The following information is based around a late model QUAD 22 pre-amplifier — If yours differs in some respects it's probably an earlier model — Considering that QUAD Hi-Fi (Acoustical Manufacturing Company) like most good manufacturers only made changes to their products for reasons of improvement [unlike today] I suggest that you bring your amplifier up to the latest spec — Download pdf Schematic

I don't recommend that you change every component just for the sake of it — You will most likely find that unless your QUAD 22 has been correctly repaired previously or manufactured within the last few years ! it will require many discrete components changing — I'll list the usual suspects in the order I remember they are likely to turn up and will try to give a description of the symptom affecting the sound

— QUAD II Power Amp fault can damage QUAD 22 Control Unit —

Capacitor C15 can get "dried out" if the system is run for some time with the above QUAD II fault especially if R32 has been replaced with a higher power modern resistor which no longer acts as a fuse! — If C15 does dry up and so becomes low in value this may show as the system "motor boating" — You're playing some music loudly or with the bass control set to maximum and the speakers start to pump furiously and sound like an old motor boat and continue to do so even after you turn down the volume to minimum — it's time to look at changing C15 along with C12 and C7 which are unfortunately in the same can

— Components Age —

The original "carbon composition" resistors R32, R25, R20, R30, R31, R18, R19, always appear to go high in value due to age and temperature but due to the nature of the design this may not be noticed until they have increased excessively in value — The high stability "carbon film" resistors R16, R17, R21, R22, R23 and R24 very rarely give trouble and should be left untouched as the main cause of their failure is the end caps being pulled off during rework

Once you have the lid off anyway you should check all the resistors with a meter — after the power has been off for some time but with the QUAD 22 still hot — and change those that are out of tolerance — Always suspect carbon composition resistors especially where they have a high voltage across them or they have high values say 220kΩ and above

The original solder joints will often have more solder than text books on the subject would recommend — they were also made with solder that had an acidic resin flux and a high lead content — Unless you are using the same original solder try to remove as much of the old stuff as you can so you are not mixing your own solder alloy with unpredictable results !

Whatever you do — AVOID LEAD FREE SOLDER — It is not suitable for repairing vintage equipment — The WEE and RoHS directives requiring low or no lead simply do not apply here - You are not manufacturing thousands of items with a view to disposal in a few years (Apple take note) you are in fact doing a more environmentally friendly repair

If your QUAD 22 has a QUAD repair sticker you will most likely find that — if nothing else has been done — the 1.5MΩ resistors R18 and R19 have been changed probably along with C8 and C9 which would originally have been 0.5µF paper types at 150Vdc or 250Vdc but get replaced by QUAD with 0.47µF 250Vdc polypropylene [PP] which should last for ever — R18 or R19 going high and or C8 or C9 going leaky will lower the gain of the respective EF86 stage which can show as a reduction in bass for the DISC input and possible distortion of both the DISC and TAPE inputs

The late production unit pictured above had been repaired after a few years with only R18 and R19 changed to carbon film types whereas the QUAD 22 shown left was repaired by QUAD in the 1980s and also had C8 and C9 changed as well as the electrolytic capacitors C5, C6, C10 and C11 — Note that C4 and the other paper capacitors were not replaced but unlike the new C8 and C9 will not last another 20 years

The original electrolytic capacitors were much larger than todays suitable replacements (compare the sizes of C6 in the inset above with those shown in the full view higher up the page) they were also the least reliable components 30-40 years ago but modern replacements should give excellent results for many years especially if they have a high temperature rating — 105°C or better — but not an excessive voltage rating — up to 16V is good enough and should ensure the capacitors remain formed

I know some people rave about using Tantalum capacitors where electrolytics have to be used — like C5, C6, C10 and C11 in the QUAD22 — If you feel the need to use Tantalum capacitors fit the tinned brass can military axial type — Not the small resin covered radial beads which look ugly and often have short leads which break off

— QUAD 22 Control Unit switches mains to QUAD II Power Amplifiers —

If C7, C12 or C15 (see above) start to "arc" the noise through both power amplifiers can sound very worrying — A similar "arc" sound may be heard if S4A or S4B (on/off switch on volume control) becomes dirty — If left without attention the switch sections will eventually fail and the respective channel will not power on

