Quad FM3 Service Notes

Quad FM3 tuner

Quad produced some 50,000 FM3 tuners between 1971 and 1982. So there are plenty of units out there, so it is well worth while developing a service approach. This article is the result of having three customer FM3s on the bench at the same time as well as my own unit, which I’ve owned for 20 years but am only just renovating now.

The FM3 should not be confused with the similar-looking FM2, which is a valve unit based on the original FM tuner that goes with the 22/II valve equipment (of which 7,500 were built between 1968 and 1971: they are now much sought after). The FM2 and FM3 are clearly marked as such on the back, and the FM2 has the stereo indicator on the left and the tuning lights on the RHS of the scale: the FM3 has them the other way around. The FM3 is a solid-state tuner with excellent performance.

There are three principal variations of the FM3:

s/n up to 5885: This series uses a two-IC front end and a Motorola MC1305 stereo decoder.

s/n 5885-9,9999: This series uses the same front end and an MC1310 stereo decoder.

s/n 10,000 up: This series uses a single-IC front end and the MC1310 stereo decoder.

Within each series there are also minor variations detailed in the Service Manual, the most interesting being that for a short period in 5885-9999 a different tuning gang and tuning scale were used, with ‘MHz’ appearing in the centre of the scale instead of at the right-hand end. I’ve never seen one of these.

Servicing these units should be confined to the following unless you have RF and IF alignment equipment (sweep generator, marker generator, stereo modulator, and oscilloscope).

  1. Check that +-14VDC is available at the red lead and the long black lead on the underside. Pre s/n 5885 adjust the PSU RVs as necessary; otherwise maybe replace the Zeners and resistors.

  2. Replace the two PSU caps with 1000uF/25V.

  3. Replace the audio PSU decoupling cap with 220uF/16V or maybe 470uF/16V.

  4. If you have high-quality poly capacitors available, you could change the audio coupling capacitors C124/125 (C116/118 pre s/n 5885).

  5. You can consider replacing the emitter bypass capacitor C100 and the composite-audio coupling capacitor C104, both located next to the MC1310, with 0.68uF/16V, but note that Quad used high-quality ‘orange drop’ capacitors in this position so personally I would avoid it unless you know they are faulty. On pre-5885 models these capacitors do not exist.

  6. Replace the stereo lamp assembly with a 5mm yellow LED in series with 1k2 (or raise R101 to 1k). The green wire should be connected to the negative (short) lead of the LED, the black wire to the positive (long) lead.

  7. Replace the tuning lamps and assembly with two 5mm yellow LEDs in series with a 1k2 resistor (0.25W will do). The left-hand lamp looking from the front is connected as (4) above; the other one is connected the other way round (because the black wire here is supplying a negative voltage). In all three cases the green wire is ground. The old lamps can be removed complete with their mounting flanges with a pair of pliers: just squeeze the ‘U’ shape together and it lifts out to the rear of the tuner. If you can figure out a way to mount an LED in the panel lamp position, you can also fit an LED there, on the same principle, i.e. also in series with 1k2. Otherwise, if you have to replace this lamp, make sure that the swinging lever is free and doesn’t contact the chassis and earth its supply voltage anywhere.

Do not adjust anything on the left-hand RF/IF board (including the part to the right of the metal divider), or anything on a pre-5885, unless you have FM alignment gear. Note that the factory instructions for s/n 10000+ assume you have an ST1000A and don’t tell you how to do it with an ordinary sweep generator.

Esmond Pitt - Australia


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