• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • The Winners for the Retevis HA1UV drawing have been selected. Click Here to see who won!

Galaxy Saturn (EPT360014B) FM audio low

Hawkeye351

Sr. Member
Jun 27, 2021
616
551
103
57
Got a Galaxy Saturn base on the desk (still waiting on cap order to finish up Osaka).

Issue:
FM audio (modulation) is low to almost non existent on both PEP and average. I know FM isn't supposed to swing, I should have said my locals can barely hear me on FM at all, they say I have a great FM carrier but almost no modulation on FM.

AM audio (modulation) fine on both PEP and average. PEP shows 53w, average shows about 25w.

SSB audio (modulation) fine on PEP but low on average. PEP shows about 40w, average shows barely 10w and not very sensitive to every syllable, even on PEP SSB output is not very sensitive to every syllable.

Recapped entire radio (last of my 10uf and 100uf caps).

Replaced Passthrough regulator.

Replaced AF regulator.

Replaced modulation transistor (beside AF regulator).

Replaced TR32.

Adjusted bias.

Tuned output.

Complete alignment.

Cleaned all controls.

Note:
In order to even barely hear any modulation when transmitting on FM, the mic has to be cranked wide open, the power mic has to be cranked wide open and the FM deviation adjustment has to be cranked wide open. Whereas on SSB and AM it sounds great (according to reports on the air) with mic gain at 12 o'clock (straight up), power mic set at 5 (goes from 1 to 10).

All frequencies lined up great.
All voltages (buffers) lined up great.
Vco lined up great.
Oscillators (both PLL and carrier) lined up great.
RF stage lined up great.
Bias adjusted great.
Receive lined up great.

Any ideas?
 

Have you had more than one local tell you the FM audio is low? The reason I ask is because if you listen to an FM signal with the receiver in AM mode you get the EXACT symptom you are describing........... you can just barely hear some modulation with everything cranked. Do you have an FM capable radio you can hook up on your bench and monitor yourself on? Or alternately, find an online SDR you can hear yourself on.
Nothing like hearing it (or not hearing it) with your own ears, rather than trusting other folks on the radio to tell you...........
 
Yes, the two guys that checked it out on the air for me both were on FM with me. Plus after they told me the modulation (deviation) was low on FM then I turned up my Yeasu FT-950 on FM mode and listened, sure enough the modulation on FM is almost non existent even if I crack mic gain wide open, power mic wide open and deviation adjustment wide open the audio can barely be heard.

I've prodded around in the radio topside and bottomside tapping on parts, wires, IF cans, ICs, etc...and solder joints, no affect whatsoever. Nothing improves it.

Could it be the mic amp?
Could it be a crystal filter (FT1, FT2, FT3)?
Could it be a final/driver/pre-driver breaking down?

No swing mods at all, and all limiters are intact. There was a diode and resistor tied together coming off the base of the modulation transistor (beside AF regulator) but I yanked that out and then replaced the modulation transistor.

R249 was clipped, I replaced it.

TR32 was removed, replaced it with a new one.

Replaced SSB modulation VR adjustment with new one of same value.
Replaced AM modulation VR adjustment with new one of same value.

Replaced all three bias adjustment VR's in front of driver and finals with new ones of same values.

Could be FM deviation adjustment, but I doubt it would cause this low of audio on FM.

I'm at a loss of what it could be, been tackling it the past couple days. Receive on FM is great.
 
  • Like
Reactions: BC Coyote
Not super familiar with that radio, but I suspect the low average SSB output is unrelated and whatever the FM problem is must be something that is only active in the circuit in FM transmit mode. That narrows it down to a fairly small number of components. Is there an IC or two that comprise most of the FM tx section? I'd start there.
 
Checked voltages on the mic amp (4558) in both receive and transmit, it shows about what it should be.

Checked TR32 in both receive and transmit, it shows exactly what it should show.

Checked FM deviation adjustment VR, value is very close to what it should be and adjusts smoothly.

