Thread: Quad FM3 tuner.
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Old 21st Apr 2013, 5:06 pm   #1
Mike Phelan
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Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
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Default Quad FM3 tuner.

This was Acoustical’s first FM tuner using semiconductors and as all their equipment, mechanical and electronic design is top-class.

This one must have been lying in a loft or outhouse as the paintwork was chipped, the acrylic scale had scratches on it and the tuning knob was reluctant to move.
Closer examination showed that it must have had a bit of a bash as the frame was bent causing the knob to rub the front panel and the station marker rail didn’t move.
The rear plastic panel had most of its pegs to attach it broken but they were all still attached to the frame. A good coating of nicotine on it, as well.

To get the tuning knob off, there’s a 4BA grub-screw. As usual, even with a long soak, heat and all the rest, wouldn’t budge so I removed the spindle and knob complete and drilled out the screw using a drill just smaller than the tapping size. Cleaned up the thread with a tap and made a new grub screw.

Repainted the case, front panel and knob.

I removed all four pulleys to clean and lubricate them, and while they were all off, straightened the front of the frame. Fortunately, the two PCBs weren’t cracked.

The "mute" preset on the rear was broken, but Keith (sTV) came to the rescue and dropped one off tor me - Thanks, Keith.

Now it’s testing time:

Of course, the last thing to do would be to apply mains for very long without doing a few tests.
The FM3 uses a -14v and +14v supply rails, and they read -20v and +8v (!) The two PCBs are (a) front end and (b) power supply, stereo decoder and tuning light system.

Hooked up an aerial and an audio lead connected to my faithful Taylor 22 Fault Finder. Nothing; not even a panel light.
A quick check showed that one of the earth rails on (a) that wasn’t earthed; a 5mm link between the panels had an almost invisible DJ.
That’s better; two 14v rails now and some audio out.

Still no panel light – the bulb didn’t like 20 volts applied to it. One of the tuning lights now worked.

These operate from a long-tailed pair (see pic). TR7 base is zero when the signal is tuned in and therefore both lights are illuminated as the two Darlington pairs turn on fully. As this would also occur if there was no signal, TR8 turns both lights off when D1 receives no output from the CA3089 IF / detector chip.

The four lights (panel, stereo and tuning) use LES 14v bulbs with curious holders consisting of a spring.
This is one of the few poor designs; the spring connects to one tag for the bulb supply, by just using pressure from the rivetted centre contact. The slightest amount of tarnish and it doesn’t work.

I took forsook originality and replaced both bulbs with LEDs and a 1K series resistor (see pic). They are just the right size to go through the plastic bush that the bulb fitted.

The setting for the 19 KHz pilot tone was off; adjusted it using a frequency counterand we now have an output on both channels, but still no stereo light.
There was a supply to the bulb and the bulb itself was fine, so no guesses as to the reason!
Another yellow LED and 1K resistor, and everything now works.

Even looks better.
Attached Thumbnails
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Name:	A- Bent chassis.jpg
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Name:	B - Front.jpg
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Name:	C - Lights.jpg
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Name:	C - Rear.jpg
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Name:	D - LEDs.jpg
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