~ QUAD FM1 Stereo MPX Decoder & Silicon Mod ~

If you are interested in repairing or modifying your 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 germanium transistor FM I 'bolt on' 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 changing to silicon transistors will not affect the sound but should give the decoder more years of active service ~ If you want good stereo FM maybe use a modern non valve tuner like the FM4

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 FM 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 ~ Swapping the two plugs of MPX decoder the Blue amplifier now powers the heaters and HT for the FM1 which will reduce the load on the Yellow amplifier that is powering the HT and heaters of your QUAD22

Pictured is a QUAD stereo multiplex decoder taken apart many years ago when it needed the electrolytic capacitors C19 and C22 changed from 25µF to 47µF [ because they measured less than 1µF ]

C2 which was okay was 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 but other than these changes nothing else needed to done

Until 2011 when Tr1 [ germanium 2G344 ] failed and was changed for a silicon BC212 following the schematic changes made by QUAD for their FM2 decoder which used the same coil set

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 sub-carrier 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 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 side-bands modulated by a 38kHz suppressed carrier synchronised 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 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 others is simply a switch which samples sections of the MPX signal synchronised to twice the 19kHz pilot tone frequency or with 26.3µs wide sample periods ~ but as the instruction manual describes this sampling 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 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 its cover 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 and look different from the other resistors

Correlation with values on the schematic suggests they may be 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 better 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 with R2 and R3 changed to 100kΩ as per the FM2 schematic which uses all 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 as the original parts often are

The decoder here still has germanium OC81 transistors Tr2 and Tr3 fitted but these were later also changed for BC556 silicon devices

 

When replacing the OC81 Tr2 with a silicon transistor the miller capacitance at its base is not enough to allow tuning L1 to 19kHz and an additional 330pF is needed across L1 pins 4 and 5 to compensate for this ~ see CL1 on my silicon mod schematic

QUAD used the same coil set for the FM II silicon stereo decoder and fitted a 330pF capacitor [ C124 on the FM II schematic ] on the reverse of the PCB ~ I removed L1 from the board and fitted the 330pF CL1 on the coil base as shown

The capacitance across L1 could be provided by placing a capacitor from Tr2 base to collector of a silicon transistor to simulate the miller capacitance of an OC81 but a polystyrene 330pF is far better as is using silicon transistors in place of germanium

 

L2 is mainly tuned by C10 across its pins 4 and 5 and 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 on the QUAD FM II schematic are C111 and R115

Another way to 'tame' Tr3 is to replicate the base to collector capacitance of the OC81 by placing a capacitor from base to collector on the silicon Tr3 replacement as shown ~ depending on the transistor used the capacitor value may need to be as high as 150pF but using C111 and R115 may be preferable

 

The output level of the standard QUAD MPX decoder is higher than the output of the standard FM1 mono output although there is no gain provided by the transistors ~ On my silicon modification schematic R4B reduces the stereo output to better match the 70mV sensitivity of a standard QUAD 22 but this may not be required

 

As stated the decoder should be adjusted with the cover in place and for safety it should also be bolted to the FM1 which 'should' ensure the ground connection is good

Here C21 has been replaced with a variable capacitor  in an attempt to balance the demodulator and reduce the amount of 38kHz ripple at each output

RV1 [ not marked ] is below L3 ~ RV1 and C21 are adjusted after L1 L2 and L3 have been set to give the best crosstalk with lowest 38kHz on the output

 

 

With the left and right outputs connected to a 2 channel oscilloscope the crosstalk and residual 38kHz can be viewed simultaneously as RV1 and C21 are adjusted ~ Note both channel are same sensitivity

Here a 1kHz left channel only MPX signal was applied to the decoder input to give a 100mV output ~ The QUAD manual states crosstalk @1kHz is better than 30dB and suppression of 19/38kHz better than 36dB but this is often not seen looking directly at the decoder output as pictured

It should be possible to reduce the 38kHz more than shown here and you may be able to balance the ripple on positive and negative cycles

The 38kHz will not be so high at the output of a QUAD 22 / QUAD II set–up which is probably where the figures in the QUAD manual were measured

 

The MPX stereo decoder M12006 in the FM II which first appeared in 1968 uses silicon transistors which are PNP type like the germanium transistors used for the FM I decoder although at the time NPN silicon would have been more common and was used by QUAD in their 1967 QUAD 33 control unit and 303 power amplifier

Note the 2 capacitors on the reverse side of the PCB one of which is the 330pF polystyrene C124 mentioned above ~ Other capacitors apart from the Mullard C280 may need replacing for correcting operation

 

 

 

When the transmission has no pilot tone the left and right outputs are combined for mono but unlike the FM I there is no provision to turn off the 38kHz oscillator to mono listening with a pilot tone but there is a separate mono output on the 5 pin DIN connector that could be used by the 33 control unit

 

 

 

" Attention all shoppers it's Cancellation Day ~ Yes the Big Adiós is just a few hours away "