[ARC5] BC-453 B dynamotor hum

[Ian Wilson]

Perhaps the spikes are on the 12A6 filament supply? They could be
coupling through the filament-cathode capacitance and would then be
happily amplified by the tube.

Perhaps check the 12A6 grid resistor (R20 - 2M<new units>ohm) also.

73, ian K3IMW


On Sun, Nov 1, 2015 at 7:24 AM, J Mcvey via ARC5 <arc5 at mailman.qth.net>
wrote:

> I had a scope on the grid but there was no DC. looking at the circuit, I
> saw that there was only the detector on the other side of the blocking cap
> so there is no B+ there anyway.Tried your suggestion and disconnected c29 ,
> just in case , to rule out human error as well. Even with the coupling cap
> C29 disconnected, the oscillation occurs ( sort of a nasty looking spike at
> approx 200 hz rep rate .) on the 12A6 plate.  The DC B+ looks relatively
> clean , so it's generated at the 12A6.
>
> The problem only occurs when the radio gets warmed up.
> Uh-oh could this be a bad output transformer arcing internally? There
> isn't much else to the 12a6 circuit! HELP!!!
>
>
>
>      On Sunday, November 1, 2015 1:58 AM, Brian <
> brianclarke01 at optusnet.com.au> wrote:
>
>
>  I've been working on Command recveivers for over 50 years. I have yet to
> find a gassy 12A6. I suspect that the capacitor providing DC blocking to
> the
> control grid of the 12A6 is leaking. To test, lift the lead from the
> capacitor going to the grid, but leave the grid leak resistor in circuit.
> Now, if my suspicion is correct, when you start up, there should be no
> noise
> and the HT should stay higher.
> 73 de Brian, VK2GCE.
>
>
> On Sunday, November 01, 2015 3:37 PM , you said:
>
> I probably should have posted the 600 m event since everybody here has a
> beacon receiver!
>
> Now back to that pesky hum:I think there is something going on with the
> 12A6. The rig is nice and quiet at zero gain when it starts up cold or even
> luke warm. one it gets hot there is an intermittent hum, then eventually a
> steady loud hum.  I monitored the grid and the cathode of the tube. I found
> that as it heated up the cathode bias went from almost 14V down to about
> 12V
> in full hum mode.The overall supply starts out at 205V , then ends up at
> about 186 when its humming.  the only thing that can draw down the supply
> like that would be the 12A6 . The 200 HZ does t appear at the grid, but no
> DC.  Tried shorting the the detector to ground with a large cap, but the
> hum
> was unaffected. I disconnected c30 and subbed another cap, but no joy.
>
>
> I'm thinking that it might be a bad tube with some kind of internal short
> or
> gas or ????
>
>
>
>
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