[Premium-Rx] WJ8888 wonder radio

[Richard Reich]

Hello List.

Following-on from the recent debate regarding what is a premium receiver, and
the need for a micro-processor to make it 'premium': 
I have recently taken delivery of my long-time 'dream receiver' a Watkins
Johnson Quad-8. To be able to afford one in here the UK, I have resorted to
buying a complete, but non-worker, and am now waste-deep in fault-finding and
repair. As a result of this, I have become completely overwhelmed by quality of
build - AND the amazing ingenuity in the design of this radio. Whoever was on
the team that put this beauty together deserves an award - and I am not
overstating here. 
Its an advanced synthesised receiver bristling with facilities, and achieves
everything a micro-processor can do - and more - but completely in hardware
using clocks and shift registers. Not a CPU in sight. It has, what can only be
described as, a token-ring network system in miniature, which continuously
shuffles data back-and-forth between all the different receiver sections. It is
'a wonder to behold' to see how this has been achieved, and of course - this
makes it very repairable (no special ROMs, CPUs, or other bits).
So if anyone asks me if the Quad-8 is a premium receiver, I would say most
definitely so.

While we are on the subject of the Quad-8. It has one or two design flaws
(nothing is perfect), one of which can have serious consequences if the failure
occurs to your receiver. The +/- 15v PSU is based around the 723 regulator (DIL
package). This component has a reliability problem (in relation to other
components) - and the fail mode is usually to present the full input Voltage to
the output - which can be a disaster. In the case of the Quad-8, the particular
PSU in question runs extremely hot. It has a high input voltage of approx 26v -
the output is 15v, and the heatsinks are inadequate due to space limitations. WJ
may have realised this, and have attempted a bodge by bonding a heatsink to the
tops of the DIL 723s! Sadly, the heatsink needs to be about 4 times the size to
make much difference. This failure has happened in my receiver - the +15v rail
being at 26v!!! This, in turn, has taken out various caps and semicons.
When the repairs are complete - I am going to replace the 723 regulators with
something else (a modern and efficient fixed-Voltage regulator), drive them at a
lower Voltage to reduce the heat dissipation, and if still necessary, add a
small PC fan to aid cooling.
By the way, operating your Quad-8 without the top and/or bottom covers
completely ruins the airflow path generated by the fan, putting heat-stressed
components under further threat.
Also beware of old tantalum capacitors - they fail to a short circuit (or low
resistance) which can also have a bad effect on your receiver!

Bye for now.

Richard

***************************************************************
Richard Reich    
Principal Engineer Hardware
SAAT Technology Ltd
Web: http://www.saatt.co.uk

DDI: +44(0)1420 545388
Fax: +44(0)1420 87259

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