Uniden 76xl rx and tx not tracking together

[Harley3315]

I have an older 76xl here when my counter shows I'm on center slot for channel 13 my rx in no where near it was looking around for test points to hook counter up to adjust the rx and tx frequency to get it back on track can't seem to find any alignment info for this radio any help would be awesome.

 

[nomadradio]

There is only one quartz crystal in that radio. If the transmit carrier is on frequency, the receiver should be as well.

Has someone mucked with the delta tune control, maybe? Can you clear up the receiver with that knob?

73

 

[Handy Andy]

Similar to a Cobra 29 - even TP spots...

Just with Uniden - Cobra "separation" the Divorce Decree said...

Uniden; To have unto it's last day, Layout of it shall appear as Right Angles on Components.​

Cobra; therefore unto it's last moments, Use current (per the Year of Decree) Lazy Draw Layout on components.​

Therefore let it beknownst to the future generations of Operators to know of it's origins be identified by board as such...

So really, the Cobra 29 board can be used as a template to help you find your way around the Uniden.

The subject of the "Line-up discrepancy" occurring on channel 13 raises an eyebrow ...

• - only on channel 13?
• or you just using that as the example and it's occurring on all channels?

Then look for near center of board...

​

See RX and TX "Tuning Splitter?" - that is where two voltages merge into L24 / L23.

TP3? That is where you look for 16~17MHz IF - you can then verify the offset...

Offset? Another word for "SHIFT" the radio uses 3 IF signals for tuning - a 16MHz+ Signal from PLL - at TP3.
Mixes with the arriving 27MHz CB band - pulls out the channel you're listening to using SUBTRACTION
Now you're left with a 10.7MHz signal...1st IF - amplified NB'd and then remixes with a 10.240MHz (x1) signal to SUBRACT from generated/used 10.7MHz...for 2nd IF of 455kHz

So it uses 16MHz IF from the PLL to make 10.7, the 10.240 Xtal is, used reapplies it again; to SUBRACT from the 10.7MHz signal to attain 455kHz.

So the "shift" is the 455kHz "offset" you look for on TP3 between TX and RX modes.

• TP3 - you can see ALL IF frequencies used here...
• TP4 - you can see the 27MHz channel signal here...
• R85 and R88 - for Varactor (L19) and Peaking L22 if needed.
• JP36 mod jumper shown but is labeled using JV

Now, if you have trouble tracking TX and RX -

Check Banded side of those diodes while keyed, and un keyed - see if a voltage is present in each mode.

IF TX voltage drops down to nil - Check to make sure TX diode is there! (Not cut or otherwise blown open)

OR if a mod was done to "open and track" TX to RX tying together - you need to look for a CONSTANT voltage to appear at L24/L23 side of these two diodes - the Banded end (in photo above it's to the right on the diodes) of either one will tell you if you have a voltage presence or not - in one mode - you do, then in another you don't - track down the reason - start with these diodes...and look a simple DC voltage and see if by turning the front "Clarifier"/"Delta-Tune"/"Play With This Knob and Go Places" - you can see and notice the change (In RX mode) where the radio tunes into across the CHANNEL it's listening to.

And now for Dessert...

​

The Foil side - showing locations of TWO varactor's that need to be present for the PLL and the 10.240 Xtal to work together...

So, no varactor, no TX or RX lineup...

 

[Harley3315]

Ok if in fact those varactors are missing what is ok to replace them with?

 

[Handy Andy]

Better show a pic of the Top and Bottom of the board to help me help you find them...

We need to know what you're up against.

 

[Harley3315]

Better show a pic of the Top and Bottom of the board to help me help you find them...

We need to know what you're up against.

I'll get ya one in the morning it's siting at the shop I appreciate the help from one of the Cb tricks Gods of radio Mr. Handy Andy you and Nomad are legends! So glad we all found a new home here after that forum went down!

 

[Harley3315]

Better show a pic of the Top and Bottom of the board to help me help you find them...

