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Galaxy Saturn (EPT360014B) FM audio low

Yanked out all diodes in that circuit and tested out of circuit. I started with D68 because it was showing 4.56v on the banded end in receive and 0.034v in transmit, so I figured it was bad. Nope, tested out fine, as did D67, D69 and every other diode in that circuit. I even checked the FM limiting diodes and D84/D85. All the diodes test as normal. Gonna throw all these diodes back in and then start the voltage checks on all the ICs. I'm beginning to believe the mic amp is shot or the audio IC is shot.
 
Hmmm, I just noticed something....

D93 and D91 are missing.

If I'm not mistaken, D93 ties in with FM deviation somehow and the emitter of the Passthrough according to the 99v block diagram.

D91 ties into TR49 (modulation transistor) according to the 99v block diagram.

Going off the 99v topside image and another Saturn plus another AM/FM/SSB ept360014b board in another galaxy radio on my desk.

Remember, there was a diode tied to a resistor in the two empty holes beside R241, which wasn't supposed to be there at all so I yanked it out. I never noticed D93 and D91 missing til just now while I was putting all the diodes back in it.
 
Well, replaced D91 and D93, same results. AM transmit audio great, SSB transmit audio great just not as sensitive on certain words as AM is, FM transmit audio is still almost non existent.

AM receive is great.
SSB receive is great.
FM receive is great.

AM transmit audio is great.
SSB transmit audio is fine.
FM transmit audio is almost non existent.

Checked voltages on IC1, IC2, IC3, IC4, IC8, IC9, IC10,...All check out ok with 99v voltage chart. Although pins 7, 8, and 9 of the PLL show higher than they should, and Pin 10 shows 8.81v on it, supposed to be 0v. Checked for 10.240mhz on PLL pin, it shows 10.2398mhz.
 
I tend to agree with M0GVZ. I cant find a schematic for your radio but highlighted the FM path I would be checking for open/shorted components. The audio chip IC4 is good as is the audio output chip IC8 which is just for the speaker/PA.
Do you have a RTL-SDR dongle/ SDRPLAY where you can observe your transmitter signal like on a spectrum analyzer? Ill bet the FM Deviation adjustment isnt doing squat due to something in that path haywire.
 

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IMG_20250204_125554565.jpg

30khz span
VBW 100hz
RBW 1khz
Center 27.205
Sweep 150ms
Attenuation 30db
Reference 0.0dbm
Mode - FM - No modulation

IMG_20250204_125527295.jpg

All settings same as above.
Mode - FM - Modulation applied
Note: mic gain wide open, power mic wide open, AMC wide open, deviation wide open.

Looks basically like an AM signal. Not seeing any of the deviated spikes as a normal FM signal would show on the analyzer.

Analyzer used:
Agilent E4411B
Attenuation - 30db

I've noticed that the C90 (supposed to be 100uf) I replaced with a 220uf is tied in with the PLL and vco, which also ties in with the deviation VR. Maybe that 220uf is too much for that circuit.

But still trying to track it down.
Mic amp shows proper voltage.
Audio IC shows proper voltage.
Balanced modulator shows proper voltage.
PLL synthesizer shows proper voltage.
Tx mixer shows proper voltage.
AGC shows proper voltage.
New Passthrough regulator.
New AF regulator.
New modulation transistor.
New 8v regulator.
Tried a new AMC transistor (TR32).
All 10v up to 35v caps replaced with nichicon brand.
AM modulation great.
SSB modulation fine.
FM modulation (deviation) almost non existent.

Receive in all modes is great.
All other functions work.
 
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I'm gonna go back over this recap work. I didn't have the following:

100uf of any voltage.
10uf of any voltage.

So I used these instead:

220uf for the 100uf.
22uf for the 10uf.

C90 and C108 supposed to be 100uf, I had none so I used 220uf instead.

C19, C20 and C331 is supposed to be 10uf, but I had none so I used 22uf instead.

Polish_20250204_144436305.jpg

Basically all the 100uf caps have been replaced with 220uf, and all the 10uf caps have been replaced with 22uf.

I need to get it back as close to factory before proceeding any further.
 
