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Old 17th Sep 2020, 6:25 am   #35
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Quad FM3 no stereo

Unknown and probably now unknowable is the outcome of any transactions that Quad had with Motorola other device suppliers about its planned uses of their products.

The +14-volt rail was probably fine for the original FM3, for both the TAA661 and the MC1305. Then the MC1310 arrived as the PLL successor to the MC1305, but with a lower maximum supply voltage of 14 volts. Plausible alternatives at the time were the RCA CA3090 and Fairchild µA758, both with >14 volts maximum supply voltages. But Motorola probably wanted to keep its existing business, and may well have OK’d use of the MC1310 with the existing FM3 power supply. Given that Quad used RCA dual-gate mosfets in the FM3 front end, I imagine that RCA had made a pitch for the stereo decoder business once the CA3090 was available. The CA3090 was the first PLL stereo decoder, around six months or so ahead of the MC1310, I think. Whether the µA758 preceded or followed the MC1310 I haven’t been able to ascertain.

Biassing the mono emitter follower from the MC1310 might also have been reviewed with Motorola. But it could have been seen as low risk. The MC1310 internal schematic was part of the data sheet, so a change that would have affected the input pin conditions would have been visible. Anyway, unforeseen consequences from “in-specification” component changes might well have been a normal operating hazard (that was certainly the case in other, quite different industries). I wonder if the bias requirement for the emitter follower was relatively uncritical, such that it would work over quite a range of voltages. In the case at interest, it appears that a failed emitter follower transistor had minimal effect on the mono signal, but did severe damage to the composite signal (or parts thereof) going into the MC1310.

Quote:
Originally Posted by Craig Sawyers View Post
According to Peter Baxandall (in a long letter to Doug Self reproduced in Baxandall and Self on Audio Power, Linear Audio Special Edition) Mike Albinson joined from Murphy Radio in about 1966/7. His first contribution was the input circuit of the 303 power amp. But the next three products were the FMII, FM3 and AM3, which I reckon were all Albinson's.

When I met him he described the way he did radio PCB's. He'd commandeered a toilet to be a circuit board prototype facility. He had a camera lens screwed in the toilet door, and he put a 4x taped up master on the end of a Dexion component store rack. This was all arranged to produce a correct size image on the wall of the toilet. He'd wait for a sunny day, go into the loo and lock the door. Tape a piece of resist coated board on the loo wall and wait a guesstimated time. He reckoned that was the only way to get an RF layout right. Iterate the layout until it worked as designed.

He'd then send the x4 master to the board house.

I asked him what audio system he had at home, expecting something Quad. "No interest in listening to audio - that is work. I really enjoy fixing up classic motorbikes"
Thanks for that. I looked up the Baxandall and Self book. Baxandall said that Mike Albinson turned out to be a quite excellent and very inventive Chief Engineer. Praise indeed I think from one in the top class.

The fact that audio was the “day job”, not a leisure interest may have helped him to be very objective at a time when objectivity in audio was heading out the door. Interesting that he designed the input end of the 303. That was somewhat unusual, both in terms of having a relatively low input impedance (22 k) for the time, and with that 22 k being the input arm of a shunt feedback arrangement.

The FMII was essentially the combination of the FM and the MPX unit in one case, self-powered, and with Si rather than Ge transistors in the MPX. The basic FM design was done by Geoffrey Horn (who also did the AM), and finished (including the twin neon tuning indicator) by John Collinson, who also did the AMII and MPX. The AM3 was a self-powered AMII, although with some changes in the output filter section.

The FM3 seems to have been mostly “clean sheet”. The front end does look as if it might have had RCA origins, though. At least judging by its literature, RCA appears to have stepped through several iterations in the early mosfet era. It started by using a single-gate mosfet RF amplifier in what had been an all-bipolar three-gang design, then retained that and went to a single-gate mosfet mixer. Next the mixer was changed to a dual-gate mosfet, then finally so was the RF amplifier, by about which time it had moved to dual protected-gate mosfets. Leak, with its Stereofetic of 1969, had gotten off that bus at the third stage, single-gate RF and dual-gate mixer.

The use of the twin-lamp tuning indicator on the FM3 may have been from a desire to continue with the same basic form as the twin neon indicator of the FM, albeit in updated form. By then, others, such as B&O, were already using the twin lamp indicator, although there had been few takers for the twin neon type. Apparently John Collinson had gotten the idea from an American magazine. A little bit of sleuthing suggests that this may have been an article in Audio, 1953 January, describing the Sonocraft ASC-2 FM tuner. Back to the FM3 though, the driving circuitry for the twin lamp indicator also offered a convenient way of providing deviation muting, which was still in the future as part of the industry-standard IF sub-system ICs. Maybe that was an example of the Mike Albinson inventiveness that Baxandall had noted.


Cheers,
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