I have done a number of upgrades as discussed in this thread on my 2235.

I made a video with all the part numbers and info needed, including the
service upgrade on the PSU section, capacitor replacement, fan install, and
510K focus resistor replacements.

Someone might find it useful I guess...

https://www.youtube.com/watch?v=lrXCVg6T-ek

Your question of how hot the heat sink gets near the MOSFET  Q9070 made me want to check a 2235 I had worked on just enough to get it to run.
This 2235 needed a lot of new power supply parts including the MOSFET Q9070 which was replaced with an ordinary IRF840 (8A, 500V) MOSFET.
After this 2235 had run for a half hour the heatsink was at 112 degrees F when checked next to the head of the screw that holds Q9070 onto the heatsink.
It was no problem to hold your finger on the heatsink at that temperature, and there was no visible evidence of heat on the MOSFET's Molex connector from this 2235's previous life.
The way I understand it, Q9070 chops up the rectified DC coming in from the line voltage as directed by the PWM chip U930 to create the 43 volt DC that powers the primary side of the transformer T948.
I believe the 43 volt supply's current limit is controlled by U930 using different values of R912 to actually set the current limit for the different models of oscilloscope in the 22XX family that use this same power supply architecture.
Getting back to your heat problem, it is hard to imagine that something is loading your 43 volt supply, and that the current limit is wrongly being set high enough to cause the temperature to be too high at Q9070, but I guess anything is possible.
I have had very good luck using the IRF840 MOSFET for Q9070, and with using the transistor TIP41C for Q947 and Q947, and I think those are the only three components on that heatsink plate.
tom jobe...




On 5/24/2019 9:44 PM, donald collie wrote:

I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which has
a cracked skirt. Willing to pay for these. Probably also used on other 22xx
types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your finger
on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald ZL4GX

I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which has
a cracked skirt. Willing to pay for these. Probably also used on other 22xx
types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your finger
on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald ZL4GX

One of the members of Tekscopes makes new replacement volt/div knob
skirts for the 2235. I have bought some of them from him and they are
very nice.
He also carries some hard to find good used/tested parts for the 2235
such as the main transformer (T948?), the high voltage multiplier
(U975?), etc.
This link shows what he has in stock for sale on eBay at the moment, he
must be out of those 2235 knob skirts temporarily.

https://www.ebay.com/sch/shockcore/m.html?item=273836542787&hash=item3fc1ee0b43%3Ag%3AHUcAAOSwZQRYf5YX&rt=nc&_trksid=p2047675.l2562
He is a great guy to do business with as his 100% eBay customer
satisfaction rating shows.
I have bought several sets of his new replacement 2235 volt/div knob
skirts and they work perfectly.
About he only way you can make a mess of the knob skirt replacement is
to use too much glue.
A few very tiny dabs of glue/epoxy at the smallest diameter of the knob
register is all you need, and there is a notch that orients the skirt to
the knob just like the original had.
The member name he uses on Tekscopes escapes me at the moment, but you
can contact him through any of his eBay auction pages such as this page
for his 2235 main transformers.

https://www.ebay.com/itm/Tektronix-120-1601-01-SwitchmodeTransformer-for-2213A-2215A-2235-Oscilloscope/273836542796?hash=item3fc1ee0b4c:g:bAQAAOSwnrJbfX~4
tom jobe...








On 5/24/2019 9:44 PM, donald collie wrote:

I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which

has

a cracked skirt. Willing to pay for these. Probably also used on other

22xx

types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your

finger

on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald

ZL4GX

I only really need the clear plastic skirt on this knob [minimum], or two
complete knobs [maximum], as I can probably repair the other one, which has
a cracked skirt. Willing to pay for these. Probably also used on other 22xx
types.
Also : I replaced the chopper MOSFET in the psu with a NZ$1-60 generic
type. It gets very hot to my way of thinking - too hot to hold your finger
on the heatsink near it- does anyone know if an exact replacement is
necessary - perhaps a faster type with lower losses. The MOSFET`s molex
socket shows signs of being quite hot over some time [browning]
Thankyou in anticipation for your
reply.........................................................Donald ZL4GX