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The loudest AM "CB" chassis ever? EPT3000-10A
- Thread starter loosecannon
- Start date
LC,
It's time you took control of that radio.
You need to seek out why that thing overheats,
One is the impedance the thing expects...the TDA's need a 2 ohm or greater impedance ohmic wind type of stuff.
You may need to pull that transformer and locate winds that give you 2 ohms or greater ohmic resistance so that the TDA's can do their job, they will overheat when their driven into loads shorted out - and 0.16 ohms is pretty much a dead short - so what is going on in there , you need to pull that T1 and locate the pins you need to drive it.
Second, once you've located the winds - you may need to pin-for-pin on the transformer to make the TDAs' to drive it properly.
We see on the schematic that It's individual winds so are they a step-up or step-down or simple isolation.
Time to take charge of that thing man...
It's time you took control of that radio.
You need to seek out why that thing overheats,
One is the impedance the thing expects...the TDA's need a 2 ohm or greater impedance ohmic wind type of stuff.
You may need to pull that transformer and locate winds that give you 2 ohms or greater ohmic resistance so that the TDA's can do their job, they will overheat when their driven into loads shorted out - and 0.16 ohms is pretty much a dead short - so what is going on in there , you need to pull that T1 and locate the pins you need to drive it.
Second, once you've located the winds - you may need to pin-for-pin on the transformer to make the TDAs' to drive it properly.
We see on the schematic that It's individual winds so are they a step-up or step-down or simple isolation.
Time to take charge of that thing man...
thanks for all the help Andy.
I have already decided that it is absolutely an issue with the transformer.
It's really the only thing it could be at this point.
What really turned the lightbulb on in my head was when you compared the output of the TDA's to a speaker voice coil.
that's when i realized that this amp acts exactly like a car stereo amp when a driver has shorted.
so the transformer is going to come apart and get inspected, and probably re-wound.
I probably won't get to that for another week or so, as money work has shown up.
I really appreciate all the insight and have learned quite a bit about how this thing ticks in the process.
I will be sure to post back with my success or failure.
LC
I have already decided that it is absolutely an issue with the transformer.
It's really the only thing it could be at this point.
What really turned the lightbulb on in my head was when you compared the output of the TDA's to a speaker voice coil.
that's when i realized that this amp acts exactly like a car stereo amp when a driver has shorted.
so the transformer is going to come apart and get inspected, and probably re-wound.
I probably won't get to that for another week or so, as money work has shown up.
I really appreciate all the insight and have learned quite a bit about how this thing ticks in the process.
I will be sure to post back with my success or failure.
LC
Hey LC,
Sorry if the above post looks a little "strong" - I was frustrated at what you are up against.
The radio is really a neat little piece of work , but what would make it AWESOME would be, if all this effort you put into it and it started to work.
What is sad, is - it isn't - yet. I haven't given up, just this project has been as hard on me as any other ones I've worked with you on. Even that little Cobra 19 LTD - the one you converted to MOSFET seems basic compared to this guy, and the problem is even simpler - just that audio amp in the darn thing - wonder if - and I know a CB is capable of it, because that's why the use spike diodes for it - to prevent the arcs' for the main reason if that is your problem right now. Perforation is my main premise of concern here...
I know that for years Cobra produced AM-only radios with continuous wind multi-tap transformers that if you had the ability to take apart, you can re-tap and re-wind the core winds and re-install and get something far different as a result. Now they've pared that down to a simpler design and took away all our fun...
I hope you can find the issue and resolve it and get that thing working again like before - I wish you all the best.
Have a good Labor Day weekend - thanks for letting me in here to post stuff, and it is my hope that in doing so, allowed me to help you get your head around this beast - it's not a monster - but it's a troublesome one.
Sorry if the above post looks a little "strong" - I was frustrated at what you are up against.
The radio is really a neat little piece of work , but what would make it AWESOME would be, if all this effort you put into it and it started to work.
What is sad, is - it isn't - yet. I haven't given up, just this project has been as hard on me as any other ones I've worked with you on. Even that little Cobra 19 LTD - the one you converted to MOSFET seems basic compared to this guy, and the problem is even simpler - just that audio amp in the darn thing - wonder if - and I know a CB is capable of it, because that's why the use spike diodes for it - to prevent the arcs' for the main reason if that is your problem right now. Perforation is my main premise of concern here...
