Hi Tom,
I believe that Tan has said that he replaced Q9070 (although he
called it
something else), and that he had put Q9070 on its own heat sink.
He thought
this ruled out the "thermal shut down board" as being the
problem.
We will have to wait and see what Tan does next, but I thank you
for taking
an interest in this 22xx problem! Normally us beginners are left
to try and
help one another on these 22xx's, so it is a luxury to get help
from a
knowledgeable person such as yourself!
tom jobe...
-----
Original Message -----
From: "Thomas Miller" <tmiller11147@...>
To:
<TekScopes@...>
Sent:
Wednesday, January 25, 2012 5:33 PM
Subject: Re: [TekScopes] Re: Power
supply problem with Tek 2220
Well, assuming that R907 is correct, the
current is not excessive. I would
guess the inverter is working correctly.
Does the overtemp sense measure the
heat from Q9070? If so, that would be
my first target. The Q908 is used to
shut down the Mosfet gate capacitance.
If that is not working right, the
gate will drop allowing the Mosfet to go
through a linear region and will
produce a lot of heat.
R908 limits
the gate current and if it has drifted high, the switch (Q9070)
will not
turn on or off fast enough.
But you are right. The first thing to check
are the two resistors, R907 and
R908.
Regards,
Tom M
-----
Original Message -----
From: tom jobe
To: TekScopes@...
Sent:
Wednesday, January 25, 2012 8:03 PM
Subject: Re: [TekScopes] Re: Power
supply problem with Tek 2220
Hi Tom,
Thank you for your highly
valued opinion on this matter!
The voltage drop Tan is seeing across his
R907 is very normal (even though
this involves the dangerous assumption I'm
making that his R907 is of the
correct value!).
Maybe Tan will look into
the health of his Q946 and Q947 transistors
before
he does anything
else?
tom jobe...
----- Original Message -----
From: "Thomas
Miller" <tmiller11147@...>
To:
<TekScopes@...>
Sent:
Wednesday, January 25, 2012 4:47 PM
Subject: Re: [TekScopes] Re: Power
supply problem with Tek 2220
Hi Tom,
I don't think I would
disable the thermal shutdown circuit if the
transistors are really getting
as hot as Tan has stated. It would be like
taking a pain killer to mask a
broken arm. But I do think you are on to
something with the leaky
transistors. Or even the main switching
transistor
could be
bad.
Regards,
Tom
----- Original Message -----
From: tom
jobe
To: TekScopes@...
Sent:
Wednesday, January 25, 2012 7:27 PM
Subject: Re: [TekScopes] Re: Power
supply problem with Tek 2220
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.
>
>
>
>
