Cobra 142GTL recap

[Handy Andy]

600W? Well, er thats a bit of overkill...LOL!

No, you just need something that will "crowbar" and keep the MB3756 from getting spiked by surges - the surges from Power line, reverse polarity as well as inadvertent overvolt from an external DC power supply.

Please note that CURRENTLY - (SIC) you don't have any true protection from any regulation issues caused by surges in the AC power supply side, only the Bridge rectifier, the DC input from External - else you have no protection from failed regulation in the Power supply itself.

If something were to happen, you have little protection from it once it's out of the power supply and on thru the power switch. After that - it's up to the protection parts that TSB describes, to protect itself.

The Fuse gives the radio the chance to survive if something power-supply wise, ever went wrong

 

[HighPlainsDrifter$]

OK I bought a resist-O-cabinet a few years back (I restored a few tube radios) which was full of all kinds of stuff, everything was labeled and both zener values were in there. I don't know any history of the previous owner but that's whats on hand, trying to use the thing I have available. The fuse addition was simple took a Cobra DC power cable and pulled the red lead (with the fuse in it I hate sloppy pieced together wiring) and replaced the red wire from the AC/DC switch to the radio main power switch. Next I will look at replacing the bias diodes I dont have any mA's on side band I do have 20mA on AM

 

[Handy Andy]

Keep us posted.

 

[HighPlainsDrifter$]

I do have a couple quick questions before I head out into the world today, first after going over the alignment instructions I see a point where I am supposed to connect the oscilloscope to the left leg of R193 but when I look at R193 it is in there backwards to the top view xray of the board, so I would be connecting to the right leg of the resistor, I don't know if this matters, but my question is will I be on the wrong side of the resistance and should I pull the resistor and turn it around, I am waiting on oscilloscope probe so I have time to do this if need be. Second in reading what I can find about this radio I found that I needed to change TR34 from a 1419 to an NTE-152, which I have done, but there is also a 1419 in the power supply, should that be changed out as well, I bought 2 seeing how there were 2 on the BOM. I appreciate all of your guidance and instruction (not to mention your patience). Thank You

 

[Handy Andy]

Good question!

Yes it does, if I remember correctly R193 "straddles" TR41 - the AM Regulator output.

​

The 220 ohm resistor provides a pull up "idle current" to help the Main Pass Transistor/Regulator operate without having to suffer from being made totally dependent on straight LINEAR voltage swing operation.

The thing (Radio; e.g - All Parts Involved) will last longer if you keep R193 in there - at least if you run it at stock (or less) Power levels.

 

[HighPlainsDrifter$]

I think I asked the question incorrectly, in my case R193 is called out in the BOM as 10Kohms and TR41 is 2SC1969-B I have confirmed that R193 is in fact a 10Kohm resistor and Tr41 is in fact a 2SC1969, so the BOM and the radio are a match. But after tracing the trace with a pencil the exposed leg of R193 has contact with the 1969 emitter, so yes the connection would be correct but the components are of different values and the connection will be on the right not the left...... I hope I am on the right track, I have very little experience with printed circuits but I cant imagine what this radio would be like to work on with point to point wiring.

 

[Handy Andy]

Sorry, thought you mean this part...R185...by description would fit A PART ACROSS another part, ahemn...

Then realized you're looking at 193
Which references R101 - Cobra 148
sorry about that - different board too..
You need info
about a PLL issue
.
​

That stem is correct...

To aid you in this... think about the "Buffering" and isolation effect - similar to the R193 on the other board, if you pull signal from the wrong lead to check, you get the wrong information on the scope.

So even though the "resistor" looks to be reversed, someone at the shop that worked on the assembly was kind enough to leave you the Test post.

If this had been a real "Factory manufacture" board, the TP would have been a Bare lead of the 10K resistor - and for them to get the tuning right, would have to be SPECIFIC-WAY of insertion and would require the QC guy to test for, locate and use TP10 to verify this chassis was correct before the radio went out.

In this board, the silkscreen is correctly set for orientation.

For R193 in the above case, test the wrong lead "side" and you'd also see 8V power all the time and it would wash out the signal you're looking for - that is if you tested on the wrong lead - so even as Radio Shops go, and Factory workers do too, need to set up the radio and the parts the right way in order to ever get a signal.

Nowadays, you are extremely lucky to even "trust" the radios tuning thru the bare lead of the Resistor.

I have had radios arrive to be fixed - simply due to a broken TP resistor or cap or the wire the "loop" is - the guy broke the part in two so all you had to do is install another and the radios' perfect - makes a guy like me feel guilty to charge someone for doing the job, being that I'm not really sure who did the damage

You're doing great - I'll hide in the bushes...