I know that for years Cobra produced AM-only radios with continuous wind multi-tap transformers that if you had the ability to take apart, you can re-tap and re-wind the core winds and re-install and get something far different as a result. Now they've pared that down to a simpler design and took away all our fun...
I hope you can find the issue and resolve it and get that thing working again like before - I wish you all the best.
Have a good Labor Day weekend - thanks for letting me in here to post stuff, and it is my hope that in doing so, allowed me to help you get your head around this beast - it's not a monster - but it's a troublesome one.
thanks Andy.
The way i read your comment was, "boy i sure hope you get that radio working, but until you get to the bottom of that transformer issue, there isn't much more i can do to help at this end."
You and i have been corresponding for long enough that i feel like i have a pretty good sense of the kind of guy you are. (that's meant in the good way LOL)
I totally forgot about that 19LTD project. It ended up going to my dad, who promptly blew the audio amp out of it telling everyone who would listen how proud he was of the radio his son built for him LOL.
I had to give him my 21LTD which was supposed to be my main mobile, but knowing it was strong as an ox and twice as durable, it was the right radio to give him.
I have really enjoyed learning about this radio, and because of what we are doing, there is now at least one place on the interwebs where someone can go if they have problems with one of these. (somehow im getting the impression that i have the only one still in existence LOL)
The added bonus is that i now know that the French word for alignment is alignement from reading through Dirland service manuals LOL.
more pics and a cheap and dirty alignment sheet are on the way!
LC
The way i read your comment was, "boy i sure hope you get that radio working, but until you get to the bottom of that transformer issue, there isn't much more i can do to help at this end."
You and i have been corresponding for long enough that i feel like i have a pretty good sense of the kind of guy you are. (that's meant in the good way LOL)
I totally forgot about that 19LTD project. It ended up going to my dad, who promptly blew the audio amp out of it telling everyone who would listen how proud he was of the radio his son built for him LOL.
I had to give him my 21LTD which was supposed to be my main mobile, but knowing it was strong as an ox and twice as durable, it was the right radio to give him.
I have really enjoyed learning about this radio, and because of what we are doing, there is now at least one place on the interwebs where someone can go if they have problems with one of these. (somehow im getting the impression that i have the only one still in existence LOL)
The added bonus is that i now know that the French word for alignment is alignement from reading through Dirland service manuals LOL.
more pics and a cheap and dirty alignment sheet are on the way!
LC
I am going to start putting little notes about this chassis in this thread as i find them.
Since i am at work and can't take the transformer apart yet, i decided to do a little math in order to get some specs on this bridged audio amplifier circuit.
the first thing i did was try to figure the frequency of the low pass filter which consists of R151 (1 ohm) and C137 (.22uf).
This gives me a frequency response of 72khz. what the heck? why on earth would they set that so high? seems weird to me.
LC
Since i am at work and can't take the transformer apart yet, i decided to do a little math in order to get some specs on this bridged audio amplifier circuit.
the first thing i did was try to figure the frequency of the low pass filter which consists of R151 (1 ohm) and C137 (.22uf).
This gives me a frequency response of 72khz. what the heck? why on earth would they set that so high? seems weird to me.
LC
Yes it does, but there is more, the ohmic response at Audio - run the values thru for 1kHz to come up with the ohmic result - lots' more.
The coulomb charge "capacity" at the higher REASONATE frequency is different (Greater) - you don't want that cycling thru the amp as a unit. So it's supposed to bypass it. (lowest ohmic value means the more of the power at that frequency is diverted)
Remember the envelope issue - a range of low frequency to high frequency - think of it as heterodyne only you make room in the lower frequency waves, for squeezing in the higher audio frequencies.
And also - a 2 to 4 ohm speaker coil has CONSIDERABLY more inductance which affects the bandpass using the L at mili-Henries not MICRO-Henries.
