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----- Original Message -----

From: Tan Chor <mailto:jonray03@...> Ming

To: TekScopes@...

Sent: Sunday, January 22, 2012 11:07 AM

Subject: RE: [TekScopes] Re: Power supply problem with Tek 2220





Ed,

Thank you for your extensive reply.

2220 is a storage 60mHz

2235 is a 100Mhz Non-storage

The power supply of the 2220 is quite similar to the 2235 which I can see
from the layout.

These are some differences on the power supply

1. 2220 has an additional -5V supply
2. Current rating in at the 43V for 2220 is higher. Based on design,
R912 is 357 ohm for 2235 and 549 ohm for 2220. This means current limit for
2235 = 0.86A and for 2220 = 1.3A
3. The added current is needed to drive the digital storage function
which is a separate board
4. Based on calculation, Shutdown voltage at R907 for 2235 = 0.18V and
for 2220 = 0.26V. Measured value on 2220 is .285V.

I have a DMM, Analog Multimeter and my other scope a 2235 has also a failed
SCR for the crowbar. I did not want to fire it up until I get the crowbar
function fixed.

I am planning to get an isolation transformer and run it at 110, before I
use my other scope if it is fixed to measure the signal of the 2220.

This is the development so far.

I have changed the following C925, C943, C942, C940 with little improvement

I have also remove the connector to Q9070 and solder it directly to the
leads. This seems to have the most improvement

Lifted the +5V supply to the digital board

Measured voltage on R907 at cold state = 0.17V (about 0.85A on 43V)

Measured voltage on R907 in warm state (after 5 mins) = 0.19V (about 0.95A
on 43V)

Scope can sustain operation for more than 15 minutes before the supply
shutdown and up again for another 5~10 min and repeat.

This shows an intermittent overload condition.

Not that bad because the scope can be used with brief shutdown in-between.

Connect the +5V to the digital board

Measured voltage on R907 at cold state = 0.22V (about 1.1A on 43V)

Measured voltage on R907 in warm state (after 1 mins) = 0.25V (about 1.25A
on 43V)

Scope can sustain operation for more than 1 minutes before the supply
shutdown and up again repeat every seconds later. This shows a sustained
overload condition.

Measured voltage on pin15 of U930 is 0.285, Clearly, the overload should not
happen since this voltage limit for voltage at R907 before shutdown happens.

This confirms that there are spike signal riding on R907 that cause the
comparator in U930 to cause a shutdown.

20Khz test

I thought maybe the pre-regular was running at slightly higher frequency
causing the switching efficiency to drop which cause Q9070 and T906 to heat
up.

Since I do have a working scope, I found an interesting way to test this
without instrument.

1. Connect a 470pf across C919 and the frequency will drop to ~15Khz
because I can heard the switch buzz.
2. Without the 470of, I could not heard the switching buzz, so I knew
it must be slightly past 20Khz
3. I confirm this because my son is able to hear the 20Khz buzz.

ESR on Cap.

I tried to change caps on C960 and C961, but there is no change to the
behaviour and leakage current on removed caps seems to be low, so I do not
suspect it is a ESR on cap issue.

The filtering caps are also warm (quite hot) which means that it is
filtering the ripple and converting to heat.

It's not an indication of good ESR though but poor ESR will have less heat.
I do not suspect the caps causing the shutdown issue.

As an improvement, I may change it later but not now, unless I am convince
that poor ESR can cause shutdown.

I am very much back to square one, because everything seems to point to a
normal working scope except that heat generated has cause an overload
condition

What's next.

1. Poor Switching

I suspect that there is a poor switching from Q9070 and CR907 which can
cause Q9070 to heat up in forward inverter.

Haven't gotten the component so will have to explore this later.

Q9070 (P6N55) needs to be at least 600V, 6A, RDSon 1.5 ohm

CR907 (BDY73G) needs to at least 600V, 1A, Reverse recovery time 50nS. Need
2 of these for 2220 in parallel, I believe 2235 uses only one.

.

2. Overload

This is a likelihood but I need a reference to know if the current measure
on +43V is too high.

