Our £110.00 service charge does not includes the
cost of blown homotaxial output transistors. Genuine NOS homotaxial transistors are now becoming very scarce. The cost of these will be added at
£7.00 per transistor to a maximum of £28.00. The cost is due to their
scarcity and the need to manually select high Vceo devices from a batch of
transistors. Please don't be put off by this. The majority of 3O3 repairs we receive
are completed at our standard price of £110.00.
On some occasions the output devices are blown or are below specification and
replacements are then necessary.
What are homotaxial power transistors?
When Quad designed the 3O3, they chose
RCA transistors for the power output, voltage regulator and driver transistors.
This was a no-brainer for the Quad
engineers because RCA in those days was one of the leading power
semiconductor manufacturers. Several years before the Quad 3O3 was launched
RCA had invented their homotaxial process, one that dramatically increased the
power performance of silicon transistors at low cost. The devices
used for the main output transistors of the Quad 3O3 (the high current end of the output
triples) are the RCA 38494. These are robust, homotaxial power devices
in a TO3 case, similar to the popular homotaxial 2N3055 but selected for higher
collector-emitter breakdown voltage (Vceo) and current gain.
Why are homotaxial types so important for Quad 3O3 repairs?
The power transistors used on the 3O3
are homotaxial types. They
have a large chip area and a thin substrate. The inter-electrode capacitance, is
therefore relatively high. For
example, the collector-base capacitance (Cob) of the 38494, at low voltages
(e.g.10V), is as high
as 1500pF. Similarly the collector emitter capacitance is around 500pF.
This meant that these robust power transistors were unsuitable for high frequency (HF)
applications. At audio frequencies, this was not such an issue. The high capacitance and poor HF performance was a blessing
in disguise for the Quad 3O3.
The positioning of the 3O3 PCBs relative to the heatsink meant that
long wires were necessary to couple the boards to the main output transistors. Normally the long wires would create
instability and ringing on fast rising/falling signals but the sluggish
performance of the output transistors at HF, along with other designed in factors,
keeps the system stable.
The problem started when the
homotaxial process was superseded by epitaxial fabrication. In this process,
even more powerful transistors could be manufactured on smaller sized dies by
diffusing deep into the die rather than across it. This meant smaller
capacitance and superior HF performance. Unfortunately, the homotaxial
process soon fell into obsolescence. Well meaning DIY enthusiasts and
non-knowledgeable repair shops, fit epitaxial types in place of the
correct homotaxial variety. We often see new (epitaxial) 2N3055s, 2N3773 and MJ150
series transistors fitted or sold as DIY kits. The result often goes unnoticed as the instability is
parasitic and above audio frequencies. The resulting sound quality though is
mediocre at best. Another indicator in some cases is an elevated heatsink temperature.
What we do
In addition to the driver and
pre-driver transistors, we check the four output and regulator transistors
of every 3O3 we receive for service. We measure the collector-emitter breakdown voltage (Vceo),
the base-collector current gain (Hfe) at 1amp and the all important 'Cob' check'
to confirm they are homotaxial. If any of these parameters are low, the
device/s are replaced with genuine Homotaxial devices specially selected for
high Vceo and current gain*. If your 3O3 has epitaxial devices fitted we will
inform you prior to carrying out further work, as we will then have to add a
surcharge. Please bear in mind, even if the whole set have to be replaced
(unlikely), the full standard service charge, including the full output set, will only be