We need to know what you're up against.

Touching back on this finally guess it was 78xl pre weather added actually checked both the vco diodes not surface mounts on this model actually in locations d13 and d16 both test fine both other diodes are around 5.5 volts rx drop to 3.5ish on tx so far have replaced the whole channel selector board the 10.240 and the 10.695 filter I believe it was starting to lean towards pll it's on on tx rx seems to be the same noise for 3-5 channels hear the same guy talking.

 

[Handy Andy]

Ok, when it does that - that 3.3V - the BANDED sides of those two diodes...



IN this board, D17 and D18 - the BANDED sides - if the radio is unmodded for slide, will show a voltage ALL THE TIME - it will change though in RX to TX and TX to RX - the fluctuation is small it's a 455kHz shift it has to accomplish so it can make the IF line up right between both modes.

Cobra 29 same thing...

This one though is modded...


So if you want them to line up - best to keep the voltage steady betwen both.

If the banded-side voltages test vary from TX to RX - try to find which one is stable - that is the one the PLL is in lock with (or trying to)

So if the voltage sourcing is steady, but the channels are not, then the issue is with (still) the varactors going bad, poor soldering joints or in some instances, a re-cap is needed to get the PLL to "regulate" with the power supply it's given, to work with.

I'll try to provide more info as time permits...

But check VOLTAGE to the PLL when it does this, Pin 7 or 11 is the PLLs' power supply. Has to be stable.

 

[Harley3315]

Ok, when it does that - that 3.3V - the BANDED sides of those two diodes...

View attachment 60232

IN this board, D17 and D18 - the BANDED sides - if the radio is unmodded for slide, will show a voltage ALL THE TIME - it will change though in RX to TX and TX to RX - the fluctuation is small it's a 455kHz shift it has to accomplish so it can make the IF line up right between both modes.

Cobra 29 same thing...

This one though is modded...
View attachment 60233

So if you want them to line up - best to keep the voltage steady betwen both.

If the banded-side voltages test vary from TX to RX - try to find which one is stable - that is the one the PLL is in lock with (or trying to)

So if the voltage sourcing is steady, but the channels are not, then the issue is with (still) the varactors going bad, poor soldering joints or in some instances, a re-cap is needed to get the PLL to "regulate" with the power supply it's given, to work with.

I'll try to provide more info as time permits...

But check VOLTAGE to the PLL when it does this, Pin 7 or 11 is the PLLs' power supply. Has to be stable.

Yes when keyed both diodes drop to 3.5 ish on the banded side of both d17 and d18 yes it unmodded for slide. How many volts am I looking for on pin 7 and 11 of the pll and what should it be on tx and rx?

 

[Handy Andy]

Pin 7 and 11 would be (or should be) about 5V - the PLL doesn't need much.

The voltage that arrives thru the RX and TX diodes - is about 3.5 - but the variance is what you need to look into.

It's not uncommon for an older radio such as the 78 to develop problems with the BCD switch where the PLL rom "locks out" the receive and TX if the pins that it's supposed to be grounded are not - due to a dirty switch contacts - I've had radios like the 78 and 29 arrive as "face plant" radios - with the knob - when rotated, you can see the "lob" it has even though the display works, the pins to select the PLL for channel - don't always follow.

The variance shows up more on RX side, because as the pins change - the ROM wants to cut out or lock out the channel - but with the dirt in the rotary selector - it can make the PLL think one moment it's ok, the next - it doesn't match - this forced lock due to a drifting pin - puts the RX side in this variable voltage scenario - the PLL is cycling in-lock and wham a pin drifts high, the ROM toggles the LOCK and the oscillator will shut down.

You don't see the RX side show this change easily - it just stops - but that doesn't turn off the Green LED - it'll turn off the RED TX side - but not the RX side.

So the channel board / BCD swtich if that has been replaced and it still does the same thing - then the PLL tracking side - needs help.

