TDA2003 amp getting hot

[Robb]

Working on a Cobra 25LTD Classic - fairly new radio in xlnt condition. TDA2003 audio amp getting hot the longer I leave it on. Replaced it once so far, because the first one failed outright. Put a scope on the audio output and turned it on and saw a steady oscillation around 7.4mhz. Don't often work on AM radios; never seen this before.

So is this a common issue?
Known faults?

Thought the GURUS would know the answer to this one.

Thanks for any input!

 

[Robb]

Are there any Cobra schematics that use the TDA2003?
Any clue as to why I would see that oscillation?
Ant direct reason for the heat?

Signed,
up a creek; no paddle.

 

[LeapFrog]

Are there any Cobra schematics that use the TDA2003?
Any clue as to why I would see that oscillation?
Ant direct reason for the heat?

Signed,
up a creek; no paddle.

Before "downloads.cobra.com" quit working I believe the 25 LX schematic was available (also 29 LX). I have a copy saved but cannot access it a.t.m.

Not the same I.C. but still relevant;
I've seen more than a few TA7222ap audio chips produce self-oscillation (and subsequently heat) when powered on.

After I recapped the radios and replaced the I.C. the problem whent away. I think failing caps made some internal damage and caused the chip to act this way.

If memory serves correct, one time I found a ground loop (previously installed echo board) had caused this symptom.

Edit:
Over driving the input of an audio I.C. could cause damage (and this issue).

 

[LeapFrog]

Here is the 29 LX schematic; i hope it helps brother.

 

[Handy Andy]

Found some 25 TDA2003 - for Serial Numbers starting with "U"...

Also for the Youda Chip is in this post too - in case more questions come up...

As far as the "Squeal" which is the self oscillation, I'd look at LOWERING the value of C67 from 1000uF to a 470uF 35V rating.

There is also an old trick they used to keep the Reactance Modulator from meltdown - instead they dropped the higher frequencies thru a 1 to 2.2 ohm resistor thru a 0.1uF - which in this radio is R118 (1 ohm) and TWO (One is C61) 0.1uF (C154 - may or may not be there they're SMD) - now considering the LOAD across the resistor for those Caps? It may have blown - up its wattage rating to a 1/2W or even a 1Watt 1.8 ohm if you can squeeze it in there to help with the loss of uF from C67 (From 1000uF to 470uF)- but obtained with the switch over to 470uF.

Originally - the load across the smaller 0.1uF caps and the resistor usually opened up causing that feedback "loop" squeal - you could tell when this occurred because not only did it get hot - it was a little too "hot" in audio too.

PDF's attached...

 

[500kpot]

Is the 29/25 audio transformer a true reactance modulator?

Thanks in advance for any information.

 

[Handy Andy]

I used to call it a Modulation Transformer - until I got jumped on for it but I'm willing to call it as I see it, which really is a Modulation Transformer...

The Stub Article in Wiki
versus

The Source of this definition
​

Here's why...

In the older days, Plate Modulation requires several known elements - two main ones being

Radio Frequency (your channel/QSO Frequency/Simplex)
and
Audio Frequency - your voice or other Audio frequency signal...

A better more elaborate synopsis of the above can be found at the link - by Clicking Here

You add one to the other - changes the output.- you have gone from a simple Carrier onto a complex form of Carrier and Audio information embedded in a range of frequencies in and around your Carrier frequency.

• If you look at the above, we use a means to mix audio into a power source to feed Collectors of two transistors in a Transmitter - in this case the Driver and Final in that Cobra 29.
• The other signal - arrives to the BASE of the driver from the Radios Pre-driver that supplies only the RF signal - the UNMODULATED Carrier

So if you'd like to start the conversation - feel free, I've tried to help others before and got jumped for the mere terminology misnomers that exist between the two effects...

One is a part, a modified choke

The other...

Is the process that Choke plays a role in...

So it the process of COVERTING the two terms, this seem to get mixed up - but to help you - in this case the process (going on inside the radio) is the one I was talking about

• - Robb is getting a signal from somewhere in the radio making the Transformer that does it's share in this process - really hot - like a self-oscillation
• which now we have to figure out if it's in the Audio amp circuit or a combination of other factors including Neutralization that has somehow failed.

It's not a "True" in the part is a self contained procees, no, the modified Choke it is part of the process - that in itself is a Modulation Transformer - the Cobra 29 in it's Class D certification - uses the Reactance Modulation as a Process to work as AM radio Transceiver

 

[500kpot]

Okay cool. Thanks for the info. I noticed in the first graphic the output was going to an oscillator not the RF so I was a bit confused on that till I read a bit more.

What isn't (or at least I haven't found the answer to) apparent is how much inductance it would take to have a true reactance modulator for a transistorized
set as opposed to a tube set. Logic tells me that the amount required for a low impedance device would be much greater than is required for the higher impedance of the tube. Like way more to where it may be impractical.

