Re: Siltronix VFO., boosting the output drive level (easy fix I just found.?!)
Although I've never had the "joy" of dealing with this particular VFO (the Siltronix model-90) I believe I found an incredibly EASY way to boost its output (some model-years that is) to SIX times its value.!
Last week, one of my customers dropped-off a pair of Browning Mark-3s, along with a pair of Siltronix 90 VFOs (freq-range #3) for repair of the radios, along with installation / modification for use with the VFO-90s.
Since I've heard there to be some problems with those VFO units not having enough RF output, I decided to first check-out the 2 oscillators before "dealing" with the antique radios...
After first setting them both for correct "ball-park" frequencies, for use with a Mark-3 SSB transmitter, I then checked their peak RF output voltage.
Before even opening them., I measured one unit to have about 50 millivolts of output., whereas the other unit had almost 300 millivolts output.!
Needless to say., they were both disassembled and inspected to discern as to why there was such a GREAT (x6.!) difference in output levels between the two...
At first., I believed it might possibly be, just a weak / aged (hFE gain wise) low output / buffer transistor (Q1).
But after a quick inspection & comparison of the two oscillator boards, I noticed that the bias (voltage-divider) resistor for Q2 (which is R6) was the schematically correct 1k resistor in the unit that was generating the 300 millivolts., while that same resistor in the low output unit (only 50 millivolts) was only 270 ohms.!!
And by the way it was installed, it was positively "OEM stock" installed.!! As to why, I have no idea., except that I know, that some transistor type radio oscillators do NOT want too much input., as then either the receiver or transmitter (depending on what the VFO is being used for) can AND will begin to get "a bit squirrely" in their operation (ie: transmitter output power and / or receiver sensitivity, also depending upon the channel)...
See the attached photo from the original poster of this thread for proof.! There should also be a schematic of the VFO attached (please let me know if the 2 images are not there, as I'm not very "internet-forum savvy)
Starting at the left side of the circuit-board (top-side) it's the 3rd resistor to the right of the 200uh coil (L1), and as can be seen, its color code is red-violet-brown (270 ohm) when it should be brown-black-red (1k ohms).!
Obviously., the with a resistor of only 1/3 the original "stated" value will then actually "pull" more of the RF generated to ground, thus limiting the oscillator's output.!
IMHO., if a user wanted to make the oscillator's output variable (to "suit" multiple radios) one would just keep transistor Q1 at its full output (keeping the 1k Base resistor) and just simply putting a variable carbon-type potentiometer (or trim-pot) at the unit's output.!
Because of the small (in value) 0.001 output capacitor (C8), I believe a high-value of trim-pot would be required as to not "load-down" its high-impedance output., possibly a 100k to 500k trim-pot.??
Anyway., I truly hope the above information I was able to come across (but only because I was lucky enough to have two of the same VFOs., but of two differing manufacturing "runs".!) that I was able to easily diagnose and correct the problem
Regards & 73s,
Eric ( KY8E )
Klein Communications
( facebook.com/radioman55 )
https://www.worldwidedx.com/gallery/photos/siltronix-vfo-90-photo-of-wrong-r6.6710/
https://www.worldwidedx.com/gallery...90-schematic-r6-needs-to-be-1k-not-270n.6711/