Challenging 2465 PS repair


satbeginner
 

Hi all,

I bought a For Parts or Repair 2465 PS on eBay.
My idea was (and still is!) to make it work again so I would have a spare for one of my 246x scopes.

It had parts missing, but in general it looked OK.

So I added the missing parts, hooked it up to my dummy load and powered it up......

Wrong idea, now it seems :-)

Some people see Magic Smoke?? Not me, I saw actual flames!!!

I still want this to be repaired, but I do acknowledge it might take some time :-)

Here some pictures: https://groups.io/g/TekScopes/album?id=263466

To be continued,

Leo


 

That's what happens when an SPS wants to do all its switching at once.
Good luck, Leo, you can do it!

Raymond


Alex
 

I bet this scope was originally being used at 120V mains, and the protection was barely being able to cope with the fault. But once put on Euro 240V it definitively was too much for the PS.


satbeginner
 

Hi,

I bought just the PS, and no scope with it, and yes I ran it on 230VAC, but in this 230VAC part of the world, it is usually the RIFA's in the net entrance that give problems.
I already removed these :-) and recapped the Power Supply.

However, I ran the PS not in a scope, but just with using a dummy load.

It's my experience these Power Supplies are quite robust, and don't give many problems in general.

In my effort to repair this PS, these are the parts that are definitely broken.
I ordered replacements and marked these in pink in this picture :-)

https://groups.io/g/TekScopes/photo/263466/3216014?p=Created,,,20,2,0,0

To be continued,

Leo


satbeginner
 

Hi all,

I am determined to repair this PS, so I removed all broken, damaged or suspicious parts and ordered new ones.
For the time being, I removed all the burned fiberglas parts of the PCB, leaving me with a nice, clean 8mm hole in the PCB and many open spaces were new parts will be placed.

After studying the schematic of this power supply, it is vaguely similar to the 22xx power supplies, but with at least one major difference.

Similar is, it has a Primary switcher, and it has a secondary switcher, but, were in the 22xx PS the secondary switcher runs at it's own frequency, in this 246x PS the secondary switcher uses the same frequency as generated by the primary switcher.
This makes it a bit more of a challenge to separate the Primary and the Secondary to test them one at the time.

The good news is, the service Manual has a procedure to do this, but extra dummy loads are needed.
I ordered these parts as well, and when they arrive I will continue this repair.

To be continued, stay safe,

Leo


Milan Trcka
 

In an attempt to keep in as much smoke as possible when powering up suspect or failed PS, I use an adjustable transformer (Variac) to slowly bring up mains voltage while monitoring for vital signs. Use isolation transformer or battery powered isolated scope when monitoring waveforms.

M


Tom Gardner
 

Since a *switching* PSU will attempt to deliver a constant output
power, a low input voltage means a high input current flowing through
transistors, diodes and some capacitors.

An abnormally high current might cause damage.

On 03/05/2021, Milan Trcka <milan.v.trcka@gmail.com> wrote:

In an attempt to keep in as much smoke as possible when powering up suspect
or failed PS, I use an adjustable transformer (Variac) to slowly bring up
mains voltage while monitoring for vital signs. Use isolation transformer or
battery powered isolated scope when monitoring waveforms.

M






 

The 2465 PSU is a switcher, so a variac is strongly counter indicated as this will typically cause the PSU to draw excess current at low voltages (below the nominal range of say 100-120V, or 200-240V over here).

D.

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Milan Trcka
Sent: 03 May 2021 16:46
To: TekScopes@groups.io
Subject: Re: [TekScopes] Challenging 2465 PS repair


In an attempt to keep in as much smoke as possible when powering up suspect or failed PS, I use an adjustable transformer (Variac) to slowly bring up mains voltage while monitoring for vital signs. Use isolation transformer or battery powered isolated scope when monitoring waveforms.