There are the usual suspects of capacitors that can't hold their values when powered - so that includes the two back to back ones that the PLL loops with as well as the Power feeder line (the cap used to filter to D19's Zener 5V to Pins 7 and 11. Just have to look for a drop - a scope might help show that drop if the PLL is toggling out of lock.

Even the AGC cap - anything that can pull down the RX can make it hard to trace down this actual cause.

So this puts us back to the age of the radio - the TX seems to hold up? (Do I have that right?) But RX fails, so this may be more of something with the TX switch in the 2SA733 PNP - got a spare? Might want to try a swap and look and change the caps that are part of the TX and RX feeder lines FROM that 2SA733.

The PLL voltage is regulated by that 5V Zener D19 - but there is another Zener D31 (Yellow band) - that regulates the voltage sent to the Display. It's by the PLL and affects the POWER line that heads off to the display board.

Due to the right angle (90drgree) rigid design of the Uniden compared to the free-flowing Cobra trace layouts - the parts lists are similar and much of the boards are alike - but they differentiate when it comes to the fringes of their designs.

Like DSS versus the Sound Tracker - they branched off using different numbering - but the BASE of the radio is very similar between the two.

So to me this is looking more and more like an old radio that needs some TLC and some caps replaced.

 

[Harley3315]

Pin 7 and 11 would be (or should be) about 5V - the PLL doesn't need much.

The voltage that arrives thru the RX and TX diodes - is about 3.5 - but the variance is what you need to look into.

It's not uncommon for an older radio such as the 78 to develop problems with the BCD switch where the PLL rom "locks out" the receive and TX if the pins that it's supposed to be grounded are not - due to a dirty switch contacts - I've had radios like the 78 and 29 arrive as "face plant" radios - with the knob - when rotated, you can see the "lob" it has even though the display works, the pins to select the PLL for channel - don't always follow.

The variance shows up more on RX side, because as the pins change - the ROM wants to cut out or lock out the channel - but with the dirt in the rotary selector - it can make the PLL think one moment it's ok, the next - it doesn't match - this forced lock due to a drifting pin - puts the RX side in this variable voltage scenario - the PLL is cycling in-lock and wham a pin drifts high, the ROM toggles the LOCK and the oscillator will shut down.

You don't see the RX side show this change easily - it just stops - but that doesn't turn off the Green LED - it'll turn off the RED TX side - but not the RX side.

So the channel board / BCD swtich if that has been replaced and it still does the same thing - then the PLL tracking side - needs help.

There are the usual suspects of capacitors that can't hold their values when powered - so that includes the two back to back ones that the PLL loops with as well as the Power feeder line (the cap used to filter to D19's Zener 5V to Pins 7 and 11. Just have to look for a drop - a scope might help show that drop if the PLL is toggling out of lock.

Even the AGC cap - anything that can pull down the RX can make it hard to trace down this actual cause.

So this puts us back to the age of the radio - the TX seems to hold up? (Do I have that right?) But RX fails, so this may be more of something with the TX switch in the 2SA733 PNP - got a spare? Might want to try a swap and look and change the caps that are part of the TX and RX feeder lines FROM that 2SA733.

The PLL voltage is regulated by that 5V Zener D19 - but there is another Zener D31 (Yellow band) - that regulates the voltage sent to the Display. It's by the PLL and affects the POWER line that heads off to the display board.

Due to the right angle (90drgree) rigid design of the Uniden compared to the free-flowing Cobra trace layouts - the parts lists are similar and much of the boards are alike - but they differentiate when it comes to the fringes of their designs.

Like DSS versus the Sound Tracker - they branched off using different numbering - but the BASE of the radio is very similar between the two.

So to me this is looking more and more like an old radio that needs some TLC and some caps replaced.

Did the electrolytic 10 volt caps already what cap is for the agc on this radio and I'll check the voltages at pll Yes the tx is great local on the phone when testing hears me 20 miles away just can't hear him respond cause the rx is not locking on the same channel. Got lot's of spare uniden and cobra parts around you mentioned caps for loop back in the pll ceramics? which two?

