IC-735 strange tone in SSB, CW receive audio
I just acquired a IC-735 at Hamvention last weekend, and it's got an odd problem that I've never encountered in a radio before.
While in CW or SSB modes (LSB or USB), it has a audio "whistle" that sounds fairly "pure" in the receive audio. It's mostly noticeable on 40/80/160. It's not a receive heterodyne from QRM, you can move the tuning dial and the tone and volume is constant. The notch filter does nothing, but using the pass-band tuning knob mostly eliminates it at extreme ranges of the passband (but then reception is shot). This tone is not observed when the radio is in FM or AM modes.
The fact that it's SSB or CW only, and it flips to opposite sides of the passband when switching SSB from lower to upper sideband initially made me think I might have some sort of BFO issue (alignment, BFO harmonic, etc, etc), but for two things:
1. The tone is apparent as soon as the radio is turned on at a higher pitch then drops lower as the radio "warms up" (as much as any solid-state rig does, I suppose).
2. Secondly, but even weirder, the tone changes pitch, and intensity somewhat when the RF POWER slider is moved up or down, with it going up in pitch as the slider is moved toward maximum. Not RF Gain, RF POWER. As in TX. My TX audio was reported as very clean by a local ham last night on 40m. Moving any other slider or turning on and off the preamp, AGC, NB, etc, does not have the same effect.
Things I have tried so far: Checking for loose internal connections, obviously burned or popped components. Tested this problem while running on battery power, and with the receivers' RX antenna port (The RCA jack the jumper connects to from the TX/RX relay) terminated with a 50 ohm resistor. I also gently thumped the radio and shook it to see if it was somehow loose-component or microphonic somehow. This does not seem to be the case.
Could this be a bad filter cap in something somewhere? I'd be chasing stuff in the receiver only if it wasn't for the RF Power slider directly affecting the issue. That's where I'm stumped (but is probably my lack of experience showing)
Any suggestions as to where I should look next from the more seasoned salts here on the Zed?
Not absolutely sure but it seems like the product detector used for SSB and CW has the normal crystal oscillator running and it is picking up something else in the rig to put out a beat note. I don't know what frequency your tone is and that might be a helpful thing to know. Perhaps the AM/FM carrier generator is on during receive and it shouldn't be.
There are other possibilities but that one comes to mind.
Does the tone vary with the volume control?
This is just a shot in the dark right now. Get back to me with this information.
P.S. do you have the service manual?
Last edited by KO6WB; 05-30-2012 at 12:47 AM.
Yes, I have a copy of the service manual.
The tone varied depending on where the RF power slider was set. Weird for it to be having an effect on RX mode, right? Turns out that the issue was closely related to voltage potentials on ground. When the voltage the pot was dumping to the ground plane went up, the pitch went up. Something was acting like a VCO, but I couldn't figure out what yet.
At that point, I contacted Scott, KA7MEF at MTS Ham Radio Repair to see if this was something he could help me out with if I shipped the radio to him, because apparently my 20-year-old bench tech skills had woefully atrophied. He very kindly told me to check in the audio amp section of the radio, he'd never seen this particular issue in a 735 before, but figured there was a bad bypass cap in the audio stage.
Taking another look at that part of the mainboard, it wasn't immediately obvious what was wrong. Then I remembered the old maxim the guy who trained me years ago taught me: "Sometimes, if you know you've got a bad component, it's sometimes not what you see, but what you smell that gives it away".
Sure enough, that did it. Something smelled...of toasted capacitor... Breaking out a handheld magnifier (as opposed to the wide-field one I've got as a bench lamp), I found it. There was a small tantalum ex-capacitor tucked right up against a big filter 'lytic. Kinda cracked and carbonized. It's C361 on the 735's parts list/schematic if anyone's curious. Said cap is a 10uF 16V tantalum that acts as the primary filter cap for power to IC14, the audio amp. As that's a fairly common value, I actually had a couple of them. 10 min of desolder/solder time later, the capacitor that went foom was replaced.
Got the chassis back together (Icom sure likes screws) and fired it up. No more audio whine.
So my prior post was fairly accurate, there wasn't anything OBVIOUSLY burned, it was hiding next to its big brother.
N8XRE hows once again that quite a few troubleshoots can be done with not very much more in the way of test equipment than the senses God gave you and a bit of savvy.
Decades at the troubleshooting bench here serve to testify that this is indeed the case.
Look, Feel, Vibrate, Smell, Listen.
Originally Posted by KE3WD
Are You working on your Spouse or a radio ? lol
Sorry i could not resist.
"Books tell how it should be, Experience tells how it really is..."
73 DE KA9JLM Don
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