The reason for the initial question was because I was looking at how to integrate a true reactance modulator in to my 29. So when you mentioned the "reactance modulator" I thought I was beat to the finish line. But I see now it's not a true version.

To me, the stock 29 modulation transformer, schematically, it appears to be a simple step-up transformer - I'm not saying I'm right.

So if you'd like to start the conversation - feel free, I've tried to help others before and got jumped for the mere terminology misnomers that exist between the two effects...

I'm not looking to "jumped" anyone here or anyone for that fact.

Thanks for the explanation

Best Regards

 

[Handy Andy]

There's a lot to be said about this process that many are not seeing...

The reason for the initial question was because I was looking at how to integrate a true reactance modulator in to my 29. So when you mentioned the "reactance modulator" I thought I was beat to the finish line. But I see now it's not a true version.

To me, the stock 29 modulation transformer, schematically, it appears to be a simple step-up transformer - I'm not saying I'm right.

Now you did touch on something that many newer radios - like those from President, Richard, and Walker - use an AM regulator design but impose the Audio onto it - much like you said with the Reactance Modulator term.

But it is confusing in terms because you look at the means to MODULATE the Regulated carrier (your AM regulator) so they use a pre-set voltage and mix that in with an AF signal - but the two are mixed at the BASE of the transistor - so we no have the AF and Bias on the base, but use the OUTPUT of the PASS Transistor as the power SOURCE for this to occur.

• Where this even gets more intriguing is WHERE you send the OUTPUT - of this small amp. It's sent to the Input side of a Darlington Amplifier (2 stage).
• In a way we use the FEEDBACK principle to provide the method of your Reactance - as the Bias and AF drive the Darlington Pair - the output of that Darlington amp rises and falls - DYNAMICALLY - in two ways, both in the DC realm, and in AF realm - at the same time.

So as Audio peaks affect the Darlington - it affects the power sourcing of the Mixing Transistor - but not the BASE region - it's sill has steady DC and the varying AF signal.

So the AM Regulator - can be defined as your "Reactance Modulator" as the device but the process occurs in the TX strip with the TX Mixer, Buffer and Pre-driver amps sending the Carrier RF signal to the Base of the Driver but we send this newly converted signal to the Collectors of the Driver and Final.

There are several AWESOME threads about this....

https://www.worldwidedx.com/threads/over-130-modulation.163831/page-4#post-458121
https://www.worldwidedx.com/threads/asymod.246057/#post-682941

You were even involved in this one too...
https://www.worldwidedx.com/threads/realistic-trc-427-audio-mod-goldfinger.237928/page-2#post-655208

So the thoughts are there, just the way Cobra, Uniden, Realistic and many others already used the "Tapped Power Choke" as the method to modulate the Collectors to a point where it's Class D and the Moniker "Plate Modulation" and Reactance Modulator somehow get thrown into this interchangeably.

So in a basic answer to your question - the 1/2 supply rail into a capacitor before it's applied to the Choke - we avoid the hassles of pre-existing conditions and support requirements of the Bias supply regulation used in an AM regulator and simply bring in the full voltage the Collectors need and let the power swings occur in the Modulation Transformer. The limitations are Phase effects, the distortion products of this conversion of treble tones and the roll-off from Hi- to Low within the Choke - altering the audio in both Phase and Frequency Bandwidth - narrowing Bandwidth and generating a Phase shift from this - and although we'd have FM or a good portion of it, we still have the envelope products of power peaking and toughing throughout the process - so we throw it thru a Diode and that changes the ballgame and help remove many of the opposing phase products and allows for the positive portions of this transformation process to pass into the collectors.

So again, the Modulation Transformer is only a Component of the process, we now introduced the Capacitor and Diode as the other two.

 

[500kpot]

I have played with the AM regulator. It was quite a learning experience, to say the least.

So the AM Regulator - can be defined as your "Reactance Modulator" as the device but the process occurs in the TX strip with the TX Mixer, Buffer and Pre-driver amps sending the Carrier RF signal to the Base of the Driver but we send this newly converted signal to the Collectors of the Driver and Final.

Yes, that is how I was thinking about that as well. It make perfect sense.

Personally myself when I think of reactance modulation I think of this


*This image is from _http://www.amfone.net/AMPX/102fig3.gif

And what threw me when it comes to the 25/29

• The primary of the 29 transformer doesn't power the audio amp
• I may not be saying this right but the secondary of the 29 transformer isn't by- passed to ground. It seems the secondary is just the "other half" of the coil.
  
• There isn't a choke (again I may not be saying that right) connected to the secondary with the B+ at the other end of the choke.

"Plate Modulation" and Reactance Modulator somehow get thrown into this interchangeably.

Sometimes the misuse of the two can be quite confusing at times.

I personally like modulation transformers over AM regulators but the transformer in the 29 not a very good one pertaining to perhaps not hifi audio but a bit more bottom end bandwidth would be nice.

Thanks for your time to type all that out. I appreciate it.

Best

 

[Handy Andy]

Ah hold on...I may have the "other half" to that question...