The Higher frequencies trying to enter into that coil will encounter a considerable amount of reactance - so they are trying to use that T1 as a FIXED inductor - so they need a path to remove the otherwise parasitic high-frequency oscillations and smooth out the waveform going into the transformer so the losses are less going thru it,
Below is a crude example of what I'm talking about...
The graphic is only trying to show SPECTRUM as a range of power - if you go back to the CB tricks days, I also talked about PHASE angle inclusive in the Asymmetrical Mods thread... you can think of the above as a indication of the DISTORTION the power input will make because not all the frequencies entered into T1 will make it out alive - at least not without considerable modification...
Because Inductance and the effects of DC bias flowing thru the transformer at the same time - skews the results - it's not symmetric and we have distortion products too (Grey / pink area added in) because of that inductor changing and "borrowing" from both Audio and Bias to make this envelope - and why a narrow bandwidth is better for to reduce the distortion products to increase intelligibility as the cost of fidelity - so they use the heavy inductance of the T1 coil winds and the amps ability to have an output of a higher frequency bandwidth - so they use more "treble" to offset the "bass" the T1 will take in more readily.
DC carrier will always be there, but the envelope forms AROUND it like a sphere - so the angle of power DEVIATES from the carrier as a means of frequency to power vector - so less higher treble equates to less DEVIATION or re-vectoring (if that can be used here) of the power around the DC steady state the carrier bias is.
Why am I telling you this?
Simple , the Cobra, Uniden, Radio Shack - any AM-Only CB radio using a Reactance Modulator has a DC blocking cap that when combined with all that inductance on the other T1 used in these AM-only units - they produce a bandwidth product of power, BIAS and Audio Envelope - throw it thru a Spike Diode to remove the negative component and there you have it...
So they are making the TDA's do the push pull with enough oomph to make the envelope without introducing the DC component to it - it's supposed to be separate - but they also generate a considerable amount of EMF kickback that is a lot like a spark plug coil - induce a large current change in a small coil induces a larger voltage change in the larger wind coil of the plug side...
So that is why the Bandwidth on the amps themselves - so they don't interact and generate a larger phase issue of similar and dissimilar (a heterodyne effect of sound some people feel as a pulsation and hear as a squeal - which can cause Cardiac Arrests due to this low-level motion and it's sonic effect- SEE: Rollercoaster The Movie...) Hence the "Smashing" moniker I used as a reference to this event I posted earlier .
Have fun for Labor Day!
The coulomb charge "capacity" at the higher REASONATE frequency is different (Greater) - you don't want that cycling thru the amp as a unit. So it's supposed to bypass it. (lowest ohmic value means the more of the power at that frequency is diverted)
Remember the envelope issue - a range of low frequency to high frequency - think of it as heterodyne only you make room in the lower frequency waves, for squeezing in the higher audio frequencies.
And also - a 2 to 4 ohm speaker coil has CONSIDERABLY more inductance which affects the bandpass using the L at mili-Henries not MICRO-Henries.
The Higher frequencies trying to enter into that coil will encounter a considerable amount of reactance - so they are trying to use that T1 as a FIXED inductor - so they need a path to remove the otherwise parasitic high-frequency oscillations and smooth out the waveform going into the transformer so the losses are less going thru it,
Below is a crude example of what I'm talking about...
The graphic is only trying to show SPECTRUM as a range of power - if you go back to the CB tricks days, I also talked about PHASE angle inclusive in the Asymmetrical Mods thread... you can think of the above as a indication of the DISTORTION the power input will make because not all the frequencies entered into T1 will make it out alive - at least not without considerable modification...
Because Inductance and the effects of DC bias flowing thru the transformer at the same time - skews the results - it's not symmetric and we have distortion products too (Grey / pink area added in) because of that inductor changing and "borrowing" from both Audio and Bias to make this envelope - and why a narrow bandwidth is better for to reduce the distortion products to increase intelligibility as the cost of fidelity - so they use the heavy inductance of the T1 coil winds and the amps ability to have an output of a higher frequency bandwidth - so they use more "treble" to offset the "bass" the T1 will take in more readily.
DC carrier will always be there, but the envelope forms AROUND it like a sphere - so the angle of power DEVIATES from the carrier as a means of frequency to power vector - so less higher treble equates to less DEVIATION or re-vectoring (if that can be used here) of the power around the DC steady state the carrier bias is.
