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Old 14th Jul 2019, 10:56 pm   #1
Anthony_M
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Default FRG-7 Quiet hum

Hi folks!

I had some troubles with my Yaesu FRG-7. One day it went silent. Found & fixed the problem - it was faulty rectifier diode.

While dealing with this issue, I've noticed quiet hum from speaker (when volume is at 0%, or antenna is disconnected). Maybe it was before - just didn't paid attention to it. It's more noticeable while using headphones or trying to record something with a laptop.

I've replaced capacitors. Not all (yet), but majority of them. Few difficult to access - left. But most essential ones (power supply) - replaced.

In everyday use - listening shortwave broadcast stations & hams with speaker - it's almost / completely unnoticeable. But using headphones at low volume levels (<5-10%) humming it's a bit annoying.

I've recorded that hum. Looked at spectrum - it's mostly 50 Hz tone (mains frequency in my country, I'm from Europe).

So... it's normal for 1970s low / mid-range shortwave receiver, or it's something else? Or it's OK and I'm way overthinking this issue?
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Old 18th Jul 2019, 6:13 pm   #2
Keith
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Default Re: FRG-7 Quiet hum

Hi Antony,


I have an FRG7 and have never noticed a hum. I almost always use it on the loudspeaker though so it maybe only a problem on headphones. I guess one option would be to use lower sensitivity headphones or load the existing ones with a resistor. You would then need to increase the audio gain but the hum level should be reduced.
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Old 19th Jul 2019, 10:55 am   #3
AlanC
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Default Re: FRG-7 Quiet hum

The power supply has a full-wave rectifier (D408 and D409) so any ripple should be at a frequency of 100Hz. So maybe you still have a bad diode?
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Old 19th Jul 2019, 11:59 am   #4
Keith
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Default Re: FRG-7 Quiet hum

As it's at such a low level, it's probably just capacitive pick-up by the audio stages from the mains wiring within the receiver.
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Old 20th Jul 2019, 4:03 pm   #5
g4aaw pete
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Default Re: FRG-7 Quiet hum

Had this problem with two Kenwood R5000 receivers.
The cause was poor eathing on the IF / audio output pwb.
Resolution was by applying Deoxit to the earth return / mounting tabs.
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Old 20th Jul 2019, 5:36 pm   #6
Jon_G4MDC
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Default Re: FRG-7 Quiet hum

It might also be worth following the low level AF wiring.

It will almost certainly be in screened cable but it wouldn't be a first time to see it routed half a turn around the mains transformer with only small spacing.

Pye managed it in one transmitter.
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Old 22nd Jul 2019, 12:02 pm   #7
Richard - F4VPR
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Default Re: FRG-7 Quiet hum

But here's another idle thought, the FRG-7, which is not exactly a Hi-Fi receiver, would have been "specified" with the Yaesu headphones designed for comms audio or a very ordinary 2" ~ 4" loudspeaker. Neither have any real response below 200Hz, but your everyday headphones designed for music and centre of the head thumping bass most certainly do. . . . . . . .
I doubt Yaesu cared too much about an ultimate low hum level. . . . . .

R.
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