Tektronix 2221a repair project

[Gianni]

Hi Guys!

I noted too late this thread and some weeks later I went thru the same story and arrived to the same conclusions about heat (I also had strong battles with the 2221 power supply and 2230, that was even incomplete; eventually it seems that I won)

I just wish to add a bit: in my case, even after having added the thermal paste, the problem went on. The cause was that the previous owner didd reverse the fan. In this way the U9208 was heated by all the heat produced inside the oscilloscope, instead of being refreshed by incoming air.

Reversed the fan, the problem disappeared.

[calibrationfixture]

Hi to All,

@ to Reply #25:

Same story with my 2221A and a 2232 of a friend. Fans are blowing to the insides. Both Scopes didn't run into trouble. But a recently serviced 2245A ran into trouble with its reverse mounted fan. After approximately 30 minutes it shuts down. Also its enclosure feels "warm". According to its Service Manual (070-6557-00 Page 2-2) air must be exhausted from the right-side ventilation holes. After the reversing the enclosure stays "cold" and no more powering-offs.

Calibrationfixture

[David Hess]

One of my 2232s just started acting up.  I will be checking the thermals first and heat sinking some of the ASICs.

[Allen49]

I have a Tektronix 2221 that I am trying to repair. The previous owner stated he was using the scope when he heard a pop and something started smelling burnt. I found that R909 was burnt and also one of the legs on Q9070, IRF710, was burnt off. The IRF710 was placed on a heat sink with a piece of heat sink tape but I don't know if it was ground through the center pin to the heat sink. Could anyone help me out with this.

[David Hess]

The tab is the drain and *must* be insulated from the chassis.

If the MOSFET is blown, then the gate driver around Q908 and switching controller U930 are probably damaged.  The entire circuit needs to be checked.

I know of many examples of that power supply having failed in that way but never had one to diagnose why.