You can also tell the later FM4s by the plastic battery enclosure. Early ones had the battery soldered directly to the pcb - dangerous when (not if) the original battery leaks corrosive goo all over the pcb. The plastic enclosure saves the day.
The little additional board is also missing from the earliest tuners.
It's also worth uprating the output coupling caps if the tuner is ever envisaged being used with a non Quad pre amp. The standard 220nf cap will roll-off LF audibly if the tuner is loaded with anything stiffer than 50k Ohms. Replacement with a 1uf film will cover all likely pairings.
The little additional board is also missing from the earliest tuners.
It's also worth uprating the output coupling caps if the tuner is ever envisaged being used with a non Quad pre amp. The standard 220nf cap will roll-off LF audibly if the tuner is loaded with anything stiffer than 50k Ohms. Replacement with a 1uf film will cover all likely pairings.
I can confirm that when I replaced C11 & C12 in my FM4, IC5 the TL094/TLC27L4ACN comparator oscillated with a ~1kHz saw wave, which was audible during quiet passages in the output of the tuner. I tried a few different capacitor types to no avail. Adding 47 ohms to the output pin 1 stopped the oscillation. This is easily accomplished by using the resistor to replace the link track on the top side of the PCB after removing the through-board links. The resistor is mounted on the underside.
That seems reasonable then, 6.14 volt between those points (Quad say 6.2 volt)
That's an important question
Is this a new issue caused by the 47 ohm mod?
If it is then I think you could try reducing the 47 ohm to say 27 ohm. The other option but you must be careful is to try adjusting L6 as Quad say it affects the frequency points. I would first see if a finger around that area and placed on the print seems to swing the range in the way you want.
Assuming L6 is a ferrite dust core then you need extreme care to alter it. Do not use a metal screwdriver, it will almost certainly crack the core. If you haven't the proper non ferrous trimmers that fit fully and perfectly then see if a matchstick will fit and grip. Ferrite cores are extremely fragile and will crack if you are not super careful.
That's an important question
Is this a new issue caused by the 47 ohm mod?If it is then I think you could try reducing the 47 ohm to say 27 ohm. The other option but you must be careful is to try adjusting L6 as Quad say it affects the frequency points. I would first see if a finger around that area and placed on the print seems to swing the range in the way you want.
Assuming L6 is a ferrite dust core then you need extreme care to alter it. Do not use a metal screwdriver, it will almost certainly crack the core. If you haven't the proper non ferrous trimmers that fit fully and perfectly then see if a matchstick will fit and grip. Ferrite cores are extremely fragile and will crack if you are not super careful.
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Its either ferrite or its iron-dust, don't think ferrite-dust is a thing. Both are fragile, but adjustable inductors/tuned transformers are usually iron-dust core as its lower loss at HF and more temperature stable. Nylon tools are used, nothing harder than that - fashion something from a plastic cable tie perhaps?