[ARC5] BC-453 B dynamotor hum
J Mcvey
ac2eu at yahoo.com
Sun Nov 1 10:24:52 EST 2015
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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