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

[loosecannon]

hey Crossbow!

Thanks for coming to the thread, i didn't want to say who it was without you knowing.

Also thanks again for this radio. it has been a lot of fun, along with some good ol' frustration trying to dig up any info.

I'm writing up a quick and dirty alignment sheet for it, but with the wax covering so many PLL components it's hard to find the test points.

I would be very interested in hearing about your experiences with this radio, and obviously any service info you might have come across.

did you cut the track on the band switch? it had to be done to make it work, but i have been wondering if that's something you figured out or if it was done by some distributor or something.

What was the main problem you were having with it? wasn't it something like the TX being 5kc off or something weird like that?

did the heatsink for the TDA2003's get hot on receive when you had it?

im pretty much interested in anything you remember from owning it.

more to come!
LC

 

[Handy Andy]

L13 peakes 10.240 - LO and 2nd IF (to obtain 455kHz)

L14 "Sets" 1st IF and your 16~17MHz window - and L15 PEAKES 1st IF and IF TX Mixer

... so L14 tunes.PLL side L13 peaks 2nd if

Per SS33 schematic

Your mileage may vary...

 

[loosecannon]

ok, replaced the 3300uf caps filtering the 13.8vdc coming in to these TDA2003's with the stock 1000uf value, and it made no difference in the heating of this circuit.

still getting burning hot in receive within one minute of turning the radio on.

i desoldered pins 1 2 and 4 from each of the 2003's so that only VCC and ground were still connected, and the heatsink did not get hot anymore.

This is leading me to believe that one or both of these things is oscillating in RX rather than just being shorted out.

still not sure on that though.
LC

 

[loosecannon]

L13 peakes 10.240 - LO and 2nd IF (to obtain 455kHz)

L14 "Sets" 1st IF and your 16~17MHz window - and L15 PEAKES 1st IF and IF TX Mixer

... so L14 tunes.PLL side L13 peaks 2nd if

Per SS33 schematic

Your mileage may vary...

thanks andy that makes more sense.

My mistake was comparing the schematic to the Galaxy 33.
LC

 

[Handy Andy]

Ok, I'm going back to the SS33 schematic, - and so I wonder if the radios IF is getting sent there.

My main concern was the PA/FM/AM switch. Please check your routings.



The radio can inadvertently send IF to the TDA2003 and they will try to process the input.

Throw the audio on the scope and check the divisions - it's my suspicion that the 455kHz derived carrier is in there too.

The Pin 1 on the TDA2003 - just see if the scope can fine tune the oscillation and at what frequency - because we many need to DEVELOP a filter to remove it.

 

[loosecannon]

that is exactly the kind of info i was hoping to hear Andy, as i could not for the life of me figure out where the signal would be coming from.

i am at work until about midnight tonight and will check this out on the scope when i get home.

i will also confirm that the routing in the radio matches the schematic.

Thanks for the reply Andy.
LC

 

[Handy Andy]

No problem.

Locate R113, you may need to scope there to see if any "Squeal" like an ultrasonic is present.

Updated graphic above to help.

It is my fear the TDA2003 are trying to amplify what exists on the Audio Line to it.

Remove C133 - scope the junction of C132, C134 side during this check.

If it's stays stable - then the two are fighting each other for some reason... because the input of one isn't even there - just its output going BACK to Pin 1 thru Pin 2.

So look (Scope) at R113 and then again at Pin 1 of the TDA 2003 input like junction of C132,C133 and reconnect the farthest one (Audio IC), but leave Pin 1 alone of the 1st on unconnected - scope that and look for it on the scope. by using C133 input side to Pin 1 of that TDA2003.

 

[loosecannon]

ok looking at the schematic i think you are talking about S503 right?

and the fact that S503-3 shows to be in FM mode, and S503-4 shows to be in AM mode.

I am still going to check it all when i get home, but right now it seems to me like i could switch the radio to FM mode and see if the heatsink cools off.

or maybe i could disconnect the wire that goes from the AM/FM/PA switch at R113 and see if that keeps the 455khz from getting in.

As i'm typing this, i am thinking that i might be dealing with the same issue i faced when trying to get the PLL and VCO sections wired up correctly.

and that is that this radio is a hodge podge of different ideas that were being thrown around over at the Dirland factory.

I believe that the PC board that holds all the front panel switches was not originally meant to be used in this chassis, but some engineer saw that it could be made to work with a few modifications, and so that's the way they made this chassis work.

they had to cut a trace on the band switch in order to be able to switch between HI and LO because the bottom and middle poles were locked together via a PC board trace.
(see the pic in my original post in this thread)

so if you look at the schematic, it shows a three position switch that goes from AM to FM to PA.

well on my radio the AM/FM switch is only a two position switch and the PA function involves pulling out on the RF gain knob.

