Tuesday, October 26, 2010

Examining Marantz 20B schematic

This tuner is very impressive from the 13 pages of the official schematic, which I have now obtained from MarantzPhillips.  Unfortunately it can't be saved or printed properly, at least I can view it onscreen for now.

The IF alone has the 8 LC filters, and of course they're all full time there is only one bandwidth.  Each filter looks to have about 8 primary reactive elements in addition to fairly complex power supply decoupling which includes chokes and capacitors everywhere.  Each of the 8 filters does appear from the schematic to be some kind of "filter module."

Funny, never has a tuner with so many filters been made so un-selective.  Legend has it the 20B is less selective than 10B, may have something to do with the gain or impedance of the active devices.  OTOH, some say the 10B bandwidth wasn't sufficiently wide for low stereo distortion, so maybe this is better for sound if not for receiving weak stations.

OK possibly it is less complex than more modern tuner because it's just brute force discrete circuitry instead of IC's.  But there is a lot of circuitry. There is also a 4 transistor circuit board "limiter".  I do not believe there are any IC's, so it's all discrete.

In the front end, there is indeed a dual gate mosfet in the first tuned circuit.  There's a jfet that appears to be part of the mixer.  3 caps one inductor in first stage, 6 caps two inductors (transformer) in second stage .  Mixer is not balanced.  BTW I think Sequerra Model One has balanced IF as well as mixer.

The MPX oscillator is quite complex, 10 transistors, one FET, one optocoupler.

I think detector is ratio detector.  Detector like most other blocks is in a shielded can.

The multiplex matrix is non trivial, 4 fets, 2 transistors, two optocouplers, and 4 diodes.  The diodes are shunted with caps and have cap+resistor loads.

About the only thing that looks a little unsophisticated is the power supply, though it is quite large.  Within the power supply itself, there are 2 unregulated voltages for scope and 3 regulated voltages for tuner.  The regulators are all shunt regulators, with 2 and 5 watt series resistors.  I notice that on the front end board itself there is an additional elaborate capacitance multiplier regulator, and there might be other sub regulators on other boards.  Most serious transistor tuners have had multiple series regulators, sometimes with active feedback.  Series regulators are far more efficient and can regulate better generally.  Shunt regulators may have lower noise.

The output stage is a flat two transistor cascade (all the countouring having already been done).  Seeing as it has a 110K input impedance it could be bypassed though capacitive coupling is required because it rides on the matrix +4.8v and -4.8v for other channel.  There is no input or interstage cap on this board, only the output 1uF cap.

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