 

[Handy Andy]

Bringing us back to here...



You can use the Cobra 29 tune ups for this chassis...

See the RED circled area? Those caps...for the loop - one of the 3 serve to fine tune the TP2 and TP1 area to smooth out the loop mixing so the PLL doesn't panic - TP1 and TP2 are just like in Cobra 29's

Yellow Arrows point to filter caps for the power

See the Blue TP3 circle? Your board uses the 17MHz just like the 29's do - so your RX instability would show up here...

Why are you getting this?

Then look around L18 right next to L19 as well as L22 the main carrier oscillator the wax plugged one by the PLL - in these areas L18 sends the signal to TP3 - so not only check for signal, but the Quality of it.

 

[Harley3315]

Bringing us back to here...

View attachment 60252

You can use the Cobra 29 tune ups for this chassis...

See the RED circled area? Those caps...for the loop - one of the 3 serve to fine tune the TP2 and TP1 area to smooth out the loop mixing so the PLL doesn't panic - TP1 and TP2 are just like in Cobra 29's

Yellow Arrows point to filter caps for the power

See the Blue TP3 circle? Your board uses the 17MHz just like the 29's do - so your RX instability would show up here...

Why are you getting this?

Then look around L18 right next to L19 as well as L22 the main carrier oscillator the wax plugged one by the PLL - in these areas L18 sends the signal to TP3 - so not only check for signal, but the Quality of it.

Yeah I had the 17 mhz at the test point I already checked that. I'll take a look at those other areas now.

 

[Handy Andy]

Yeah I had the 17 mhz at the test point I already checked that. I'll take a look at those other areas now.

Ok, then if you're getting the 17MHz, then the 10.240 MHz window to generate the 455kHz is L24 -into- L23 With L24 fine tuning (Voice lock / Clarifier / Delta Tune) FOR L23 and the PLL base clock.

This may mean the 10.7 IF Crystal filter is drifting off, that may need a cap to straddle across the Xtals in and out contacts to see if RX can be restored - at least you can use it as a diagnostic. a small value of 68pF is sufficient.

​

L 7 "peaks out the signal at FET2 - which is receiving the 10.240 signal from L23

The 455kHz result is at the TR8 and VR1 trimmer mess into the dual amp from TR8 and heads into TR9 -TR10 with L8 and L9 peaking that out.

C27, and C20 are sensitive electrolytics. But that is on a Cobra 29 - I don't have a pic of your board to help you from here. The Uniden switchover to more chinesium occurred as the 76 XL was becoming the 76XLW and Vietnam was origins of this at a time.

 

[Harley3315]

Ok, then if you're getting the 17MHz, then the 10.240 MHz window to generate the 455kHz is L24 -into- L23 With L24 fine tuning (Voice lock / Clarifier / Delta Tune) FOR L23 and the PLL base clock.

This may mean the 10.7 IF Crystal filter is drifting off, that may need a cap to straddle across the Xtals in and out contacts to see if RX can be restored - at least you can use it as a diagnostic. a small value of 68pF is sufficient.

L 7 "peaks out the signal at FET2 - which is receiving the 10.240 signal from L23

The 455kHz result is at the TR8 and VR1 trimmer mess into the dual amp from TR8 and heads into TR9 -TR10 with L8 and L9 peaking that out.

C27, and C20 are sensitive electrolytics. But that is on a Cobra 29 - I don't have a pic of your board to help you from here. The Uniden switchover to more chinesium occurred as the 76 XL was becoming the 76XLW and Vietnam was origins of this at a time.

Found it loose small electrolytic by the pll the trace was broken were the cap had been moved around or something. Thanks for the help Andy I never would have found it had you not pointed those 3 caps out as a possible issue. Now on to the next thing Channel mod for trc-485