LK2 Service Manual
Page9
Copyright © 2000 Linn Products Limited. All rights reserved.
Contents
Paul O'Neill
Fault Finding on the LK2 – hints & tips
The LK2 circuitry is notoriously difficult to fault-find on in the traditional way (tracing signals, measuring
voltages etc), so this manual seeks to help you to avoid this ordeal. Check the fault tables below firstly, to
see if your fault is included and if not, use the information in this section.
Trip
One issue that may prevent you from fault-finding is the inclusion of the “Trip” or protection circuitry which will
shut down the entire board if it detects a problem that might cause damage to the amp or the speakers. If trip
occurs, it makes fault finding very very difficult as no power reaches the amplifier section of the board. If this
happens it is best to try to find the fault with the power off – use the information found in the Fault Tables
below and in this section.
Isolating Faulty Components in the LK2 Circuit
Common Faults
Around 60-70% of faults in an LK260 or LK275 board
only
(i.e. not LK280) are attributable to just 4
components. See “
LK2 Reliability Modifications
” (accessible via Linfo – Product Information) for
instructions on how to replace these components and prevent re-occurrence. This modification is a good first
step for a faulty PCAS 003 board and should even be carried out to working boards as a preventative
measure.
Transistors
The majority of remaining faults on an LK2 board are caused by faulty transistors, so this is a good place to
start. If the transistors develop a fault, they normally develop a short circuit (zero ohms) or low impedance
across one or more junctions, so it is usually quite a simple job to trace the faulty transistor by measuring
across every junction of every transistor – while the transistor is still in circuit. This might seem daunting,
given the number of transistors on each board, but it is definitely a worthwhile exercise as it may save you
many hours of conventional fault–finding and does not take as long as you might think (about 20 minutes per
board).
Just measure the impedance across all 3 junctions (Emitter – Bass; Emitter – Collector; Bass – Collector) of
every transistor on the faulty board.
In general a working junction will measure many Kohms or even Megohms, whereas a faulty junction will
tend to measure almost short circuit – perhaps a few ohms – possibly up to 500ohms at most. If in doubt,
compare with the same junction on the good channel and replace transistor if you suspect faulty.
It is advised that you check every single transistor on the board, i.e. don’t stop if you find a faulty one, as
quite often there is more than one faulty transistor on the board.
Remember to include the big transistors (the 4 that are clamped to the heatsink) when measuring.
The above test, while not absolutely guaranteed to find every faulty transistor, will find the vast majority.
Remember that it is quite common for more than one component to be faulty, as often a faulty component
will cause damage to others around it in the circuit. See below for advice on how to check for this.
Other components
Other components are slightly less straightforward. The same principles as with Transistors above can be
applied (measure using an ohmmeter) but there is a lot more work involved and a lesser chance of success
as these parts are less likely to go faulty and less likely to follow such a straightforward pattern as the
transistors.
If you have a fault that is not caused by any of the faults listed in this manual – i.e. the fault tables and this
section - then try to narrow the fault down to an area of the circuitry using conventional fault-finding and then
measure and/or replace the components in that area.
Resistors can of course be easily measured for impedance but are very unlikely to go faulty unless another
component failure has damaged them, so look for telltale signs like scorching etc on or around the resistor
that might indicate this.
The Capacitors on this circuit can also be checked using an ohmmeter as above. Sometimes capacitors will
internally short-circuit so worth a quick measurement.
Check the components (as transistors above) using an ohmmeter.
Содержание LK260
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