Thank you Tom for taking the time to point out specifically the
likely cause of the problem.
I know it takes effort to go through the details and I really appreciate
it.
Just to let you know where I come from, so that there is
less to assume.
I am an electronics engineer from Singapore
above 50 so very much familiar with vacuum and solid state electronics.
I do not have the schematics for 2220 so like you I am assuming it
is quite similar to 2235 which I have it downloaded.
I have both 2220 and 2235 and there appear to be quite similar
though the PCB layout is slightly different.?
I have complete understanding of how the power supply works.
Pre-regulator - PWM driving Q908 to produce +43V across TP940 and
TP950
Final Regulator ¨C DC-DC push-pull inverter through Q947 and
Q946.
I have another scope 2235, unfortunately, it too went down
at the same time, so the only instrument I have is a multimeter.
Anyway, I do not have an isolation transformer so even with the
2235 working, I will need to get an isolation transformer before I could take
any measurement.
Any idea what VA rating is needed for the isolation transformer?
Based on the power needs, it appears to be at least 200VA.
Initially, I was asking about R907, not because I was
suspecting the resistor but I intend to check the load current by putting an
ammeter in series with the resistor.
For that to be useful I need to know what is expected.? Like
you. I am suspecting that the voltage at R907 has gone up after the set warms
up thus shutting down U930. ?
I will probably go with the cap changes you have recommended
since I have some time during the Lunar New Year break.?
Unfortunately, I do not have ESR meter, but I don¡¯t think
that a big issue just replacing the caps.
I am aware of the direct 43Vdc injection, but thank you for
telling me the detail that Q908 needs to be disconnected.
Unfortunately, I do not have a power supply with this voltage
output.? I only have a power supply of 18V max 2A.? So I will have to
hold off this approach.
One more thing which I did not mention earlier is that I
detached the +5.2V connection to the digital board (for the storage function of
2220) and behaviour remains unchanged.
Rgds,
Chor Ming
From: TekScopes@...
[mailto:TekScopes@...]
On Behalf Of tom jobe
Sent: Saturday, January 21, 2012
7:30 AM
To: TekScopes
Subject: Re: [TekScopes] Power
supply problem with Tek 2220
?
I saw your
2220 question the first time you posted it a few days ago, and I
did not have a nice short answer for you. Since you have not gotten the
answer
you were looking for, let me comment a bit on this.
First off, I don't own a 2220 or have the service manual for it, and none of
the download sites I checked seem to have it either, including
ArtekMedia.com having it for purchase. So I will assume it is similar to the
better 22xx's such as the 2235, at least around the power supply. I will
also assume that you do have a service manual to work with. U930 monitors
the voltage drop across R907, and in your case it probably thinks the
current is too high after a few minutes, so it shuts down.
I have worked on lots of 22xx scopes and I have not seen the value of R907
drift and cause this problem, but others have reported that as a
possibility. Odds are, that this is not your problem.
An over current situation can be caused by anything after R907 within the
inverter, the transformer itself or on the secondary side of the main
transformer.
How you go about finding the problem depends on what equipment you have
available, and what your testing preferences might be.
If you don't have much to work with, just start changing components. In my
experience the components in the right rear corner of the mainboard cause
the most grief so I would start there with C925, C942 and C943. If you have
some decent soldering equipment you can change all three capacitors quickly
without removing the mainboard.
If you have access to an ESR meter you could check all of the aluminum
electrolytic capacitors in circuit. Most of the electrolytics give no
trouble, especially the six(?) 840 uF caps in front of the heat sink for
Q9070 (but you would want to check all of the electrolytic caps if you had
an ESR meter handy).
Another approach would be to put in the 43 volts DC from an external power
source until something got hot or went up in smoke. If your power supply
allowed you to adjust the current limit you could sneak up on it and just
get the problem part(s) nice and warm. You will need to at least disconnect
Q9070 when you apply the external 43 VDC at TP940 and TP 950 from the bottom
or top of the mainboard. I lightly tack on two wires to the bottom side to
make the 43 VDC connections. You can use the scope in its normal way when it
is powered with the external 43 VDC.
The external 43 VDC idea came from a fine Tekscopes member named Hakan, many
years ago, and he has an excellent document you should read at:
A couple of years ago I made up a document listing some commonly available
components you might use to replace some of the original Tektronix part
numbers that often fail. I can send you that directly or you can find it in
the Tekscopes Message archive if you like.
Another thing you might do is take resistance readings from chassis ground
to each of the voltage test points before you get very far into this repair
(with the scope not connected to the mains of course!).
There are lots of other ideas to add to this, but this should get you
started.
This will be a simple fix, so just take your time and understand what you
are doing.
tom jobe...
----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Friday, January 20, 2012 6:45 AM
Subject: Re: [TekScopes] Power supply problem with Tek 2220
> Has anyone seen this behaviour repairing Tek2000 series scope power
supply?
>
>
>
> Behaviour
>
> 1. When power on from cold, it will fire up properly and stay on for a few
> minutes
>
> 2. Once it warms up, the power supply shut down and try to on again and
it
> continues in this on/off mode
>
> 3. Pre-regulator out = 43V (between TP940 and TP950)
>
> 4. All voltages when the unit is stable for a few minutes are +8.6, -8.6,
> +5.2, +100 are all ok.
>
>
>
> What are likely problems before I start ripping the components from the
> circuit?
>
>
>
> ttesenq@...
suggested that it could be the pre-regulator MOSFET
> P9070.
>
>
>
> I though that it could be a electrolytic cap but it doe not look likely
> because electrolytic failure would not have allowed the unit to fire up
for
> a few minutes
>
> It is more likely a active component that weaken with increase
temperature,
> so ttesenq@...
could be right.
>
> This would mean Q947 and Q946 are also likely components
>
>
>
> Any other possible lead of similar experience.
>
>
