help with Cobra 142GTL

[KD2GOE]

friends of mine are getting in to CB and then moving in to ham after we get all the stuff down experience and such so a friend of mine got a cobra 142GTL on evil-bay now it is my job to get it right before we take it to some one with tools to get it all tuned up I just have a freq counter.. saving up for more tools I want to get the easy stuff sorted out before he has to pay some one to fix it

transmit and reserve are vary bad...

transmit was only doing 2 watts but after touching up the solder joints it is doing 4 with the AM power "cranked up" and only 2-3 watts of swing SSB is only swinging 4 watts

some one swooped out the driver and pre-driver, and final with

2SC1957 < pre-driver
2SC2166 < driver
2SC2312 < new final

what's up with that and is that an acceptable swop? I don't see why the driver, and final wont work but what is that pre-driver??


and if any one has a 11.1125Mhz crystal I need one.

 

[Robb]

2SC1957 < pre-driver was originally a 2sc1973
2SC2166 < driver is OK
2SC2312 < new final is OK

Is this an 858 PLL chassis?

Dunno if the 1957 is a straight swap for the 1973; but it seems to work.

Set the driver bias first and then the final bias.
Then peak the TX coils L26 through L29 in SSB mode.

 

[KD2GOE]

2SC1957 < pre-driver was originally a 2sc1973

Set the driver bias first and then the final bias.

the lowest I can get on the driver bias is 134ma

the final bias is 500ma!!! I am going to look for bad CAPS

 

[IMD262]

I don't think the 142 ever came with the 858 PLL, or did they ????

 

[Robb]

I don't think the 142 ever came with the 858 PLL, or did they ????

You are right, it was the 139XLR that was a 858.

mkube:
The bias must be tested in SSB mode only w/mic gain set at 0.

 

[sonoma]

I would change all the 10 volt caps out to 16 to 25 volt since the radio is so old the 10 volt caps are drying out. I have 2 I just changed them out in to get them working right. I would do the cap updates as a 1st item to do. will help bring the receive and TX back up to par.

 

[KD2GOE]

mkube:
The bias must be tested in SSB mode only w/mic gain set at 0.

I am embarrassed.... I was on AM ok I set it and now there with in spec

Connect DC Volt meter / oscilloscope to TP9 (top leg of R72, just below the VCO card)

Set radio to channel 1 on the lowest band, adjust L13 so that the base voltage just lifts (should be set to around 2.0 VDC).

Check that the radio will transmit at the set voltage, if not adjust L13 to increase it a little until it does transmit.

Go to channel 40 on the highest band (or the highest channel available), check that the voltage is below 6.8 VDC and that the radio will transmit.

I got it set to 2 volts on ch 1 but on 40 it is reading only 3 volts

voltage regs are reading 13.8 and 8.24

 

[KD2GOE]

I would change all the 10 volt caps out to 16 to 25 volt since the radio is so old the 10 volt caps are drying out. I have 2 I just changed them out in to get them working right. I would do the cap updates as a 1st item to do. will help bring the receive and TX back up to par.

I did that with my uniden Washington it had all rubycon capacitors. I put in Nippon chemi-con and nichicon I have a few extra and I save all the old rubycon's
I have been putting them in no change as of yet

 

[Robb]

I am embarrassed.... I was on AM ok I set it and now there with in spec






I got it set to 2 volts on ch 1 but on 40 it is reading only 3 volts

voltage regs are reading 13.8 and 8.24

Do you remember what the voltages were before you changed them?

A tip: always write them down in case the PLL unlocks ('unstable freq readings') due to too little/much voltage.

I would set the VCO voltage/L13 @ 4.5v on ch 40.

BTW - after doing the bias on the driver/final, the next step would be peaking the TX coils in SSB mode - L26, L27, L28, & L29.

 

[KD2GOE]

Do you remember what the voltages were before you changed them?

A tip: always write them down in case the PLL unlocks ('unstable freq readings') due to too little/much voltage.

I would set the VCO voltage/L13 @ 4.5v on ch 40.

BTW - after doing the bias on the driver/final, the next step would be peaking the TX coils in SSB mode - L26, L27, L28, & L29.

it was 2.98v


ok I am done with this thing... my friend just needs to take it in to a shop with the right tools. on top of all this it wont even receive a station unless it is my CB in the same room....

thanks guys for the help I tried to help he needs some one that can probe around with a scope to fined the faults.....

 

[KD2GOE]

Ok now that i got some sleep I started looking at it..
I got the receiver working TR9 was done I put in a trsnsister from a cobra 29.. now I can hear people

There has to be more bad transistors I have to crank up the am power to get 4 watts dead key and it only swings 2 or 3 watts..

I hope this is not to late but tr34 had been cut out and the alc "vr7" is not working at all

 

[KD2GOE]

Now that the receiver is working the sgnal meter is working good on AM but not so well on ssb.. ssb sounds fine compard to my uniden..

 

[Robb]

mkube396 wrote:

There has to be more bad transistors I have to crank up the am power to get 4 watts dead key and it only swings 2 or 3 watts..

Set it at 4w dead key.

Replace TR34, and then VR7 should work again.

Tuning the TX coils -as I described in earlier posts- is what will peak the output watts.
If you don't have the equipment and don't know how; then it is time to pass it off.

 

[KD2GOE]

I did tune the TX coils you listed, I used my pc headphone jack output modfyed hand mike. the computer was putting out 1khz tone the radio was in to a 50ohm dumby load.I first did it with just AM dead key then on Lsb

Is that right or close to how it is done?

 

[Robb]

I did tune the TX coils you listed, I used my pc headphone jack output modfyed hand mike. the computer was putting out 1khz tone the radio was in to a 50ohm dumby load.I first did it with just AM dead key then on Lsb

Is that right or close to how it is done?

Replace AM Limiter/TR34 if it was removed - first!

Use ch 20 in either USB or LSB modes only; not AM.

Need a wattmeter set on peak, a sound source for the mic - like what you mentioned above, and a dummy load.

Peak each coil starting with L26 first, let the radio rest a minute/two between each next coil peaked.

STEADY hand with the mic close to the computer speaker when keying; no other background noises.

DO NOT use a jeweler's screwdriver; nylon/ceramic tool ONLY!