How could power ESR cause overload?

Poor ESR will cause poor functional performance because there is high ripple
on the DC but will it cause an overload?

Background.

Both scope were working scope which I have not used for more than a year

Final Action if all else fails.

I am quite sure if I run the 2220 at 110V the shutdown issue will go away
because of lower generated heat with 110V.

If the problem persist, I will just have to switch to a lower line voltage
supply on an isolation transformer.

Rgds,

Chor Ming


_____


From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of Ed Breya
Sent: Sunday, January 22, 2012 2:43 AM
To: TekScopes@...
Subject: [TekScopes] Re: Power supply problem with Tek 2220



Yes indeed, the 2220 is virtually identical to the 2230 in most respects -
just a notch down to fill the price v. performance market position at the
time. The 2200 series models share the same basic power supply topology,
tweaked to get more power for the digital storage add-ons.

Your equipment appears to be very limited, but you do have a working DMM and
a "working" scope available for a few minutes at a time. You can use it to
look at its own supplies and other circuitry, at least up to the point of
shutdown.

Before delving further into the supply, I recommend that you first check
your line voltage to see if it's on the high side, or possibly has
transients or surges that may be causing the scopes to self-protect. I don't
recall if the 2220 has the "trigger view" feature, but if so, set up for
line triggering, and activate that to see roughly what the line looks like.
This is band limited, so won't show fast transients. You can also look at
the line voltage directly and more accurately with a X10 probe tip to the
hot side. DO NOT connect the ground clip to anything, or preferably,
disconnect it from the probe whenever poking around line voltage.

As I recall, the scopes should run on any line voltage between the specified
limits, with no range switching needed. If you have access to a variac or
step-down transformer you can try running at lower line voltage. If not, you
can try loading down the branch circuit with extra lighting and appliances
to see if it drops significantly, or even try something wired temporarily in
series, like a space heater, coffee pot, or a large incandescent lamp.

Some background info should help:

1. Were these scopes working and then stopped, or did you just get them, and
they didn't work? It seems from your initial post that they both failed at
the same time. If so, one big thing in common would be line voltage
problems. Do both scopes exhibit the same symptoms?

2. Is the available run time always about the same before shutting down?
Does recycling the power make it run for the same length of time, or does it
need to cool down first?

If the shutdown timing is fairly predictable, it may be good to monitor the
outputs closely and watch to see if they sag or rise just before shutdown.
Also, watch the display closely for dimming or blooming - evidence of same
on the high voltage.

Also, look at the 20 kHz AC waveforms on the secondaries, ahead of the
output rectifiers to see if they are nice, rounded squarish waves, and see
if that shape changes just before shutdown.

That's probably enough for now.

I agree with the suggestion to revisit the 2235. It may be easier to fix, or
you may luck out and find it's problem - then you'll have one fully working
scope - a big improvement.

Ed

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com> ,
David <davidwhess@...> wrote:

When I was searching for a replacement channel swtich, I found that a
number of the 22xx series oscilloscopes have the same main board which
has the power supply including the 2213A, 2215A, 2230, 2232, 2235,
2236 but not the 2213 or 2215. For the analog only models, the extra
outputs which would go to the digital storage board are tied to ground
to maintain bias on the channel switches. The 2220 is much rarer with
no online service manual so I never checked it.

On Sat, 21 Jan 2012 06:28:59 -0800, "tom jobe" <tomjobe@...>
wrote:

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@... <mailto:TekScopes%40yahoogroups.com> >
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%40yahoogroups.com>

[mailto:TekScopes@... <mailto:TekScopes%40yahoogroups.com> ] 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@...
<mailto:jonray03%40singnet.com.sg> >
To: <TekScopes@... <mailto:TekScopes%40yahoogroups.com>

<mailto:TekScopes%40yahoogroups.com> >

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@... <mailto:ttesenq%40tiscali.co.uk> 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@... <mailto:ttesenq%40tiscali.co.uk> could be

right.

This would mean Q947 and Q946 are also likely components



Any other possible lead of similar experience.