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Very Weak Modulation Galaxy DX77 HML

W

wildchild455

Active Member
Mar 19, 2006
100
83
38
I am about to pull what little hair I got out over this issue. I was told that I have a great carrier but my modulation is very weak. PC Board EPT360014B. So heres what I have replaced so far (Now being I bought this radio a few months after Galaxy discontinued it). All parts were replaced with what was on the part list on CB Tricks.Com

1. Replaced IC8 the Audio Power Amp with the factory TA7222AP
2. Replaced TR41 2SA1869 Voltage Regulator
3. Replaced TR51 2SB754Y (What was on parts list) AM Modulation Amp (Component that was removed was a 2SB827)
4. Replaced TR50 AM Mod. amp 2SA473 part list showed the 473 but when it was removed the component was 2SA1869 which is the better component.

Still the same problem very weak modulation. Now I am going to throw this out there could it be a weak 2SC2166 driver and the Pre Driver 2SC2538.

Wanting to put the radio back on the table so I can replace the lamps in my Magnum 257. If anyone knows where or what type of lamps I need to get please let me know.

Any help would be appreciated
Thanks
 

Robb said:
Did you try another mic?
Did you check IC4?
Does it do the same thing on SSB as it does AM?

Doubtful it is the driver or final if you are seeing the right output.
Click to expand...
Yes we tried 3 different mics one stock galaxy one superstar
and a astatic D 636 L all the same results
Power is a little down on SSB from 35 watts PEP to 20 watts PEP power on AM is like it has been set for years Low 1.5 watts Med 3.5 watts High 8 watts get barely any forward swing no matter what setting, turned VR12 SSB ALC you can turn it to the end both ways and same power same low modulation and VR14 AM Mod. don't make a bit of difference stays at about 45 to 50% even with mic gain turned all the way up checked with a meter . Just checked this not too long ago.
Havent checked IC4
Talked to a tech that's retired he says it very well could be the driver getting weak so I am at a loss here
 
Robb said:
Did you peak the transmit coils too?

Check/set the bias mA's on the driver and final?
Click to expand...
Nothing has been don't to it since the parts were replaced. Nothing has been done to it in about 7 years took it off the table about 18 months ago to use the Magnum 257 since its a better SSB radio. Took it in for a alignment don't have the equipment to align it was told that it couldn't be aligned brought it back home started to search for solutions
Havent checked the mAs yet But will do that in the morning
 
Peaking the coils is essential for proper saturation.
Need a 50 watt/50 ohm dummy load, watt meter, and a means to inject a steady audio signal into the mic.
Done in SSB mode after the driver/final have been adjusted properly:

http://www.cbtricks.com/radios/galaxy/dx77hml/dx77hml_trans_adj.htm

Be sure to check the audio quality on another radio to ensure that isn't any distortion. The TX coils can also be over-saturated - even though you might be seeing a greater output on the watt meter. Can be misleading if one isn't aware of that fact. Check it out . . .
 
Last edited:
Definitely sounds like it is in need of a TX chain alignment. Off the top of my head, I believe it's L44, L43, L42, and L40.

Replace TR51 with a TIP36C.
Replace TR50 with a TIP42C.
Replace TR41 with a TIP42C.

These transistors are much beefier than the originals. They are also more cost effective too. This isn't necessary, but helpful, and you will see more output, if the radio is aligned properly.

The TX chain alignment is necessary, though. As Robb suggested, you'll need something that can produce a steady 1khz tone, a ceramic or plastic screwdriver, and a good wattmeter. Adjust those 4 cans and that should net you more output, IF they are out of alignment.

My Superstar 3900G had the same issue as yours until I did a good TX alignment on it, now it does 50W PEP on SSB and has excellent forward swing on AM.

Good luck. :)

~Cheers~
 
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Yep, forgot to mention using a plastic tipped adjustment tool for the coils.

DO NOT - under ANY circumstance - use a metal jewelers screwdriver.

It WILL break the delicate ferrite within the coil, AND THEN YOU WILL BE IN WORLD OF HURT!

Just - don't do that!!!
 
Last edited:
I might also add that it's good practice to pull the finals and driver and reapply some fresh thermal heatsink compound on them, and on the backside of the insulators, so they can thermally transfer the heat properly. That will also help things.


~Cheers~
 
Also - use the low power setting when adjusting these coils.

Running the radio at full output will put a huge strain on them and might cause them to fail. Wait one full minute between each coil adjustment to let the finals cool - too.
 
Robb said:
Also - use the low power setting when adjusting these coils.

Running the radio at full output will put a huge strain on them and might cause them to fail. Wait one full minute between each coil adjustment to let the finals cool - too.
Click to expand...
Robb said:
Did you peak the transmit coils too?

Check/set the bias mA's on the driver and final?
Click to expand...
Checked the bias and could not get it up past 35mA on the driver stopped after that didnt do anything else.
 
Robb said:
Also - use the low power setting when adjusting these coils.

Running the radio at full output will put a huge strain on them and might cause them to fail. Wait one full minute between each coil adjustment to let the finals cool - too.
Click to expand...
I did try to adjust the output on low power at 1.5 watts and did use another radio still sounded very low modulation
 

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