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Cobra 148 GTL mod
- Thread starter larryb
- Start date
1. --- Clip R-44, D-52.
2. --- Jumper R-175.
3. --- Move orange wire on Clarifier to Pin 1 of IC-4 (hole beside R137)
4. --- Jump R-174 or clarifier WILL NOT center up.
NOTE : L-23 (AM), L-59 (USB), L-22 (LSB), will have to be readjusted for center slot on Clarifier control - some units slide more than others.
2. --- Jumper R-175.
3. --- Move orange wire on Clarifier to Pin 1 of IC-4 (hole beside R137)
4. --- Jump R-174 or clarifier WILL NOT center up.
NOTE : L-23 (AM), L-59 (USB), L-22 (LSB), will have to be readjusted for center slot on Clarifier control - some units slide more than others.
Very old post, I know but this mod looks interesting to me. So Switch Kit, with this mod there is no need to disturb the 8v wire (red) that comes from the clarifier? If I'm reading the post correctly I would only have to move the white wire from next to VR3 into the spot next to R147 and remove D52 and that's it?Auh yes ! those sliders from hell !! thank God there at least on the older Grants and 148 5 pin models (the best) Why ? cause they were very stable on SSB and had very little drift after a slight warming (some a little longer then others) Yes those big ass sliders in the motor car can be a real pain in the ass ! (and dangerous for ones health behind the wheel)
Would always suggest a small slider in the motor car , just makes life that much easier on sideband. But hell !! these great radios can have 10 up and down Khz's without a sweat from center slot ! Touchy ? Yes , but stable all the same. I find these big dog sliders best suited for base station operations rather then the motor car. (and a 10 turn verner dial rocks and rolls on these bigger sliders !)
Of course as mentioned in this thread , there's at least half a dozen ways to open for RX and TX on these chassies , this mod I find to be perfect for motor car operations , about 1.5 khz down and up from center slot , not touchy at all and locks in very nicely.
Getting up to speed....we're looking at clarifier/voicelock mod for less slide...
Actually - went home [while I still have one] and photo'd the old Cobra 148 GTL that I still have.
For those that just want a simple open clarifier do the following.
Orient radio 's front face towards you - speaker/component side up [assuming you've taken the covers off].
Look in centerline of radio - separate right and left - locate voicelock/clarifier [Grant had same board] and it's color code [e.g. Red - Blue - Orange]. Locate VR 3 - by meter dispaly right corner closest to you. Note white wires here. There are two.
Using a soldering iron, remove white wire CLOSEST to VR3. leave other white wire there - see above photo.
Locate PLL by channel selector BCD switch. Note ribbon cable - locate R147 next to it. To ribbon cables' right.
At R147, to it's left, you will see an empty hole - hole closest to PLL right next to R147 - the white wire is to be soldered into this hole [constant 8V] BUT NOT YET! In this photo - white wire is installed there - DON'T PANIC
Locate D52 and D75 - these steer and compensate TX 8V line. See photo - they are by Xtal that sits right next to the clarifier wires. See photos'. Clip or remove one of these diodes - You may want to unsolder it instead - there's more below.
NOTE:
If you want less drift - place diode you unsoldered - on the end of the white wire going to hole located by R147. Solder BANDED end to White wire lead - UNBANDED side of the diode goes in hole.
IF YOU DON"T REALLY WORRY ABOUT DRIFT skip diode and wire directly:
Solder in - and double check your work for blobs, loose wire/frayed ends - or and potential problems on soldered joints and check for shorts.
This white wire is the 8V line from the RX side - you now have put in with the same power supply feed that goes to the PLL. The PLL and the white wire get their voltage from the same source and can use the same filter network - see photos.
(and thank you H.A. for your time)
I seem to remember that removing one diode, and then taking loose one end of a wire ON THE MAIN CIRCUIT board was the simplest way. Not the wire on the clarifier lug, but one with both ends attached to the circuit board.