This leads me to believe that the AM/FM switch was bodged together, and if someone used the schematic to do it, it is possible that they got it wrong.

hmmm...
LC

 

[Handy Andy]

LC,

The main concern is to make sure FM is FM ok? The Discriminator IC does get switched off when in AM, only the signal is routed differently (Bypassed)

Remember my post earlier - one pole shows FM while the other is connected to AM?

Time to take that comment seriously - take those arrows out - erase them - re-work the switch to send FM TX from the Mic IC - 4558 chip to those two diodes by the PLL - FM RX comes from the TR12 - BUT POWER IS SENT TO POWER IN OF IC3 Discriminator only IN FM MODE AND ONLY IN FM MODE! (the other half of the S503 - 3 stays off in AM - DVM it to make sure it's is off in AM mode and PA modes) FM is "quenching" parts of the AGC in TDA6324

The AM side - DEMODULATES the IF present back at the FM detector input BEFORE IT GETS THERE it's taped off - the two 1N60 diodes rectify it and send it thru the smoothing filter - means it doesn't SEND ANY SIGNAL to the TDA2003 system - in RX - so if they heat up it means something is getting in there in RX mode.

(...My XYL interrupts me at inopportune times so sorry for the massive edits - like right now...)

 

[loosecannon]

andy i'm having a bit of trouble understanding this specific section of your post:

"FM RX comes from the TDA6324 POWER IS SENT TO POWER IN OF IC3 Discriminator only IN FM MODE AND ONLY IN FM MODE!"

would you please check this section for typos? it could just be me.
LC

 

[Handy Andy]

Re-wrote...see above you got me in-between XYL interruptions...

Since I still have something of an "Edit" window before the post timer expires...

I just wanted to add in how the FM sides' wiring issue throws a wrench into the works.

I had to do a double take on how the FM signal routes thru and why I saw so much "branching" going on when the radio is in FM mode.

Then it hit me.

This has yet to be Determined - stay tuned will update in a later post - :+> Andy <+:
Remember, this board is BEFORE Galaxy took on all that electronic switch issues and routed IF by branching thru caps, and the other side of the cap was the circuit that turns on or off thru the front panel and all that trace wiring - a place to send the signal but if you chose the cap values correctly - the branch in the circuit was a small signal drop versus the one shown in the schematic above - being brute force method of toggling and sending power and signal to where it needs to go.

I was going over an older Galaxy 77 and I found places they "fudge factored" bias across several lines either of signal or internal IF - just for the purpose of when the power was applied and the other circuit was turned off, the voltage drop from the power branch kept the circuit off but allowed IF signal to re-route to the other parts of the board that needed it for that mode.

So in a way, the TDA6324 was "quenched" by power from FM mode to ensure the radio would receive all FM signal - which if AGC was on too - it may "demodulate" an FM signal that had an AM component or a lot of flutter that would caused the Receiver to "pulsate" when it tried to track the FM signal at an audible or signal flutter rate - you just wanted the receiver to work straight thru with no attenuation unless it needed it - thru other means just not the OP amp chips use of the AGC and audio / carrier bias issues associated with AM signal.

I just need to know if turning off power to the IC is not what they intended due to the error in the schematic...we are figuring this out as we go...

 

[Crossbow]

Sorry LC, I don't remember a lot about it other then it was getting hotter the a fire cracker. The radio is factory and there was never no mods or anything done to it. Between you and Andy I have faith that you two will get her back going. I do remember it was one heck of a talking radio! Sad part is not much info out there on this model. When I told ya I had never seen anything like it well now ya know what I was talking about. One I knew I did not want to tackle! Good luck, I will be watching and learning along with ya guys.

 

[nomadradio]

This is only the latest radio with this underlying design.

Can't remember the model or brand, but we saw one like this in the late 80s. Had two 2SC1944 finals, that enormous modulation transformer and two MB3712 audio-power chips feeding into it.

The Superstar 3000 appeared in the late 90s, but there was never a shred of technical info for it. Nada. Had a single 2SC1969 final and delivered honest 50-Watt modulation peaks.

After that was the Ranger RG-33. Same trick, but with two TA7222P audio chips.

None of them ever caught on, but they had about twice the audio power needed to get 100% modulation.

Loud radios, but stepchildren models.

73

 

[Handy Andy]

Crossbow!

If you come across any old memories about this chassis - please by all means chime in!

Thanks for being on board with us.

 

[loosecannon]

im just going to post this to make sure i have it right for later.
If i do have it wrong, please let me know.

S503-3:

AM mode: N/C
PA mode: N/C
FM mode: connects pin 4 of the TA6324 and pin 8 of the 4558 (through D22) to the anodes of D19 and D18. (yes, there are other connections involved)

S503-4:

AM mode: cathode of D12 grounded.
PA mode: N/C
FM mode: R113 and R149 junction grounded.


one thing i want to remind myself to do is to see if C220 actually exists in my radio.
LC