Hi Tan,
I see that JC has reported that his properly working 2230 has 0.23 volts
across R907.
I remembered that I had a 2230 so I found it and a note on it said that I
had worked on it in 2009 and that I had put some new parts in the power
supply.
I took the outer covers off of the 2230 and ran it on the bench. R907 had
0.245 volts while cold, and 0.246 volts after 30 minutes. The side of the
chassis where the heatsink for Q9070, Q946, Q947 is attached, barely got
warm to the touch in 30 minutes (maybe 2 or 3 degrees C more?).
I also checked R949, it was 0.683 volts cold, and 0.687 volts after 30
minutes.

The voltage on R949 came up on Irwin Zosa's 2235 when he discovered that one
of the inverter transistors (Q946-Q947) had a leaky emitter to collector
junction (in one direction only). Irwin replaced the transistor pair with
some new TIP41C transistors and saw some improvement in his heat shut down
problem.

Some other test results from the past include:

Tektronix 2232,
R907 = 0.278 volts(!), R949 = 0.776 volts (R949 is 0.51 ohms on this scope)

Tektronix 2236,
R907 = 0.189 volts, R949 = 0.497 volts (R949 is 0.51 ohms on this scope)
This is a late model 2236 and R912 is 575 ohms compared to 549 ohms for the
earlier 2236's.
The higher ohms on R912 increases the shutdown current threshold across
R907.

All of the above and your symptoms, makes me wonder about the health of your
Q946 and Q947 transistors?
Also, could you disconnect W950-1, W950-2 and W950-3 to completely
disconnect the thermal shutdown board A18?
Your voltage readings across R907 seem to be quite normal.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Wednesday, January 25, 2012 1:46 AM
Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220

> Hi Tom,
>
> Thank you for the detail of the problem which is similar to my tek 2220.
>
>
>
> I am still unable to nail down the problem. I believe the scope is
working
> normally, because when it fires up it works normally.
>
> My feeling is that it poorly designed thermally for 230V operation
>
>
>
> Do you have voltage measurement for tek 2235 on R907?
>
> I am getting 0.198V (with the +5V to the storage board disconnected,
> effectively turning off the storage function).
>
> This means that without the storage function connection, the current on
+43V
> is slightly below 1A.
>
> I am not sure if this is a normal condition but it should be similar to
the
> tek 2235.
>
> Under this condition, the scope could operate for quite a while without
> problem
>
>
>
> Once the +5V is connected to the storage board, voltage at R907 goes up to
> 2.3V~2.5V or about 1.15 to 1.25A on +43 and the power supply will shut
down.
>
>
>
> Following done with little effect or improvement
>
> - Change 3 filter caps (only those that are feels hot) - C960, C962, C964
> (for -5V for 2220 only)
>
> - Change NMOS P9070. When changing P9070, I used a separate heatsink for
> P9070 and still such down happens which means it is unlikely to be caused
by
> the thermal shutdown board.
>
> - Shutdown is more likely cause by spurious overload in R907
>
> - Change C907
>
> - Change CR907 (two diode in parallel)
>
> - Added C919 with 1000pf in parallel. Lowered switching freq but still
> having a heated P9070 which shuts down. Not sure what freq but it is
> definitely audible.
>
>
>
> Rgds,
>
> Chor Ming
>
>
>
> _____
>
> From: TekScopes@... [mailto:TekScopes@...] On
Behalf
> Of tom jobe
> Sent: Monday, January 23, 2012 11:17 PM
> To: TekScopes@...
> Subject: Re: [TekScopes] Re: Power supply problem with Tek 2220
>
>
>
>
>
> Hi Tan,
> This morning I remembered about an interesting 2235 repair that was done
by
> a very crafty person by the name of Irwin Zosa during 2007 and 2008.
> Irwin lives in your part of the world in the Philippines where it is
almost
> as hot as in Singapore. His basic problem was the same as yours, the 2235
> would shut itself down with a thermal problem.
> There was not much interest in his 2235 problem on Tekscopes at that time,
> so the email exchanges we had went off group as he worked his way through
> the problem. Once he had the problem solved, he posted a nice summary of
his
> findings to the Tekscopes group.
> Irwin's repair summary is in message number 36518 and it was posted on Jan
> 6, 2009.
> He found a number of problems as he went along, but his final discovery
> might amaze you.
> It sure amazed me because I would have never found it!
> tom jobe...
> PS I will paste in Irwin's 2235 repair summary below to save you the
trouble
> of getting it from the Tekscopes Message archive.
>
> Hello to all: Way back in the first quarter of 2007, I had this
> problem of my 2235 shutting down after a few minutes from switching
> it ON. After a lot of interaction from forum members I finally
> nailed down the problem. Here is a brief summary of this process:
> First, I checked all the voltages according to the service manual and
> they were OK. I also checked the ESR of the capacitors and they read
> good but the unit being about twenty years old or so, I decided to
> replace the secondary caps (C960, C961, C962, C963, C968, C970) with
> ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
> so I used 1000uF. The problem still did not go away although the
> length of time-to-shutdown increased. I also replaced the FET
> switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
> replaced Q946 and Q947 with TIP41 units. At every change that I
> made, slight improvements were observed. I also replaced R912 (357
> Ohms)with a 390 Ohm part to increase the shutdown threshold of this
> 2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
> CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
> replacements that I used were the MUR160 ultra-fast rectifiers. All
> of these were not done "shotgun" style and in the exact order as
> described but rather, followed logical steps of checking one section
> at a time. I even thought of thermal runaway because T944, T948, and
> Q9070 would heat up so much (really hot!). I also noticed early on
> that the frequency of the pre-regulator section (U930) was about
> 72KHz and in the service manual it is listed as about 60KHz. It
> suddenly dawned on me to try to reduce the operating frequency of
> this section so I replaced R919 and now it is running at 60KHz. The
> excessive heat has gone and no more shutdown. I tried to run the
> scope for about half a day and still no shutdown. This 2235 has all
> of the power supply improvements already installed from the factory
> (but why was the pre-regulator section running at 70KHz ?). I would
> like to thank the forum members who gave their insights. Special
> thanks goes to Tom Jobe, who stayed with me all the way in this
> repair project. He took the time to take measurement readings for
> me, gave his analyses, and even offered parts that I may need for
> free. At some point I was considering another brand of oscilloscope
> but I held on to this (even keeping it in storage for almost a year
> because of this problem) because from my research, these TEK scopes,
> during their time, were among the best. I think they still are.
>
>
>
>