[Boatanchors] BC-312/342 audio problem

[Ian Wilson]

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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