Bound to have some pics at work. With any luck I'll get some slack time to look for it tomorrow.
73
Bound to have some pics at work. With any luck I'll get some slack time to look for it tomorrow.
73
Ahh I wonder if this is the mod from the previous page of this thread that Switch Kit posted. That requires taking the white wire that ties to R44 and moving the opposite end to the open hole next to R147. If course then I would have to also remove D52. Could that be the one that you're thinking of? It just seemed so simple that I was second thinking it.
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Sure sounds like it. Got slammed at work today, never did look for the working file for this mod. But yea, pulling one end of a wire loose and moving it to a different spot, and unhooking one diode sounds too simple.
But really there are two basic elements to this no matter which radio is being 'fixed' to work on sideband. First, disabling the transmit-only side. Second, activating the front-panel control while transmitting.
Gotta find our working file and refresh my memory to be sure.
73
But really there are two basic elements to this no matter which radio is being 'fixed' to work on sideband. First, disabling the transmit-only side. Second, activating the front-panel control while transmitting.
Gotta find our working file and refresh my memory to be sure.
73
In light of the mod, - well firstly - do you realize how (creepy?) it is to see your work here - before you posted it here?
To Switch - Kit - Bless you - you saved me a lot of typing in the first place.
But the "white wire" spoken of, the one that goes to the TX Trim pot?
That wire heads off to the diodes and meets with RX from the Clarifier. DON'T PANIC
The removal - relocation of that wire - breaks the RX side of the merge from the TX - you remove the diode to break the TX side of the merge from the TX Trim pots' variable which is still connected to the pots'; foil ground lug - people say well that is a diode and it's one way, you're forgetting too - the RX feeds thru into this.
All you have to do after this, is remove one of the two diodes
You didn't tell me you were going to kill it!
The board in the radio shown from the videos that @Robalo links to, is from the newer boards.
You don't have the point to point wiring.
You have to remove parts.
( I noticed the post this references was removed so - I'll let this stand for now as a guide)
In the VID is shown a Blue wire routed from the board into the Loom - since I don't have that radio in front of me - the basic premise is to locate the wire that routes to COMMON at the header - that is the RX side - just set it to the PLL 8V Constant.
You still break the connection to the Diodes to prevent TX and RX swings of drift - just make the oscillator steady by supplying it with 8V constant thru the Clarifier / Delta - Tune / Voice Lock (The reason for so many different uses of the same term is because each radio BRAND that can utilize "same power rail as PLL - slides" use these terms - so even though this is a 148 - same can be done for Uniden and nearly every model out there using clarifier to RX track - tying them together.
To Switch - Kit - Bless you - you saved me a lot of typing in the first place.
But the "white wire" spoken of, the one that goes to the TX Trim pot?
That wire heads off to the diodes and meets with RX from the Clarifier. DON'T PANIC
The removal - relocation of that wire - breaks the RX side of the merge from the TX - you remove the diode to break the TX side of the merge from the TX Trim pots' variable which is still connected to the pots'; foil ground lug - people say well that is a diode and it's one way, you're forgetting too - the RX feeds thru into this.
All you have to do after this, is remove one of the two diodes
- (that forward voltage drop BTW is the EQUIVALENT loading the RX and TX side does to each other in a thermally stable environment)
- - the diodes provide some compensation of drift by providing the voltage drop across their junctions to offset a thermal profile condition between the external pots' own resistive composition and the internal TX trim pot thermal condition.
- Remember too, that is is to offer a little less slide to compensate for fast tracking or bad/cold temperature issues with older radios' with rather large surface-area carbon composition which is not the most thermally stable in mobile conditions.
You didn't tell me you were going to kill it!
You don't have the point to point wiring.
You have to remove parts.
( I noticed the post this references was removed so - I'll let this stand for now as a guide)
In the VID is shown a Blue wire routed from the board into the Loom - since I don't have that radio in front of me - the basic premise is to locate the wire that routes to COMMON at the header - that is the RX side - just set it to the PLL 8V Constant.
