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Re: 2215A LVPS repair

tom jobe
 

Hi Leo,
Yesterday I found the Fluke thermocouple and did some tests on a 2215A.
First I disconnected the optional fan, then let the scope run with two traces like you did.
The temperature of the Mosfet Q9070 ceramic heat sink piece stabilized at 54.0 degrees C in a 29.5 degree room.
I removed Q9070 and discovered it was a TP3N40 Mosfet that I doubt was original, The only datasheet I found for it suggested that it was a 3 Amp part rated at 400 volts with an RDS(on) of 3.3 Ohms.
The Mosfet replacement I have used for years on 22xx's is the common IRF840, so I put one of them in for Q9070.? According to the datasheet, this is an 8 Amp 500 Volt part with an RDS(on) of 0.75 Ohms.
Later in the day, the 2215A stabilized with a temperature of 56.5 degrees in a 30 degree room.
Maybe I did not wait long enough the first time, because the results were not quite what I expected?
In any case, I consider this 2215A to be functioning normally so this range of Mosfet temperatures must be what we should expect to see.
I also believe that this 2215A has all of the standard component values you would expect it to have for R909 etc.
tom jobe...

On 8/10/2018 10:14 AM, satbeginner wrote:
Hi all,

Hi Tom,
I tested for temperature this afternoon again, after I performed a full calibration on the scope.
So, the scope had been running for several hours when I did this measurement.
I used a 80-TK Fluke surface temperature probe, holding it flush with the small heatsink just next to the FET, that was the hottest spot I found.
At that point in time the scope was just running with both traces visible, where trace A was showing the "calibration" signal.

Hello Ed,
in the end I consider my modification of the PS minimal, I used a lower ON-resistance FET (better, but maybe a slightly higher "switch-on" current?), and I reduced the Gate series resistor, where Tek in some models removed it even completely. (maybe the faster current pulse is not filtered as it should be, so maybe some may be injected into the grid?)
In my humble opinion is adding a fan also in the category minimal, again, because Tek used several FAN configurations in basically the same Power Supply.

Hola Vincent,
I think even the 2-or 3-wire PC fans have electronics inside.
Usually these fans are brushless, so they need some form of rotating magnetic field, and this is generated by a cheap and simple oscillator and coil driving circuit.
Some information can be found here:

I added some pictures of the temperature taking in the album: /g/TekScopes/album?id=64919

Un saludo,

Leo
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