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2246A Power supply failure turning on / off


JorgeTop
 

Hi!

Some time ago I got a used 2246 oscilloscope which was working quite well until recently. First it began to show a single point on the screen and then it would return, or you just had to turn it off and turn it on again.

Soon after, it started to cycle on and off and sometimes I needed to turn it off for a moment and turn it on again to get it working again.

Now it kept turning on and off cyclically without stabilizing. So far, I have changed things like some electrolytic capacitors (I read right here that it is a common fault), however I have not found all. I have also changed zener diodes and some transistors without success.

With the service manual I have tried to measure some points however since it turns on and off the voltage is erratic. I have not yet found a similar power supply for sale so at the moment it is not an option and I would like to be able to repair it.

Any suggestion?


Milan Trcka
 

A lot of potential problems seem to have been eliminated. Next I would
start looking for a potential overload on the power supply which may (or
may not) cause the PS to go into a shutdown to protect itself (electronic
fuse?). Another scope is likely needed to poke about the various sections,
starting with the first reference (5V?) and going up the voltage ladder.
This for checking time-related changes (caps charging etc.). Maybe
disconnecting the loads from the power supply might give you a clue whether
the problem is load related and which section is the offender on either
side. Unloaded supply may be easier to check for faulty components.
Unfortunately this level of troubleshooting needs to be done under power so
be careful as to not letting the smoke as well as not getting hurt. (FWIW)

Milan

On Wed, Aug 19, 2020 at 7:50 AM JorgeTop <jdavidtop@...> wrote:

Hi!

Some time ago I got a used 2246 oscilloscope which was working quite well
until recently. First it began to show a single point on the screen and
then it would return, or you just had to turn it off and turn it on again.

Soon after, it started to cycle on and off and sometimes I needed to turn
it off for a moment and turn it on again to get it working again.

Now it kept turning on and off cyclically without stabilizing. So far, I
have changed things like some electrolytic capacitors (I read right here
that it is a common fault), however I have not found all. I have also
changed zener diodes and some transistors without success.

With the service manual I have tried to measure some points however since
it turns on and off the voltage is erratic. I have not yet found a similar
power supply for sale so at the moment it is not an option and I would like
to be able to repair it.

Any suggestion?





 

On Wed, Aug 19, 2020 at 04:50 PM, JorgeTop wrote:


Any suggestion?
The two most common faults causing the P/S to go into burst mode I've seen on the 224x supplies
have been one or more shorted rectifier diodes on the secondary or a bad HV multiplier.
Another not so obvious fault is that the 0.1 ohm current limiting resistor has increased slightly
in value causing the supply to shut down prematurely.

/Håkan


JorgeTop
 

By now I have removed almost all the electrolytic capacitors and have observed that they all have much higher capacitance than stated, for example 1000uF have 1400uf, 470uf go for 600-700uF. How troublesome can this be?

For the diodes I have removed one of the pins to perform the diode measurement with a multimeter in diode function and they seem normal. I have not found open or shorted diodes. Zener diodes, I have changed most, as well as some transistors.

It remains for me to change the MC310 and the IC that I have not found locally.

I'm going to review the 0.1Ohm resistor you mention.

Thanks, I will update this week


Brian Cockburn
 

This https://www.rc-cam.com/forum/index.php?/topic/4165-tektronix-2245a-oscilloscope-smps-repair-re-cap/ may be of some interest.

On a side note to everyone; is there a cannonical design or schematic for a dummy load suitable for 224x PSUs?

Cheers, Brian.


tekscopegroup@...
 

Well known problem with rectifier diodes going "soft". It is recommended to do the procedure bellow on any 224x or 225x scope, as preventive measure even if no faults are showing yet, and ideally along with recap.

Replace any rectifier diodes with ZM or ZS marked on them, they tend to get leaky after some time, sometimes problem will only show up years after.
In my case all the diodes had a ZM marking so all 17 went out and where replaced. I used MUR160 diodes as replacement (Mouser PN 821-MUR160) 600V 1A 50nS.

Original diode: 152-0400-00 MB2501 (total 17 diodes) at these locations:

-CR2202
-CR2204
-CR2205
-CR2208
-CR2209
-CR2210
-CR2211
-CR2212
-CR2213
-CR2214
-CR2215
-CR2216
-CR2218
-CR2227
-CR2228
-CR2235
-CR2236

Hope that helps.


tekscopegroup@...
 

Oh, and in case its not obvious to someone, DO NOT use common 1N400x series diodes as a replacement for these application, they are way too slow and will not work in this case that requires fast switching. MUR-160 or any 1Amp and 200-600PIV equivalent fast switching diode will work just fine.


JorgeTop
 

On Thu, Aug 20, 2020 at 08:00 AM, <tekscopegroup@...> wrote:


N400x series diodes as a replacement for these application, they are way too
slow and will not work in this case that requires fast switching. MUR-160 or
any 1Amp and 200-600PIV equivalent fast switching diode will work just fine.
Thank you for the information, I will search for the diodes tomorrow.


Giuli Lauto
 

I d check also the on off switch, very common failure.


JorgeTop
 

Thanks, I already check the switch and it is working fine.

Update: I already change almost all the normal diodes and zeners, almost all the electrolytic capacitors also, but the problem is still present.

I was searching in my country the MC34060 (NTE1753) and the SCR2117 but I cannot find them here.

The next step that I will do is to disconnect all the outputs of the transformer, low and high voltage, isolate them and start the power supply to check if is a problem of the load.

0.1 Ohm seems to be ok.

My other option, is to buy an old 2245-2246-2247 that stills turns on to extract the power supply and have other components for future repairs. I already check in ebay the power supply but only appears in Greece and Israel, and over $180 + shipping, and the old oscilloscopes in US are from $60.

Thanks for the support


JorgeTop
 

Another update, I have disconnected the HV section and left without insert the pins in the lower PCB, the problems is still present, I hear a sound like a capacitor loading and a tick but I cannot determine which component make the tick.


Brad Thompson
 

JorgeTop wrote on 8/24/2020 8:20 PM:

Another update, I have disconnected the HV section and left without insert the pins in the lower PCB, the problems is still present, I hear a sound like a capacitor loading and a tick but I cannot determine which component make the tick.
<snip>
Hello--

Dunno where you're located, but here in the U.S., an imported tool seller (Harbor Freight)
sells a "mechanic's stethoscope" for $4.99; I can't determine whether the device's probe is metallic
and hence conductive, but substituting a hollow plastic tube would be a safety improvement.

73--

Brad  AA1IP