Many thanks to all who responded. I swapped out the calibrator board with a known good one from my other R-4C and it made no difference. The birdies really appear to be related to the audio amplifier section. I tried the suggestion of putting a 100 ohm resistor in series with the headphones which reduced the audio level, but the birdies are still present. They are also audible on an MS-4 external speaker.
As I said in my original note, they are definitely appearing in the passband of the receiver, as they can be peaked with the preselector but change in frequency with the position of the volume control. I suspect something is "flying" somewhere in the power supply or audio amplifier section that is "tunable" with the volume control. I will try to borrow a scope and obtain some capacitors to start my search for the offender. Thanks again. 73 de Bill, K9RZ ----- Original Message ----- From: "Bill Jackson" <[email protected]> To: <[email protected]> Sent: Friday, February 05, 2010 10:01 PM Subject: [Drakelist] R-4C Audio Problem > I have an interesting problem to present to the group. > > I'm in the process of restoring a later model R-4C that has a peculiar audio > problem. The receiver is working fine with excellent sensitivity on all bands, > except that on 80m when I turn on the crystal calibrator, I hear what appear to > be weak, but very raspy spurious birdies in between the 25 KHz markers. When I > vary the volume control, the birdies sweep through the passband of the > receiver. Also, when listening to the audio output with a pair of headphones > when the receiver is muted, I hear the usual low level hum that is typical for > the R-4C, but it changes characteristics over a relatively small range around > the 9 o'clock position of the volume control. The hum does not change in > volume, it just sounds rather raspy over this small range, similar in > characteristic to the birdies I hear in between the 25 KHz markers. Is it > possible the audio output stage is self oscillating at some odd frequency and > is somehow mixing with or modulating the output of the crystal calibrator? > > I don't have access to a scope, but I do have access to a good quality Fluke > DVM and an IFR service monitor. I would appreciate any suggestions on where to > start looking. This particular receiver was in storage for about 15 years, so > I wouldn't be surprised if there are some electrolytic capacitor issues. > > Thanks in advance. > > 73 de Bill, K9RZ _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
