[Drake] 2B mute problem.

[Tony Sokol]

Bingo!  V1 the 6BZ6.  I put the DMM on the AVC terminal and watched as I 
pulled tubes.  I pulled V1 and the voltage immediately shot up (or down 
in this case) to under -2.0 volts.  I set it to -1.5 per the manual and 
viola!  FYI I compared the readings with my Fluke 77 DMM and then with 
my HP-410B VTVM and they were almost identical so the loading of my DMM 
must be pretty light.
     Anyway thanks for the lead.  I can tell that someone previously 
spent a lot of time chasing the problem and ended up jury rigging it.  
It's nice to have it up and running.

Thanks
     73
Tony - W9JXN




On 5/27/2014 8:32 AM, Garey Barrell wrote:
> Tony -
>
> The 2-B has an extremely high impedance AVC circuit.  Even an 11M 
> input impedance VTVM 'loads' the circuit slightly.  If you are using a 
> DMM with a lower input impedance that "may" be part of your problem.
>
> That aside, leaky caps are the most likely cause.  C68, C69 and C80 
> are all candidates.  Of course resistors tend to go up in value over 
> time.  One obscure problem is leakage from the wiper of the RF GAIN or 
> BIAS ADJ pots to the case/ground. Gassy tubes can also cause this, you 
> can check by pulling the AVC controlled tubes, V1, 2, 4 and 
> particularly V5 one at a time to see if they are loading the AGC buss.
>
> Replacing that 6U8 by a 6EA8 will improve overall receiver performance.
>
> You might want to join us on DrakeRadio at Yahoo groups.  A far more 
> active group with over 1200 members, and some very experienced 'help'.
>
> <http://groups.yahoo.com/group/DrakeRadio/>
>
> (You met my Daughter and Grandson at the St Charles R/C airfield a few 
> years ago!)  Leslie and Ryan.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
> and TR-4/C Service Supplement CDs
> <www.k4oah.com>
>
> Tony Sokol wrote:
>> Hi Gang,
>>     I picked up a 2B at Dayton this year that was less than perfect.  
>> In fact, I probably should have worn a HazMat suit to handle it but 
>> close examination revealed a minimum of scratches and no dents.  
>> Under all that grime was an almost perfect panel. The little receiver 
>> cleaned up really nice and with the help of a new Hayseed Hamfest 
>> filter cap the rig is performing really great.  The last problem I am 
>> trying to chase down is the lack of adequate bias voltage to mute the 
>> receiver.  The RF gain works perfectly so the bias supply is working 
>> fine but I can not get more than about a volt at the AVC terminal on 
>> the back. Somewhere along the line someone hay wired a pot to bring 
>> up the voltage on the terminal strip but I removed that and am 
>> attempting to solve the problem rather than band-aid it.  I have 
>> cleaned up a lot of the poor solder connections where I can see that 
>> someone had unsuccessfully tried to solve it. Any thoughts?
>>
>>     Thanks
>>        73
>> Tony - W9JXN
>>
>> PS:  I tried replacing the 6U8 per an on-line article.  No joy.
>>
>>
>>
>>
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