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ISO: Tech help, RX issue on an RCI 2990

ExitThirteen

ExitThirteen

Grumpy and Cranky
Apr 18, 2008
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Cornpatch of Iowa
SO... a tech is needing a little help on an RX issue on an RCI 2990. The issue is that the RX gets very distorted or "blown out" on close signals, almost sounds like the mode is set on AM and the signals sound like they're on SSB, even tho the mode switch is actually set to SSB. My guess is that it would be an AGC issue, but I'm not 100% sure where to start looking. First time encountering this kind of a problem on this particular chassis. Any info on where I should initially look to start troubleshooting would be greatly appreciated. Thanks in advance!


~Cheers~
 

Just did the AGC mod, no change, still very distorted/overdriven audio. Perhaps it might be a bad capacitor somewhere, this radio is 30+ years old, and it has the "10V cap blues" thing going on. I'll do a re-cap on it in the near future. For now, I'll just set it aside until I can order up a cap kit for it. Thanks!


~Cheers~
 
I remember that any Galaxy/Ranger radio I've ever worked on that had receive overload issues, I would always check the first receive tuning can and make sure that it wasn't more than 1 turn from the bottom. They will pick up fluorescent lights and give you a 9db reading as well as receive overload if so. Just saying . . .
 
Xit13, from the way you describe it, im wondering if you have a carrier that should have been filtered out but wasn't.

does this sound like a possibility?
LC
 
Well, here's the deal, if I turn down the RF gain, I can get it to clean up, but it has to be barely cracked open, otherwise anything past about 8 o'clock on the RF gain and the RX starts to distort. And by this, I mean actual voice audio distorts. That's why I'm almost thinking that it could be a failing capacitor. But then again, I'm not 100% sure.


~Cheers~
 
check voltages on IC1, the quad op amp.
one of its functions is to control the gain of Q18, the 1st RF amp.
LC
 
There was a batch or two of these that had two disc capacitors with the wrong value.

One of them feeds carrier into the receiver's SSB (only) detector. This one was too small, and reduced the carrier level needed to turn that SSB signal back into audio. Can't remember where the other cap went. Turning down the RF gain far enough would always clear up this symptom.

Haven't done one of these in a while. I'll see if I have some notes on this next time I'm at work.

73
 
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Try as I might, a record of this fix does not turn up. I sure thought I had a bench file on it.

Looking at the schemo, the problem had to do with two ceramic disc capacitors, C89 and C90.

C89 feeds the SSB IF signal into Q16, the sideband detector. C90 feeds the carrier into it. In the Galaxy DX22B schematic, both parts are shown as 5 pf. In the mid-90s radios with the distorted SSB receive trouble C89 had a part with a (wrong) higher capacitance value, probably 10pf. Seems to me that making sure that C89 is 5pf and changing C90 to 10pf was the fix.

But that's just from memory, jogged a bit by looking at the diagram.

If C89 is 10pf (wrong) and C90 is 5pf, simply swapping them should make a noticeable improvement. C90 is the one that needs to have the larger capacitance.

73
 
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