[ARC5] BC-453 B dynamotor hum

[Ron Barlow]

If something was causing the 12A6 to conduct more heavily, after extended operation, the 12A6 cathode voltage would increase, not decrease, as per your description. Aside from that, the coupling cap to the 12A6 grid, does not have B+ on it, so leakage would not cause an elevated grid voltage, on the 12A6 stage. Also, the cap in the 12A6 plate circuit can be eliminated, as its' "cold" end is not grounded.  If the tube heaters are wired for 12 volt operation, you should be able to pull out the12A6 (and other tubes), one at a time, in order to disable the associated stage, and aid your efforts to determine the source of excessive current drain (and oscillation). If the heaters are wired for 24v operation (original configuration), you would be disabling 2 stages, when you remove any of the tubes. This may still provide some  evidence that could direct you toward the fault.              GL & 73 de Ron  n4gjv       


     On Sunday, November 1, 2015 12:43 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:
   

 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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