Slightly less common — and only affecting the blue channel — is failure of S1A, which is a mains rated switch linked to the STEREO button — If either S1A or a section of S4 are faulty swapping the power amplifiers will show the problem to be the QUAD 22 control unit and not a power amplifier

The switches S1 and S4 can be cleaned with a good contact cleaner such as Electrolube EML — Aim the tube provided where the solder tags meet the insulation and the oil will get drawn into the switch — EML is also excellent for cleaning the volume and tone control pots and other switches

Total failure of S4 often occurs and no amount of switch cleaner can fix it because the insulated arm inside becomes worn and cannot operate the switches — It is possible to remove the switch section from the volume control and repair it with epoxy or a new paxolin arm hand crafted with a small file or maybe stolen from another old switch

If S4 is beyond repair or you just want to fit a new "better" volume control that does not have a mains switch, it is possible to wire the STEREO/MON switches as an on/off control — S1A is normally used to switch off the blue channel for MONO listening on the yellow channel only but nowadays it is likely that this feature is not required and the STEREO switch can be used as an ON control, the MON switch used as an OFF control and 2MON becomes on in MONO if required

— QUAD 22 Control Unit combined Volume and Balance controls —

The balance control is wire wound and often goes "noisy" before the volume control shows problems — It is not easy to use contact cleaner on either of the controls because they are usually sealed types but if you remove the front panel as shown above you can rotate the balance fully to its extremes — The balance control is wire-wound in 3 sections and that part in the limited centre range is fine wire with a total resistance about 9kΩ — Either side of centre is coarse wire which if you rotate the balance back and forth to its extremes several times can actually clean the wiper and clear the problem

Another mechanical problem affecting the entire control including the switch is the insulated wiper arms wear on the shafts and become sloppy — The only cure is to disassemble and thoroughly clean the parts and then glue the wiper and switch arms to the shaft with epoxy resin — This is not easy as unsoldering the sections is difficult and you have to ensure that the sections of volume control are physically aligned as the glue sets but if done carefully you will have no more problems

If the volume control is unbalanced only at low level and mechanically is OK measure the residual resistance at minimum volume but not off — if there is a great difference in the two readings with one more than 3 times the other — you could use a resistor in the ground connection of the lowest residual section to make the 2 readings as measured from the wipers to ground the same — This will better balance the two sections throughout the whole range — Also check the ground wire soldering and beware that heating the tags of the volume control may change the residual resistance

If you can find a volume control with a 1/8" spindle (and better still with a mains switch) you could try to implement a "slider" balance as fitted in the QUAD22 volume replacement KIT R22VOLM which unfortunately is no longer available from QUAD — Even if you only have a pot with a 1⁄4" spindle you can still make a slider balance control but will have to machine the volume control knob — Such changes along with the modifications below can make the 22 a well featured "modern" design

Note Yellow and Blue are reversed from the QUAD document because the yellow channel is now the Left channel after fitting Red and White replacement phono connectors

500kΩ pots with switches are now hard to find and with 1/8" shafts even harder — A 6mm shaft dual 500kΩ without a switch could be used with S1A used to switch the mains

As an alternative to this you could fit a dual 500kΩ [or 1MΩ] without switch and fit a balance pot on the back again using S1A to switch the mains or not have mains going to the 22 — see mod below

— QUAD 22 Control Unit Phono or DISC equalisation —

The intention was that you screwed the plate inside your Hi-Fi cabinet door for reference — The DISC button alone provides stereo RIAA equalisation but there is additional equalisation available in Mono on the Yellow channel or both channels when using the 2MON button and the Mono DISC input

If you refer back to the picture of the QUAD 22 control unit underside you can compare the diagram below with the physical layout and note the large area of "switch real estate" used for the DISC input to accommodate mono equalisation on the Yellow channel only — The EF86 amplifiers V1 and V2 are also used for the TAPE input and although almost all the 7 pin TAPE adaptors are flat response 1x gain — type N — allowance was made to connect the low output from a stereo tape head directly to the QUAD 22 to amplify and equalise the signal via the TAPE input but this could only be done for one speed or reply characteristic unless adaptors were changed

