Cobra 2000 Can’t adjust L19

[Brian G]

Seems to have synthesizer issue in this Cobra 2000. I am getting 8 volts at Test point 9. It should be around 2.7 volts and adjustable through L19. But L19 turning L19 doesn’t change the reading.
When i connect scope to test point 10 for L21 adjustment or to test point 1 for L20 adjustment, turning L19 here does affect those readings. But i can only get L21 to about 650 mv when it should be about 3.4 volts. And I can only get L20 up to about 36 mv when it should be about 190 mv.
All VCO voltages seems to be right on.and PLL seems to be ok as well. In case it matters this PLL is MB8719 and radio is mod for extra channels
Could L19 be bad?

any Help is appreciated!

 

[Handy Andy]

Several approaches here...

It may be the Varactor in the UHIC007 chip (Flat-pak SIPP) may have failed. Not much can be done except replace the SIPP itself.

(c) EURO Radio Co.
​

Several Electrolytic's are used here, any old ones can cause problems - might want to replace these older Can caps with fresh ones.

Did you check so see if L19 was open?

Maybe all it needs it resoldering one of the Coil winds back onto it's post.

This is INTERNAL to the Coil - so you should unsolder it from the board and remove it - pull off the can shield, and examine the Coil - this Coil an LA219 - is specific to this type of VCO and Chassis - So that doesn't rule out you can't fix it, just that it's rare - no spares - so use caution on disassembly. Inspect it carefully.

 

[Brian G]

Several approaches here...

It may be the Varactor in the UHIC007 chip (Flat-pak SIPP) may have failed. Not much can be done except replace the SIPP itself.

View attachment 42965
(c) EURO Radio Co.
​

Several Electrolytic's are used here, any old ones can cause problems - might want to replace these older Can caps with fresh ones.

Did you check so see if L19 was open?

Maybe all it needs it resoldering one of the Coil winds back onto it's post.

This is INTERNAL to the Coil - so you should unsolder it from the board and remove it - pull off the can shield, and examine the Coil - this Coil an LA219 - is specific to this type of VCO and Chassis - So that doesn't rule out you can't fix it, just that it's rare - no spares - so use caution on disassembly. Inspect it carefully.

Thanks I will look at L19 and see if that’s anything. I replaced all the electrolytics a few years back so I don’t think those are the problem. Hopefully if I have to replace the VCO that I can find one!
Thanks.

 

[Brian G]

Thanks I will look at L19 and see if that’s anything. I replaced all the electrolytics a few years back so I don’t think those are the problem. Hopefully if I have to replace the VCO that I can find one!
Thanks.

I have

Several approaches here...

It may be the Varactor in the UHIC007 chip (Flat-pak SIPP) may have failed. Not much can be done except replace the SIPP itself.

View attachment 42965
(c) EURO Radio Co.
​

Several Electrolytic's are used here, any old ones can cause problems - might want to replace these older Can caps with fresh ones.

Did you check so see if L19 was open?

Maybe all it needs it resoldering one of the Coil winds back onto it's post.

This is INTERNAL to the Coil - so you should unsolder it from the board and remove it - pull off the can shield, and examine the Coil - this Coil an LA219 - is specific to this type of VCO and Chassis - So that doesn't rule out you can't fix it, just that it's rare - no spares - so use caution on disassembly. Inspect it carefully.

Several approaches here...

It may be the Varactor in the UHIC007 chip (Flat-pak SIPP) may have failed. Not much can be done except replace the SIPP itself.

View attachment 42965
(c) EURO Radio Co.
​

Several Electrolytic's are used here, any old ones can cause problems - might want to replace these older Can caps with fresh ones.

Did you check so see if L19 was open?

Maybe all it needs it resoldering one of the Coil winds back onto it's post.

This is INTERNAL to the Coil - so you should unsolder it from the board and remove it - pull off the can shield, and examine the Coil - this Coil an LA219 - is specific to this type of VCO and Chassis - So that doesn't rule out you can't fix it, just that it's rare - no spares - so use caution on disassembly. Inspect it carefully.

I have a feeling I will not be able to find a replacement for the VCO. Initial look online and I don’t come up with one anywhere. Can those things be worked on and replace just the varactor or no? If so, which one is it?

 

[Brian G]

I have


I have a feeling I will not be able to find a replacement for the VCO. Initial look online and I don’t come up with one anywhere. Can those things be worked on and replace just the varactor or no? If so, which one is it. D1?

 

[Brian G]

Several approaches here...

It may be the Varactor in the UHIC007 chip (Flat-pak SIPP) may have failed. Not much can be done except replace the SIPP itself.

View attachment 42965
(c) EURO Radio Co.
​

Several Electrolytic's are used here, any old ones can cause problems - might want to replace these older Can caps with fresh ones.

Did you check so see if L19 was open?

Maybe all it needs it resoldering one of the Coil winds back onto it's post.

This is INTERNAL to the Coil - so you should unsolder it from the board and remove it - pull off the can shield, and examine the Coil - this Coil an LA219 - is specific to this type of VCO and Chassis - So that doesn't rule out you can't fix it, just that it's rare - no spares - so use caution on disassembly. Inspect it carefully.

