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

[Low_Boy]

I notice no change adjusting L24. But at CT6 which is associated with X6 1128.50 crystal and I get 1128.45. All good. Then my signal stops. I have a signal at D45 but nothing at D43 or D41. I can not by the schematic see how the signal path goes. Also all Crystal's check good.

 

[nomadradio]

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

 

[Low_Boy]

Thanks Nomad. I will be back on it tomorrow And will check some more signals. And FET 4. Spent all day helping a friend and will not be doing anything tonight.
Thanks

 

[Low_Boy]

So I triple checked L24 and it shows adjustment. Before the previous post I checked some of the transistors in The PLL section all were good so this time I checked FET 4. it showed a short on one leg. Unfortunately I think I shorted out my last 3SK45 testing it. I put two leads off the tester on one leg and may have screwed up the new one. I tried it anyway but I am getting the same frequency. The only reason I am thinking I cooked the new one when I pull one of the three leads and move it to the forth leg the tester shows no component detected. And I have no more 4 leg FET's to double check my testing.

 

[nomadradio]

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]

I could not screw up that way.

 

[TM86]

I could not screw up that way.

I bet I could!

 

[Low_Boy]

I installed a new fet 4. Same problem. I get 22.300 MHz on all channels. TP6 is getting 33.855 MHz there is a 10.240 MHz signal on pin 9 of the PLL.Pin 1 Lock detector is getting .02V and manual says that is correct. I went through the PLL section and everything aligned correctly except CT2 and CT3 but I am assuming that because I am getting a carrier on sideband and those are the offsets. One more question someone had isolated pin 18 from the board and It is going back to ground where it belongs. That Pin is P5 a divider. May the PLL be damaged? all PLL voltages are correct.

 

[nomadradio]

all PLL voltages are correct

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

 

[Low_Boy]

Pin 1's voltage stays the same from Ch1 through Ch 40. it is .20V. There is a frequency counter in line with the coax. All channels are 22.300 MHz Approx. There is a dead key and a swing with modulation on AM. But for some reason and I am not sure if this has anything to do with it there is a high dead key on ssb. about 9W where there is a 4W dead key on AM. The VCO is 2V and is correct. it even appears to have factory wax on it. IC5 voltages were correct also.

 

[nomadradio]

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]

At TP7 there is a nice steady voltage. Increase from 2.070V to 2.740V.
So I believe you are saying all the coils have been turned and are out of alignment?
I do have a Rigol DSA705 Spectrum Analizer that may do the same thing?
So I am thinking Someone messed with L1, L2, L3, L4, L5, L6,L7. And I see that L4 is the 27 MHz coil. And I see L2 slugs are cracked so someone was there before me.
Edit::: sorry those are receive coils.

 

[unit_399]

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

 

[nomadradio]

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

 

[Low_Boy]

I will try that. I also remember in the beginning I noticed wax removed from L30 TVI trap.
And there was no signal at all until I started adjusting that. I will see what L37 does and keep you posted. I appreciate the help. This is a very clean radio and sure would like to revive it.