M


Siggi
 

On Mon, May 3, 2021 at 3:54 AM satbeginner <castellcorunas@gmail.com> wrote:

After studying the schematic of this power supply, it is vaguely similar
to the 22xx power supplies, but with at least one major difference.

Similar is, it has a Primary switcher, and it has a secondary switcher,
but, were in the 22xx PS the secondary switcher runs at it's own frequency,
in this 246x PS the secondary switcher uses the same frequency as generated
by the primary switcher.
This makes it a bit more of a challenge to separate the Primary and the
Secondary to test them one at the time.
Switchers will run just fine on DC input, and if you have a suitable bench
supply to run it from, that gives you two big benefits:
1. Limited energy input, the current limit could be set low enough that
nothing blows up.
2. Mains isolation, so you can now peek around anywhere with your scope.

Unfortunately the DC bus on this scope runs at 2-300V (I see a 264V
annotation on the schematic), using an AC voltage doubler when on 115V
input, and not many bench supplies have that sort of voltage output. I do
find that switchers will often start way below their supposed minimum input
voltage, which may work just fine for diagnosis. You really don't want to
load them - as others have mentioned - with a low input voltage.


-
 

I do the same but I have an amp meter attached to my Variac and I start
out at about 1 VAC and watch for excessive current. If the device under
test is rated to draw say 5 amps at 115 VAC and I apply say 3 VAC and it's
already drawing 3 Amps then there's probably a short somewhere. But the
low amount of power (9 Watts) isn't as likely to cause damage as applying
115 VAC and 20 Amps (the normal limit of 115 VAC circuits) would.

On Mon, May 3, 2021 at 11:46 AM Milan Trcka <milan.v.trcka@gmail.com> wrote:


In an attempt to keep in as much smoke as possible when powering up
suspect or failed PS, I use an adjustable transformer (Variac) to slowly
bring up mains voltage while monitoring for vital signs. Use isolation
transformer or battery powered isolated scope when monitoring waveforms.

M






-
 

The equipment that I usually work on is usually too old to use a
switching PSU. Also I've NEVER had a switcher fail by using it on a Variac.
IMO there is a big difference between "it's theoretically possible to" and
"likely too". I've *operated* literally thousands of smaller switching PSUs
at extremely reduced voltages and never seen one that damaged itself by
drawing "excessive" current. Sure, they will draw more current at reduced
voltages but, in my experience, *never* enough to damage themselves. I say
"operated" loosely, since at very reduced voltages (< ~60%) they will not
operate and their current draw drops to near zero. Let me also point out
that as part of their Performance Tests, HP and others frequently call for
attempting to operate their equipment at both reduced and excess voltages.
Also I don't ever recall seeing a warning in a HP manual saying that
attempting to operate their equipment below XXX voltage *would* cause
damage.

YMMV

PS does anyone have a few switching PSUs that they're willing to
possibly sacrifice, in order to test and see how low their input voltage
can go before they stop operating and if very low input voltages will
damage them? Maybe we can finally prove or disprove the "under-voltage =
damage" theory.

On Mon, May 3, 2021 at 11:59 AM Tom Gardner <tggzzz@gmail.com> wrote:

Since a *switching* PSU will attempt to deliver a constant output
power, a low input voltage means a high input current flowing through
transistors, diodes and some capacitors.

An abnormally high current might cause damage.

On 03/05/2021, Milan Trcka <milan.v.trcka@gmail.com> wrote:

In an attempt to keep in as much smoke as possible when powering up
suspect
or failed PS, I use an adjustable transformer (Variac) to slowly bring up
mains voltage while monitoring for vital signs. Use isolation
transformer or
battery powered isolated scope when monitoring waveforms.

M










Harvey White
 

The undervoltage damage theory isn't a theory under certain circumstances.

It all depends on how the switching is done.

If the pass element always switches, and has a current limit, then it's protected.

Some supplies don't.  I've see designs where there is a minimum required voltage to start switching, and the transistor is full on until the switching starts.  If there's no protection, then you can get damage.  Bad design?  Yes, but it's possible to do and you may not be aware that you have one.

