Bang & Olufsen. Distortion on FM at peak signal strength - UK Vintage Radio Repair and Restoration Discussion Forum

Dillen_de · 12th Dec 2007, 6:54 pm

Hi,

None-UK, I know, but I am currently restoring a wonderful 1960 Bang & Olufsen radio and I have run into a little problem;

If, on FM, I tune in to a station so that I get max signal according to the magic eye, the sound is distorted and a bit low in volume as if
the sound "struggles" to get through.
If I then tune the radio to a slightly higher frequency, just off
center according to the eye, the sound "clears up" and the volume increases and everything is fine.

Valve lineup; PCC85(new), ECH81, EF89, EBF89, ECC83, PL84(new) and PM84 (EM87 is mounted).
I have the original service manual.

The anode voltages are a bit low on both triodes in the ECC83, new
anode resistors has been mounted, the old ones were a bit high, and
a new ECC83 has been test-fitted but no improvement and I'm not sure that
the fault lies here.
New cathode capacitors has been mounted on the ECC83 and PL84.

The radio has a metal rectifier and to me it seemed that it dropped
a bit more voltage than I liked but a swap for a diode with a 200ohm
series resistor didn't change anything.

Am I going to trim the whole thing or do I have a marginal valve or ?

Martin

Steve_P · 12th Dec 2007, 7:07 pm

A lot of stations now are using greater mod depth and compression than in years gone by. So loud passages simply do sound more distorted.

But try realignment of IF stages first.

http://www.vintage-radio.com/repair-...alignment.html

Cheers,

Steve P.

Dillen_de · 12th Dec 2007, 8:57 pm

Thanks Steve,

I am normally very reluctant to touch any adjustments unless they
were obviously already played with, but I followed your
advice and already at the first touch of the 1.IF coil
found inside the VHF screening can), the sound turned much clearer
and stronger.
It took less than 1/8th of a turn of the core to put everything right.

Thanks again, greatly appreciated !

Martin

12th Dec 2007, 6:54 pm	#1
Dillen_de Tetrode Join Date: Jun 2007 Location: Copenhagen, Denmark Posts: 62	Bang & Olufsen. Distortion on FM at peak signal strength Hi, None-UK, I know, but I am currently restoring a wonderful 1960 Bang & Olufsen radio and I have run into a little problem; If, on FM, I tune in to a station so that I get max signal according to the magic eye, the sound is distorted and a bit low in volume as if the sound "struggles" to get through. If I then tune the radio to a slightly higher frequency, just off center according to the eye, the sound "clears up" and the volume increases and everything is fine. Valve lineup; PCC85(new), ECH81, EF89, EBF89, ECC83, PL84(new) and PM84 (EM87 is mounted). I have the original service manual. The anode voltages are a bit low on both triodes in the ECC83, new anode resistors has been mounted, the old ones were a bit high, and a new ECC83 has been test-fitted but no improvement and I'm not sure that the fault lies here. New cathode capacitors has been mounted on the ECC83 and PL84. The radio has a metal rectifier and to me it seemed that it dropped a bit more voltage than I liked but a swap for a diode with a 200ohm series resistor didn't change anything. Am I going to trim the whole thing or do I have a marginal valve or ? Martin

12th Dec 2007, 7:07 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Distortion on FM at peak signal strength A lot of stations now are using greater mod depth and compression than in years gone by. So loud passages simply do sound more distorted. But try realignment of IF stages first. http://www.vintage-radio.com/repair-...alignment.html Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

12th Dec 2007, 8:57 pm	#3
Dillen_de Tetrode Join Date: Jun 2007 Location: Copenhagen, Denmark Posts: 62	Re: Distortion on FM at peak signal strength Thanks Steve, I am normally very reluctant to touch any adjustments unless they were obviously already played with, but I followed your advice and already at the first touch of the 1.IF coil found inside the VHF screening can), the sound turned much clearer and stronger. It took less than 1/8th of a turn of the core to put everything right. Thanks again, greatly appreciated ! Martin Attached Images