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Galaxy DX-2547 no AM output, SSB ok.

Hawkeye351

Hawkeye351

Well-Known Member
Jun 27, 2021
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Got another weird one on the desk.

Galaxy DX-2547 (no mods other than frequency expansion).

Issue:
No AM output

Checked:
AM receive - great
SSB receive - great
SSB output - great both modes
Alignment - great, all freqs deadon.
All switches/knobs work as new
Meter and all lights work as new
No heating issues

Talks great on either SSB.

Tested:
Passthrough - 2SB827 (weak, replaced with new 2SB817).
Q53, tested fine but replaced anyway.
Q56, tested fine but replaced anyway.
All diodes in the passthrough area (3, all tested fine).
Checked values of resistors in passthrough area (several, all are fine).
Checked traces and solder connections, all look fine.
Both AM power VR's (14 and 18) checked fine on resistance.

Passthrough voltages in AM xmit:
Base - 13.24v
Collector - 0.16v
Emitter - 13.65v

Adjusting either hi or lo power VR's (14 and 18) doesn't change the collector voltage on the passthrough regulator. Varying the RF power knob has no affect on the passthrough regulator either. RF Power works fine on SSB.

I can short out the base and collector and get an 18w deadkey though. Acts as though the passthrough isn't working.

Any ideas guys/gals?
 
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Q54 and Q55 form a darlington pair. If the failure of Q54 involves the base-emitter shorting or the collector opening, all of the current for the blown finals tries to go through Q55, the smaller transistor driving the passthrough regulator Q54. When finals blow, it draws max current through the regulator, and if the regulator failure involves anything but the opening of the base-emitter junction, the darlington driver gets smoked too. And with that amount of current, you can expect that failure in the passthrough driver to be an open. With Q55's emitter open, it cannot pull the base of Q54 down to turn it on. That 10k resistor going around Q55 is too high of value given the lower Hfe of the passthrough to turn the new one on much at all, which is what you are seeing.

Edit: just realized that has to work for SSB to work, sorry
 
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Already replaced Q53, Q54, Q55 and Q56. I also checked D102, D103 and D109 out of circuit, they checkout fine on tester. And resistors in this area all checkout fine. Haven't checked the disc caps in this area though.

I can short out the collector and base of Q55 and get a full 18w carrier on AM.
I can short out the collector and base of Q53 and get a full 18w carrier on AM.
I can short out the collector and base of Q56 and get a full 18w carrier on AM.
I can short out the collector and base of Q54 and get a full 18w carrier on AM.

Without shorting any of those transistors, I get nothing on AM, no carrier at all and no modulation at all.

Both sidebands work great on transmit.
 
Hmmm, interesting...

I rechecked VR13 and VR14 again. Must have skipped VR14 the first time around, but this time in circuit showed no ohms on both outer leads. Although VR13 showed 1,093 ohm (1K) on the outer leads. Yanked out VR14 to check it out of circuit and still no ohms (no indication on mm) on the outer leads. Took a new 502 and verified the outer leads, yep shows around 4.98k ohms.

Tired, I'll pop in this new VR14 tomorrow and get back with you guys.
 
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Update - FIXED:

Replaced variable resistor (VR14) with a new VR. Reinstalled passthrough reg, AF reg, etc...i had removed to check.

Now:
AM - carrier varies from 1w with RF power all the way down, to 3.80w with RF power all the way up.
AM swing around 18w peak, at about 8w average.
SSB - 18w.

Realigned radio, tuned it out, it's now working great.
 
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Update 2:
After talking on it a few hours I noticed the meter seemed a little floppy and would at times not show the correct signal on my locals as it was just a few moments earlier. I also noticed a slight change in the sound of the receive.

For the meters, I figured I should spray them with some cleaner. Well, after spraying them and working them back and forth, I noticed the sound of the receive would get staticy, then get quiet again. I replaced both VR1 and VR2 with new ones, then the meters would barely come off zero and wouldn't adjust up any higher. I turned the radio over and the receive sound came up and the meter pegged out. Set the radio right side up again since receive and meter seemed to work, but then they both went out again. I knew then it had to be a bad solder connection. From following Andy, nomad, shockwave, tmz and a few others on this site when they would be helping others out with similar issues, I then knew it had to be around the AGC/NB IC. Sure enough, there were 3 leads on the AGC/NB IC that were soldered to the board but the traces were cracked right at the leads of the IC. Soldered all 3 leads with jumpers going to their respective locations, BAM issue solved. Realigned receive and adjusted meters, haven't noticed any other issues since. Meter now functions properly all the time and the receive doesn't change anymore, even after picking it up and shaking it around aggressively.
 
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