That's what I'd say would be the problem.  Pass transistor on (especially if it's a boost supply and not a buck supply).

Be aware, just in general, that some of the inexpensive boost or buck modules coming out of China may behave like this.  Running them on a current limited supply will let you know.

Does your supply do this?   Don't know.  Could it?  Maybe...

If I run a scope or piece of test equipment off a variac, I'm watching the input current, and also a bit of the outputs to see what the regulator is up to.

Harvey

On 5/3/2021 1:47 PM, - wrote:
The equipment that I usually work on is usually too old to use a
switching PSU. Also I've NEVER had a switcher fail by using it on a Variac.
IMO there is a big difference between "it's theoretically possible to" and
"likely too". I've *operated* literally thousands of smaller switching PSUs
at extremely reduced voltages and never seen one that damaged itself by
drawing "excessive" current. Sure, they will draw more current at reduced
voltages but, in my experience, *never* enough to damage themselves. I say
"operated" loosely, since at very reduced voltages (< ~60%) they will not
operate and their current draw drops to near zero. Let me also point out
that as part of their Performance Tests, HP and others frequently call for
attempting to operate their equipment at both reduced and excess voltages.
Also I don't ever recall seeing a warning in a HP manual saying that
attempting to operate their equipment below XXX voltage *would* cause
damage.

YMMV

PS does anyone have a few switching PSUs that they're willing to
possibly sacrifice, in order to test and see how low their input voltage
can go before they stop operating and if very low input voltages will
damage them? Maybe we can finally prove or disprove the "under-voltage =
damage" theory.



On Mon, May 3, 2021 at 11:59 AM Tom Gardner <tggzzz@gmail.com> wrote:

Since a *switching* PSU will attempt to deliver a constant output
power, a low input voltage means a high input current flowing through
transistors, diodes and some capacitors.

An abnormally high current might cause damage.

On 03/05/2021, Milan Trcka <milan.v.trcka@gmail.com> wrote:
In an attempt to keep in as much smoke as possible when powering up
suspect
or failed PS, I use an adjustable transformer (Variac) to slowly bring up
mains voltage while monitoring for vital signs. Use isolation
transformer or
battery powered isolated scope when monitoring waveforms.

M











-
 

Thanks for the added info Harvey. That's one of the reasons why I always
start my Variac at very low voltage (~1 VAC) and watch the Ammeter for a
good minute before I start *slowly* start increasing the voltage. If I get
any current that approaches the rated operating current (at 110VAC or
whatever) then I KNOW that there's a problem but as long as the current
draw stays *below* the rated current then it shouldn't cause any damage.

On Mon, May 3, 2021 at 2:14 PM Harvey White <madyn@dragonworks.info> wrote:

The undervoltage damage theory isn't a theory under certain circumstances.

It all depends on how the switching is done.

If the pass element always switches, and has a current limit, then it's
protected.

Some supplies don't. I've see designs where there is a minimum required
voltage to start switching, and the transistor is full on until the
switching starts. If there's no protection, then you can get damage.
Bad design? Yes, but it's possible to do and you may not be aware that
you have one.

That's what I'd say would be the problem. Pass transistor on
(especially if it's a boost supply and not a buck supply).

Be aware, just in general, that some of the inexpensive boost or buck
modules coming out of China may behave like this. Running them on a
current limited supply will let you know.

Does your supply do this? Don't know. Could it? Maybe...

If I run a scope or piece of test equipment off a variac, I'm watching
the input current, and also a bit of the outputs to see what the
regulator is up to.