The TDA2003 Pin 5 as you know, supplies the main power for the amp to use, but Pin 4 is output...

​

That is all tied back to Pin 2 and R7 the Amps internal Gain resistor - which if left alone would be the entire gain of the circuit - but it uses the EQ and Resistor divider network to offset Pin2's ability to amplify - as in you don't give it as much gain ability as you would for Pin 1 else you wind up with a comparator - not an amplifier.

• It just supplies power offset (as a reactive dynamic changing element of the output of a filter) as a means to control the power available for Pin 1 to drop for a factor of gain. (A negative feedback principle)

​

As a side note, at CB Tricks there used to be a Battle between Bill and CB Doc over the aspects of the Cobra 29's Mod Transformer (to shorten it) and there was CB Docs' Modulator effort to work on an NPC mod for the 29 - only his was requiring the very device you are talking about which makes this impractical being that the better effects are achieved usingTube versus Solid state.

Bill countered this by saying that the Modulation Transformer, even with it's limits for Bandwidth - provided the "oomph" that make the Modulation Transformer approach to this a desired trait over the simple conversion across the Battery rail supply and ground of a transistor driving the load.

​

Doc just wanted a Modulator - similar to what was discussed in the threads listed above, for the purpose of replacing the Mod transformer with that is used in the 148/Grand XL Outputs from their Audio Amps for AM modes - that 1419 / 945 stuff...

Anyhoo...gotta' go get Dinner done...

Happy Wife Happy Life - Don't forget that...

 

[500kpot]

Thanks for the breakdown on the TDA2003. I'm familiar with opamps and understand how to set gain and so forth. C44 is an interesting add as this part isn't shown as required on the mfg. spec sheet. Again, I could be wrong but it seems that is part of the EQ as well.

Bill countered this by saying that the Modulation Transformer even with it's limits for Bandwidth - provided the "oomph"

Transformer oomph is word left to interpretation. Is it hifi, mid-fi, or communication style audio. Is it 100% + modulation? Of all the youtube videos I've watched, I've never seen a 29 go over 100% with out massive distortion. Appears to be worse with the TIP120 swinger mod.

On the other side of the coin the darlington mod offers a ton more available audio bandwidth. I've personally witnessed a 29 modulating a 2hz sinewave. It's not cost prohibitive but takes a medium skill level to make work. Unfortunately the extra heat is something that needs tending to.

Happy Wife Happy Life - Don't forget that...

I'm super lucky as my GF supports my radio enthusiasm. Her father and uncle are hard core hams so she grew up around radio - just not CB. We met in high school in AP Algebra class a few years ago. Her math skills are super even rivaling, dare I say mine. I'll never admit it to her thou. I've been attending monthly meetings at my local ham club (with my GF).

Thanks again for all your education.

 

[Handy Andy]

I wish you the best!

@Robb - you may have your answer with C44 if it's even in there...

Which I've asked that too...

Check Foil side for evidence of "Heat and scorching "
​

EDIT:
Adding more:

Just wanted to pass along the "C44" is the part they just decided to put in place of the 1 ohm and a disc cap of various values between 0.039uF (393) on up to as high a 0.47uF (474) even in Electrolytic polarized values.

This Cap is what I mentioned earlier in the R114 and C61 filter section. They used a resistor then they dropped in and now just use a straight cap in it's place of the two component filter.

This is a hi-cut filter that rolls off the higher tones before it enters into all the inductance on the other side of the Transformer.

 

[Robb]

I haven't had a chance to get back on the radio as yet. But I do have to.

I did not see any bodged in smd cap on the board as yet, and certainly not on the audio IC.

Oscillations occur - check me if I am wrong - when there is sufficient inductance and capacitance to create a resonant freq. Again, I am just a beginner. But it seems to me that a resonant point has been reached in either the audio circuit itself; or an oscillation is being fed to it from a point upstream of the audio IC. Dunno if it happens downstream; figured you could tell me that.

But more importantly, is there a known problem area with this tda2003 or this radio that creates an oscillation? Otherwise, it makes no sense to me why the audio IC would heat up - at all.

Isn't there a filter network in the radio to keep oscillations from reaching the audio IC? I would think that there must be a cap to filter off oscillations or noise to ground - that would otherwise trigger oscillations.

 

[Handy Andy]

It a shame we can't get this problem solved easily. Seems there is something wrong right from the start.

But to know what is wrong requires some comparisons to previous methods and what they did to "interface" the TDA2003 to take the place of a older 7222AP chip - with more pins.

So to me, that oscillation - is a feedback loop THAT IS MISSING - so I'm looking for a schematic that fits that board but right now what I've given you is all I got.

If you can take pics of the Top and Foil side of that Audio Chip we can reverse engineer the design and help you make the right choice, or changes, or repairs - to make the oscillation stop.

I'm going to be doing a restart, so there are other articles I've been working on and since I've been seeing some of my posts disappear - not sure why - better put something here to make someone rethink in deletion or at least the ones that may need this info - get it while its here before some one or some thing takes it away...