Guess tomorrow I'll check the audio IC (7222AP), balanced modulator IC, tx mixer IC for any irregularities. I may recheck around in the Passthrough regulator area where that diode/resistor swing mod was tied onto the base of the modulation transistor beside the AF regulator was at to see if they changed anything else in that section that went along with that swing mod.

I don't get it, I thought AM and FM audio shared the same chain in the circuit.

By the way, thanks for chiming in BC. The extra brain is very helpful.
 
  • Like
Reactions: BC Coyote
A 'scope on pin 1 of IC4 would reveal if the mike amp is delivering a couple of Volts or more peak-to-peak mike audio. Should be closer to 4 Volts. If not, C128, a 1uf 50V electrolytic is famous for going open and reducing the gain if IC4.

73
 
  • Like
Reactions: Hawkeye351
C128 or C129? Reason I ask is C128 is a .22uf 35v tantalum capacitor. Whereas C129 is a 1uf 50v electrolytic capacitor. I replaced C129 with a new 1uf 50v when I recapped it the other day.

I checked C128, it checked out fine but I replaced it with a new one anyway. I do hear a slight more modulation on FM after replacing C128, but it's still extremely low.

I probed pin 1 of IC4 with just my volt meter for now in both receive and transmit. In receive I get 4.65v on pin 1 and in transmit I get 4.60v on pin 1.
 
I wonder if C108 or C90 being 220uf instead of the stock 100uf would have anything to do with my issues on this radio.... I had no more new 100uf left to replace these two, I figured it wouldn't hurt to use 220uf instead, but maybe that's the issue.

C90 is between the PLL and FM deviation adjustment.

C108 is next to the PLL oscillators and carrier oscillators.
 
Try scoping both sides of c126, a 2.2uf that leads from ic4 pin1 to the back to back diodes d32 and d31. Check those diodes also, and vr5 and d94.

A circuit descript per Lou Franklin:
IMG_20250127_080755296~2.jpg
As soon as mic audio is amped in IC4, it takes a different path than AM or SSB. I still don't think those 220uf caps are gonna hurt, but I am wrong often enough to warrant a check.
 
As another thread on this site stated in which the same kind of radio was exhibiting the same issue, I removed TR32 and checked FM audio (it actually came up some enough to use). Both SSB sound great without garbling or warbling and the modulation sensitivity improved dramatically on both SSB. I can't leave that out though, I don't like running radios with no limiters, I did this just to test if the audio would improve by removing it, this tells me something is screwing with TR32 that it won't function properly.
 
Gonna replace that TR32, then I'm gonna check voltages in receive and transmit on all the ICs and transistors that could affect the audio. I'll post voltage numbers in a bit.
 
I'd check the base of tr32 during am and then FM. See if it's turning on harder in FM than AM during voice peaks. That would mean your suspicions of tr32 being to blame is right, but more of an effect and not the cause. The path from pin1 of the 4558 around through the diodes that help limit FM audio and it's circuit, to tr32, should be checked.
1737993972749601334336115620440.jpg
 
  • Like
Reactions: Hawkeye351
Could be FM deviation adjustment, but I doubt it would cause this low of audio on FM.
Absolutely could be FM deviation, it has a massive effect on received audio of FM transmissions. Take a ham HT that runs on narrow (2.5kHz) or wide (5kHz) FM deviation. If you are receiving a narrow transmission on a FM radio set to receive wide deviation it will sound very quiet. If you're receiving a wide transmission on a radio set to receive narrow deviation it'll sound very loud and distorted.

Legal FM deviation in Europe is 2kHz, on UKFM it's 2.5kHz. Because of how strict Germany is in enforcing that most CB manufacturers set the FM deviation to 1.8kHz out of the box which makes the received audio sound quiet, especially on UKFM. Knights CB here in the UK increase the FM deviation on every unit they sell to 2.5kHz.
 

dxChat
Help Users
  • No one is chatting at the moment.