Just what exactly are the properties of BeO that make it suitable for this application?
Very high thermal conductivity with low electrical conductivity.
Beryllia 209-330 W m
-1 K
-1Diamond 1000
Copper 401
Aluminium 237
Alumina 30
Resistivity of berylia is 10
12 - 10
15 ohm m.
Just what exactly are the properties of BeO that make it suitable for this application?
It very high thermal conductivity for an insulator.
I'm not sure if a modern less carcinogenic solution would be good enough here. Hard to know if a someone simply picked the best thermal conductor without looking at options.
I honestly don't see why it was used here. The input bias is fairly stable inside the meter's specified operating range even with an air gap.
-- It only started to drift when the meter got cold. - Even then it eventually warmed up and was stable.
Of course in 1978 there were not as many options. Could be a case of an over specified design staying in because it was too expensive to change or simply because the person who made the choice was no longer on the team.
In 1978, making a change meant redrawing the schematics and PCB layout by hand. Expensive and time consuming. There are some questionable choices but I can see why it wasn't changed.
What is your opinion aboot the change in mass of the junction? It seems to me the BeO disc was chosen for both thermal conductivity and electrical insulation properties...
The whole point of the assemblage is to keep the two transistors as near the same temperature as possible to keep input offset voltage from the temperature sensitivity of VBE as close to fixed as possible. Any changes here are going to affect offset voltage and offset voltage drift with temperature. The offset voltage can be trimmed with the front panel offset pot, drift is minimised by maximising the thermal coupling of the two transistors. That this comes with a front panel offset adjustment suggests that the whole thing was not Mr. Stable in the first place so there's clearly no long term 'baked in' stability to worry about. With that in mind, going to the expense and trouble of a BeO washer attached to the tops of the cans in hoping to replicate a monolithic matched pair (that's how it's drawn so we must presume that at some point in its production history there really was a monolithic matched pair there) with just a matched pair smacks of magical thinking.
I found myself wondering aboot
this myself... we were taught that this package
needs wraparound heat sinks, and specifically to avoid the convenient options that clip on the top. I would expect similar from this application... but that would require something like a BeO sleeve around both parts... which would invite mechanical damage and exposure to BeO dust and...
Yeah, okay. So it was a half-assed (okay, maybe 3/4-assed...?) attempt to solve 2 engineering problems
and be economical to produce. I guess we should be glad it works as well as it does.
mnem
Well, we don't know where the die attach actually happens in those cans but I'd be really rather surprised if I were to learn that it was to the top of the can - it's not impossible, but it's wildly unlikely. I suspect that the die is on a lead frame and is just floating inside the can in a glob of epoxy or, at the extreme, glass. The lead frame might touch the case. The problem with all the non-hermetic TO-x cases is that the internal construction is usually a mystery. If they've got a metal base then you can be pretty sure that is where the die is attached.
The
hp handheld 2000 count multimeter
went a bit silly, way more than I was prepared to pay considering the crap pictures & high potential for a pile of battery corrosion inside.
https://www.ebay.co.uk/itm/144244364494David
It's my understanding that Gorrila Glue (the classic one) is a gap filler that foams as it sets. Foam is the last thing I'd use for a themal connection.....
Super glue or epoxy woud be my choice.
Well, we don't know where the die attach actually happens in those cans but I'd be really rather surprised if I were to learn that it was to the top of the can - it's not impossible, but it's wildly unlikely. I suspect that the die is on a lead frame and is just floating inside the can in a glob of epoxy or, at the extreme, glass. The lead frame might touch the case. The problem with all the non-hermetic TO-x cases is that the internal construction is usually a mystery. If they've got a metal base then you can be pretty sure that is where the die is attached.
Q3, has a brown epoxy? base. Q5 is a mystery. Removing it to have a look is not an option.
I don't think any of the leads on Q3 are directly tied to the case. I did a check and the resistances were far too high. I didn't do a diode check though doh!
Almost forgot to post this here....
David
The hp handheld 2000 count multimeter went a bit silly, way more than I was prepared to pay considering the crap pictures & high potential for a pile of battery corrosion inside.
https://www.ebay.co.uk/itm/144244364494
David
the 2000 counts mean you are counting at minimum 2000 Webshops to get the batteries for your 427.
So, if they are fresh inside, the 100+ pounds may be allready
The OKi / METCAL PS800 that Mnem forced me to buy arrived today.
Fast delivery, very well packed, All complete including barely used tip and tip removal pad. Bonus items were a brass "scrubber tip cleaner and a anti-static curly cord
It all works. Even more importantly the coil is the latest PS-CA3. Theis is the same as i used on the current PS-900 so the current SFV series tips fit.
The handle and coil alone cost twice what I paid.
HP427A
open the empty battery ( Eveready 763 Type, = 22,5V ) ,
remove all what is inside, but the cover with the connection knobs is to re-use.
Insert 2 pcs. of Lithium 9V Blocks made for smoke detectors, they provide 10,4V each, in Series this works best.
Close the batterycase and put it back in the 427A.
The original Battery will suck 50$ from your bag + postage ..
HP427A
open the empty battery ( Eveready 763 Type, = 22,5V ) ,
remove all what is inside, but the cover with the connection knobs is to re-use.
Insert 2 pcs. of Lithium 9V Blocks made for smoke detectors, they provide 10,4V each, in Series this works best.
Close the batterycase and put it back in the 427A.
The original Battery will suck 50$ from your bag + postage ..
Sounds like a job for 3DP.
mnem
*drops back and punts*
The hp handheld 2000 count multimeter went a bit silly, way more than I was prepared to pay considering the crap pictures & high potential for a pile of battery corrosion inside.
https://www.ebay.co.uk/itm/144244364494
David
the 2000 counts mean you are counting at minimum 2000 Webshops to get the batteries for your 427.
So, if they are fresh inside, the 100+ pounds may be allready
The meter has Option 01, mains power. Mine too, even so modern it has a C14 inlet. I removed the completely dead battery, and am running it from mains ever since. It's a desktop meter, it can be mains powered! If one insists, I believe
threetwo 6F22 in series would be a sensible bodge, IIRC.
a Question about 427A:
The banana jacks don`t fit standard banana plugs perfekt,
seems to be larger holes there, not exactly 4mm.
Is the a special set of wires for the 427A?
It's my understanding that Gorrila Glue (the classic one) is a gap filler that foams as it sets. Foam is the last thing I'd use for a themal connection.....
Super glue or epoxy woud be my choice.
Yes, it's an addition cure polyurethane. It cures as it absorbs atmospheric moisture and produces CO
2 as a waste product. It's basically the same formulation as the gap filling polyurethane foams beloved of bodging builders the world over (including myself, the gaps between my living room window frames, the wall next to them and the drylining in front of them are
full of the stuff).