[Drake] Drake 2B Hum

[Kurt]

My schematic shows an 8BN8. As I recall when I tried a 6BN6 there was a 
noticeable increase in hum.

Regards, Kurt    KU7RT
----- Original Message ----- 
IFrom: "Bill Stewart" <cwopr at embarqmail.com>
To: "Drake" <Drake at mailman.qth.net>
Sent: Friday, February 14, 2014 4:02 PM
Subject: [Drake] Drake 2B Hum


>
> GA all,
> It's hair pulling time here on ye ole work bench.
> I have a 2B rcvr which has a noticeable hum. When first powering up,
> the hum comes up in volume, then dies back down to a steady hum...not
> blasting but noticeable a couple of feet from the spkr. The vol.
> control does not change it. The can lytic is disconnected and modern 
> lytics
> are in place. I have replaced all the paper bypass caps.
> Here are other things I have done/tried:
> - I clipped in other lytics in place of the first replacement ones....no 
> change.
> - Makes no diff. whether it is in prod. det. or diode position.
> - Disconnected from the det/diode switch at vol. control...no change.
> - Changed out both audio tubes...no change
> - Chd'd coupling cap 6BN8 to 6AQ5, .005...no change.
> - Replaced 6AQ5 plate bypass cap (.001)...no change.
> - Replaced 6AQ5 cathode bypass lytic...it was cracked, but....no change.
> -Changed the 6X4...no change.
>
> The last thing I have done is to remove all tubes, except the 6X4 and 
> 6AQ5. The
> hum is still there and still very noticeable a couple feet from the spkr.
>
> I've probably got tunnel thinking, but it appears to me that the pblm is 
> in the audio
> section. In the 6AQ5 ckt, I have temp. replaced C71, .01 across the audio 
> xfmr;
> the.001 from pin 5 to gnd; & replaced the cathode 10uf lytic. Abt the only 
> thing left
> is the audio xfmr and some resistors. Looks like the only inputs to the 
> 6AQ5 are via
> the screen and B+ lines (with all tubes pulled but 6X4 & 6AQ5). 
> Unfortunately, no
> scope available.
>
> Any suggestions would be very helpful. Tnx...
> 73 de Bill K4JYS
>
>
>
>
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