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Galaxy Saturn base radio - no transmitted audio.

26TW06

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May 1, 2020
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Galaxy Saturn base radio - no transmitted audio.


I have a Galaxy Saturn which I discovered that the AM limiter had been removed before i brought it. It also drifted terribly so I haven't used it a great deal.
I decided that it should be pretty straight forward to source a replacement transistor and solder it in as I don't believe that there is anyone in the UK that will work on this model of Galaxy.
I soldered in the transistor, and hooked up the radio to a modulation plus deviation meter.
Whilst the covers were off, I de-soldered the solder bridge from the crystol X2 to its adjacent can, after researching that this can help reduce the frequency drift. And it has! It had dropped in frequency a little, but I corrected that (and also had to slightly adjust the frequency display). Now the displayed frequency does not change on the front of the radio, hot , cold, or in between! the problem I have is that the radio has no transmitted audio now.
Zero modulation on AM, but there is a carrier. FM transmits, but with no audio, and SSB does nothing. If I remove the transistor tr32, audio is restored. transistor appeared to check out on a meter, but I ordered two so I put another 945 transistor into the board and lost audio again!!
The PA on the radio works with or without the TR32 installed, so I know the mic hasn't just gone bad, and correct me if I'm wrong, but suggests that the audio chip is working.
I have properly run out of talent re this problem, and wonder if anyone out there has experience (or repaired) something similar. I would really like to use this Base radio as it appears to deal with local interference on RX much better than the resent Chinese multi-modes do, and its in real nice cosmetic condition too!
Any suggestions or info with be greatly appreciated!

Cheers,

Darren 26TW06
 

There are a lot of "clip this resistor" tips on the internet. Sounds to me as if one of those is forcing current into TR32 when there should not be any.

If the SSB ALC pot is turned to one extreme, this can kill the mike audio that way. The SSB limiter is supposed to shut off in AM mode, but cranking the trimpot to one extreme can

TR32 has three inputs. First is the 'compressor' function that serves to hold the mike-amp chip below the distortion point. Second is the AM limiter, third is the SSB-only ALC.

One of those three inputs has 'gone rogue' and is shutting down TR32 when it shouldn't.

See if turning the sideband limiter's trimpot the other way doesn't make a difference. And take a very close look for clipped resistors.

73
 
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Without getting too specific, the board has a lot of OLD caps on it.

Locate your 4558 IC - that's your mic amp...and TX power GOOD to allow you to key up and talk.

On the older SSB bases, they had C132 - if that cap is dried out, it may SHORT out - sending a voltage to TR32 and turn it on- leaving it on. (Acts like a dead sort - plates corroded and touching)

Remove that cap to see if TR32 stops conducting.(killing your audio) then replace that cap.

The value varies, but you can even sub in a 0.1uF Non polarized Disc Cap to avoid problems later.

The Circuit is novel in that it "samples" the audio from the Mic circuit, rectifies it, using a doubler - then smooth's it out (C133/C135) to generate a drive current into the limiter (TR32) so it's conducting nearly all the time.

The Circuit consists of (Per Galaxy SSB EPT3600-13/14 Board)
  • R172 - 1K
  • C132 - 1uF 50V Electrolytic
  • D67, D68 - 1N4148 x 2
  • C131 - 10uF 10V << EDIT - Update!
  • C135 - 0.0047uF Disc (472)
  • D69 - 1N4148
GalaxyLimiter.png

So TR32 to "act upon and work hard" this circuit does that - takes EVERYTHING from Pin 1 Mic audio - multiplies it and puts it into TR32 - so when you are having problems with it "killing audio" look to see if a voltage appears from C132 - that may be your reason for the problem, a shorted - blown cap.

To play with it, change the value of C131 - the 10uF to a smaller value and even change R178 to a 1.5K to help with "limiter recovery"

Had to update the Graphic, for C133 is the Roger Beep Length of Delay cap BELOW this circuit - so even though both use the same value - they are two different circuits.

C131 is your other smoothing cap for the Limiter.

My apologies...


Now, you may not see these part numbers on your board, but if you can locate the output of Pin 1 and trace it back thru this series of components back to TR32 - those parts are your "chain" to work the limtier.
 
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Hey Mr Nomad,

Thanks for the info. I tried moving the SSB ALC pot to its extremes, neither setting made any difference I'm afraid.

