[Boatanchors] BC-312/342 audio problem

[Dave Brown]

Maybe a 6DC6-
73
Dave, ZL3FJ
PS no original thort here- Collins uses 'em so they're probably OK..........


----- Original Message ----- 
From: "Ian Wilson" <ianmwilson73 at gmail.com>
To: "Boatanchors" <boatanchors at mailman.qth.net>
Sent: Wednesday, July 24, 2013 11:25 AM
Subject: Re: [Boatanchors] BC-312/342 audio problem


> Found it - when I replaced the front panel I not-so-cleverly shorted out
> the cathode bypass for the 2nd detector/1st audio 6R7. The audio was
> in fact getting heavily distorted as the (ganged) volume control was
> in the last 1/4 or so of its revolution. For some reason that was hard to
> spot with an audio tone.
>
> Much better now with 10MHz WWZ quite clear and its thunder-and-
> brimstone neighbor rolling in loud and clear.
>
> I am pondering a reversible mod: replacing the 1st RF (6K7) with
> an octal plug and something with a lower noise figure (and higher gain,
> although that's secondary). I wonder whether the 1st stage gain can be
> increased enough to compensate for the very noisy 6L7 mixer.
>
> One article I've seen used a 6SG7 (6BA6 is almost the same as a mini).
> I can almost do this without component modifications except for reducing
> the cathode bias resistor (500 ohms in the BC-312). I thought I could
> add a resistor from cathode to ground but this would kill the manual RF
> gain function. Back to the data sheets .. something that will have decent
> gm at low plate current ...
>
> 73, ian K3IMW
>
>
>
>
> On Tue, Jul 23, 2013 at 7:52 AM, Ian Wilson <ianmwilson73 at gmail.com> 
> wrote:
>
>> Hi Joe, Bry -
>>
>> I did replace the audio coupling cap on the small PCB lurking
>> behind the front panel. One advantage of having removed the
>> front panel :)
>>
>> The audio chain seems OK - I found some cables and the Heathkit
>> audio gen, fed it into the grid of the 6R7.
>>
>> Also found a signal generator that is more portable than the URM25-D
>> so was able to inject 470kc into various stages. Also seems clean. I
>> can see about -6.5V on the RF amplifier grids when in AVC mode.
>> Don't know what sort of level to expect there but it would seem that the
>> remaining Micamolds in the RF cans (AVC decoupling) are OK.
>>
>> Have to look for a VT-87 in The Box I guess.
>>
>> 73, ian K3IMW
>>
>>
>>
>> On Tue, Jul 23, 2013 at 7:41 AM, Joe Connor <joeconnor53 at yahoo.com> 
>> wrote:
>>
>>> Ian:
>>>
>>>      Did you replace that one cap (.01?) lurking behind the panel 
>>> between
>>> the 6F6 tube and the panel? That's an easy one to miss, and it's a 
>>> usually
>>> a notoriously unreliable Micamold. I believe it's an audio cap.
>>>
>>>                                     Joe Connor
>>>
>>>   ------------------------------
>>>  *From:* Ian Wilson <ianmwilson73 at gmail.com>
>>> *To:* Boatanchors <boatanchors at mailman.qth.net>
>>> *Sent:* Tuesday, July 23, 2013 10:02 AM
>>> *Subject:* [Boatanchors] BC-312/342 audio problem
>>>
>>> I've recapped this radio, apart from the AGC decoupling
>>> caps in the RF/mixer cans.
>>>
>>> Have replaced a few underperforming VT-86's and aligned
>>> the IF stages and the RF stages on 40m.
>>>
>>> Everything is basically working. However, the audio is poor,
>>> with a lot of high-frequency noise and distortion above about
>>> 75% on the volume control. Both MVC and AVC are like this
>>> but behave somewhat differently. Voltages on the RF gain
>>> pot and the various grids all check out OK as I vary the gain
>>> in the two modes.
>>>
>>> The audio is poor both on AM and SSB. I should look for an
>>> audio signal source, I guess, to check out the audio stages on
>>> their own. Am using an audio output transformer of unknown
>>> impedance ratios to a decent speaker.
>>>
>>> Anything obvious I should look for?
>>>
>>> 73, ian K3IMW
>>> ______________________________________________________________
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>>
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