Revised Quad FM3 Service Notes, nov 2010

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 and a very sweet sound, well suited for areas with good signal strength and not too many stations in the band.
There are three principal variations of the FM3:

1: s/n to 5884: This series uses a two-IC front end and a Motorola MC1305 stereo decoder.

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

3: 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. You can update pre s/n 5885 to the later power supply, by removing R106/7, shorting out RV102/3, and changing MR101/2 to 15V zeners. I have seen FM3s fitted with 7812/7912 voltage regulators but the MC1410 really needs more than 12V for lowest distortion: do not imitate; undo if found.

2. Replace the two PSU caps with 1000uF/25V. Quad used 680uF occasionally in one or other of these positions: this was strictly a stock or economy measure; do not imitate.

3. Replace the audio PSU decoupling cap C121 with 470uF/16V.

4. The audio coupling capacitors C124/125 (C116/118 pre s/n 5885) are rather small at 100nF, especially if you’re not using the FM3 into a Quad 33 or another preamp with 100k input impedance. Change them to 470nF high-quality poly capacitors. This will improve bass response and phase into lower impedances.

5. From s/n 5885+ only, you can consider replacing the resistors in the final multi-pole Sallen-Key filter chain: R115-120 with 10K 1% metal; R121/1 with 1K 1% metal, and R123/4 with 4k7 1% metal. If you can, check the values of C115-120 and C122/3, for tolerance within 5%, and replace with quality poly as necessary. Whatever you replace, replace in both channels.

6. There have been suggestions to replace resistors in the earlier RF and IF stages ‘to reduce noise’. This is complete nonsense of course, as the signal is FM-modulated at this point, and resistors produce noise in the amplitude domain only. However if you find a burnt or out-of-tolerance resistor there (you won’t), replace it.

7. Replace Tr107/8 with BC549C.

8. You can also consider replacing the composite-audio coupling capacitors C100 and 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.

9. Adjust the output level at RV3 to be no more than 250mV on normal reception, or on 75% modulation if you have an FM RF generator. Too high a level can cause distortion inside the decoder chip.

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

11. 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.

12. Using either a digital counter (preferred) or a DVM with a kHz range, adjust RV100 so that exactly 19kHz appears at pin 10 of the MC1310 with power on and no signal at the aerial input. Adjust to within +-2Hz if your meter is accurate enough. If you have s/n 5885-9999 and you can’t quite get to 19kHz, change R111 to 15k and if you can’t get accurate enough change RV100 to 10k linear as per 10,000+. This step aligns the stereo decoder, reducing distortion, and lights up the stereo lamp if it wasn’t on before. Pre s/n 5885 you need a stereo modulator to adjust all those coils for the MC1305 decoder: just forget it.

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 Sound Technology ST1000A and don’t tell you how to do it with an ordinary sweep generator.

Dada Electronics Australia, Esmond Pitt, may 2010