~ QUAD FM1 Stereo MPX Decoder Silicon Mod ~

If you are interested in repairing or modifying a QUAD multiplex MPX decoder then first download my re-drawn schematic which is an annotated version of the original plus a modified drawing showing the changes to make the FM1 decoder into a silicon transistor version similar to that used in the QUAD FM2 tuner

The 1965 instruction manual has a short description of the circuit function which along with my schematic notes should explain how the decoder functions and how using silicon transistors will not affect the sound but should give the decoder more years of active service

On my re-drawn schematic I make the comment "For stereo only listening . . . Yellow and Blue plugs can be swapped at the QUAD 22 . . . " ~ Almost all broadcasts nowadays are stereo so there is no need to have the HT on the Blue plug switched by the QUAD 22 MON button to make the QUAD multiplex decoder respond to mono transmissions without a 19kHz pilot tone ~ By swapping the two plugs the Blue amplifier powers the heaters and HT for the FM1 which will reduce the load on the Yellow amplifier that is powering the QUAD22

Pictured above is my QUAD stereo multiplex decoder many years ago when it had the electrolytic capacitors C19 and C22 changed from 25µF to 47µF and C17 changed to 4.7µF 63V non electrolytic as shown on page 2 of my schematic

On the left is the same decoder which in 2011 developed a fault with Tr1 and had several out of tolerance resistors changed ~ The original Tr1 Texas Instruments germanium 2G344 was changed to a silicon BC212 and R2 and R3 were changed to 100kΩ and all appeared okay

Looking at the schematic Tr1 performs a few roles ~ It acts as an emitter follower to provide a high impedance load to the FM1 detector output and a low impedance drive to the balanced demodulator at pin 6 of L2 ~ The collector load is complex with L1 selecting the 19kHz subcarrier to pass on to Tr2 and RV1 providing a variable L+R sum or mono signal in anti-phase to the multiplex MPX signal sent to the demodulator via Tr1 emitter

The balanced demodulator diodes MR7-10 are switched at 38kHz synchronised to the 19kHz pilot tone by the full wave rectified output from L4 and MR1 MR2 ~ A negative d.c. voltage from MR1 MR2 via R30 and C20 switches diodes MR5 MR6 to connect the demodulator outputs only during stereo transmissions to maintain a good signal to noise for mono transmissions which then had to be enabled by selecting MON+STEREO on the QUAD 22 control unit

The process of encoding and decoding 2 stereo channels for broadcasting by the standard Zenith–GE system adopted by the BBC and most of the worlds broadcasters since the early 1960s is described here which saves me a lot of paraphrasing ~ Figure 22.27 shows the baseband of the multiplex signal made up of a L+R mono signal and the L–R sidebands modulated by a 38kHz supressed carrier sychronised to the 19kHz pilot tone

With the Tr3 L2 oscillator synchronised to the 19kHz pilot tone ~ diode pairs MR7+MR8 and MR9+MR10 are switched at 38kHz and connect the MPX signal at L2 pin 6 alternatively to the Left and Right channel outputs ~ The circuit phase is arranged that when MR7+MR8 are conducting the output at R24 is (L+R)+(L–R) and the output at R25 is switched off and when MR9+MR10 are conducting the output at R25 is (L+R)–(L–R) and the output at R24 is off

The QUAD multiplex de-modulator is simply a switch which samples sections of the MPX signal at twice the pilot tone frequency or with 26.3µs wide sample periods ~ but as the instruction manual describes the process is not perfect

In each sample period the L+R signal is a continuous function and the L–R signal is a half sine ~ The normalised difference between them is π-2/π so a negative π-2/π x(L+R) signal ~ derived from the MPX signal across RV1 with L–R filtered out by R26 C11 and R27 C12 ~ is introduced to maximise the crosstalk figure

The use of a balanced demodulator should ensure that the 38kHz switching frequency does not appear at the audio outputs but there is clearly a capacitance imbalance around L2 which is compensated for by capacitor C21 ~ Making C21 a variable capacitor can reduce the residual 38kHz on the outputs to a very low level but to get the best results it needs to be adjusted once the decoder is assembled with the screen can in place

For a good description of the stereo system introduced in the UK in the mid 1960s and an early DIY decoder see this article which uses transistors as the synchronous switching devices ~ The QUAD decoder uses a balanced diode bridge

While on the subject of electrical balance it's worth noting the odd looking resistors around the balanced ring demodulator which are marked R20 to R27 but look different from the other resistors ~ Correlation with values on the schematic suggests they are 5 band resistors with 2 violet bands which does not make sense or would make them 0.1% 5ppm ! ~ They are marked on the parts list in the instruction manual as ±7% TE Welwyn F20 and as it turns out they are a special type of vintage resistor known as Total Excursion which nowadays could be replaced with a 1% metal film type like the MRS25

High level -6dB series C1 later R4B


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