 

[HighPlainsDrifter$]

OH Dude .... you dont have to hide in the bushes .... LOL. Like I said I think I asked the question in the wrong way. Post #19 was actually 2 questions, 2 things I was thinking about with morning coffee, it's actually MY bad. I should have addressed 1 topic at a time, but sometimes my mind is on more than just 1 .... most of the time I think. The alignment info I have concerning TP10 I got from radiomods, (Australia I believe) maybe right is left in the land downunder .... LOL. The info about 1419 being changed for an NTE-152 came from CB tuneup Manual 1, according to my notes, not sure where I came across that, but I wrote that much down. Like I had said before, I have very little experience with printed circuits, heck changing the electrolytics and the tantalum's was quite a challenge, lota stuff in a small area. When finish I want to make sure I test from the correct locations. Spring Sprung so outdoor work is starting to pickup. I appreciate your help Thanks

I think things are starting to make a little more sense to me here

 

[Handy Andy]

The 1419 and NPN Linear Switch - and later revised to a 1012 - as a simple PNP Audio Amp - both are part of a Darling design the Radio maker did up to drive - impose - one signal onto another signal.

In this case, a simple DC straight voltage and combine and amplify an Audio Signal onto the line - At The Same Time...

Not as easy as it looks...

Which is why they went with a Darlington design - one transistor takes in the DC voltage from the AM Power (Carrier) to act as a "Center point" in which another signal can swing it's power levels around - and still provide a DC level current output and an Audio Signal - CENTERED on the DC Bias the transistor is set for.

Two things are happening in the one transistor...

A operation point is set by a trickle power used to turn on and set the transistor into a Class A - low power mode, then an audio signal is sent into the same lead, at a given power level that the Transistor works with - this AC signal - floats at the input level of the DC Bias voltage - so the up and downs are set to work with the main voltage present The Bias - as a STABILIZATION region - the radios "Ground Zero" for the Carrier and Audio signal being mixed together.

This gives the next component - a high power linear switch amp - a mixed signal of this Bais and Audio Signal to then carry this still too weak to be used, signal and impose this signal - the DC Bias and Audio - onto a higher voltage - heavy current source to provide this power to the Driver and Final.

A secondary event also is taking place.

The output power of this circuit to maintain the "center point" has to be able to sense the output as a measure of power and means to restore order by locating - stabilizing to - and working with - the output power of the main amplifier in the Darlington in a way to provide Feedback and error correction control to restore DC bias and the "Center point" and also sense and respond to the Audio Signal to correct itself from too much drive or if DC Bias rises and Falls below a set level of time predetermined by a set of filters, one being the sense lead of the output, the other from a series pass circuit used as a Gain cell - between these two filters a level of power is detected, and used - applied back into the Darlington pair - as a form of control, or REGULATION.

I mention filters because one filter is taking in power by tapping the output back into and to be used by the other filter as a means to control the Audios Envelope - or bandwidth - as a process of regulation.

You can think of it as a graph - your typical chart - but you need to extrapolate or step out of the typical 2 Dimensional graphic and look into this a 3D event, as more than one type of power is being processed.

Audio signal can be thought of as a spectrum above Zero - Zero being no voltage - yet is a power in itself as a AC signal - having an inherited trait being energy but can have positive and negative swings that cross over a Zero level - yet are above Zero as a level of reference.

AC being what it is, can be thought of Alternating Current - but also you can see this as a different type of reference - using mixing with DC - being Direct Current or a steady voltage.

So how do you place AC and DC together without changing both - you can't avoid the change but what you can do is compensate for the two values by restricting them into a circuit that then can be detected then amplified.

This is what loops back to your comments about in you check the part for a signal but use the wrong side - you will get the wrong information you're looking for.

 

[HighPlainsDrifter$]

Amazing what can be done with these little devises. What purpose does the 1419 serve in the power supply, I would imagine a regulator as well. If the 1419 was found to be unreliable in the AM circuit ... is it also unreliable in the power supply? Would seem like it to me.

 

[Handy Andy]

The 1419 was easier to utilize, because of it being cheaper...

Most of the time, the 142 would poke along consuming idle current between 600mA to maybe 1A in RX.

When in TX, maybe 2 Amps...

Note the use of a 2SC945 NPN used to drive another NPN - just cheaper for them.

The 1419 is rated to 3 amps (more like 5 but read this thru first then you'll know why - it's De-Rated) - doesn't mean it can deliver it.

