Cobra 142 GTL high DK, no TX audio?

[Deleted]

This is a piece of good news. Friend got one for $10, and said if I fix it, I can buy it for $20. Not a bad deal.

Have not opened it up yet. High dead key like 5 watts, no TX audio. RX appears good. Could this be one of those 10v cap jobs?

How are these? How does the receiver compare to the 2000?

[cbrown]

The capacitor is the usual problem on the 142's.

[Sandbagger]

Sept 1, 2010 8:17:46 GMT -5 @vinnies said:

This is a piece of good news. Friend got one for $10, and said if I fix it, I can buy it for $20. Not a bad deal.

Have not opened it up yet. High dead key like 5 watts, no TX audio. RX appears good. Could this be one of those 10v cap jobs?

How are these? How does the receiver compare to the 2000?

Vinnie,

Good to see you back. You seemed to be a bit scarce around here for a while.....

Anyway, the Cobra 142 is a similar radio to the 2000, except that it is a single conversion receiver. It's not quite as selective as the 2000, but performance otherwise will be similar.

That vintage Uniden chassis had numerous issues with "the 10V blues", but there are some checks along the audio chain that you can do with a scope to see where your modulation disappears, which might help you narrow it down. You can also eliminate a bunch of common early mic audio stages if you get normal modulation on SSB. Your biggest friend will be the schematic and a good scope probe....

[Deleted]

Thanks, Dave. I haven't been totally away, but the warmer weather is my slow period for working on radios or such. I have been after my DXCC which I started last Dec 29th. I got 115 worked and 94 confirmed. I go back to the CB for nightly rag chew. Kind of weird, but I use the ham bands for DX, and CB for rag chew.

So I am readying myself for the upcoming busy period. I till trace a signal thru this radio. I will follow up. Thanks!

[Sandbagger]

Sept 1, 2010 16:56:19 GMT -5 @vinnies said:

Thanks, Dave. I haven't been totally away, but the warmer weather is my slow period for working on radios or such. I have been after my DXCC which I started last Dec 29th. I got 115 worked and 94 confirmed. I go back to the CB for nightly rag chew. Kind of weird, but I use the ham bands for DX, and CB for rag chew.

So I am readying myself for the upcoming busy period. I till trace a signal thru this radio. I will follow up. Thanks!

Using CB for local rag chews can be a lot of fun if you have a decent group that knows how to have a little radio fun, and you don't have sociopathic butt-heads trying to ruin the fun.

I still have yet to put out some sort of antenna for the lower HF bands. We have been having a weekly Sunday night net on 10 meters, and I can get away with tuning the 11 meter antenna. But that doesn't work all that well on anything lower than maybe 15 meters.

Let me know how you make out with that Cobra 142. You should be able to get it running.

[Deleted]

Dave,

I went thru the early part of the chain. Found TR 32 cut. Replaced it. Followed the signal thru IC3 pin 7 with a scope, and it was still there. I got tired and these components are packed in like sardines. I have to look at the R104 t make sure it is intact.

Should I just replace the 10v caps anyway?

[Sandbagger]

Sept 2, 2010 22:47:00 GMT -5 @vinnies said:

Dave,

I went thru the early part of the chain. Found TR 32 cut. Replaced it. Followed the signal thru IC3 pin 7 with a scope, and it was still there. I got tired and these components are packed in like sardines. I have to look at the R104 t make sure it is intact.

Should I just replace the 10v caps anyway?

I've never been a fan of "shotgun" troubleshooting. Sometimes you hit the bad part, and other times you might mess something up in the process of changing out parts. But in the case of an old radio with known cap issues, it might make sense to change them all out. But there are a bunch in there, so it could become tedious.

But I would do it AFTER you track down the original problem and correct it.

[Deleted]

Here is the problem I have. I do not have a SAMS. The schematic I got at CB tricks is unreadable when printed. I have to trace by going from my bench, to the office room to look at the schematic on my laptop, and back to the bench. I do not have anywhere to put my laptop near my bench. This is making it difficult to trace. Also, I would really like a block diagram, but cannot locate one.

[Sandbagger]

Sept 3, 2010 17:33:34 GMT -5 @vinnies said:

Here is the problem I have. I do not have a SAMS. The schematic I got at CB tricks is unreadable when printed. I have to trace by going from my bench, to the office room to look at the schematic on my laptop, and back to the bench. I do not have anywhere to put my laptop near my bench. This is making it difficult to trace. Also, I would really like a block diagram, but cannot locate one.

Let me see if I can give you a little help tracing the mic audio.

TR32 is the modulation limiter, which is why that was cut.

TR29 is the first mic amp. The signal passes out of the collector of that transistor on to TR28 which then passes the signal on to IC3 on SSB. On AM the signal passes through TR27 on to IC4. From IC4 pin 9 the signal passes through C118 and C119 on to the modulator TR35 and TR34.

Now knowing this, if your radio has normal receive audio and there is modulation on SSB, you've pretty much eliminated TR29, TR28 and IC4 and their associated support components. You should try looking at pin 9 of IC4 when transmitting on AM. If there is no signal there, try flipping it to PA mode. If signal is there in PA, then suspect TR27 or something near it. Make sure the switching voltage is there on the base of TR27 on AM transmit.

