Revox B261 FM demodulator - UK Vintage Radio Repair and Restoration Discussion Forum

djsbriscoe · 2nd Apr 2018, 5:08 pm

Just had a look at the Revox B261 service manual. I'm curious as to what significance the coaxial delay line has in the FM demodulator. How does it work with the MC1496 chip and what do the LF357 and LF353 chips do?
This receiver is very interesting as it also seems to use a dual balanced mixer in the front end. Can someone satisfy my curiosity? Thanks.

Nuvistor · 2nd Apr 2018, 6:32 pm

I can take a guess but there will be better heads than mine on the forum that may give a correct answer.
I believe the detector is a type of quadrature detector, the 68ns delay line will give an input to the detector 90 degrees out of phase with the other input.

LF353 take the Signal strength and Centre tuning signals from the IF board for processing by the microprocessor.

I have not seen the circuit before so take my suggestions with caution.

Radio Wrangler · 2nd Apr 2018, 7:15 pm

Yes, that describes it well. It's a sort of quadrature detector using a fixed delay line. It isn't the 'pulse count' discriminator, which can be built with a delay line as a sort of monostable.

A quadrature detector works with a 4-quadrant multiplier multiplying a signal with a 90 degree shifted version of itself. The use of a broadband delay line changes this a little and the rate of change of phase versus frequency increases the sensitivity of the detector. A delay of an odd number of quarter wavelengths increases this effect further, and the B261 uses a delay line which is three-quarters of a wavelength long at the nominal IF centre of 10.7MHz.

The RF mixer is single balanced.

Notice also the LC filter in the IF rather than the usual ceramic jobs, and the stereo decoder is MC1496 stuff as well.

Sounds great.

David

kevinaston1 · 2nd Apr 2018, 7:32 pm

Never seen this unit, but I found a seven page description at

https://arxiv.org/pdf/1610.01631.pdf

Kevin

Nuvistor · 2nd Apr 2018, 7:36 pm

Service manual
https://archive.org/details/studer_Revox_B261_Serv

Radio Wrangler · 2nd Apr 2018, 7:41 pm

Not sure I can see any similarity in that reference. the B261 demodulator is not a closed loop. Signal and delayed signal are fed into the two ports of a 4-quadrant multiplier. The outputs of the multiplier are lowpass filtered to recover the baseband.

David

2nd Apr 2018, 5:08 pm	#1
djsbriscoe Pentode Join Date: May 2015 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 133	Revox B261 FM demodulator Just had a look at the Revox B261 service manual. I'm curious as to what significance the coaxial delay line has in the FM demodulator. How does it work with the MC1496 chip and what do the LF357 and LF353 chips do? This receiver is very interesting as it also seems to use a dual balanced mixer in the front end. Can someone satisfy my curiosity? Thanks.

2nd Apr 2018, 6:32 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Revox b261 fm demodulator I can take a guess but there will be better heads than mine on the forum that may give a correct answer. I believe the detector is a type of quadrature detector, the 68ns delay line will give an input to the detector 90 degrees out of phase with the other input. LF353 take the Signal strength and Centre tuning signals from the IF board for processing by the microprocessor. I have not seen the circuit before so take my suggestions with caution. __________________ Frank Last edited by Nuvistor; 2nd Apr 2018 at 6:44 pm.

2nd Apr 2018, 7:15 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Revox b261 fm demodulator Yes, that describes it well. It's a sort of quadrature detector using a fixed delay line. It isn't the 'pulse count' discriminator, which can be built with a delay line as a sort of monostable. A quadrature detector works with a 4-quadrant multiplier multiplying a signal with a 90 degree shifted version of itself. The use of a broadband delay line changes this a little and the rate of change of phase versus frequency increases the sensitivity of the detector. A delay of an odd number of quarter wavelengths increases this effect further, and the B261 uses a delay line which is three-quarters of a wavelength long at the nominal IF centre of 10.7MHz. The RF mixer is single balanced. Notice also the LC filter in the IF rather than the usual ceramic jobs, and the stereo decoder is MC1496 stuff as well. Sounds great. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done Last edited by Radio Wrangler; 2nd Apr 2018 at 7:34 pm.

2nd Apr 2018, 7:32 pm	#4
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: Revox B261 FM demodulator Never seen this unit, but I found a seven page description at https://arxiv.org/pdf/1610.01631.pdf Kevin

2nd Apr 2018, 7:36 pm	#5
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Revox B261 FM demodulator Service manual https://archive.org/details/studer_Revox_B261_Serv __________________ Frank

2nd Apr 2018, 7:41 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Revox B261 FM demodulator Not sure I can see any similarity in that reference. the B261 demodulator is not a closed loop. Signal and delayed signal are fed into the two ports of a 4-quadrant multiplier. The outputs of the multiplier are lowpass filtered to recover the baseband. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done