I checked the voltages on T32 and compaired them to the service manual.
In AM RX the emitter was zero, the collector was zero and the base was just a little under the 0.7v quoted. In AM TX, the emitter and collector showed 3mv instead of the zero quoted in the manual. The base was high at 0662v with the manual quoting 0.1v .
what I don't know for sure is if that 0.662v in transmit is considered within spec or if its over. If I had to guess I would think over.
I then checked the IC4 mike amp voltages on AM RX and TX to compare with the service manual once again. They where all just slightly low of the quoted voltage in the manual, e.g: pin 1 AM RX is 4.45v when the manual quotes 4.57v. Manuel instructed that these readings should be taken on channel 40 band D, which is what I did. I probably should explain (if it isn't obvious) that I'm not a tech, and may well be punching above my weight here!
I cannot see any clipped components on the board despite inspecting for sometime using a magnifying glass.

Once again, any info is greatly appreciated.
 
apologies Andy, I didn't see your message. just reading through it now.
 
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The base was high at 0662v with the manual quoting 0.1v .

I think you found your answer.

Check the parts especially around TR32 - the R178 resistor - and the RX "quench" diode for Mic audio "idle" Noise may also have shorted - if you get too much reverse bias current - that diode should hold it back - but if it went leaky it can "rectify" adding another "color" to the pallet of problems age usually shows up with.
 
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The 0.7 Volts on TR32 base explains why TR32 is killing your mike a audio. This should be zero volts until you start to talk. Might have a look around TR53, especially if it's been removed. A solder blob across that one will do this. You'll never guess how I learned that.

The other thing that can do this is a fault in the transmit/receive switching circuit.

The 'receive-only' 8 Volts DC coming from the center lead of TR36 should be zero when you key the mike. If this voltage only drops to around 1 Volt or so, this causes the sort of trouble you're seeing.

Those are the next two items on the hit parade.

73
 
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Thanks once again folks for all your assistance. It's really appreciated. Hoping to get some time tomorrow to look at the Galaxy radio. I will report back as soon as I have looked at it.
Cheers,
Darren
 
Hey Folks, I had little time look at the Galaxy Saturn this evening. Not as much as I would have liked but hey!

So, this is only a small update: I removed C132, and checked to see if there was any audio. Unfortunately there was none. Checked C132 and it reads 1.081uf.

TR53 is installed and there are no solder blobs etc, looks like its never been messed with.

R178 which should be 4K7, measured as 2k2, but it was measured in circuit.

D67,68,69 appear to check out although I measured these in circuit also (each only displayed a reading one way).

Finally before I ran out of time, I checked out TR36. Centre pin had the 8 volts RX. In AM transmit, the meter reading was 0.68mv which is fairly close to the 0v required.

I intend to remove the diodes tomorrow from the circuit to test properly. I've sourced some brand new replacements, so will replace the diodes anyways.

Cheers,

Darren
 
Ok, before this goes into a "Shotgun" approach...

Locate TR31 - TR31 is a NPN '945 - designed to re-route audio.

When it's on, it takes all the audio present on the opposite side of C126 - the output cap that is sending Audio Signal to FM and SSB sides, and sends it to ground.

It's base is hooked up to two diodes, that route power to the Base lead of that transistor. When you're in AM mode or FM mode, you can't be sending audio to the SSB side which is your AN612 - Balanced Modulator - IC3 - so the purpose of he transistor is to "quench" or send any audio signal that is not supposed to go to IC3 to Ground.

Again, age being a factor - if the caps are going bad, as if shorted, they can suck all the signal to ground no matter which way it's routed - not saying to remove it to troubleshoot, but keep it in mind to double check.

One thing I have not heard from you about...

There is a CW key jack, have you verified this works ok?

There is also a Roger Beep, do you even get any "Beep" when you unkey?

As far as I can tell, when the PA works, so you have Mic audio - but I don't know if you verified the Mic amp is still getting power when in TX mode...

Refer to the schematic above, that little smippet...

Here's another look...
upload_2020-9-22_21-50-20.png

Go here...
http://www.cbtricks.com/radios/galaxy_other/ssb_base/index.htm

That link is what I use for most older Galaxy SSB radios to help you. It's not perfect, but the main board is what I usually work with the most.

Ok, did you know that the PA amp is on a little tiny corner of the main PCB? It has it's own amplifier for the PA and routes to the Audio chip - this is why I needed to ask if you even get power to the Mic amp when in TX? It's TR901.

I hope this is something as simple as an overlooked wiring jumper or header misplaced...Because it seems you'd have the same audio if the PA side worked, you should also have it routing to the main board - connection #46 where that tap is.

So - there may be enough power for TX mode, but the Mic amp side needs a voltage too - to operate its' own op-amp amplifier It uses the voltage feed that goes to Pin 8 - but applies it thru two resistors set up in a divider design - R174 and R163 - so the amp's "input" sees 1/2 supply voltage or between 3 to 4 volts, on Pin 3 so the Base of the amp "transistor" inside it's connected to - of that input leg, will have power to utilize and amplify the Audio signal present at that pin.

