I'm currently working on final touches of restoring an old military tube HF
amplifier that has very useful mechanical memories. It's constructed in a
way that there is no need to retune within the same ham band (except the
top of 10m, mind this is Europe so 80m is much shorter here). Which makes
it very nice for remote work.
To make it recall one of its memories it needs a -27V signal onto one of
the control lines.
The way I originally set it up is with a 12 way rotary switch and a
momentary switch that applies power once a selection was made.
This has been working well enough for me to make quite a few contacts and I
have no complaints. But I'd like to make this switching remotely operated
so I've added a computer connected control box.
The control box contains a USB connected microcontroller that sends 3.3V
logic signals out (that part is working fine). These then drive NPN
transistors which control miniature 12V relays. These relays are then
connected "on top of" the rotary switch essentially bypassing it if the
relay closes.
The problem is I'm getting RF getting it's way into the bases of these
transistors and activating the relays when they shouldn't be. Here is the
schematic , more info below. This is all through hole construction BTW.
https://ibb.co/ZJGDvK9
There are also 2 optically insulated inputs, but they are irrelevant. None
of this RF is bothering the microcontroller, nor the PC. Just the BJT
transistors. BTW, transistors are 2n2222
As you can see I've added capacitors to ground to try shorting any RF.
Leads going to ground are pretty short, but the problem is getting worse
the higher the frequency so grounding inductance is probably playing a
role. I'm not sure how as the whole thing is grounded to the metal frame
and also signal ground is sent within the shielded wire bundle too and
connected to it (these two grounds were originally separate, connecting
them made no difference ).
What made a difference was to add 6 split ferrites to the connection
between the old manual and new pc controlled control boxes. Even one made
it work fine on 80m, 2 on 40m, but even with maximum number (more will jot
fit) at high power on 20m the rf exceeds about 1.5v on the bases and
switches activate. Same happens on 10m at barely 200W.
I can see this RF getting in through both the ground (connecting an
oscilloscope probe between the ground and metal frame shows about 2V pp,
adding a 10nF cap only drops it by 20%). Also via the control lines (same
voltage). And here too, adding capacitors dropped the voltage by only 20~30%
An obvious thing to do would be to add capacitors between bases and ground
right? I guess so, but it requires ripping out all the assembly while my
experience with caps here so far tells me they fail to short the RF. No
doubt due to some inductance to ground.
So I decided to ask here. Perhaps I'm not aware of some better method
etc.(other than running every single line in its own coax like +12v is
using)
I'm thinking about adding these capacitors as 100nF or 10nF (depending
which one will fit) smd parts right between the base and emitter tht pads.
Do you think that will work? If not, what would you do?
73,
Łukasz - SP4IT
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