Harvey


On 5/3/2021 1:47 PM, - wrote:
The equipment that I usually work on is usually too old to use a
switching PSU. Also I've NEVER had a switcher fail by using it on a
Variac.
IMO there is a big difference between "it's theoretically possible to"
and
"likely too". I've *operated* literally thousands of smaller switching
PSUs
at extremely reduced voltages and never seen one that damaged itself by
drawing "excessive" current. Sure, they will draw more current at reduced
voltages but, in my experience, *never* enough to damage themselves. I
say
"operated" loosely, since at very reduced voltages (< ~60%) they will
not
operate and their current draw drops to near zero. Let me also point out
that as part of their Performance Tests, HP and others frequently call
for
attempting to operate their equipment at both reduced and excess
voltages.
Also I don't ever recall seeing a warning in a HP manual saying that
attempting to operate their equipment below XXX voltage *would* cause
damage.

YMMV

PS does anyone have a few switching PSUs that they're willing to
possibly sacrifice, in order to test and see how low their input voltage
can go before they stop operating and if very low input voltages will
damage them? Maybe we can finally prove or disprove the "under-voltage
=
damage" theory.



On Mon, May 3, 2021 at 11:59 AM Tom Gardner <tggzzz@gmail.com> wrote:

Since a *switching* PSU will attempt to deliver a constant output
power, a low input voltage means a high input current flowing through
transistors, diodes and some capacitors.

An abnormally high current might cause damage.

On 03/05/2021, Milan Trcka <milan.v.trcka@gmail.com> wrote:
In an attempt to keep in as much smoke as possible when powering up
suspect
or failed PS, I use an adjustable transformer (Variac) to slowly bring
up
mains voltage while monitoring for vital signs. Use isolation
transformer or
battery powered isolated scope when monitoring waveforms.

M
















satbeginner
 

Hello all,

thanks for the input while I was waiting for parts to arrive.

Most of them are here now, so I started with building the dummy-loads for testing the Power supply.

My weird mind has a question: the service manual says to use 2 pcs of 2 ohm resistors to load the 5VD output, however, the manual does not say whether they should be in series or parallel?
I put them in parallel, because if they needed something like 4 Ohm, they just would have used a 3.9 Ohm resistor.

For the 3uF I used 3x1uF in parallel.

I added a picture of the dummy-load(s) here: https://groups.io/g/TekScopes/album?id=263466

To be continued,

Leo


SCMenasian
 

I am completely ignorant about this instrument; but I think you might consider putting the resistors in series. In a 2465B/2467B service manual I found on the web, I found in the circuit description (page 3a-52) "+5 V Regulator" that current limiting is set to limit current to 2 A. 4 ohms would load the supply to 1.25A and 1 ohm would load it to 5 A - well above the current limit.


Ozan
 


My weird mind has a question: the service manual says to use 2 pcs of 2 ohm
resistors to load the 5VD output, however, the manual does not say whether
they should be in series or parallel?
I put them in parallel, because if they needed something like 4 Ohm, they just
would have used a 3.9 Ohm resistor.
--
Hi Leo,
In 2465 schematic 5VD has a 5A fuse so 2//2=1ohm would be too small (too much current for the supply). Interestingly 2467B has a similar PSU and the load instructions say: "TEST LOAD. Connect a 2-ohm, 25 watt resistor (Tektronix part number 308-0205-00) from the +5 VD pins of J303 and J232 (on the Inverter Board) to ground.". Almost the same instructions as 2465 but "a 2-ohm" instead of "two 2-ohm".

A single 2-ohm might be intended if this is a typo; however, it is better to start with two 2-ohms in series to be safe,
Ozan


-
 

Tek probably specified two 2 Ohm resistors instead of one 4 ohm resistor
since they would allow twice as much heat dissipation. Yes, you could
probably use a larger wattage resistor but the one larger resistor might
have cost more than two small ones. Or two small resistors was what they
had on hand when they developed the procedure so that's what they wrote
into the instructions. I've seen similar *unnecessary* requirements
written into thousands of TE requirements for military systems. Things like
very high precision HP power supplies being used to simulate an ordinary
battery simply because that was the type PS that the contractor had in
stock in their tool crib when they were developing the test procedure.

