Jeff Liebermann wrote in message
...
On Fri, 21 Jan 2011 04:37:26 -0800 (PST), b
wrote:
Pet hates: excessive amounts of screws holding covers of TVs etc.
together. this seems to have got worse with flat panels.Often you
spend as much /more time assembling and re-asembling than the repair!
-B
Screws are fine. I can live with having too many screws because half
of them are usually stripped out and the remainder hold things
together. What bugs me are snap together clamshell cases, such as LCD
monitors and TV's, where you have to remove a mess of screws *AND* pry
the case apart. While snap together plastic is probably easier to
assemble because it doesn't require much fixturing to position the
robotic screwdriver, it does suggest that the case was never intended
to be opened or the unit repaired.
I recently repaired a Yamaha P70 electric piano. I didn't count, but
I'll guess about 60 large "sheet metal" type screws holding it
together. I don't use an electric screwdriver, but this is one time
that I wish I owned one. Even with switching hands, my hands ached
after I as done. The problem with such "sheet metal" screws is that
they offer high frictional resistance on every turn, while more
conventional screw threads, only offer high resistance when tight (or
smeared with thread lock).
My guess is the large number of screws was to prevent mechanical
resonances in the plastic case or to keep them from falling out from
vibrations. Still, metal thread inserts, screw threads, and steel nut
plates, would have been much easier to handle.
On the silicon grease front, I agree with most of the comments. Thin
works much better than globs of silicon grease. One should remember
that the purpose of silicon grease is NOT to bridge gaps. It's to
fill in the surface roughness, groves, and crevasses. Most of the
heat transfer is metal to metal contact, not through the silicon
grease.
In a past life, I used to design marine radios. The problem was that
the power xsistor packages of the day (1970's) were generally thermal
disasters. Either there was insufficient contact area to obtain
sufficiently low thermal resistance, or they were not flat. I solved
the first by building pyramid like structures of copper washers to act
as a heat spreader. I solved the latter by polishing the mounting
base of the power transistors on fine emery cloth. I hated to polish
away the gold plating, but that's what it took to get the heat out. I
made numerous tests and measurements trying to determine the optimum
amount of silicon grease, and eventually concluded that ultra thin is
best. Instructions were to smear a tiny amount onto the area, and
then wipe ALL of it off with a plastic scraper. What remained was
silicon grease in the remaining surface roughness, which was all that
was necessary.
I recently repaired an IFR-1500 service monitor. The power supply
section was intermittent. The 0.062 aluminum power supply case, was
butted up against the large aluminum heat sink that covered the entire
rear panel. In between was a huge amount of silicon grease. The
sandwich was held together by two large 10-24 screws, which probably
explains the silicon grease overdose. Two screws is not going to bend
the aluminum case so that it lays flat. So they tried to fill in the
lack of flatness with silicon grease. That doesn't work.
It took me considerable effort and alcohol to clean up the mess, but I
still managed to get it all over everything on the bench. After the
repair (large copper wires on torroids were not soldered properly), I
reassembled it with only a little silicon grease around the two large
screws, and left the rest to it's own devices. Works fine with no
obvious overheating (checked with an IR thermometer and thermocouple
probe). My guess is all that silicon grease did nothing useful.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
I suspect we are as engineers are just as likely to be mislead by
manufacturer's claims as joe public and general ads. I could not convince
myself that silipads were better than mica (no patents so litterally dirt
cheap) - so experiment called for. These days I own a matchbox size remote
IR pyrometer and would use that to compare device body temps in before and
after situations , not fingertips
Mica versus Silicone pad insulators
I was not convinced that for an existing used amp with 4x TOP66 power output
devices that the silipads were better than mica.
Each of the 4 white insulating pads had shrunk about 5mm at the tops
(hottest)
compared to bottoms , ruffling the original outer edges, heat damage ?.
I'm wondering if they can chemically change over time and/or excessive
temperature , downgrading to be more of a thermal insulator.
They are not discoloured or hardened or anything different in the
ex-compressed area by sight or flexing, just permanently deformed , the
ruffling is permanent.
I replaced all 4 with mica and thin films of thermal grease.
Before doing so I powered up the amp with 400 Hz continuous sine giving 20
watts in a dummy load. No fan cooling for this amp, just
convection/radiation.
Laid a brass barrel protected thermometer on the heatsink and took
measurements. Stabilised at 33 deg C over ambient after 50 minutes.
Replaced with mica and redid the load test.
For same ambient , same testing position/attitude, power in load etc it now
took 30 minutes to stabilise at plus 32 deg C over ambient.
More graphically , but less scientific, - the finger test.
After half an hour of heating with the mica setup I could hold a fingertip
on each tranny for about 5 seconds before finding it uncomfortable.
Previously half a second of fingertip touch was enough.
I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silipads had aged, been affected by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to me
to be otherwise.