Alignment Question - Problem

[sdmahr]

I've got a Malaysian Cobra 148gtl I'm doing an alignment on. I have a problem with getting USB lined up.

I've got the counter on TP1 and the radio on CH19 per Cobra's service manual.

The frequency here should be 34.9865, but the lowest I can get it with the L59 can is 34.9906.

Not sure what to do......it won't go any lower.

AM and LSB are perfect..........USB I cannot get it there.

HELP !

 

[Robb]

Think you forgot to mention that the clarifier has opened up all of the way . . .

When this was done, you can no longer center the clarifier to get that freq on USB. So, you must turn the clarifier knob until you get the desired freq for USB, and then align AM and LSB to fit. Either that, or change the slide back down to the stock range and then the clarifier can be centered. Take your pick.

 

[sdmahr]

Robb,

Clarifier wasn't "cranked open". Just lifted D58.

AM and LSB is perfect.

Can you explain more ?

THANKS ! !

 

[Robb]

Check and see if R175 has been jumped. Check and see if R174 has been jumped and clipped. Check if D52 has been clipped/removed. If so; restore all of them. Check R44. Check the loop osc crystal to have a 11.325mhz value. Or your USB loop osc can is out of spec. Use a schematic and trace it all through - to be sure. Also check those resistors values against the schematic.

 

[sdmahr]

Everything checks out. Not sure where to head next. Where would I get a "Can" ? I tried a shorter and longer slug. The longer one helped.....but not enough.

Any help appreciated.

 

[Robb]

Check:
L25 for 220uH
R236 for 8200 ohms
Replace D33 with another 1N4148
Make sure that crystal/X3 is a 11.325mhz.

You will need a decent digital multimeter to test the first three components.

L59 is a variable choke; but the schematic doesn't give its value.

*EDIT*
Sometimes the jumpers are on the trace side of the board; not always on the part side.

BTW - the scheme for the clarifier control:

 

[Tucker442]

I have complete junker 148 board here with no parts missing yet, let me know what your needing and I would be more than glad to send it to you for the shipping charge

 

[sdmahr]

Tucker442.

Thanks for the offer ! I have replaced everything in the USB circuit with the exception of the L59 tuning can.

Its the one closest to the front of the radio.

You have PayPal ? I'll get you the payment for your trouble.

Let me know......and THANKS

 

[sonoma]

I did not look at the schematic but did you change the little disk caps in the usb circuit by the L59 can? small disk pf caps that help in the circuit. I have had one bad before causing the same problem you are having and a new can did not help.

 

[sonoma]

did you align L20 and L21 with a scope as it states to do. if they are off it can cause a problem also. was just looking at the schematic on it.

 

[sonoma]

also check what the freq reading is at the 11.325 crystal in usb,lsb and am and check on each side of the caps and L59 to see if you can pin point where you are losing the little bit of freq you are looking for.

 

[sdmahr]

Sonoma,

Thanks for the input. I have replaced almost all of the small caps around L59 that appeared connected to the USB circuit.....including resistors in that circuit as well. Same result.

I have a freq counter and scope so I will start at the crystal and work my way out to see where it falls apart. I still have a feeling its the can......but its totally possible I may find something else. With each component changed I'm hoping I'll see a different result.

I only need to "shift" down a little bit. Spec calls for 34.9865 and the lowest she'll go is 34.9907 with L59.

 

[Grinder74]

My Cobra 2k has the same problem but I don't have a scope just the freq counter. I have the L59 bottomed out still just short. I am going to change caps and follow the circuit also.

 

[sdmahr]

Grinder74:

If and when I find a solution, I'll post it ASAP. This has been driving me NUTS !

Sounds like you are the opposite of mine.....I am moving the core UP and run out of change. You are bottoming out.......strange.....

 

[unit_399]

Try removing the core and check it out to see if a small piece has broken off. If it has, check to see that the piece isn't stuck in the bottom of the can. Also, sometimes removing the slug and turning it over can make a difference. Don't have a clue as to why. Good luck...73s.

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