Hi Tan,
Some one told me that the 2220 is the 60 MHz version of the 2230 100 MHz
scope. If this is true, the 2230 manual might be your best bet. It is
available from several places including:



Now that I hear about your experience level and what equipment you have on
hand, I would fix the 2235 first as it has no top board limiting your access
to the power supply (and almost everything else in the scope) while the
scope is actually running.

You mention the need for an isolation transformer. You probably do not need
an isolation transformer at this point, as its only value is to be able to
"float" the scope so you can use another scope to look at the inverter side
of the power supply. The inverter is already making the 43 VDC so what would
you need to check anyway? I know the service manual calls for an isolation
transformer (so I bought one) but my experience finds it to be fairly
useless in the 22xx repair process. Also, once you "float" the scope the
chassis of the scope is now at some elevated voltage, which is a little
scary for an amateur like me.

You mentioned that Q908 was up on the heatsink but it isn't. Take a closer
look as it only is involved in running the "gate" of Q9070.
Q9070 is up on the heatsink. Q908 is a small transistor with minor currents
through it, and Q9070 does the real work of creating the 43 VDC for the
inverter.

If you ever decide to do the external injection of the 43 VDC, you need to
disconnect Q9070, not Q908.
tom jobe...

----- Original Message -----
From: "Tan Chor Ming" <jonray03@...>
To: <TekScopes@...>
Sent: Friday, January 20, 2012 8:37 PM
Subject: RE: [TekScopes] Power supply problem with Tek 2220

> 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 - 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@... 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.
> >
> >
>
>
>
>