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-   -   FRG-7 Quiet hum (https://www.vintage-radio.net/forum/showthread.php?t=158051)

Anthony_M 14th Jul 2019 10:56 pm

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?

Keith 18th Jul 2019 6:13 pm

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.

AlanC 19th Jul 2019 10:55 am

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?

Keith 19th Jul 2019 11:59 am

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.

g4aaw pete 20th Jul 2019 4:03 pm

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.

Jon_G4MDC 20th Jul 2019 5:36 pm

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.

Richard - F4VPR 22nd Jul 2019 12:02 pm

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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