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The loudest AM "CB" chassis ever? EPT3000-10A

crossbow,

i was looking back at our conversation regarding this radio, and noticed that you said you got the TX working.
do you happen to remember what broke or how you fixed it?
that might help.

also the VCO thing was just mis-wiring. probably confused by that toggle switch on the inside. I know i was. i removed it LOL.
the other side of the band switch needed to switch ground or B+ to circuit point "13".
once that was figured out it worked!

also, i wanted to let you know that my offer of a free radio service still stands. what a great discussion you have fostered.

Nomad,

Mine is swinging to about 22-25 watts before alignment, which only adds to the conundrums. thanks for checking the thread out.
LC
 
LC - you gave me a pause for concern, were looking at that switch of S503 - backwards...

What if the pivot was "power" side, and the pointer went away - to the AM/FM and PA...

IkeepgoingbacktoHerepart3reviseda.png

Why because in the way that they have it done up - the error is we lose the AGC when it's in AM mode because the power is turned off to the AGC, Squelch and Meter because we killed the power to Pin 4 ...

Because we don't know if ACG will be needed - but we can fix that later - let's get the AM and FM metering and squelch to work right...
 
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Thanks for the offer LC. I am so sorry but I don't remember it right off hand. Ever since my cancer operation and treatments last year has made my memory go very weak. Add me back to that conversation so I can jar my brain here please. Thanks.
 
ok, so i have confirmed the switching of the AM FM switch.

and my radio matches the schematic for S503-4. (switching ground between D12 cathode and R113)

however, for S503-3 (connects IC2 ((mine is actually an NJM324D)) pin 4 to the junction of R33/R34/R100.

yes, that is normal, what is not normal is that the trace that connects these points also has two legs of L5 soldered to it. yes, only in FM mode.

i have desoldered these two legs and confirmed that they do not connect to eachother internally nor to any of the other legs of L5,

its like they needed to anchor the two empty legs of L5 somewhere and it just ended up being right in the middle of where they needed to run this trace.

back with more later.


oh, and the thing heats up in FM receive also.
LC
 
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oh and Crossbow,

hover your cursor over the envelope just to the right of your user name at the top of the screen, then click on 'conversations' when it appears.

the one we had going was titled "cobra".
you should still have access to it. the post i was referring to was about half way down the first page.
LC
 
well i found something strange.

C228 is silkscreened on the PC board with the negative lead going toward pin 2 of the TDA2003, while the schematic shows the positive lead should be there.
It's the 10uf electrolytic. i installed mine with the silkscreen when i recapped the radio, and that means that the old cap was installed this way also because i always make a note of the actual orientation of an electrolytic as opposed to what is on the board.

20180826_175522.jpg
LC
 
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oh and Crossbow,

hover your cursor over the envelope just to the right of your user name at the top of the screen, then click on 'conversations' when it appears.

the one we had going was titled "cobra".
you should still have access to it. the post i was referring to was about half way down the first page.
LC

Yeah I know where conversations are but I had cleared out all mine from the past. Not sure it will allow you to add me back into it or not. If not no big deal.
 
well i'm about out of ideas at this point.

my thought right now is that the transformer winding going between the pin 4's of the 2003's has a problem.

the ONLY reason i think this is because if i de-solder the two transformer leads that go to pin 4 of each 2003, then the heating does not happen.

I know that does not automatically mean that the transformer is the culprit, but it's the only thing i've tried that makes it not heat up.

tried removing pin 1 and pin 2 inputs, still heats.

tried replacing the darn things thinking that it might just be faster. still heats.

remove the transformer link between the outputs, and no heat.

im stumped ATM.
LC
 
Is there a voltage on the winding?

That may be the whole problem - it' shorted a wind to the power feed

SS33PowerChokeOrTransformer.jpg

(again, sinking power from the "expected speaker" to 1/2 rail voltage can make it overheat.)

So are there supposed to be caps in line with the output of Pin 4 - not shorted directly? (Should we put them in?)
 
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I was running out the door when i posted that, and didn't have time to re-solder and check the winding for a short to the middle winding with the voltage on it.

It certainly seems plausible that it could have gotten damaged in the past.

now the question becomes, if it is bad, can i use that other winding that doesn't seem to be connected to anything but ground on one end?

otherwise i am looking at re-winding the transformer. yeeesh...

I will do a check for voltage on that "pin 4" winding tonight when i get home.

I honestly don't know which scenario to hope for LOL.

thanks again for all the help Andy.
LC
 
6.61 volts dc.

yep that's what it measured from either pin 4 connection to DC ground in receive.

measuring coils just for fun yields results in the millihenries.

doesn't sound right to me.
LC
 
No, disconnect the TDA's from it, then power up and remeasure....

Because 6.61V is 1/2 supply rail (13.8V), and there is no blocking-DC cap - I'm wondering if the winds are shorted to the other that has the spike diode in line with it.

Here's how I see this...

IF the spike diode wind has shorted to the Audio Bridge line, the TDAs will see on their Pin 4's 12~13 volts they have to drop - so they may be able to drop and regulate that thru the ohmic results of the winds themselves. The transformer gets very hot in this case too, in it's case.

If the transformer is broken - as in a shorted wind, we have two choices - find a cap value that will work with the circuit - or replace the transformer. The caps go in line at Pin 4 outputs towards the transformer - because as the other circuits show, and related to RF, are there jumpers in or along Pin 4's route to the Transformer?

But, here's the rub...
Pin4.jpg

Are those two big caps - are those not the ones that come off of Pin 4? OR are they the input side?

I see jumpers too, your soldering and traces, I haven't seen any trace side so I cannot see how the transformer is even hooked up to these two amps.

So if you're (SIC - the Audio Amp IC's are) heating up - those large caps generate a separate oscillation issue - they are simply too large for the inputs. But if they are for Pin 4 output - then those caps may have been inadvertently shorted during one your tests. OR we have a trace wiring issue.

Don't have trace or clue as to how the transformer is hooked up or if we even have the right one (multi-tap versus distribution isolation style).
 
yep you're right Andy.

The trace shows 6.75 volts but the actual coil leads of the transformer do not have a DC voltage on them.
the anti spiking diode line is sitting at about 5.5vdc.

the transformer never got hot, just the TDA's.

The big filter caps you asked about are off of the pin 5's and i think are just there for DC filtering.

the reason i didn't immediately think that the transformer was bad when i disconnected it from the TDA's was because i figured that without an inductive load to work into, the amplifier wasn't doing it's thing.

as for the connections, they all match up with the schematic. its just that those two big caps are drawn in a slightly deceiving location making them look like they come off of pin 4.

LC
 

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