[Drakelist] R4C Hum and audio transistor
I have a very nice R4C it had a hum with bad audio distortion so I decided to investigate, whilst taking it apart I noticed that if I touched the noise blanker the hum would alter, I removed the NB board and the hum was still there, I found eventually that the audio transistor had desoldered itself, do they really run that hot? so I resolded the wires, now the radio does not work anything like as well and has a high amount of white noise, has that audio transistor died and if so where do I get a new one, is there an equivalent for that power transistor? 73, Max M0GHQ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R4C Hum and audio transistor
Max, I'll get crucified for saying this, but that class A transistor stage was poorly designed -- and I think we can dispense with the but they didn't have cost-effective transistor technology back thirty years ago as an excuse. A two-transistor complementary-symmetry PP pair running class AB and a moderate amount of negative feedback would have sounded substantially better, and produced much less heat. As an example, look at the Marantz, Sansui, Kenwood, and other mid-fi Japanese audio amplifiers being produced during the same period. Those designs, or a variant, could easily have been leveraged into the R-4C. Eliminate the audio output transformer and tap the symmetry pair mid-point, then couple with 500+ uF of series coupling C. I don't know what Drake's cost was for the audio output transformer, but I have to believe the production cost difference between the transformer and an extra audio output transistor, and surrounding passive parts would have been reasonably minimal. I jettisoned the R-4C AF stage after the volume control and won't be going back. Paul, W9AC - Original Message - From: Max Cotton To: drakelist@zerobeat.net Sent: Wednesday, October 31, 2012 2:10 PM Subject: [Drakelist] R4C Hum and audio transistor I have a very nice R4C it had a hum with bad audio distortion so I decided to investigate, whilst taking it apart I noticed that if I touched the noise blanker the hum would alter, I removed the NB board and the hum was still there, I found eventually that the audio transistor had desoldered itself, do they really run that hot? so I resolded the wires, now the radio does not work anything like as well and has a high amount of white noise, has that audio transistor died and if so where do I get a new one, is there an equivalent for that power transistor? 73, Max M0GHQ -- ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R4C Hum and audio transistor
I tried one, but to my ears, there's too much hiss with some high-end headphones with the AF control at low settings and even full CCW. I've tried the LM383, LM380, and TDA2002. The hiss can be reduced with an in-line attenuator, then running the AF control at a higher level to compensate. I would rather just deal with the root issue and solve it. Paul, W9AC - Original Message - From: kc9...@aol.com To: w...@arrl.net ; drakelist@zerobeat.net Sent: Wednesday, October 31, 2012 2:39 PM Subject: Re: [Drakelist] R4C Hum and audio transistor Why not install a Sherwood audio kit ...I have one in mine..works FB 73, Lee -Original Message- From: Paul Christensen w...@arrl.net To: drakelist drakelist@zerobeat.net Sent: Wed, Oct 31, 2012 2:37 pm Subject: Re: [Drakelist] R4C Hum and audio transistor Max, I'll get crucified for saying this, but that class A transistor stage was poorly designed -- and I think we can dispense with the but they didn't have cost-effective transistor technology back thirty years ago as an excuse. A two-transistor complementary-symmetry PP pair running class AB and a moderate amount of negative feedback would have sounded substantially better, and produced much less heat. As an example, look at the Marantz, Sansui, Kenwood, and other mid-fi Japanese audio amplifiers being produced during the same period. Those designs, or a variant, could easily have been leveraged into the R-4C. Eliminate the audio output transformer and tap the symmetry pair mid-point, then couple with 500+ uF of series coupling C. I don't know what Drake's cost was for the audio output transformer, but I have to believe the production cost difference between the transformer and an extra audio output transistor, and surrounding passive parts would have been reasonably minimal. I jettisoned the R-4C AF stage after the volume control and won't be going back. Paul, W9AC - Original Message - From: Max Cotton To: drakelist@zerobeat.net Sent: Wednesday, October 31, 2012 2:10 PM Subject: [Drakelist] R4C Hum and audio transistor I have a very nice R4C it had a hum with bad audio distortion so I decided to investigate, whilst taking it apart I noticed that if I touched the noise blanker the hum would alter, I removed the NB board and the hum was still there, I found eventually that the audio transistor had desoldered itself, do they really run that hot? so I resolded the wires, now the radio does not work anything like as well and has a high amount of white noise, has that audio transistor died and if so where do I get a new one, is there an equivalent for that power transistor? 73, Max M0GHQ ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R4C Hum and audio transistor
I must admit this receiver will probably benefit from the Sherwood audio board, however comparing both my R4C's one is perfect with no hum and very good audio whilst this one is shushy and has bad hum, maybe the main caps need replacing but when I couple another cap accross them no change so I am a little reluctant to ditch them quite yet. swapping the tubes between each made no difference either. It does do an odd thing though, gradually as it warms up the audio and signal gets worse until it is distorted and with a lot of white noise. When switched on, the hum is really bad immediately and does not go away. both receivers are 26000 serials within about 700 from each other. Ideas? 73 Max M0GHQ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R4C Hum and audio transistor
- Original Message - From: Max Cotton airra...@dsl.pipex.com To: drakelist@zerobeat.net Sent: Wednesday, October 31, 2012 11:10 AM Subject: [Drakelist] R4C Hum and audio transistor I have a very nice R4C it had a hum with bad audio distortion so I decided to investigate, whilst taking it apart I noticed that if I touched the noise blanker the hum would alter, I removed the NB board and the hum was still there, I found eventually that the audio transistor had desoldered itself, do they really run that hot? so I resolded the wires, now the radio does not work anything like as well and has a high amount of white noise, has that audio transistor died and if so where do I get a new one, is there an equivalent for that power transistor? 73, Max M0GHQ Which transistor desoldered itself? Its possible it was overdriven because the bias was wrong. The three transistors in the audio amp are direct coupled so a problem with any of them will affect all. Check the power supply voltages and the amount of ripple on each. While a scope is the best tool for looking at ripple any AC voltmeter capable of reading perhaps ten percent of the DC voltage will do. Distortion is likely if one or more of the transistors is bad. Check the caps around the audio amp. You will have to check them for leakage rather than ESR. You will probably have to substitute them. Unfortunately, the only schematic I have for the C is from BAMA and is so low res that I can't even read the voltages or part designators. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] R4C Hum and audio transistor
Hi Max, I replaced the original audio amp in the R4C with a Velleman audio kit board with an TDA2003. The Velleman kit # K4001. I believe I paid around $ 15.00. Works great. If you're interested I can send you pics. 73 Bert, VE3NR _ From: drakelist-boun...@zerobeat.net [mailto:drakelist-boun...@zerobeat.net] On Behalf Of Max Cotton Sent: Wednesday, October 31, 2012 3:12 PM To: drakelist@zerobeat.net Subject: Re: [Drakelist] R4C Hum and audio transistor I must admit this receiver will probably benefit from the Sherwood audio board, however comparing both my R4C's one is perfect with no hum and very good audio whilst this one is shushy and has bad hum, maybe the main caps need replacing but when I couple another cap accross them no change so I am a little reluctant to ditch them quite yet. swapping the tubes between each made no difference either. It does do an odd thing though, gradually as it warms up the audio and signal gets worse until it is distorted and with a lot of white noise. When switched on, the hum is really bad immediately and does not go away. both receivers are 26000 serials within about 700 from each other. Ideas? 73 Max M0GHQ ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
