"N_Cook" wrote in message
...
Arfa Daily wrote in message
...
"N_Cook" 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.
Arfa
Thanks for that - both of those caps are bulging, I had not noticed up to
now. I will replace those and the TO92s. A tip in reply for these and
other
apparatus with that grey interconnect ribbon. Before removing any such
boards I run some hot-melt glue down the join of the cables at and to the
board. So any bending, you have to do, is then only in the cable run which
consists of multistrand but soldered wire, so will easily break where not
supported by the grey plastic sheathing, ie at the ends, where it
otherwise
would choose to bend.
I would have thought the ducted air design was about ideal for a given fan
size, but I will bend inwards the outer fingers between the +/- dc
carrying
adjascent heatsinks. Perhaps the airstream takes the path of least
resistance and does not pass through the 44 vane version of this type
http://www.elfa.lv/images/tn/d9b8dde...5056b94d7a.jpg
staggered fingers of those square format heatsinks and goes around outside
instead. Perhaps bending inwards all 20 internal run fingers of each h/s
would be a sensible mod, less vortexing/blocking, more open, would make
less
of a dust trap as well
Having a great deal of experience of these, I feel they are a fundamentally
well-designed and constructed amp - both electronically and mechanically,
and I don't think that I would contemplate altering any aspect away from the
way it was designed, including the arrangement of the heatsink fins, and the
airflow paths. The heatsinking and cooling of the heatsinks *is* adequate as
designed, but becomes a bit marginal if the units are thrashed hard on a
regular basis, whilst the air intake vents are allowed to clog. If the
transistors / heatsinks of both channels are cleaned down and repasted
during the repair process, and the owner is alerted to the importance of
just scrubbing a soft paintbrush around the vents once in a while, they are
unlikely to give further trouble. I've never had any repeat failure issues
with them, anyway.
I take your point about the inner 'row' of fingers, but I don't think I
would feel inclined to bend them in more than a few degrees, otherwise, they
might actually block convection off the transistor cases ??
Arfa