On Thu, May 20, 2021 at 5:42 PM Ozan <ozan_g@erdogan.us> wrote:



My weird mind has a question: the service manual says to use 2 pcs of 2
ohm
resistors to load the 5VD output, however, the manual does not say
whether
they should be in series or parallel?
I put them in parallel, because if they needed something like 4 Ohm,
they just
would have used a 3.9 Ohm resistor.
--
Hi Leo,
In 2465 schematic 5VD has a 5A fuse so 2//2=1ohm would be too small (too
much current for the supply). Interestingly 2467B has a similar PSU and
the load instructions say: "TEST LOAD. Connect a 2-ohm, 25 watt resistor
(Tektronix part number 308-0205-00) from the +5 VD pins of J303 and J232
(on the Inverter Board) to ground.". Almost the same instructions as 2465
but "a 2-ohm" instead of "two 2-ohm".

A single 2-ohm might be intended if this is a typo; however, it is better
to start with two 2-ohms in series to be safe,
Ozan






Ozan
 

On Thu, May 20, 2021 at 06:45 PM, - wrote:


Tek probably specified two 2 Ohm resistors instead of one 4 ohm resistor
since they would allow twice as much heat dissipation. Yes, you could
probably use a larger wattage resistor but the one larger resistor might
have cost more than two small ones. Or two small resistors was what they
had on hand when they developed the procedure so that's what they wrote
into the instructions.
--
Each resistor is 25W capable so even one would be sufficient in terms of power handling. Reading it again they may wanted to say one 2-ohm from J303 to ground and one 2-ohm from J232 to ground.
Ozan



On Thu, May 20, 2021 at 5:42 PM Ozan <ozan_g@erdogan.us> wrote:



My weird mind has a question: the service manual says to use 2 pcs of 2
ohm
resistors to load the 5VD output, however, the manual does not say
whether
they should be in series or parallel?
I put them in parallel, because if they needed something like 4 Ohm,
they just
would have used a 3.9 Ohm resistor.
--
Hi Leo,
In 2465 schematic 5VD has a 5A fuse so 2//2=1ohm would be too small (too
much current for the supply). Interestingly 2467B has a similar PSU and
the load instructions say: "TEST LOAD. Connect a 2-ohm, 25 watt resistor
(Tektronix part number 308-0205-00) from the +5 VD pins of J303 and J232
(on the Inverter Board) to ground.". Almost the same instructions as 2465
but "a 2-ohm" instead of "two 2-ohm".

A single 2-ohm might be intended if this is a typo; however, it is better
to start with two 2-ohms in series to be safe,
Ozan






Jean-Paul
 

Leo: Very fine work....haved designed SMPS since 1970s...and debugged 246x PSU....

1/ Virtually all SMPS have UVLO and OVLO = undervoltage and overvoltage lockout, the input V is sensed and switching inhibited till the DC bus is in range.

I have never had issues in using a variac to test SMPS. Use of Variac reduces chances of blowups if a part has a short.

2/ These are indeed complex designs but very reliable. The flowchart and debug procedures in the TEK serv manual are excellent.

3/ Blow switch transistors will often be symptoms of other issues and the driver IC or transistors in the switch circuit are at risk.

4/ There was discussion of the proper loads and connectors for dummy testing..long ago.

5/ I have indeed used a DC bus from a variable lab supply to test these, but this is rather dangerous and the lab supply current limit will not guarantee safety if the SMPS has faults!

6/ Besides RIFA X/Y line caps, all lytics (ELCO) can be at risk, especially in the LV rewct section. The HV bus caps (360V bus) seldom fail.

7/ The schematics PCB layout have a reversal of two caps in the LV rect section, that can result in problems when recapping.

8/ The rectifiers and switch transistors are easy to check in circuit.

Hope this note is of use,

Bon Chance,

Jon


SCMenasian
 

If I read the correct manual correctly (see my previous post) , even a single resistor as low as 2 ohms would put a 5 Volt supply into current limit (2.5 Amps draw / 2 Amps current limit). I still vote for 2 in series.