Tube biasing help

[Dmans]

Champo,
You will be tuning for an average since A14/15 "acts" on a total of 6 - 10.xxx Mhz crystals and A16/17 "acts" on a total of 4 - 8.xxx Mhz crystals.
Try rolling through the channel selector while measuring and find an average.

The RX crystal (M4 in both schematics and 7.5 ish Mhz) should have a trimmer capacitor in series with it. It is shown as C31 in the Range Gain schematic. It is C31 but not labeled as a trimmer in the Range Gain II schematic but the call out is "4-20pF".

May try to narrow your sweep on the VNA to get a more exact measurement.
(Seems you need about a 500Khz bandpass??)

73
David

 

[Champo]

Champo,
You will be tuning for an average since A14/15 "acts" on a total of 6 - 10.xxx Mhz crystals and A16/17 "acts" on a total of 4 - 8.xxx Mhz crystals.
Try rolling through the channel selector while measuring and find an average.

The RX crystal (M4 in both schematics and 7.5 ish Mhz) should have a trimmer capacitor in series with it. It is shown as C31 in the Range Gain schematic. It is C31 but not labeled as a trimmer in the Range Gain II schematic but the call out is "4-20pF".

May try to narrow your sweep on the VNA to get a more exact measurement.
(Seems you need about a 500Khz bandpass??)

73
David

OK perfect. I'll give it another go with the narrower bandpass tonight. I'll make sure I use more markers to show the band edges. I should have done that last night but it didn't cross my mind.

I'd like to confirm that I aligned it correctly so let me describe the process to make sure my methodology is correct. We'll use A19/20 and A21/22 for examples. So I connected the VNA across A20 (4,1) and adjusted for a peak value of 27.205 (ch. 20). I used 27.205 for the peak since I plan on using the radio with a matching PAL VFO. This would center me across all 40 channels...or so I believe. The PAL instructions say to use ch. 23 (so you can go above 40) but I have no plans of freebanding so I used ch 20. I made sure that all 40 channels were equally balanced across the peak (half on one side, half on the other) while maintaining a uniform dip and rise on either side of the peak. I then moved on to A22 (4,2) and repeated the process adjusting for a peak of 27.205 and so on. Let me know if this was the correct method.

At this point I've stuck to just trying to align transmit. Until I get the transmit working I didn't want to throw another variable in the equation unless receive will impact transmit in which case I'll do that as well.

Thanks again for all of your help and patience with this noob!

Jacob

 

[Dmans]

Jacob,
There is some "shared" use of crystals for both transmit and receive. (See the attachment below) So "tuning and peaking" (I hate that phrase!) the receive section should be done as well.

Using the schematic as a guide, I would start as close to the crystal bank as I could and proceed to the output section (including the load and tune capacitors) and all the way through the receive section (to A1/A2). Then I would do it again and maybe a third time. There may well be some interaction between the transformers. I would expect each "trip" through tuning will require less and less adjustment but it's the only way I know to get everything spot on.

As far as your tuning method, I would try connecting between pins 2&1 and 4&3 (using A19/A20 as your example) and see if that gets you any closer. There is an unlabeled cap on the output of Pin 4 of A19/A20 to the anode of V11. (Range Gain II schematic identifies it as .5-1.5Pf. I believe I would check transformer output on the V11 side of that cap or at the anode (pin 6 of V11). Same is true for Pin 1 of A19/A20. I would check it between R75 and pin 8 of V11. This way you are tuning the transformer output circuit, not just the transformer.

Don't forget that A19/A20 and A21/A22 should be tuned using a scope with a signal injected at C77 [C79 per Range Gain schematic]) for a bandpass of 26.95Mhz - 27.26Mhz. (Though the VNA may be able to measure this! )

Your Nano VNA will be most useful (WITH POWER OFF) tuning the TX and RX oscillator transformers much as you described above.

73
David

 

[Champo]

No luck with the alignment. My original method from two days ago didn't work (output went to 0) and attempting the other method last night netted the same result.

Jacob

 

[Dmans]

Back to the drawing board. I will do a little more studying and thinking on it.

Edit:
Have you tried tuning the transformers using a good frequency counter and oscilloscope?

73
David

 

[Champo]

No oscilloscope but I do have a frequency counter. I really, really wish I had a scope at the moment lol!

Thanks,

Jacob

 

[Champo]

I just wanted to post a quick update about where I am at the moment. Guitar_199 hooked me up with some instructions on how to build an rf sniffer and how to use it with the VNA to check the oscillators. I was also able to max their output or align their output across all 23 channels. As it stands I'm getting the correct mixed frequencies from the first two oscillators all the way to the third oscillator (V11). I am not getting the 7.5mhz crystal mixing with the other two signals to give me a frequency in the 11 meter band though. The 7.5mhz crystal shows that it is at 7.255mhz when installed however when the crystal is removed it tests correctly at 7.5mhz. The original crystal, which was working prior to all output dieing and was on frequency per a frequency counter confirmationas well as an on air radio check, also presents the exact same way. So, what would prevent the 7.5mhz crystal from mixing with the first 2 frequencies and also why would the crystal show that it is low when installed? I also checked frequencies as if it were indeed off by 245khz but still nothing registered on the VNA. I also tried a second tube at V11 with the same results. Thanks!

 

[Dmans]

The 1st receive conversion frequency is 260Khz. That is approximately the same amount your 7.5xxMhz crystal is reading low. Approximately 1/4 of 1 Mhz.

You should be tuning in the 260Khz range on the receive side from tube V6(Det Noise Limiter) back to tube V3(2nd mixer tube) transformers A1-A6. Much easier to do with a signal generator and scope.

73
David