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
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