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Your spectrum analyzer is a great tool. Do you have an O-scope you can beg, borrow, steal? Do you have a schematic of your radio you can share so we all are on the same page? I would take a scope probe and check the audio signal from IC4 pin1 along the FM path till I lost it. or half the circuit at a time. The AM SSB and FM audio paths are all different. You have shot-gunned alot of parts. Also did you turn VR5 both ways, sometimes all the way to the right isnt max
 
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Yes, I have a fnirsi 1014d oscope, dual channel 100mhz. It also has the FFT feature. I also have an old HP 150mhz dual channel oscope, not hooked up though.

Yes, I noticed the FM path goes through c26, D67, D68, D69, d94 and a few other parts in a different path than SSB or AM.

The Saturn schematic I have is large, in PDF format included with the service manual. I always use just Galaxy DX-99v info on Saturns. Basically same radios.
 
I'll do that in the morning. Although I do have an old Heathkit signal tracer I need to learn how to use. It should be simple. It works fine, it came from our late local tech before he passed 4yrs ago.

Basically all I should have to do is turn it on, let it warm up til the tube turns green, then use the probe in AF mode to touch both sides of each component in the FM chain while keyed up with a tone running through audio line and listen for the tone until it drops, where it drops will be the bad spot. Isn't that correct? Sorry to sound stupid, just always used the old school shade tree trouble shooting route.
 
DX99v FM Path.jpg

FM Audio Path from IC4 Pin 1
C126 - 2.2uf @ 25v (+ side towards IC4 Pin 1)
R160 - 15k
D32 - 1N4148 - Switch to pass FM TX Audio to VCO
D31 - 1N4148 - Switch to pass FM TX Audio to VCO
R75 - 68k - FM +8v to turn D32-D31 on
VR5 - 10k(B - Linear Taper Pot) - Adjust for FM TX AC voltage from IC4 Pin 1 to superimpose onto PLL VCO buffered voltage
C87 - 222 (.0022)
C86 - 100pf
R119 - 220k --> VCO Buffered voltage from PLL

Take the "Mirror board" out to disable the Driver and Finals, using an Oscilloscope, and injecting a 1000 Hz tone or some other audio into the Mic input, you "should" be able to trace this path to see where the audio drops off, it will become lower as you go down this path, but completely drop off when you go past the bad component. It could even be D32 or D31, now that I think about it.

R172/C132/D67/D68/D69 are the Positive voltage peak detection circuit for Limiter - in all TX modes - highly unlikely.
D94 - SVC251 - additional Varactor diode in parallel with D30 (FC54M) to increase VCO lockup range - highly unlikely.

I would check R75 for +8v on one side (in FM Mode), this turns on diode switch D32/D31. The "high" side of VR4 should show maybe about a volt or 2? D31 Cathode ties in here.
C126 (2.2uf) could be bad.
C87 (222) could be open and not passing FM TX Audio.
C86 (100p) could be shorted to Ground.
 
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Well, I just used my signal tracer for the first time. Simple and fast. Probed pin 1 of IC4, then J21 (jumper that runs from pin 1 of IC4 over to C126). Audio blarred me out on pin 1 of IC4, J21, positive side of C126, but anything in that circuit from the negative side of C126 over to the FM deviation adjustment has very little audio.

This was done with radio in FM mode and in transmit mode.

So, I guess C126 is shorted. I thought the same as you unit399, I checked over all that the other day. All caps are oriented the way they should be. But if I did the signal tracing correct then it looks like C126 is bad, I can get FM transmit audio on the J21 side of the cap, but very very little on the other side of that cap, D31, D32, D94, vr5, etc...in that circuit has very little if any FM transmit audio when tracing with the tracer, but from pin 1 of IC4 over to the c126 contact closest to J21 has great audio.

I'll swap it out when I get back and let you all know what the outcome is.
 
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That did it. Used the signal tracer through the FM transmit audio chain, had great FM transmit audio on the J21 side of C126, but almost non existent on the opposite side of C126.

Yanked out C126, tested it out of circuit in my tester and it showed as a 710pf with an extremely high ESR. Replaced with a new 2.2uf 50v (shows on tester as 2.3uf in tester), FM transmit audio is back, and also the SSB audio became a lot more sensitive to every word.

Thank you guys... Now I need to pull out some of my 2950 parts radios and grab a part number for someone else.

This tracer is the ticket for tracing down signals. Now I just need to learn how to use it in the RF stage for a few radios that have great final bias voltage and amperage, but the drivers have good bias voltage but not bias current.
 

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