Replaced the VCO, no change-still cannot adjust L19. It shows 8 volts in the meter. And I cannot get L20 or 21 up to voltage specified. They do adjust but not nearly enough. I did find an electronics supplier that has L19 in stock so I will replace that when it come in. However I did u solder the one that is in there now and I don’t see a problem with it. This one only has one winding that goes to the VCO. The other legs look like they have no function and are just grounded. I am doubtful that will fix the problem but who knows. Will see once it comes in.

 

[nomadradio]

Is the tuning slug of either L21 or L22 showing it's "peak" with the slug DEAD FLUSH with the rim of the hole?

My intuition is that the root of your trouble lies with the 3rd harmonic of the 11.325 crystal you should be getting from L21.

If this signal from L21 is weak or absent, L19 won't do anything at all.

I'm accustomed to just checking pin 17 of the PLL with the 'scope. Yeah, okay they call it TP10.

If you see this "slug dead flush with the hole" when any of the small tuning slugs show a peak, this tells you a capacitor has failed inside that RF transformer/coil.

A shot in the dark, but easy to check.

73

 

[Brian G]

Is the tuning slug of either L21 or L22 showing it's "peak" with the slug DEAD FLUSH with the rim of the hole?

My intuition is that the root of your trouble lies with the 3rd harmonic of the 11.325 crystal you should be getting from L21.

If this signal from L21 is weak or absent, L19 won't do anything at all.

I'm accustomed to just checking pin 17 of the PLL with the 'scope. Yeah, okay they call it TP10.

If you see this "slug dead flush with the hole" when any of the small tuning slugs show a peak, this tells you a capacitor has failed inside that RF transformer/coil.

A shot in the dark, but easy to check.

73

Thanks I’ll check. Only problem is if that coil is bad the chance of finding a replacement is probably about zero.

 

[999]

Thanks I’ll check. Only problem is if that coil is bad the chance of finding a replacement is probably about zero.

I have an old 148 board that I can grab parts from.

 

[Brian G]

I have an old 148 board that I can grab parts from.

I would be much obliged. Will pay you if we can work it out.

 

[Brian G]

I have an old 148 board that I can grab parts from.

Would like L20 and 21.

 

[Handy Andy]

Replaced the VCO, no change-still cannot adjust L19. It shows 8 volts in the meter. And I cannot get L20 or 21 up to voltage specified. They do adjust but not nearly enough. I did find an electronics supplier that has L19 in stock so I will replace that when it come in. However I did u solder the one that is in there now and I don’t see a problem with it. This one only has one winding that goes to the VCO. The other legs look like they have no function and are just grounded. I am doubtful that will fix the problem but who knows. Will see once it comes in.

This isn't your coil, but I wanted you to examine the core and the winds carefully.

\​

I just wanted you to visually inspect the L19 coil, making sure it's wires from the Coil form, were properly soldered onto the "stems" that reach thru the Phenolic into the main PCB - I know that on some of the radios that have crossed the shops bench - the coil or it's embedded mess, failed - opening the coil or somehow they "tweaked" the slug to detune it so bad - the form it used, broke internally from the stresses.

The Slug too, that is the most important - broken slug or parts of it inside the form - can make your day...

Sounds like L19 passed those tests, so the Coil seems ok, so that means it's serviceable - we just have to find the reason for the missing 3rd harmonic, sounds like @nomadradio has got you on the right path...

 

[999]

Would like L20 and 21.

Sorry I haven't gotten back to you, been a bit busy. I'd be glad to get them off the board for you. Shoot me a pm and we'll work something out.

 

[Brian G]

Sorry I haven't gotten back to you, been a bit busy. I'd be glad to get them off the board for you. Shoot me a pm and we'll work something out.

Hi not sure how to send a PM?

 

[Brian G]

This isn't your coil, but I wanted you to examine the core and the winds carefully.

View attachment 43069 \​

I just wanted you to visually inspect the L19 coil, making sure it's wires from the Coil form, were properly soldered onto the "stems" that reach thru the Phenolic into the main PCB - I know that on some of the radios that have crossed the shops bench - the coil or it's embedded mess, failed - opening the coil or somehow they "tweaked" the slug to detune it so bad - the form it used, broke internally from the stresses.

The Slug too, that is the most important - broken slug or parts of it inside the form - can make your day...

Sounds like L19 passed those tests, so the Coil seems ok, so that means it's serviceable - we just have to find the reason for the missing 3rd harmonic, sounds like @nomadradio has got you on the right path...

Ok so I can now tune L19 (i replaced with a new one). Set to 2.70 volts on channel 40 as spec. . However I am having trouble with L21. It’s supposed to be peaked at about 3.4 pp volts but I can only get it to 2.3 Volts. And I cannot tune L22/23/59 on the frequency counter to spec. Not sure if maybe L21 is not good and that is preventing me from peaking to 3.4 volts? Also I noticed that the spec say to peak L20 for max RF or about 190 Mv. But peak on the scope shows about 1.6 volts which is way higher. Should I just put it on 190 mv either way or does that suggest an issue? Thanks.