You still break the connection to the Diodes to prevent TX and RX swings of drift - just make the oscillator steady by supplying it with 8V constant thru the Clarifier / Delta - Tune / Voice Lock (The reason for so many different uses of the same term is because each radio BRAND that can utilize "same power rail as PLL - slides" use these terms - so even though this is a 148 - same can be done for Uniden and nearly every model out there using clarifier to RX track - tying them together.
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For 8v, since at one time there was an issue with guys ruining the factory regulator, wouldn’t building your own be a wise idea?
I got a headache from reading this thread and it made me rethink my own 148, think I’ll build a dedicated 8v source.
I got a headache from reading this thread and it made me rethink my own 148, think I’ll build a dedicated 8v source.
You can, just remember to keep the voltages separate - the PLL can act weird if it sees one supply feed from the Clarifier as a different source - which shouldn't be a problem, until the voltages between the PLL source and the Clarifier source - if the Clarifier output voltage rises above the PLL supply - POOF to the PLL.
That has happened and can get expensive. Those RCI chips are not Fujitsu. The RCI 8719's on board regulation - if you can call it that - is not as "strict" as the older Fujitsu.
That has happened and can get expensive. Those RCI chips are not Fujitsu. The RCI 8719's on board regulation - if you can call it that - is not as "strict" as the older Fujitsu.
copy rights ! sorry
I done about 4 radios that way and it work from 27.379 to 27.391 and is stable mmm the fact is it work for me.
Once again Andy thanks for all the detailed info. So I performed the mod as Switch Kit suggested and it works perfectly. One question I do have that you may be able to clarify...Switch Kit recommends to add D52 in line with the white wire when connected to the 8V constant supply to improve stability. I am assuming he's referring to thermal drift. If that is done it will drop the voltage by about 1V and now you are only supplying ~7V. Wouldn't that present a problem?
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Here's something you may not have known...
A lot of that deals with "loading" - and how a diode, it's thermal profile "works" in this kind of circuit.
You really don't suffer any losses. And much of the effects that I talked about in earlier efforts was incomplete.
Sorry for this because as I was dumping more information into the site for the PC-122 - I got sidetracked by - well, by the economy in 2006~2008 - need I say more?
One thing though I can now explain a little further - now that you've brought this up...
Diodes, yes - have a voltage drop - as seen by any typical meter - that uses a load across the meter.
It is when you have a Reversed biased junction - does the Diode exhibit another trait - Reverse Transfer Capacitance.
Yes, this means that nearly every PN junction diode out there, in some form or another, is a Varactor.
It exhibits capacitance.
Place a (Standard - Regular PN-type) diode in a high impedance Circuit (Read: Low Current draw - or flow) the Diodes' Voltage Drop tends to disappear - due to the effects of Transfer Capacitance - the lower the draw, the more that capacitive effect has on the ability for Electrons to move their charge across a junction to REMOVE the difference in charge - to equalize the "plates" to make them equal in charge. This occurs inside the Diode itself
So look back on the Clarifier circuit- the D51, and the overall effect of Forward biasing of power across the diode to keep the VOLTAGE component from becoming a LOAD upon the circuit from the Clarifier. Then the Circuit exhibits this trait of low-load transfer of power and then become more efficient at TUNING - you're not sucking out power from the Tank circuit back to the Clarifier.
They already figured out just how much of a "Load" they need to put on the Varactor to keep the Voltage from rising, R149 is applied ACROSS the Varactor (18K or Thereabouts) this helps with keeping the Oscillation stable and just how much LINEARITY is needed in voltage (power across the Reversed Biased Varactor Diode) to keep that voltage even more stable.
