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Early Zachary T- no Tx or Rx, VCO problem.

Night Stalker10 said:
Well with C153 the frequency on G1 is 31.2xxx and without the cap its 32.8xxx. I did lose the 3v at TP5 which would be expected.
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I was hoping that removing it shifted it up further. No amount of capacitance will bring it above what it is without a cap there, so trying different combinations will not work.

I see no choice besides reducing the turns in L20 or using a different VCO. No missing signal can account for the VCO operating that low so I suspect the replacement VCO has more capacitance than the original VCO or the replacement inductor has more inductance.

Edit: If there was a way to put a capacitor in series with L20, that would bring it up, but it would require trace cutting (and I don't want to recommend that until all options have been exhausted).
 
nomadradio said:
Only way I ever learned to do this is first check for 10.24 MHz at TP1. If it's present see if you have 1.09 (I think) MHz on pin 11 of the uPD858 PLL chip. The symptom suggests that one or the other is missing.

73
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The only thing that influences the VCO frequency is the tuning voltage and L20, so the absence of other signals shouldn't change this unless the error is within the adjustment of the varactor. I have been working on the assumption 6MHz is too far for it to be a PLL/varactor voltage issue, but I could be wrong.

Maybe the thing to do is give the varactor (TP5) 5v and then turn L20 to see how high it will go. Just to see if it is capable of hitting 37MHz with the components presently installed.

If it were me, I would do this by temporarily jumping pin 5 to pin 8 and then monitor the VCO frequency while adjusting L20. If it goes above 38MHz, the parts are fine. If it can't reach 38MHz that way, the replacement parts are not compatible as is.
 
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brandon7861 said:
The only thing that influences the VCO frequency is the tuning voltage and L20, so the absence of other signals shouldn't change this unless the error is within the adjustment of the varactor. I have been working on the assumption 6MHz is too far for it to be a PLL/varactor voltage issue, but I could be wrong.

Maybe the thing to do is give the varactor (TP5) 5v and then turn L20 to see how high it will go. Just to see if it is capable of hitting 37MHz with the components presently installed.

If it were me, I would do this by temporarily jumping pin 5 to pin 8 and then monitor the VCO frequency while adjusting L20. If it goes above 38MHz, the parts are fine. If it can't reach 38MHz that way, the replacement parts are not compatible as is.
Click to expand...

With my dc meter on TP5, I can turn the voltage down using L20 to .07V and the frequency reads 31.5xxxxmhz on pin 10 of ic5. Turning the core on L20 to the top of the can, raises the voltage to 4.77v which is the Vcc voltage. At 4.77v the frequency moves up to 32.2xxxxmhz.

Now when I jumpered from R117 the 4.7v side to TP5 like you recommended, the frequency adjust from 32.2xxxxx up to 37.01xxxmhz. then my S-9 signal came blaring through the speaker, and the S/RF meter shows S-9, and TP5 shows .722v.
Plus I was able to adjust TP6 to 37.8804mhz. Not quite 37.880, but close enough.
Now I'm afraid to try and adjust L20, TP5 for 3v... Ha. Also pin1 of upd858 shows to be locked in at .016v. Also its transmitting too, just over 4 watts on the bird.
 
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Since 5v doesn't raise the frequency enough, I believe the coil is the wrong inductance. Where did the replacement L20 come from?

I believe the PLL and the NAND buffers are good for two reasons. The total current draw is 30mA as determined earlier (nothing shorted), and it is changing the varactor voltage when you change the frequency using L20. Normally, once locked, a change in L20 will not change the frequency as long as it is within the range it can lock, just the voltage, so I believe the phase detector is trying to lock to the 4th harmonic (40kHz instead of 10kHz) because that's the closest thing it has. I believe once we get the VCO to operate at 37MHz, everything will clear up.

Is there any way you can fix the old L20? It could also be the different VCO since the tank capacitors are built into it. Are you sure the old one is bad?
 
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I've spent a lot of time on this radio over the last couple of months, and I can't thank you Brandon and Nomad enough. I've learned a lot about the VCO 10 pin sip modual. I have no idea what part was effected by jumping the 5 volts and adjusting L20 and seeing the frequency increase, but it worked. Thanks again.
I was able to adjust TP5 to 3v with no problem. I believe we can call this fixed. Hopefully it will help someone else who might have a similar problem and find this Zachary T info on this forum. Warren.
 
that is definitely NOT fixed. Putting 5v there was just to see if the varactor had enough pull to get the VCO to 37MHz. It will not function properly like that. Are you saying that after removing the jumper, it started working at 37MHz? The voltage doesn't matter until the frequency is right.
 
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brandon7861 said:
that is definitely NOT fixed. Putting 5v there was just to see if the varactor had enough pull to get the VCO to 37MHz. It will not function properly like that. Are you saying that after removing the jumper, it started working at 37MHz? The voltage doesn't matter until the frequency is right.
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The 1khz signal came through the speaker shortly after applying the 5 volts as I was adjusting L20 and the frequency was I think around 36mhz or somewhere in there.
 
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I can't explain it, but that's awesome. Maybe with 5v there and turning the slug, it jumped past the 10k harmonics. Try going from ch1 to ch40 and make sure it works at the band edges.
 
Great!!! Not sure what we should coin this new trick. Maybe jump starting a VCO lol? Next time I encounter a VCO stubbornly running low, this will be the first thing I will try.
 
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brandon7861 said:
Great!!! Not sure what we should coin this new trick. Maybe jump starting a VCO lol? Next time I encounter a VCO stubbornly running low, this will be the first thing I will try.
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It sure surprised me when the tone came through my speaker. I'm not sure what to tell you, but I have 37.880mhz at TP6 and the collector of TR17 is around 2.6v and the pll pin 1 is in lock mode. I guess I will wait awhile before I put the covers on just to be sure.
 
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