开云体育

On Aug 4, 2018, at 5:08 PM, tom jobe <tomjobe@...> wrote:

Hello,
Do not forget that scopes such as the 2215A and the 2235 have the fan drive circuitry you need already printed on the mainboard, but it is un-documented in the service manual.
You just populate the half wave fan circuit with a few components, and put a 60mm 12 volt fan on the back panel, on the two fan mounting holes that are already there.
This Tektronix fan circuitry drives the fan at a much reduced voltage, so noise is not a problem.
About those large resistors under the CRT that often scorch the mainboard... on the later production 22xx scopes Tektronix raised those resistor up so air could get in underneath them.
You can easily raise your your existing resistors by removing them and attaching some simple extensions to the resistor's leads. Do check the value of your existing resistors to make sure they have not been cooked too much and had the value changed before you raise them. Also consider raising them at alternating heights so you don't make a "wall" of side by side resistors that is just up higher.
My guess is that the burnt mainboards are in scopes that were run 24/7 and had the scope installed in a level position, so there was no 'chimney' effect to make the heat rise through the scope.
I have some photos of the above modifications that I can send to anyone directly on request. If you want these photos to be posted elsewhere, you can do it.
tom jobe...

On 8/4/2018 6:58 AM, Vincent Trouilliez wrote:
No idea about FET temperature, as I have no way to measure temperatures yet ! A thermocouple for my DMM and a simple IR "gun" are on my list of things to buy, but the thing is sooooo long, no idea when I will get round to it ! ;-)

As for discoloration/overheating, I don't see any sign of it on my scope, anywhere. All power resistors look sparkling new and so does the PCB in their vicinity. I guess that only means my scope happens to be a low hour unit.

These beefy resistors overheating in the 2215 and 2215A ans 2235 scopes, as per your findings, is probably due to the fact that these 3 scopes are among the few 22XX scopes which did not have a cooling fan.... what a "coincidence" ! ...

Well I can only speak for my personal 2215, which definitely does not have a fan. However I just downloaded the service manual of both the 2215A and 2235 to have a quick look at their mechanical parts list, and they indeed do not seem to have a cooling fan. All these 3 scopes being so closely related technically (the 2215 being an updated 2215 AIUI, and the 2235 just being a 2215 with improved B/W), I guess it's consistent.

Probably why some people advise to add a cooling fan, especially since the back of the cabinet/chassis already provides the "grill" where a cooling fan would go. So it's not that big of a job as far as mechanical integration goes, at least.

All that being said, unless your scope is not working properly (other than SMPS issues I mean), then I don't see any emergency to replace those resistors. Unless of course your aim is to spend time and money to make the scope as good as it can get, and as reliable in the long term as can be.
If so then yes, I guess adding a cooling fan and replacing those resistors wouldn't hurt. While doing so, you could fit higher wattage resistor (a size up), since space is not at a premium in this area of the scope, from what I can see on my 2215. Also you could try and raise the resistor off the board by half an inch or so, to improve cooling a bit and keep them from burning the PCB any further.
I remember a discussion on here not so long ago, about dedicated tools used to form the leads of (these very ?) power resistors to raise them from the PCB.
Don't ask me to dig out that thread though...my searching skills are not that good... but it's there... somewhere...
I remember it because I found it cool to have a nice little tool to be able to form the leads "cleanly", in a professional looking manner. Will definitely get one of these tools some day...

As for my 2232, was involved indeed, especially since it was my first "meaty" repair experience (definitely more than just replacing a blow fuse I mean ! LOL) , and had zero working knowledge of the 22XX scopes not even SMPS in general...
Still, I need to get back to that scope, to recap the PSU, as well as try to get rid of the SMPS annoying whine issue, and another issue also, where the trace on the CRT look "dotted" at some sweep speeds. Never a dull moment with this old 2232 ! ^^ But that will be for later...


Regards,


Vincent Trouilliez