Why am I telling you this?
Simple , the Cobra, Uniden, Radio Shack - any AM-Only CB radio using a Reactance Modulator has a DC blocking cap that when combined with all that inductance on the other T1 used in these AM-only units - they produce a bandwidth product of power, BIAS and Audio Envelope - throw it thru a Spike Diode to remove the negative component and there you have it...
So they are making the TDA's do the push pull with enough oomph to make the envelope without introducing the DC component to it - it's supposed to be separate - but they also generate a considerable amount of EMF kickback that is a lot like a spark plug coil - induce a large current change in a small coil induces a larger voltage change in the larger wind coil of the plug side...
So that is why the Bandwidth on the amps themselves - so they don't interact and generate a larger phase issue of similar and dissimilar (a heterodyne effect of sound some people feel as a pulsation and hear as a squeal - which can cause Cardiac Arrests due to this low-level motion and it's sonic effect- SEE: Rollercoaster The Movie...) Hence the "Smashing" moniker I used as a reference to this event I posted earlier .
Have fun for Labor Day!
took the transformer apart today.
posting the 'specs' in case i lose my cocktail napkin/datasheet.
outermost winding is the 'voice coil' winding that is tied to the pin 4 outputs of the TDA2003's.
Consists of 45 turns of 22ga. magnet wire. measured inductance was 94 millihenries.
resistance was 0.2 ohms.
middle winding is the 'unused' winding that has one end at DC ground and the other not connected to anything.
it consists of 55 turns of 26ga. wire. measured inductance was 33 microhenries.
resistance was 0.3 ohms.
innermost winding is the voltage line going to the driver and final. This is the winding we are inducing the modulation on to.
consists of 42 turns of 22ga. wire. measured inductance was 7 millihenries.
resistance was 0.2 ohms.
have to order some magnet wire so it will be a week or so before i can re-wind it and try the measurements again.
I really don't know much about core saturation, but i do wonder if that could be the problem with this one, as i tried to inspect the wire and never found any burn marks or obvious damage.
LC
posting the 'specs' in case i lose my cocktail napkin/datasheet.
outermost winding is the 'voice coil' winding that is tied to the pin 4 outputs of the TDA2003's.
Consists of 45 turns of 22ga. magnet wire. measured inductance was 94 millihenries.
resistance was 0.2 ohms.
middle winding is the 'unused' winding that has one end at DC ground and the other not connected to anything.
it consists of 55 turns of 26ga. wire. measured inductance was 33 microhenries.
resistance was 0.3 ohms.
innermost winding is the voltage line going to the driver and final. This is the winding we are inducing the modulation on to.
consists of 42 turns of 22ga. wire. measured inductance was 7 millihenries.
resistance was 0.2 ohms.
have to order some magnet wire so it will be a week or so before i can re-wind it and try the measurements again.
I really don't know much about core saturation, but i do wonder if that could be the problem with this one, as i tried to inspect the wire and never found any burn marks or obvious damage.
LC
Were the winds (layers) separated by any sort of tape or vinyl wrap?
Else we may have just hit a wall...
Innermost winding - 7 mH?
Outer winding just 3 turns more - 94mH?
Else we may have just hit a wall...
Innermost winding - 7 mH?
Outer winding just 3 turns more - 94mH?
Andy,
when i saw the values in the milli-henries, i knew all bets were off.
I would have expected to see something in the MICRO henries or something around there.
honestly my first thought was that my meter couldn't measure them properly for some reason.
Thank you for continuing to check in with this thread. I know it's up to me to put in the work, and all the stuff im posting now is more for record keeping, so i won't mind if you decide to wait me out until i actually get something happening. (That doesn't mean i want you to stop LOL!)
somehow though, i get the impression that you won't be able to keep yourself from turning on the lights for me as i wave my arms around in this dark hallway
.
LC
when i saw the values in the milli-henries, i knew all bets were off.
I would have expected to see something in the MICRO henries or something around there.
honestly my first thought was that my meter couldn't measure them properly for some reason.
