This is where it gets dangerous, for it will drift terribly after reading that mod...
You'll need to make some decisions.
You want to put in a different pot...?
One, When you do this mod, are you able to keep - or wish to keep - center slot? Then all parts must STAY in place - in one form of ohmic reading or another.
Two - You are using a different pot than what was specified - since this is a Malaysian model, I cannot guarantee the ability to use that described mod for your work.
Three - VERIFY the pot you're swapping in. Did you measure the resistance across the 2 FIXED terminals of the Pot, the two that are not adjustable? What was your reading?
If it's 40K or more, then you will have to change R174 from 12K to 10K to offset the divider difference between 20K pot of the ORIGINAL, to the 50K Pot of the replacement - being higher - changes the LINEARITY or "center" of the pot to the EXPECTED Center slot the board thinks you should be at.
- 1/2 being Center would be 10K either side, of the ORIGINAL 20K pot.
- now? IT's more like 20~22K either side, divide down the R174 by half to obtain a better linearity of turn (rotation) for Up and Down. You can try 8.2 K - but 10K is better - See R124 afterwards to help with "Center"
- You will need to "adjust R174" to fix the "divider range" in expected ohmic resistance across the Varactors' to provide more linear rotation. This also helps you with Drift issues later.
- If you use a 10K pot, you'd have to GO UP in value for R174 to 18K to offset this ohmic loss of tuning range.
There is a better way, you have to keep R124 in place - R174 is a "Divider" that makes the rotation more even both up or down from "center". IF you don't care, then REMOVE R174.
HOWEVER you'll have to "retune" the USB LSB and AM to "Center slot" when you remove R174.
R124 is needed to even obtain Center Slot - keep it in there - for both Drift and linearity to either side up or down from Center. R124 can also provide you an easier WAY OUT of the "center slot tuning mess" you'll be in when you apply for open clarifier and make too many changes.
Fourth - The test, you will have to KNOW which way the Clarifier "center" will change.
- IF all parts are in place and you just want to replace the 20K with a 50K - then R174 is a 10K, R 124 is now a 1.2K.
- You are going DOWN in ohmic value to offset a LOADING condition the Varactor will see, since it's literally twice as much as before (If you switch to a 50K) - DROP- LOWER R124 to 1/2 Original value
Locate VR402 going back to the Main PCB.- Remove/Unbolt any echo boards, Frequency counters - to allow access.
EASY - Beginners Way - When you KEEP the Original Potentiometer
- also the most beneficial and least stressful way...
Behind the front panel - locate the Voice Lock Pot and it's color code, trace it back to near -centerline- more to front, you should be able to see an outline or header labeled VR402 - this is the location.
Find R124 towards the back of the header on the main PCB, behind it there will be a wire, trace this wire - follow back towards the Right Front - by S/RF meter - remove it- unsolder or clip - from this point...
Locate D52, unsolder - lift one leg - to disconnect the 8 volt TX
- - Why unsolder? Easier to restore normal operation later if you need to return this back to stock for Resale - SEE Ferengi
Locate MODE switch. A series of wires head to BACK to the MAIN PCB IN FRONT OF the VR402 location by D52 and D75 - it's by the edge of the front of the board. Locate the 8 volt Constant at that Jumper J12A (?) - I've posted a graphic below to help.
Solder wire to this jumper that has your 8V constant - you're done!
Does It Have To Be This Complicated?
It's when you want to add in a different ohmic reading potentiometer
and all of the associated headaches caused by using a less than ideal,
non-linear functioning and more drifting problematic potentiometer
of a different value than what the manufacturer used/installed there,
in the first place....
Yes, because you haven't even verified that you can successfully unlock the Clarifier using it in it's stock form and observing results to even see if it tunes the way you want to go.
See graphic above and note the Color coding - this follows your comment on the B20K pot you scavenged from another Galaxy-based Donor.
It's Pin out of leads are - two outer lugs are your FIXED resistance, the inside one (Middle) is your variable.
Orange and
Violet, if it's doesn't rise and fall in the right direction of turn to suit your tastes, swap Orange and Violet around
AT THE POTENTIOMETER . -
Green wire is your Wiper/Variable in the above Graphic.