During the 1950s many manufacturers produced open frame tape decks [as opposed to tape recorders] for people to build into their own cabinets — These were often playback only and many had replay amplifiers with equalisation linked to the speed control but for those that didn't pre-amplifiers like the QUAD Hi–Fi QC22 could be used to play tape

Tape recorders tended to provided an equalised output for each speed anyway and as the recording process also required such a machine the idea of equalisation at the pre-amplifier never caught on and the flat response TAPE N adaptor became prevalent — The 7 pin adaptor was also offered in a MIC N version with level response and 1.5mV sensitivity for home recording or PA use but again if you had a tape recorder it would most likely already have a microphone input

Magnetic cartridges produce a voltage output that doubles for every octave increase in frequency (+6dB/octave or 20dB/decade) — Ceramic cartridges with the correct high impedance load produce a constant output with frequency — During the mastering of a record disc the magnetic cutter head excursion reduces at -6dB /octave response and so as frequency increases lateral excursions become smaller and recorded signal to noise (S/N) becomes worse

To correct for the cutter head response equalisation is applied — Below 50Hz the signal to the cutter is reduced at 6db/octave so that low frequency signals do not make large excursions and to compensate for the poor S/N with rising frequency a step of about 12dB is introduced between 500Hz and 2kHz

When using a magnetic pick-up its natural +6dB/octave response has to be corrected with an opposite RIAA 3180µs replay time constant which in the QUAD 22 is provided by R1 and C1 + C5 in the DISC adaptor — It may not be obvious at first but the value of C1 is multiplied by the gain of the EF86 (miller effect) so it appears as 46nF from grid to ground and C1 + C5 = 47nF * R1 = 68KΩ gives the 3180µs replay time constant — For the 4mV adaptor DISC A the gain is all that the EF86 can deliver and is about 255 x 180pF = 46nF with a good valve — For the 10mV adaptor DISC B C1 is higher at 560pF and the gain is reduced by R2 = 10MΩ to about 82x — If there is a fault around the EF86 stage or the valve is low then the Bass will be reduced — It is also possible to "overload" the TAPE OUT and also reduce the Bass severely

For the Ceramic pick-up the constant lateral amplitude above 50Hz gives a constant amplitude output which has led some to suggest that RIAA equalisation is not required but there is more to it than the 3180µs as you will see below — Crystal or Ceramic cartridges require a high impedance load and capacitive coupling -3dB at 50Hz to correctly equalise at low frequencies — The QUAD adaptors DISC E and F have high impedance by virtue of R1 = 1.5MΩ and 2.2MΩ respectively and gain is lowered by R2

Most discs and all discs cut from 1954 onward have the 3180µs recording time constant where the signal sent to the cutter head increases at +6dB/octave above 50Hz but this increase of amplitude with frequency cannot be allowed to continue because the lateral excursions would then be difficult to cut at high frequency — Therefore 2 other time constants are introduced in the cutting process such that the amplitude is made to fall before 1kHz is reached and then level out again beyond 1kHz

The additional RIAA recording time constants used for stereo 45rpm and 33rpm recordings are 318µs and 75µs which relate to -3dB points at 500.5Hz and 2122Hz as shown on the graph below — The amplitude step between 500.5Hz and 2122Hz does not look significant when viewed on such a graph but once the 6dB/octave at 3180µs is corrected for the step is seen to be 12.5dB — If the 318µs and 75µs are neglected as often happens with Crystal - Ceramic or Strain Gauge cartridge amplifiers because they do not require 3180µs correction then the replay response between 20Hz and 20kHz will drop 12.5 dB or ± 6dB either side of 1kHz

Even when using a Crystal or Ceramic cartridge the QUAD 22 applies all 3 time constants required for a correct disc replay response — Referring back to the schematic extract above the RIAA time constants at 318µs and 75µs are cleverly implemented by reducing the feedback and thus the miller effect of C1 which increases the gain as required between 500Hz and 2122Hz giving the 12.5dB step

For the yellow channel — (R7 + R10) * C2 = 75µs and ( R7 + R10 + R8//R9) * C2 = 318µs

Note that R8//R9 ≈ 47kΩ and R7 + R10 ≈ 15kΩ and for frequencies below 100Hz C2 can be considered open circuit and beyond 10kHz C2 can be considered short circuit so the amount of feedback is reduced by the potential division — 15kΩ/(15kΩ + 47kΩ) = 0.24 ≈ 12dB

