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Old 20th Feb 2020, 4:30 pm   #1
Mike Noble
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Default Roberts RD50 Aerial Problem

I now have three RD50's with the same fault. The radio works fine but when you touch the aerial the radio stops. With the circuit out of the case the same thing happens but when you touch the earth with your hand or attach a scope probe earth then all works fine even when you touch the aerial.

I was wondering if anyone else has found this with an RD50 and what the fix is.

I've checked the aerial lead and other connections and they all seem good.

Thank you

Last edited by Mike Noble; 20th Feb 2020 at 4:40 pm.
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Old 20th Feb 2020, 6:32 pm   #2
regenfreak
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Default Re: Roberts RD50 Aerial Problem

It is not a fault. Your body/hand distributed capacitance detunes the RF tank circuit assuming you touch the insulated wire of the antenna. When you earth the radio, the RF oscillation energy is stronger, the capacitive effect of your hand is weaker relatively. Thats the theory anyway. Hand effect is not an exact science.

Last edited by regenfreak; 20th Feb 2020 at 6:42 pm.
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Old 20th Feb 2020, 11:43 pm   #3
pip5678
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Default Re: Roberts RD50 Aerial Problem

I have had this issue too with an RD50 radio, from memory the signal disappears for a few seconds.
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Old 21st Feb 2020, 9:09 am   #4
Mike Noble
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Default Re: Roberts RD50 Aerial Problem

The fault is only on DAB and does not present itself on FM. If the aerial is extended the signal level drops. I do have four RD50's and the first one I bought does not have this problem.
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Old 21st Feb 2020, 4:32 pm   #5
Mike Noble
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Default Re: Roberts RD50 Aerial Problem

I've found a fix but don't understand why it works. I soldered an 8.2kohm resistor across the aerial input on the pcb. The value does not seem to be critical as I tried values right down to 100 ohms. Most odd!
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Old 21st Feb 2020, 6:06 pm   #6
Graham G3ZVT
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Default Re: Roberts RD50 Aerial Problem

Quote:
Originally Posted by Mike Noble View Post
I've found a fix but don't understand why it works. I soldered an 8.2kohm resistor across the aerial input on the pcb. The value does not seem to be critical as I tried values right down to 100 ohms. Most odd!
That would be consistent with a static discharge. Perhaps upsetting the DAB processor, rather than the analogue front-end.
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Old 21st Feb 2020, 6:31 pm   #7
Julesomega
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Default Re: Roberts RD50 Aerial Problem

I'd say any static discharge from fingers would last only a few µs and the effect on the tuner would be permanent

I can't explain why the shunt resistor would work but bear in mind that a telescopic aerial for DAB is usually ½-wavelength and has a high impedance at the bottom so should be left untouched or it is effectively shorted out.
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Old 21st Feb 2020, 7:22 pm   #8
G6Tanuki
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Default Re: Roberts RD50 Aerial Problem

If the mains-PSU is one of the usual type it will have a series-pair of capacitors across the incoming supply, with the centre-point being used as 'ground' for the DC output.

Which means that your radio's ground-rail will be sitting at something like 120V [albeit at a high source-impedance] with respect to ground - so touching the antenna can inject a bit of rough noise into the receiver's front-end and so confuse the digital side.
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Old 21st Feb 2020, 9:03 pm   #9
Mike Noble
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Default Re: Roberts RD50 Aerial Problem

It's running off an older style psu with a bridge rectifier and regulator.
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