They knew of the Drifting issue, if you took away the D51 - you then ADDED more of a load and worsened the effects of the loading because now the Voice Lock/Clarifier/Delta-Tune is now a working R149 as a PARALLEL resistor - but as a VARIABLE resistor with a severe voltage drop-off when you turn the knob to slide - go too low - the varactor loses linearity. (It's why you have R44, R174, and R175) - it's not just for LINEARITY - but to keep the best part of the Varactor - the part that changes the greatest in capacitance - near the center slot to even allow you to move above and below the center of the channel. That's for impedance, loading - voltage stabilization and to keep the Clarifier (Voice Lock - et al...) from "bottoming out" the voltages used to tune the Varactor.
Hope this helps!
A lot of that deals with "loading" - and how a diode, it's thermal profile "works" in this kind of circuit.
You really don't suffer any losses. And much of the effects that I talked about in earlier efforts was incomplete.
Sorry for this because as I was dumping more information into the site for the PC-122 - I got sidetracked by - well, by the economy in 2006~2008 - need I say more?
One thing though I can now explain a little further - now that you've brought this up...
The Following Is not for kids, more for Adults that play with Electrons.
Ok - Rabbit Hole Time!
Since you asked...Ok - Rabbit Hole Time!
Diodes, yes - have a voltage drop - as seen by any typical meter - that uses a load across the meter.
It is when you have a Reversed biased junction - does the Diode exhibit another trait - Reverse Transfer Capacitance.
Yes, this means that nearly every PN junction diode out there, in some form or another, is a Varactor.
It exhibits capacitance.
- When you initially put power thru it, in Forward Biasing - the Junction itself drops a tiny amount of voltage - because the push necessary to transfer power across - only so many electrons are able to move.
- However - Keep the needed "push in" power low - little to no loading - the Electrons don't have to push so hard - a field effect (sort of - by proximity in the boundary layer) can take place to help with the transfer.
- Thermally, this also affects HOW WELL the electrons can move their charges across - reverse transfer capacitance is also a thermal issue.
- in some oscillators of the GHz and Terra-Hz ranges the effect tends to act like a parasitic - sucks out too much power because the capacitance RISES in certain conditions - affecting the ability of the oscillation to even occur (SEE Tunnel Diodes on this)
Place a (Standard - Regular PN-type) diode in a high impedance Circuit (Read: Low Current draw - or flow) the Diodes' Voltage Drop tends to disappear - due to the effects of Transfer Capacitance - the lower the draw, the more that capacitive effect has on the ability for Electrons to move their charge across a junction to REMOVE the difference in charge - to equalize the "plates" to make them equal in charge. This occurs inside the Diode itself
So look back on the Clarifier circuit- the D51, and the overall effect of Forward biasing of power across the diode to keep the VOLTAGE component from becoming a LOAD upon the circuit from the Clarifier. Then the Circuit exhibits this trait of low-load transfer of power and then become more efficient at TUNING - you're not sucking out power from the Tank circuit back to the Clarifier.
They already figured out just how much of a "Load" they need to put on the Varactor to keep the Voltage from rising, R149 is applied ACROSS the Varactor (18K or Thereabouts) this helps with keeping the Oscillation stable and just how much LINEARITY is needed in voltage (power across the Reversed Biased Varactor Diode) to keep that voltage even more stable.
They knew of the Drifting issue, if you took away the D51 - you then ADDED more of a load and worsened the effects of the loading because now the Voice Lock/Clarifier/Delta-Tune is now a working R149 as a PARALLEL resistor - but as a VARIABLE resistor with a severe voltage drop-off when you turn the knob to slide - go too low - the varactor loses linearity. (It's why you have R44, R174, and R175) - it's not just for LINEARITY - but to keep the best part of the Varactor - the part that changes the greatest in capacitance - near the center slot to even allow you to move above and below the center of the channel. That's for impedance, loading - voltage stabilization and to keep the Clarifier (Voice Lock - et al...) from "bottoming out" the voltages used to tune the Varactor.
Hope this helps!
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Very good explanation Andy. Once again you've definitely help me wrap my head around something that I couldn't understand. I also just read what you posted in the other forum on the PC-122 clarifier. Another great little article related to this topic. Thanks again!!
Tom
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