Thank you for continuing to check in with this thread. I know it's up to me to put in the work, and all the stuff im posting now is more for record keeping, so i won't mind if you decide to wait me out until i actually get something happening. (That doesn't mean i want you to stop LOL!)
somehow though, i get the impression that you won't be able to keep yourself from turning on the lights for me as i wave my arms around in this dark hallway
.LC
When in a dark hallway of your home, it's difficult at times to see your way, even though it can be a straight shot just to get thru it.
I wonder if you're thinking - what I'm thinking?
If we can't wind enough resistance onto those winds, care to try and add a resistor in series from pin 4 to their respective terminal? Like say...2 ohms? 1W? Carbon comp of course...
You were getting boxcars - so it tells me we were pushing pretty hard...
Expect a hiccup or two until we can get a value to balance out the twins.
Start out even as high as 10 ohm just to see reactions...(it would equate to 20 ohm - but a 10 ohm each side)
This method may work when it comes to "trimming" the volume and drive levels - just sub a resistor in to meet the waveform-envelope-power (whatever floats the boat) equally on both sides to match.
I wonder if you're thinking - what I'm thinking?
If we can't wind enough resistance onto those winds, care to try and add a resistor in series from pin 4 to their respective terminal? Like say...2 ohms? 1W? Carbon comp of course...
You were getting boxcars - so it tells me we were pushing pretty hard...
Expect a hiccup or two until we can get a value to balance out the twins.
Start out even as high as 10 ohm just to see reactions...(it would equate to 20 ohm - but a 10 ohm each side)
This method may work when it comes to "trimming" the volume and drive levels - just sub a resistor in to meet the waveform-envelope-power (whatever floats the boat) equally on both sides to match.
Yes Andy, i actually do happen to be thinking what you are thinking LOL.
What i was thinking of trying was a 4 ohm wirewound resistor, about a 5 or 10 watt white square one, with 42 turns of 22ga. wire wrapped around it.
I don't know what the idea of the third unused winding is, or if it needs to be there at all.
i figured it was just already a part of whatever transformer they found for this chassis.
and who in the heck allowed this thing out of the factory looking into 0.2 ohms!?
I figure after i re-wind the transformer, if it ends up with all the same readings im getting now, i'll be willing to try anything in order to induce some modulation from this amp to the driver and final.
LC
What i was thinking of trying was a 4 ohm wirewound resistor, about a 5 or 10 watt white square one, with 42 turns of 22ga. wire wrapped around it.
I don't know what the idea of the third unused winding is, or if it needs to be there at all.
i figured it was just already a part of whatever transformer they found for this chassis.
and who in the heck allowed this thing out of the factory looking into 0.2 ohms!?
I figure after i re-wind the transformer, if it ends up with all the same readings im getting now, i'll be willing to try anything in order to induce some modulation from this amp to the driver and final.
LC
In all that - I remember my ex's owned a Mongoose - one of those gold-plated jobbies of a 120 channel radio that came stock as a 40 channel - move two jumpers and the radio had 120 channels ...
Anyways, the board in it was "reworked" to fit a Modulation Transformer that used dropping resistors on the leads to it. 2.2 ohms 1 Watt I believe. But it had only one KTA7217 audio chip - strange radio.
It's nice to see it had cousins' - some form of relations...
Anyways, the board in it was "reworked" to fit a Modulation Transformer that used dropping resistors on the leads to it. 2.2 ohms 1 Watt I believe. But it had only one KTA7217 audio chip - strange radio.
It's nice to see it had cousins' - some form of relations...
Just re-read the post above. The 55-turn winding measured 33 MICRO Henries.
The 42-turn winding is 7 MILLI Henries. Roughly a 212-to-one difference.
The 55-turn winding only needs to have the enamel on the wire get hot, and allow two adjacent turns on the winding to short.
Only that would explain a difference that large.
And a resistance check would not reveal a difference in resistance that small.
Sure would explain the overheating issue.
73
The 42-turn winding is 7 MILLI Henries. Roughly a 212-to-one difference.
The 55-turn winding only needs to have the enamel on the wire get hot, and allow two adjacent turns on the winding to short.
Only that would explain a difference that large.
And a resistance check would not reveal a difference in resistance that small.
Sure would explain the overheating issue.
73