35 point whatsit MHz is better than measuring the radio's channel frequency.

Do you have a counter in line with the coax? For that matter, does it have transmit power?

Just not getting a picture here.

73

Wait a minute. You got 22.3 MHz coming out of the antenna socket on transmit?

The list of things that have to be aligned all wockerjaw to achieve this is not to be underestimated.

I thought you were reading 22.5 on the internal test point where they say to get 35 MHz. Not the same thing.

If pin 1 reads zero, this means the PLL chip thinks that it is locked. The tuning voltage at TP7 is what I was interested in. Seeing how it behaves when changing channels gives me more confidence than just the state of pin 1. If pin 1 is low, the PLL must be locked.

Or the PLL chip has gone psycho. Seeing the tuning voltage advance smoothly channel by channel from 1 to 40 is a more-convincing proof. The transmitter should refuse to show any power if the PLL is not locked.

Seeing 22.5 MHz on the antenna socket in transmit means the entire transmitter chain needs to be aligned.

Do you have a radio you can set to the same channel as this one, and listen for any energy coming out on the correct channel? Quick-and-dirty way to get those slugs peaked back to 27 MHz instead of 22 is done by watching the S-meter on the monitor radio, turning down its RF gain to keep the meter reading below 3 or 4. Makes the apparent peak sharper.

73

LOW_BOY -

I seem to remember this same thing happening on an 858SSB chassis that I was working on. Turned out to be a misadjustment of coil L37. This is a dual coil and it was the end of L37 closest to FET6. Bring the slug all the way up to the top of the coil and SLOWLY adjust it down until you get the highest peak reading. If you were to keep going, you will get a 2nd peak, but it will be on a harmonic. The set I worked on was stuck at 23.2 mhz.

- 399

I think 399 is dead-on this time. The spec-an is the perfect tool to get L37 back where it belongs. L39 and L32 would also deserve some attention once L37 is peaked for 27 MHz again.

The mixer circuit used here has harmonics of the two input signals coming out of it. Third harmonic of the 7.8 MHz (plus/minus) carrier crystal is 23.4 MHz. Seems to me a badly-screwdrivered radio may have done this for me 30 or more years ago.

Slept since then.

73