If there is signal on Pin 9 of IC4 in AM mode then check the other side of C119. VR6 sets AM carrier, and if that works, then that tends to prove that TR35 and TR34 are working and C119 is the most likely culprit. C118 must be ok if you have normal receive audio from the speaker, but it could be bad is the volume is low. If VR6 does not work correctly, the there may be a problem with TR35 or 34 or possibly a shorted D42.


As for a block diagram, there's nothing listed on CB tricks for the 142. BUT, we know that the same chassis is used on several other radios. If you download the factory service manual for a Realistic TRC-450, that includes a block diagram that will do the job for you. Not to mention a full explanation of many of the other circuits in that rig.

[Deleted]

That is great, Dave. The block diadram and circuit description will help grealy, as did your description. I found a couple of cut resistor, too. I did replace the 10V caps, anyway, only because it was a matter of time.

[Sandbagger]

Sept 4, 2010 9:26:21 GMT -5 @vinnies said:

That is great, Dave. The block diadram and circuit description will help grealy, as did your description. I found a couple of cut resistor, too. I did replace the 10V caps, anyway, only because it was a matter of time.

Hope it helps you. Let me know how you make out.

[Deleted]

All the transistors check out. I pulled some of them because they were in too tight. I will next go after the support components.

[Sandbagger]

Sept 4, 2010 11:41:25 GMT -5 @vinnies said:

All the transistors check out. I pulled some of them because they were in too tight. I will next go after the support components.

I would trace the audio signal through the stages, and not just shotgunning transistors. The point where you lose signal is where I'd start looking closer.

[Deleted]

That is what I am going to do. I was just checking them while I had the chance. I did not have excess time to do it yesterday. So I just tested some transistors. Tomorrow (Monday), I should have plenty of time.

[Deleted]

OK, Dave. I made progress I purchased a small audio amplifier from Radio Shack to trace the audio stages. Works well. I went thru the signal tracing you recommended, and noticed something.

At the beginning of this thread, I had no swing or modulation, and about 4.5 DK.

Now, with a 1 KHz tone at the mic input, I get 100% modulation. Here is the kicker. If I whistle or talk, I get nothing. No swing or modulation. However, the high amplitude signal of the audio generator seems to flip a "switch". I was also able to use the variable and hit that exact point where I get the modulation.

I am pretty sure that before I changed the caps, the tone did not move the needle at all. However this time, it moves the needle.

In essence, I get modulation when I use a high amplitude audio signal, and none if I use a mic. I did try several mics. Same deal.

OK, more info. I just put the output of the antenna to my scope. Not a pretty waveform. Distorted and look like the wave was cut in half going from left to right (with the right half missing).

[Sandbagger]

Sept 6, 2010 19:12:40 GMT -5 @vinnies said:

OK, Dave. I made progress I purchased a small audio amplifier from Radio Shack to trace the audio stages. Works well. I went thru the signal tracing you recommended, and noticed something.

At the beginning of this thread, I had no swing or modulation, and about 4.5 DK.

Now, with a 1 KHz tone at the mic input, I get 100% modulation. Here is the kicker. If I whistle or talk, I get nothing. No swing or modulation. However, the high amplitude signal of the audio generator seems to flip a "switch". I was also able to use the variable and hit that exact point where I get the modulation.

I am pretty sure that before I changed the caps, the tone did not move the needle at all. However this time, it moves the needle.

In essence, I get modulation when I use a high amplitude audio signal, and none if I use a mic. I did try several mics. Same deal.

OK, more info. I just put the output of the antenna to my scope. Not a pretty waveform. Distorted and look like the wave was cut in half going from left to right (with the right half missing).

Ok, a couple of things.......

First, were you able to verify whether you have normal modulation/output power on SSB with the normal microphone? This is an important step. If SSB is good, then it eliminates a whole bunch of parts.

Secondly, your signal generator is putting out far more level than you need to drive a mic circuit. A normal level would be in the 30-100mV range. That generator, if it's like mine, runs up to 5V of output. You are probably "blasting through" the mic stages and on to the modulator.

I remember you said that you replaced TR32 because it was cut. Maybe you should remove it temporarily, until you troubleshoot the rest of the audio problems. It's possible that due to past modifications and/or additional problems, that the limiter is turned hard on and it's shunting your mic audio to ground.

Lastly, Unless your scope is designed to be used as a transmitter monitor, you shouldn't feed your radio directly into it. The impedence of most scopes is very high, which will present a bad SWR to the radio. Some scopes have a 50 ohm setting, but the power of a transmitter will likely fry the load resistor. Normally what you do to see the modulation waveform is to make a "sniffer box". 2 PL-259 connectors, with a piece of buss wire between them, and a 1 or 2pf cap from the buss wire to a 3rd connector (usually a BNC) that you run to the scope.

[Deleted]

I use an RF sampler when I connect to a scope. That is how I adjust modulation.

[Deleted]

Dave,

Is this the same radi as the Undien Washington? I have a SAMS for the Washington. It would be much easier to use the schematic.

[Sandbagger]

Sept 20, 2010 7:46:46 GMT -5 @vinnies said:

Dave,

Is this the same radi as the Undien Washington? I have a SAMS for the Washington. It would be much easier to use the schematic.

Depends on which version of the Washington. If it's the "late" version with the MB8719 PLL chip, then yes, it's the same. The "early" version of the Washington used the 858 PLL board.