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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th Jul 2019, 8:29 pm   #1
veedub565
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Default Odd problem with HF set

Hi, I have a bit of an odd problem with my Plessey badged TRP 8255 HF set. On receive only it works just fine, but as soon as I turn on the TX I lose the RX.

For example a good clear contact on 40m disappears into the noise when the TX is activated

Please note I am not keying it up at all, just turning on the TX side of it.

I'm trying to work out if this is a fault in the unit, or maybe some other problem. I have the Automatic ATU connected up with a 20M long wire. Rig and ATU are grounded to an earth rod.
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Old 17th Jul 2019, 8:47 pm   #2
merlinmaxwell
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Default Re: Odd problem with HF set

It would, there is no point in trying to RX your huge TX signal when you know what is being transmitted! And it saves the RX from overload. Also most older sets didn't have break in keying.
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Old 17th Jul 2019, 9:32 pm   #3
G3VKM_Roger
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Default Re: Odd problem with HF set

Is it possible that a fault on the transmitter is generating noise or is somehow reducing the receiver gain, when you power up the TX? Do the TX and ATU both work?

Cheers

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Old 17th Jul 2019, 11:07 pm   #4
g4aaw pete
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Default Re: Odd problem with HF set

Hello Ben

this transceiver looks a serious piece of equipment:-
http://www.greenradio.de/htm2/e_skanti.htm

I note that reliability is perhaps an issue with these.
Wonder if you have a manual with voltage test-points given?
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Old 18th Jul 2019, 7:33 am   #5
AlanC
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Default Re: Odd problem with HF set

I wonder if the signal path is switched in some way when you switch the TX on... this sort of symptom is classic for relay problems!
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Old 18th Jul 2019, 8:45 am   #6
veedub565
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Default Re: Odd problem with HF set

Thanks for the replies

On this particular set the TX side can be switched on or off without actually transmitting anything. Also it's capable of full duplex, so can TX on one freq and RX on another at the same time.

That said the way it is currently configured is to use the same antenna to both TX and RX. It can also be configured to use separate antennas for TX and RX. I still wouldn't expect it to mute the RX without actually keying it up though.

I did wonder if there was some fault reducing gain on the RX when the TX side is switched on. I'm unable to test the TX works as I'm still working on my foundation license so I can't actually TX anything at the moment. There is a "Tune" function though and this activates the ATU and tunes it according to length of the antenna and frequency set. This does work.

It is quite a serious bit of kit, it's probably not as good as some modern stuff, but I find it interesting and I enjoy using it (receive only for now of course) I do have the manual with circuit description and diagrams. it is 30 years old now so there's every chance of a fault somewhere. I want to try and rule out anything obvious though before I get into pulling it apart and fault finding.

The ATU does have a relay board inside it, this allows it to use the same antenna for TX and RX. The ATU circuitry is a bit easier to get at for fault finding. one thing I could try is disconnecting the ATU and connecting the rig up directly to the antenna and see if the fault persists. this would be interesting to eliminate or incriminate it as the source of the problem.
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Old 19th Jul 2019, 9:01 am   #7
veedub565
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Default Re: Odd problem with HF set

Ok I had a little free time to look at this a bit more. The ATU has a switch inside it for Auto or Manual operation. Switching it to manual mode eliminated the problem. However as soon as I start manually switching in the capacitor and inductor banks to tune the antenna, then the problem comes back and I lose the RX into the noise.

In auto mode it uses relay drivers to switch the various banks in or out, in manual mode there are dip switches to set for the same purpose. It seems as though this is working ok, but either in auto or manual when the cap/inductor banks are switched in to tune the antenna then it introduces loads of noise and/or attenuates the recieved signal.

SO a little bit further along with this, at least now I know the problem is most likely with the ATU and not the rig.
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Old 19th Jul 2019, 9:31 am   #8
Herald1360
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Default Re: Odd problem with HF set

Quote:
Originally Posted by veedub565 View Post

I did wonder if there was some fault reducing gain on the RX when the TX side is switched on. I'm unable to test the TX works as I'm still working on my foundation license so I can't actually TX anything at the moment. There is a "Tune" function though and this activates the ATU and tunes it according to length of the antenna and frequency set. This does work.

Surely you can run the TX into a dummy load? That way you can measure its behaviour without actually transmitting.

In any case, a few minutes of test signals is hardly likely even to be noticed, let alone tracked down and prosecuted!
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