For 78 rpm discs the other 2 time constants vary between manufacturers but from 1954 onward the common standard became known as the "coarse groove characteristic" with time constants of 450µs giving a response -3dB at 354Hz and 50µs such that the response levels out beyond 3183Hz — The QUAD 22 also provides 450µs with 25µs and 318µs with 100µs to equalise a range of 45rpm and 78rpm discs as indicated in the "Record Equalisation Guide" card above and in terms of their time constants as shown in the next table

So taking the coarse groove equalisation as an example with the DISC and TAPE buttons pressed simultaneously and only for the yellow channel or mono pickup input — (R4 + R10) * C2 = 47µs and ( R4 + R10 + R9) * C2 = 457µs

Although the 450µs and 50µs characteristic became known as coarse groove for commercial 78rpm discs it was also used for fine groove 33rpm recordings for archive use and transporting radio programmes between broadcasters between about 1951 and 1954 before the RIAA curve was ratified — These discs being for professional use would often have the speed and playback time constants printed on the labels so there was no confusion

— QUAD 22 Pre-Amplifier DISC Adaptors —

PU Adaptor A — 4mV Magnetic — 68kΩ load
PU Adaptor B — 10mv Magnetic —68kΩ load
PU Adaptor E — 300mV Ceramic — 1.5MΩ load
PU Adaptor F —150mV Ceramic — 2.2MΩ load

TYPE	R1	R2	R3	R4	C1	C2	C3	C4	C5	C6
A	68kΩ	—	—	—	180pF	—	—	—	1nF	—
AA	68kΩ	—	—	68kΩ	180pF	180pF	—	—	1nF	1nF
AB	68kΩ	—	10MΩ	68kΩ	180pF	560pF	—	—	1nF	—
AE	68kΩ	—	330kΩ	1M5Ω	180pF	1nF	—	47pF	1nF	—
B	68kΩ	10MΩ	—	—	560pF	—	—	—	—	—
BA	68kΩ	10MΩ	—	68kΩ	560pF	180pF	—	—	—	1nF
BB	68kΩ	10MΩ	10MΩ	68kΩ	560pF	560pF	—	—	—	—
BE	68kΩ	10MΩ	330kΩ	1M5Ω	560pF	1nF	—	47pF	—	—
E	1M5Ω	330kΩ	—	—	1nF	—	47pF	—	—	—
EA	1M5Ω	330kΩ	—	68kΩ	1nF	180pF	47pF	—	—	1nF
EB	1M5Ω	330kΩ	10MΩ	68kΩ	1nF	560pF	47pF	—	—	—
EE	1M5Ω	330kΩ	330kΩ	1M5Ω	1nF	1nF	47pF	47pF	—	—
F	2M2Ω	1MΩ	—	—	330pF	—	33pF	—	—	—

The earlier Mono Quality Control Unit II — QCII — used a similar style 9 pin B9A based plugin for Pick-up and Microphone matching but these were Marked R1 to R13 or M1 to M13 — The QC II Phono/Microphone amplifier stage used an EF86 but the equalisation feedback was applied in a more "traditional" manner similar to the LEAK and many other 1950s pre-amplifiers — The QUAD QC22 phono circuit was not one of these "me too" designs

In the schematic extract above I also included the Radio input switching to show how it goes directly to the volume control — unlike the TAPE and MIC and DISC inputs which use the EF86 amplifier stages and are responsible for the much of the wiring between the back plate and the switch-bank and the cross–talk between inputs and channels

The TAPE input today would to be a nominal 100mV or 300mV sensitivity and be switched directly to the volume control like the original RADIO inputs were — The MIC + RADIO 2 input sockets can be used for a 300mV CD input which again is switched directly to the volume control

For "modern" use only the DISC stage requires the extra gain and equalisation provided by the EF86s and the components needed for RIAA (or other) equalistion can be removed from the switch-bank and new components fitted around the EF86 valve bases to make a more reliable and modern QUAD 22 based on the schematic below

Note there are some changes to component values on the above schematic to make the RIAA accurate while using readily available 1% standard value components and also to give the cartridge a lower 47kΩ load