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

[OldTech03]

thank you Andy.

i will look at other applications for this circuit and try to glean some understanding about it.

also thank you for the explanation of the transformer action. that was a cool read.

now i have a question for the group.

C58.

a 1uf electrolytic located just above the 4558 mic preamp chip on the schematic.

it shows positive to ground, and the cap i pulled out of there was indeed installed this way.

looking at the schematic for the galaxy DX33 (EPT3600), that part would be C71 and it shows negative to ground.

so, what say you all?
should i put it in as it's shown on the schematic? OR was it put in backwards because of a mistake on the schematic that was corrected in later versions?
LC

Are you sure about those part numbers and where you say they are? It may be it's just to early in the morning for me to be coherent but I can't find them on the schematic in a place that makes sense with respect to where you say they are. There that proves I'm not very good at 3:43 AM I'm going back to bed.

Oldtech03

 

[loosecannon]

the cap in question is in the AM / FM switching circuit.

It is above the 4558 in these schematics:

SS3000: http://www.cbtricks.com/radios/rci/ss_33/graphics/ranger_ss33_sch.pdf

DX33HML: http://www.cbtricks.com/radios/galaxy/dx33hml/graphics/dx33hml_sch_main.pdf

thanks for the reply OT!
LC

 

[loosecannon]

pics as promised:

 

[loosecannon]

 

[loosecannon]

 

[loosecannon]

LC

 

[Handy Andy]

The C58 is the ANL filter cap, note the 1N60 and the AM FM switch - the AM side is recitified at processed like an AM carrier with audio detect into the ANL circuit - leave positive lead of cap to board please - it's a floating negative - a filter. R112 is the FM product.



The FM side of it, note the location the switch goes to ground - so when in FM mode, the audio out is grounded at that location from the chip - instead it's sent to the PLL's VCO and "vibrates" the carrier to the audio frequency in and around the carrier IF


R112 sends it there, R113 gets cut off.

S503 and it's the bane of many a technician - is the default switch setting - shows FM on one pole but AM on the other - so it is confusing. When multiple pole switches are used, they all should point to the same setting they are drawn on one pole - and treat all the same way - not just put this on FM and this on AM - draws too much confusion. R75 (1Meg) powers the bias, the R76 68K removes (drains) cap charge so the ANL can recover signal after a spike. D13 and D12 set up a NEGATIVE going carrier region - where it can be below ground potential - So R75 supplies some power to flow into the circuit to keep the negative bias from being excessive, and R76 helps that by providing a work effort (resistance) to the recovery effort so the spike and whatever else noise is in there is removed but you get audio back.

:+> Andy <+:

 

[loosecannon]

thank you Andy.

I wonder why they chose to change the polarity of that cap when they evolved to the EPT3600.

just about to power it up for the first time since re-cap.
LC

 

[Handy Andy]

Good question, usually with the reversal you'd get funfetti everywhere but in this case, the 1 meg and the drain resistor don't allow a lot of current to flow thru the cap - just tiny bits of energy so the degassing issue and subsequent decompression (explosion) is moot - the cap can dissipate the energy well enough. The delay and reversal is not very strong and with the audio signal - the signal itself is an AC component - so there is little effort the cap can do except rise and fall with the residual carrier charge and noise level.

Keeping finger crossed - nice chassis in there BTW.

 

[OldTech03]

The C58 is the ANL filter cap, note the 1N60 and the AM FM switch - the AM side is recitified at processed like an AM carrier with audio detect into the ANL circuit - leave positive lead of cap to board please - it's a floating negative - a filter. R112 is the FM product.

View attachment 24647

The FM side of it, note the location the switch goes to ground - so when in FM mode, the audio out is grounded at that location from the chip - instead it's sent to the PLL's VCO and "vibrates" the carrier to the audio frequency in and around the carrier IF
View attachment 24646

R112 sends it there, R113 gets cut off.

S503 and it's the bane of many a technician - is the default switch setting - shows FM on one pole but AM on the other - so it is confusing. When multiple pole switches are used, they all should point to the same setting they are drawn on one pole - and treat all the same way - not just put this on FM and this on AM - draws too much confusion. R75 (1Meg) powers the bias, the R76 68K removes (drains) cap charge so the ANL can recover signal after a spike. D13 and D12 set up a NEGATIVE going carrier region - where it can be below ground potential - So R75 supplies some power to flow into the circuit to keep the negative bias from being excessive, and R76 helps that by providing a work effort (resistance) to the recovery effort so the spike and whatever else noise is in there is removed but you get audio back.

:+> Andy <+:

Please tell me that S-503-3 and S-503-4 mean what they have for the last million years and that both of those switches are part of the same switching meaning they move in unison when the shaft is turned. And then tell me what fancy cam system would be needed to make that happen. Anyway, I never went back to bed from 3:AM this morning so it could be that schematic is perfect, please let me know?

 

[Robb]

Wow, you really did find a cool old oddball.

