Gareth Magennis wrote in message
...


"Ron" wrote in message
...
On 05/01/2010 09:59, Arfa Daily wrote:
wrote in message
...
They have an otherwise original assembly method of the final contact
to
the
pins of TO3 power transistors by 2 zero ohm "resistor" links, so 2
current
paths to the pcb traces. Anyone else observed bad solder joints to
these
links on the pcb? and cause? believed conventional solder not PbF





I repair many of these, and can honestly say that I have *never*
observed
any bad joints at the place you mention. By far the commonest problem
is
failed output transistors. This occurs because the heatsinks are only
just
about adequate with proper cooling. As soon as the fan air intake vents
start to clog up with fluff, the outputs start to run too hot. Over a
period
of time, this dries out the heatsink paste to a powder, leading
ultimately
to transistor failure.

Whenever I get one, I always remove the other pair of transistors as
well,
clean down their heatsinks, and re-paste them, not forgetting the
flatpak
transistor that's in contact with the underside of one heatsink on each
channel.

The manufacturers recommend that when the outputs are replaced, two of
the
BC546Bs nearby are replaced as well (T7 / 8 on one channel, 10 / 11 on
the
other). Check also C3 and C21 to make sure that they are not bulging.

Other than this, these amps are very well behaved, and new outputs and
fuses
will, in 99.9% of cases, effect a complete cure. Note, however, that
they
have proper differential inputs, so are not that easy to drive
correctly,
unless you have a proper balanced XLR source, and that they don't like
earthed test equipment connected to their outputs / inputs
simultaneously. I
usually hook a completely isolated speaker to them for final check, as
the
music shop which sends these to me for repair, often remove the amp
chassis
from the cab, to ease the transport, and save me having to strip it all
out.
He now tells customers when they collect the repaired unit, that they
should
brush out the air vents at three monthly intervals.

I agree with Arfa, I`ve repaired dozens of these and always found the
solderwork to be excellent. Perhaps someone else has been in there
before
you. There was a mod for early units, different output transistors and
the
addition of a couple of 1N7007 on the print side of the board.

Ron(UK)


Well, I agree with Arfa and Ron. I've repaired lots too, since the
cooling
is also IMHO inadequate, which is quickly made worse by fluff around the
fan
vents.
Never had any dry joints, but seen plenty of the bulging caps Ron
mentioned.
I replace them with 105 degree types regardless of apparent condition.

One warning - it is easy to accidentally short the solder tags the big
diodes are soldered to, to the zero ohm links as you tighten the nuts. If
you power up fully in this condition with 4 amp fuses you will blow the
Darlingtons again.
You have probably noticed both speaker output tags have amplifiers on
them,
there is no grounded speaker connection, though each output is referenced
to
ground.



Gareth.


I see no circuitwise reason for those repeated cap failures. The Darlington
heatsinks are elevated off the pcb (trackside) with spacers and although
the minor component side of the pa pcb is un-fanned I would not have thought
pcb and working caps would get to 85 deg C
Will make airflow mods and RTV-stick non-resettable thermochromic dots
around various items , for if it bounces back.