You also need to know if audio from that #46 connection header is getting there to Pin 3 also.

EDIT: update connection #...
 
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Hey Andy, I had a quick look this evening. Re: TR31, measured as: AM tx 0.72v tx 0.73, same for FM and 0 on LSB and USB. Sounds like TR31 is behaving?

Had some trouble locating TR901 I'm afraid, so haven't looked at this yet.

The mic amp IC is getting 8v at pin 8 and pin 3, just under 4.5v.

Didn't check audio from #46 connection header is getting there to Pin 3. I presume I can test this with a simple speaker (like the type found on a mobile) and some wire. one wire on #46 and the other to ground? Need to dig out a spare speaker as soon as I can find it!

I haven't checked the CW key jack so far as I do not have a cw key, but I guess it should be easy to wire up a switch to a 3.5mm plug. Assuming that it will be sufficient to test it with? Or even just short the wires right, instead of a switch??

I checked the beep. when dropping the key, nothing is heard in AM,FM or SSB. The radio maintains transmit for the duration of the beep (except SSB, there is no indication of a transmission). I switched over to CW, and discovered that I could hear the beep when releasing the PTT!
Not as crisp and clear as under normal working condition, but it is there. I returned to the AM mode, no beep. If I remove tr32 again the beep is restored with audio (but unlimited).

I rechecked PA mode by plugging in some very old headphones into the PA socket.
I realised that although I could hear myself clearly through the headphone speaker, it was quite quiet for an alleged 3W PA stage. Mic gain at max.

I will have another look for TR901 tomorrow and check #46 and pin 3 for audio.

Cheers,


Darren
 
Ok, you helped a lot with that last post.

That "PA Amp" that uses a cap off that mic line, so you are noticing what I'm trying to get at The Mic line is having a problem - it's not getting to the Amp TR32 - yes it's the SYMPTOM, the cure is with the output of Pin 1 thru down and over to TR32.

Every time you remove TR32 - the audio is restored full bore volume. So the PA isn't the REAL problem, but if TR32is stuck on all the time, what "area" is causing it?

Now, I need you to check D71's BANDED end - what is your voltage in TX then to RX?

Then go to PA mode, and do this again D71 TX and RX...

Why? R177 and R176 form a bias on that line to help KEEP the Mic line quiet - TR32 stays on all the time, as in full on, IF that D71 is shorted out or the RX line it feeds from is NOT powering down keeping TR32 on all the time.

That 0.7 to 0.56 volt feed on the Base is all it needs, for that is the Forward Voltage drop across Base to Emitter - so that is a Bias diode in a way because the Base is forward biased straight into the Emitter - but that is why you're seeing that ON voltage - now look for D71 and see if it falls in TX, if it don't look at the RX TX switching circuits TR40, TR38, TR37 and TR36 - if those transistors are already checked, I need to know the Voltages on TR38 - PNP and TR37 NPN. In RX and TX.
 
Ok, finished dinner, had to run back and finish in a separate post...

Ok, here's the thought...

IF RX voltage is NOT turning off, that tells TR32 to stay on even in TX - leaving you with a muted radio. Those Pin diodes and RX side is quenched when in TX mode - but not in RX. You'd have receive.
upload_2020-9-24_21-11-58.png

So something is keeping a line high for some reason - hence the D71 - again. To this seems to be the culprit but if you pulled that diode and TR32 stayed on, then the problem actually relies on TR38's ability to conduct to the TX line - but at that same time the RX line needs to toggle Off - using TR37.

This is an either or or even both situation for if TR38 is shorted, you have TX all the time - at least trying to - because of the PNP junction the Transistor is.

In TR37 - it needs to see that output line from Pin 7 of that 4558 Chip - RISE - goes to about 7.5 volts, plenty enough to offset the Zener effect of D76 - since TR37 in an NPN it will now turn ON, but it needs to TOGGLE - as in full on to full off and back ON again like a switch - so TR36 solves this "threshold" problem, it's a BUFFER - it flows from 8 volt regulated thru itself into the RX line. That Zener effect is important - if the Zener is shorted - ANY trickle current will trigger TR37 on.

TR38 on the Other Hand, is a PNP - meaning that it works the opposite of NPN - so it needs to see Pin 7 of that 4558 chip FALL below 1/2 the voltage of the Regulated supply line or 4 volts to power the TX line with enough voltage to start the TX strip working.

So if TR32 should work ok, is the RX and TX voltages that are supposed to route to TR32 working correctly?

While you're in there, look for and "clip" D85, don't remove it - I'll tell you why later...

Locate TR34 2SA733 - check it, test it - it is also a PNP
.
 
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