BTW - what's the story on that panel switch mounted inside near the PLL?
Wha????

 

[loosecannon]

well i got to power the beast up last night right before bed, and it did turn on and pass the smoke test.

It still only transmitted on one freq though. 25.1290xx or something like that.
yes, it would go into TX with full power, but channel selector and band switch had no effect.

this is somewhat expected, as i still have three wires from that slide switch that is internal to the radio.
Robb, it supposedly switches the radio from a 40 channel radio to a 240 channel radio.
only info i found on it was on some french site.

the three wires went to the HI LO band switch and there was a trace cut, i took pics but haven't studied them yet to figure out what to connect to what.

My best guess is that this was one of those radios that got modded for full bandwidth by a distributor or a shop somewhere before the sale, but i can't figure out for the life of me why the solder connections on the switch side of those three wires look factory, but the other side of the wires were bodged on to the back of an existing switch that had an obvious non-factory trace cut.


havent adjusted anything yet, but the radio keyed up 2.5 watts on LO and swung to 18-20 watts. It did 6.5 watts on high and also peaked at 20 watts at this level.

it was over modulated and showed big time box cars on the scope.
I will give it a very close look and make sure all AMC related parts are in place, but i figure the trimmers are just cranked.

the thing that bothered me a bit was how fast the heatsink on the two audio amps got in such a short time period.

after just a minute or so of testing, with time in between keys, it was too hot to keep a finger on. the only change i made to this circuit was to up the value of the filter caps on the 8 volt rail from 1000uf to 3300 uf.

going to start looking at how the switch plays into the programming of the PLL tonight and hopefully will have it transmitting on all freqs soon.

as always, im open to all hypotheses, conjecture, and wild @ss guesses.
LC

 

[Handy Andy]

You are aware that the heat sink CANNOT be grounded?

The tab then needs a mica insulator

The amps themselves need to be electrically isolated from the back panel. Else the two will try and "drop" each others power feeds.

The power on, idle current - did you see how much amps it draws?

 

[Handy Andy]

Reason for the question is to make sure those audio amps are NOT in an ultrasonic oscillation.

Although Pin 3 is grounded to tab, you also use pin 3 on the main PCB - the rear panel can generate a feedback antenna for ripple to appear and the amp draws up to it's full current in the squeal - you should also check the parts and traces below - for heat - can give you a clue as to what parts are the feedback paths to make that amp seem to run hot.

 

[Handy Andy]

Also - did some research, wanted to point out some stuff that may have given you such a headscratching...

I think I found it's Achilles' Heel...there is a parts swap you may wish to try - it's only for the sake of balancing that amplifier...

​

And I need to issue a correction - firstly - I presume it was Bi-amp - basically two amps driving two parts of the same piece - like one speaker to one and a totally different speaker to another - in concept you are BRIDGING two amplifiers into one unit.

So again I apologize - this supersedes the original premise of bi amp and I will firmly state Bridge amp.

​

Why the change, because of the same design - I located here - but there is a fault in the design you have in that 3000010 board...

Note C3...

Because it ISOLATES not just the amplifier - but notice the Pin 4 output - IS NOT TIED as shown blow...

​

Although both amps are isolated from ground - yes... but look closer...

It's an INTERNAL issue...

Note Pin 2 - takes in the feedback for DIFFERENTIAL amplification. Which means - when you use a capacitor - to ground - the reference level is set between the two amps and floats as an AC signal between the two.

We have a mistake though...remember the differential part...

If pin 2 is held too low, the loop senses full output - which I guess is what you want - but the signal arriving from the other amp as part of this feedback loop - is tied off to a low value resistor - sending a lot of the signal you want to push and pull - into ground or reference to ground. R191 is that restrictor...

So that resistor will need to be changed - I recommend that we BALANCE the bridge first to see if we can keep it stable and power output acceptable - both sides will be driven, but the amp will still perform in a push and pull configuration - just not full tilt until we get the balance right.

​

Another circuit I found, and is closer to what you have now. Note R2 and R5 are "reference to ground" while the R6 and R3 are the reference to output of amp.

The above is a good way to show how this amp will work - but R5 and R6 "float" above R2 and sum into the overall amplification of the input signal side - the left one - while they work as a push pull - but the gain totals may need to be more balanced.

So that 180 (EDIT KEEP THIS VALUE), the 470 change that to a 220 ohm - and the 16 ohm reference - we need to bring that down too - I'd say about 5.6 ~ 10 ohms. This is so the differential amp KNOWS how to cross from one to the other without excessive power amplification in the loop itself. So what is shown in the SS33 schematic is needing to be brought more even...It is my belief the R191 being a 16 ohm - is tipping the cart too far to one side and makes the amp run hotter than it should.

I will post more - later. Gotta' feed mom and the pups din-dins...

I'll let you munch over the stuff I found and see if it can apply some changes to make that amp run cooler and better for the needs...

:+> Andy <+: