Siltronix VFO 90-6 Info Needed

[SuperLid]

I need the LC values for the elusive model 6 VFO. The factory documents do not have this info. L2 turns and C13, C14 and C15 (or whatever cap combination that was used).

Thanks,

SL

 

[brandon7861]

I believe it is called a Seiler oscillator. Wikipedia has a brief example that mentions how the frequency is determined.

If you know the inductor value, the cap value should be fairly simple to approximate. Whatever the inductor's reactance is at your desired frequency, calculate the pF needed to equal the same reactance and then subtract the pF from the cap the tank connects to (27pF). Should get you close enough to tweak values the rest of the way.

edit: and I never understood why they drew the schematic upside down. This is a little easier to look at.

 

[SuperLid]

I believe it is called a Seiler oscillator. Wikipedia has a brief example that mentions how the frequency is determined.

If you know the inductor value, the cap value should be fairly simple to approximate. Whatever the inductor's reactance is at your desired frequency, calculate the pF needed to equal the same reactance and then subtract the pF from the cap the tank connects to (27pF). Should get you close enough to tweak values the rest of the way.

edit: and I never understood why they drew the schematic upside down. This is a little easier to look at.
View attachment 75313

Nope....L value is unknown.

SL

 

[brandon7861]

I assume you are trying to convert a 5 to a 6 based on the other thread. Can't you just add a cap in the C15 spot and increase it until the frequency drops to what you want (7.8MHz)? If I try to calculate what inductor might be there based on the cap values shown for the 5, I would guess about 100pF or so is a good place to start. I think you mentioned 127pF in that thread.

 

[nomadradio]

Version six is sort of an orphan. They were dreaming to try and make it stable at 23 MHz. All the other models run at lower frequencies and drift less. The schematic for the 6 is different, so the widely-published schemo show capacitor values for the various versions, but skips from version 5 to version 7.

Oops.

The main tuning cap is too big to cover only the range of frequencies on the dial at 23 MHz, so a disc cap is placed in series with it to reduce the max capacitance. Pretty sure there's at least one other difference. Only thing we do with that model is convert it to a "3" for Browning Mark 3 use. The "6" just isn't stable enough, even after the usual measures are taken.

I'll have a look and see if we have any deeper detail at work next time I'm there.

73

 

[brandon7861]

Version six is sort of an orphan. They were dreaming to try and make it stable at 23 MHz. All the other models run at lower frequencies and drift less. The schematic for the 6 is different, so the widely-published schemo show capacitor values for the various versions, but skips from version 5 to version 7.

Oops.

The main tuning cap is too big to cover only the range of frequencies on the dial at 23 MHz, so a disc cap is placed in series with it to reduce the max capacitance. Pretty sure there's at least one other difference. Only thing we do with that model is convert it to a "3" for Browning Mark 3 use. The "6" just isn't stable enough, even after the usual measures are taken.

I'll have a look and see if we have any deeper detail at work next time I'm there.

73

Based on SL's other thread, I believe it only has to be stable at 7.8MHz due to the tripler inside the radio. If it does have to be running at 23MHz, it might be possible to add a tripler and still use 7.8MHz. Just not sure how this would scale on the dial.