Re: [Drakelist] R-4C Audio Problem - RESOLVED!
Thanks to Garey and his documentation CD, my birdie problem is resolved! I was able to determine that somewhere in one of my receiver's previous lives a capacitor was added across R83 on the audio board. This provided a feedback loop which caused the audio board to oscillate at approximately 330 KHz. This waveform was clearly visible on the audio output transistor using a scope. It was also backfeeding on the +14 volt buss to the crystal calibrator board which modulated the crystal calibrator output, causing the birdies in the passband of the receiver. Adjusting the volume control changed the frequency of the fundamental oscillation in the audio amp just enough to change the frequencies of the birdies in the passband. My receiver is now working fine and will be taking its place on my operating table shortly. Without Garey's documentation CD in hand, it would have been a long and tedious search for that capacitor. Thanks again to all! 73 de Bill, K9RZ ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R-4C Audio Problem
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" To: 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 ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R-4C Audio Problem
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" To: 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 Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R-4C Audio Problem
Bill - The Calibrator problem is almost certainly the electrolytic C179 on the Calibrator board itself. This puts a little hum modulation on the calibrator signal. It's also possible that C167 has gone up a little in ESR as well, which might also account for some of the audio hum you are hearing. Another possible contributor would be C202. Best idea is a kit from "hayseedhamfest.com", but he is temporarily out of stock. He has the full kit with ALL electrolytics to drop in. One other possibility for the audio hum is just the sensitivity of your 'phones'. They're much more sensitive than your speaker and typically have a lower frequency response as well. The audio stages, like most, run "wide open" or maximum gain, all the time. The output level is determined by how much signal is put INTO the AF stages via the volume control. As a result, the s/n ratio is not very good at the very low levels, where you have minimum signal and maximum noise, (always maximum noise/gain,) as opposed to "some" signal with the same "noise" at the higher levels. It's best to have the AF GAIN control at 10-11 o'clock, which is normal for the speaker. One way around this is to put a series resistor either in the R-4C or in the headphone plug to drop the sensitivity of the phones to something similar to the speaker. This way you can run AF GAIN up where the s/n is better without blowing out your eardrums! I'd start with a couple hundred ohms or so and select whatever lets you set that gain control up into the "good" region. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs Bill Jackson wrote: 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 Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R-4C Audio Problem
Bill, The tube Drakes where not designed for 8 ohm headsets if that is what you are using, install a 100 ohm resistor in series with the headsets. I use aviation headsets that are around 300 ohm. As far as the calibrator issue try replacing the filter the capacitor on the calibrator baord. 73, Gary___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[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 Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[drakelist] R-4C Audio Problem
"Bill Jackson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang -- Greetings, I am working on an R-4C with a strange audio problem. When I rotate the volume control between the 9 and 11 o'clock positions, I hear a faint birdie oscillation at a couple of different spots on the 10m band. The frequency of the oscillation will change with the position of the volume control. Also, when I use a pair of headphones, I hear a distinct change in the quality of the audio in the same range of the volume control that produces the birdies. Any suggestions on what to look for? Thanks de Bill, K9RZ -- Submissions:drakelist@www.zerobeat.net Unsubscribe:[EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost:[EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net --
