Sudden failure: Leader LBO523 oscilloscope

[Menticol]

Hello guys!

Today I turned on my analog oscilloscope and sadly found that it was operating erratically.

Strange, because yesterday it was operating well. It wasn't exposed to high voltages, short circuits, humidity or shocks.

I've filmed a small video to show you the symptoms.

The focus look blurry because I intentionally reduced it, to reduce the wear of the phosphorous.

PS: Right now I'm looking for the service manual, but I don't know where to start with. Since the image looks crisp and it deflects on both axis I suspect there's a more complex problem inside.

 

[Inquisitive]

We can't see it.

 

[Menticol]

Oh that's odd, here it says "public"!

Maybe Youtube is still processing it, I've uploaded it just minutes ago. Please let me know if you can see it now, Inquisitive.

 

[Inquisitive]

Try this link for Leader LBO 522 and Leader LBO 523 Manuals.

https://bama.edebris.com/manuals/leader/lbo522/

 

[Inquisitive]

The Youtube link works now.

 

[Menticol]

Outstanding Inquisitive, thank you very much!

While I was studying your pdf, I hooked up the oscilloscope to its built in CAL. I'm happy to see some life coming out of it

Y axis for both CHAN1 and CHAN2 appears to be working good.

However, is funny that the graph only departs from the screen center to one side of X axis at a time. It's like having half of the oscilloscope screen. Take a look.

 

[JimB]

I just downloaded the manual from the link by Inquisitive.

My best guess would be something in the horizontal amplifier.
I would start by checking the power supplies coming on to the board.
If they are OK look at the voltages around Q10, Q11, Q12, Q13.
I hope that your problem is not in IC15, that looks like it could be expensive, or it is made from unobtainium.

JimB

 

[Menticol]

You are wrong Jim! As you can see on the pictures IC15 is not made from unobtainium, but an alloy made from unicorn piss, dragon nail trims and silicon from Cinderella's lost heel.

Last time I saw one of these packages was inside an audio equalizer from the 80's era. What is inside? The bumps looks like discrete components epoxied together, am I right?

Anyway, I'm praying for the voltages at the indicated test points are wrong! Changing a transistor or any other discrete component is better than finding replacement for that IC. Tonight I will know...

 

[JimB]

IC15 is not made from unobtainium, but an alloy made from unicorn piss, dragon nail trims and silicon from Cinderella's lost heel.

Oh, come on! Didn't you know that that is one of the ways used to make unobtainium in Japan?

JimB

 

[Menticol]

Hello guys!

Little update here. All voltages entering to the board are normal, except the +270V which is only 90V.

Strange thing is, is not getting the correct voltage even from the transformer.

Maybe the transformer went nuts?



Connecting / Disconnecting the P1 molex doesn't cause any change in the trace.

Could the missing +270V be the reason why the trace doesn't fully deflect to the side of the screen?

 

[Menticol]

Strange: voltage at the AC side of D1 reads zero. Voltage at the DC side of D1 also reads zero. But when I check D1 DC positive DC output and a GND from some place at the PCB, it shows 91 Vdc. I'm now in doubt about my multi-meter capabilities to this type of task. It's a UNI-T UT61A

 

[JimB]

This is one of those odd ones where you have to be carefull otherwise it can confse and/or shock you!

What you have is a 170 volt PSU sitting piggy back on a 100V PSU.

It looks like the 100v is working OK and the 170v is broken.

Connecting / Disconnecting the P1 molex doesn't cause any change in the trace.

Sounds reasonable, so measure (AC volts) at J1 coming from the transformer. There should be 145V AC there.
Measure at pins 11 and 12 on the transformer, if there are no volts there, your transformer is broken.

If you have volts at J1/P1, check the rectifier, what is the voltage measured at C1 ?

Check R2, has it gone open circuit or high resistance?
If it has check C2 and also measure the current drain at P6 pin1, has something gone short circuit on the Sweep PCB?

Could the missing +270V be the reason why the trace doesn't fully deflect to the side of the screen?

Yes!

JimB

 

[Menticol]

Looks like the transformer is dead, Jim.

Transformer show 83vac open circuit, 0 vac when connected to the pcb.

Bridge recifier output: -1.3 vdc. Replaced the bridge rectifier thinking it could be defective, but the condition persists.

R1: 69.6 KΩ

Voltage at C1: -1.3 vdc as expected.

Re-winding the oscilloscope transformer may be an option (there are very experienced technicians at the city who make their living from that). But they will charge at minimum $50.000 COP, when the whole unit costs 200.000 COP.

I'm thinking about getting a separate transformer, not very easy due the required specs: 120 vac In, 135 vac Out.

Or maybe a DC-DC converter, hooked up to the working 100 vdc circuit?

Is worth mentioning that the pcb tracks are very flimsy and tend to separate from the pcb easily. I shouldn't trusted the hippie who sold me this oscilloscope

 

[JimB]

Yup!
Looks like the transformer is broken.
Commiserations.

JimB

 

[DerStrom8]

**broken link removed**

 

[tvtech]

This is my kinda thread. This is what I like to see. Solid advise and good/bad outcome solved.

My 2 Cents.

 

[Inquisitive]

tvtech where you been?

 

[tvtech]

tvtech where you been?

Stuff to work through. Like serious stuff.

Thank you though for thinking of me. So much good here on ETO that it never ceases to amaze me. I love this Forum and it's members with all my heart.

And I mean it

tv

 

[JoeJester]

Transformer show 83vac open circuit, 0 vac when connected to the pcb.

Put a 10k resistor across the transformer terminals and measure the voltage. 10k will draw approximately 14.5 mA. See how far the voltage drops with a constant load. If it goes to zero ... you know there is a problem with the winding. Make sure you are measuring AC volts. For a quick check, measure your mains. It will give you a quick ballpark calibration check on your meter.

Then you can power down and do a resistance measurement on J1-2 and J1-3. If you place the negative probe on J1-2 and the positive probe on J1-3, you are forward biasing the diodes and might read a little above 70k. Reverse the leads and you should read open.

Attached is a file illustrating what you said ....