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2467 U800 heatsink
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Ondrej Pavelka
Hi,
I just received today my new/old scope 2467B, I investigated the insides and I need to swap the electrolytic caps on the A5 board and RIFA caps in the PSU as well as replace the Dallas chip with fresh one. This scope was made in 1995 second half, I can see some chips labeled as late as 9540. I take it this is among the last 2467Bs made? Is it lt known what was the last serial number made? This one is 53626 I noticed somebody glued heatsink to the U800 chip, is that a good practice? I have read a bit about U800 failures and how Maxim vs Tek made ones are different but the information is sometimes contradicting. Is it good practice to add extra cooling? I have my desk facing a window which has a radiator underneath so my shelf with test equipment is unfortunately heated from underneath by raising warm air. If there is a imminent danger to my 2445B or 2467B in this setup I need to think about adding some extra airflow. |
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Ondrej Pavelka
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Jean-Paul
Hallo, where are you located?
The fine photos reveal the U800 is precarious as the bolts carrying the wire lash up are at 87V and the heatsink is apt to fall out or move. Suggest that you removed the HS carefully, see if the U800 is Tek original or Maxim and followed other threads on U800. Very deep issues re heat, heatsinks and failures. I am sure that Chuck Harris or Dennison can give you better advice Kind Regards Jon |
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Ondrej Pavelka
Heatsink is glued to the U800, the wire on the top is a secondary safety
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measure. It doesn't really touch anything but the question is how much can extra heatsink help. I am also hoping for people like Chuck to give me the answer because there seam to be many contradicting advices. On Tue, 2 Feb 2021, 21:10 Jean-Paul, <jonpaul@...> wrote:
Hallo, where are you located? |
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Chuck Harris <cfharris@...>
U800 is a strange and unique thing in the 2465 scopes.
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It is fairly likely that early in the design, when the IC packages were selected, so the main board could be laid out, U800's designer wasn't very far along in his design, and estimated U800 would dissipate a lot more power than it ultimately did. The U800 package is made to mount over a centrally located heat sink that is sandwiched between the IC and the circuit board. The IC's leads either passed along side of the heat sink, or passed through holes in the heat sink. The IC was made to be clamped to the heat sink using mounting studs, or screws on either end of the package. In preproduction testing, they must have found out that the finalized chip drew less power than expected by quite a lot, and they decided to leave the heat sink off for the full production life of the 2465 family. It was never added in any form for any of the models, at any time. Because the heat sink tab is the -5V power connection for the IC, they had to do something to make the electrical connection. If they had included the under chip heat sink, it would provide the electrical path, but they didn't. Instead, they punted, and just stacked a pair of star lock washers over the mounting studs to lift the package up, clear the extra long leads, and make the connection. If the nuts that mount the package are over tightened, or the star lock washers are omitted, the slightly longer than usual DIP package leads will force the IC's lead frame to crack the epoxy package, and allow moisture infiltration.. which is never a good thing. This is probably the cause of the so called unreliability problems with U800. The 2465 family is designed to operate at ambient temperatures as high as 50C. U800 is designed to operate as it is applied at that temperature as well. The fan must work, the scope must be clean, and have room around it for air circulation, but that was the design temperature. There are even holes in the scope's case bottom right there to feed fresh air to U800. I can see no reason to put a heat sink on U800, and I have seen quite a lot of damage caused by poorly applied heat sinks, like are shown in your picture. They break free, and rattle around in the scope shorting this, and that. I suspect that the installer of the heat sink on your scope read some of the damage reports, and decided to mitigate the problem on your scope... probably after the fact. If it was necessary, tektronix would have made an ECO and a kit available to install one. -Chuck Harris Ondrej Pavelka wrote: Heatsink is glued to the U800, the wire on the top is a secondary safety |
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I had a 2465B that had already failed the U800. Now that I think about it I had two like that. One I replaced with a new one from our friend in Greece. The other one would work for a few minutes and then fail.. I glued a computer cpu tiny fan on top of it and it worked ok after that. However I did wonder if the temperature change caused by the fan could have effected the calibration.
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Jim Ford
I believe it about the thermal design, Chuck. A few years ago I was working with an RF power amplifier that dissipated almost 2 Watts. The package was 4 mm square, IIRC, so we thought it was a lot of heat in a small area and bought several types of heatsinks. Well, I had laid out the PCB such that it had a bunch, maybe 25, of thermal and RF ground vias directly under the package, and I had the fab shop plate the copper to shut the vias. Well, there was so much heat wicked out of the PA by the copper in the board that the difference in temperature at the package on the top side was essentially the same with or without a heatsink on the bottom side. I was surprised how well the PCB operated without any heatsink. Copper is your friend!
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Jim Ford ------ Original Message ------
From: "Chuck Harris" <cfharris@...> To: TekScopes@groups.io Sent: 2/2/2021 1:37:47 PM Subject: Re: [TekScopes] 2467 U800 heatsink U800 is a strange and unique thing in the 2465 scopes. |
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