Locate TR401 - it may be bolted to a large plate on the power supply module - wired in. Cobra 29's use a similar for Regulation of 8 volts main PCB for RX/TX and a step down to help with PLL Regulation that taps this for 5V. Also uses a 2SD188 NPN type only the Package was changed to protect the innocent.

TO-220

(2SD880 Y)
Used in Cobra 29's and PC-78
Even 25's and 66'/68 series...
​

That is your Regulators' Pass Transistor - it would be more like a larger TO-220 or a TOP-Hat (3055) onto a TO-247 SQUARE pack - the linearity problem here is how to dissipate the 18 or so volts the output of the Transformer and Bridge Rectifier and cap system - down to the 13.8V or so the Radio runs on (Expects).- so that can place several watts of heat extra on the part just to drop to the expected, and Regulated...Idle RX current is one thing - TX Demands as amperage needed is another.

But you SHOULD have a 3rd transistor TR401 - they use a 2SD188 - that is your actual pass transistor Rated more for 10 amps but that handles the ENTIRE chain and the rippled output of the Bridge Rectifier and Cap combo....

May look like this...

​

You can use an NTE 36 (NPN) if needed​

Look for it, for if you don't have it - TR401, you may have some extra work to do.

 

[Handy Andy]

I realize I jumped ahead of myself for I wanted to point out something that you had asked earlier - that I needed to address as an upgrade - forgot to mention that...

A secondary event also is taking place.

The output power of this circuit to maintain the "center point" has to be able to sense the output as a measure of power and means to restore order by locating - stabilizing to - and working with - the output power of the main amplifier in the Darlington in a way to provide Feedback and error correction control to restore DC bias and the "Center point" and also sense and respond to the Audio Signal to correct itself from too much drive or if DC Bias rises and Falls below a set level of time predetermined by a set of filters, one being the sense lead of the output, the other from a series pass circuit used as a Gain cell - between these two filters a level of power is detected, and used - applied back into the Darlington pair - as a form of control, or REGULATION.

I mention filters because one filter is taking in power by tapping the output back into and to be used by the other filter as a means to control the Audios Envelope - or bandwidth - as a process of regulation.

What they do in the 142 and 2000/148 lines use a AMC/ALC together at the mic amp to control the VOLUME of audio - doesn't do much for the development and center point for recovery does it?

That's where I jumped ahead...

Your typical Cobra 2000/148/142 AM output - is not truly regulated, more like USES regulated voltage as that bias set point for Carrier - the only feedback being the AMC/ALC stuff using R166 and R165 as a divider combo to sense a pre-determined mid-point and then Limit the modulation using this method.

I jumped ahead because other radios now use a Sense circuit in AM Regulator section - using the output of the regulator as the means to set and regulate itself. Not depending and or dependent on the 8 volt regulation signal sent to the TR42 to be mixed with Audio and amplified to swing current at TR41. It's still there to set mid point, But the LEVEL of audio drive, it's gain is set to a specific level - which; in the Case of the 2000/148/142 - it is not - you can throw several watts of audio to swing the current output at TR41 using the input at TR42 to do this.

Just review this thread...
https://www.worldwidedx.com/threads/over-130-modulation.163831/page-2#post-457911

Speaks volumes more about it than I can...

The part about the 220 ohm resistor then becomes more clear, as it' really is not needed, but it is there to offset a linearity issue that TR42/TR41 have regarding dissipation and clipping - they felt at the time it was more important to pre-set a power level across a known load, across the Pass transistor as a method to keep the Driver and Final more Forward Biased - then they found the Regulation method used and is common nowadays - it is not used in the 2000/148/142 series.

Compare this schematic to the one about 4 posts ago...

​

But this concept of self-centering AM Regulation that was incorporated into the 146GTL - was their last one of their last truly USA designed and FCC type accepted radios - none of their other radios produced since then ever used the AM Regulation design like the 146GTL - the 148/2000 series is their last effort at what the Link above describes.

Only Galaxy and RCI have taken the AM Regulator section a few revisions and upgrades since then...

EDITs:

Updated Graphic Blandishment​

 

[HighPlainsDrifter$]

This is the power supply in question, from what I read it is intact. Lots to digest I will have to read it over again.

 

[Handy Andy]

Power supply appears to have all the parts you need. The 1419 is the TO-220 and the TR401 is your 2SD188 on the rear panel...pretty much stock so far.

How is the Voltage?

 

[HighPlainsDrifter$]

Using chassis as ground the red power wire at the main switch says 13.83V