Galaxy Melaka weak AM receive, SSB fine

[Hawkeye351]

Got a Galaxy Melaka on the desk with a receive issue:

Replaced all 10v, 16v and 25v caps in the entire radio.
Complete alignment.
Complete receive alignment.

Single sideband receive is great. Transmit in all modes is great. All controls work as they should smoothly.

AM receive is down quite a bit. Locals who normally put over 9 units on me, only give me about 7 units at the most. Receive audio on AM is great though, I can hear everyone, even those I've never heard before. Just the am receive meter won't go past 7 units at the most. I can't get the meter to adjust up to 9 units when adjusting receive with the RF signal generator, SSB turned out fine.

Another thing on AM receive:
If a nearby station keys up, the receive will have a 2 second delay before recovering after they unkey. Also, flipping to AM after being on SSB, the AM receive takes about 2 seconds to recover.

I replaced C19, C20, C40, C44, etc .. in the receive route (pretty much all the caps in the radio except the 50v caps), slight improvement, but not much. The AM receive meter adjustment won't adjust the meter no higher than 7 units.

I got transmit bangin, lol... Both SSB and AM rock on transmit. Single sideband rocks on receive. Receive not so great on AM.

Would the 455 crystal filter cause just AM receive to be weak without affecting SSB receive?

 

[nomadradio]

Would the 455 crystal filter cause just AM receive to be weak without affecting SSB receive?

If it went bad, yeah. It's only in the signal path for AM receive.

C19 is where I point people first when this happens. It serves to prevent negative feedback from reducing gain. But you already changed it.

I would first look for "flush with the rim of the hole" tuning slugs in the 10 mm cans with the skinny slug especially L10,11,12. . Likewise if either of the 455 kHz cans L3 or L4 doesn't display a convincing peak that would be worth investigating further.

Either FL1 or FL2 will limit the AM sensitivity if it fails. Not a common thing to see, but not impossible. Tuned IF transformers have a higher rate of failure, just the same.

73

 

[Hawkeye351]

Thank you nomad for jumping in.

Yes, I replaced C19 along with pretty much every other cap in this radio except for the 50v caps. None of the IF cans flush out to the top or bottoms out. They are all no more than 2 maybe 3 full turns from the top is where they peak (2 maybe 3 full turns down from the tops).

I checked voltages on the following transistors:
TR17
TR18
TR13
TR12
TR30
TR31
TR9
TR10
TR11
TR36
TR37
TR52 (This one was all wrong)

All tranny's showed around the proper voltages, even when switching modes and from receive to transmit. The only one that showed out of whack was TR52 the sideband switching tranny.

It showed 13.5v on the collector (center pin) in SSB and in AM. The chart shows the collector as being 0 in AM, but mine shows 13.5v. The base shows the proper voltage in AM. The base and emitter show proper voltage in SSB, but the collector shows 13.5v.

I checked the voltage on both sides of D84 while in receive on AM. It shows .96v on the front leg (near front edge of board). And it shows around 1.5v on the back leg (up against a wire junction plug).

Would TR52 actually affect AM receive? I thought that tranny was just a SSB switch.

 

[Hawkeye351]

I replaced TR52 and now AM receive is back right.

Hmmm, never thought the SSB switch would affect AM receive, oh well, lesson learnt, lol...

On to the next one, a classic, 148 GTL-F with RFX95HD added.

Thanks guys for helping out, I appreciate any info ever passed my way.

 

[nomadradio]

I replaced TR52 and now AM receive is back right.

I would never have thought to suggest that one. But a short in TR52 could maybe turn on sideband and AM receive both at once? Not gonna bust my brain on that one for now. But I'll file it away for future reference.

Thanks for keeping us updated. Definitely a weird gremlin this time.

73