~ 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 'upside down' annotated version of the original plus a modified drawing showing the changes to make the FM I decoder into a silicon transistor version similar to that used in the QUAD FM II or 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 is my QUAD stereo multiplex decoder taken apart many years ago when it had the electrolytic capacitors C19 and C22 changed from 25µF ~ that measured closer to 1µF ~ to 47µF and C2 which was okay changed to 22µF as 12µF or even 15µF were not readily to hand

C17 which measured low was changed to a 4.7µF 63V non electrolytic as shown on page 2 of my schematic other than these changes nothing else needed to done until 2011 when Tr1 ~ a Texas Instruments germanium 2G344 failed and was changed for a silicon BC212 using the schematic changes made by QUAD for their FM2 decoder

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 Zenith–GE system adopted by the BBC in the UK and most of the world 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 suppressed carrier synchronised by 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 so 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 like many 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 this process is not perfect

In each sample period the mono 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 an anti–phase 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

For a another description of the Zenith-GE 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 known as a balanced demodulator which can also be used as a balanced modulator

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

While on the subject of electrical balance it's worth noting the odd looking resistors used in the QUAD balanced 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
My QUAD stereo multiplex decoder in 2011 after the faulty Texas Instruments germanium 2G344 PNP transistor Tr1 was changed to a silicon BC212 and R2 and R3 were changed to 100kΩ as per the FM2 schematic which uses silicon transistors

Tr1 is not visible in this picture but you can see that the 330kΩ resistors around the diode demodulator have been changed because they were way out of tolerance ~ The decoder was now still working with its OC81 germanium transistors for Tr2 and Tr3 but these were also changed for silicon devices

Originally Tr1 would have been a 2G344 or OC44 and Tr2 and Tr3 either 2G303 or OC81 but finding and fitting silicon replacements for these was not straight forward ~ The OC44 is an 'RF' transistor and should be okay amplifying the baseband MPX signal but the same cannot be said of the OC81s which depending on the make can have up to 70pF or more collector to base capacitance
When replacing the OC81 Tr2 with a suitable silicon transistor the miller capacitance at its base is not enough to allow tuning L1 to 19kHz so an additional 330pF or so is needed across L1 pins 4 and 5 to compensate for this ~ see CL1 on my silicon mod schematic

Clearly QUAD used the same coil set for their FM II silicon stereo decoder and a 330pF capacitor ~ C124 on the FM II schematic ~ was clearly added after the PCB was made as it is mounted on the reverse of the board ~ see picture below

I removed my L1 from the board and fitted 330pF CL1 on the base as shown

L2 is mainly tuned by C10 across its pins 4 and 5 ~ Using a silicon transistor for Tr3 can make this 38kHz self oscillating circuit unstable ~ The QUAD solution for their FM II decoder was to place a CR network across L2 pins 1 and 7 and these are the 1nF capacitor in series with a 3.3kΩ resistor shown in green on my schematic and C111 and R115 on the QUAD FM II schematic

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