81 - Réparons un oscilloscope Hameg HM-303

Let's repair a Hameg HM-303 oscilloscope

Hi, today, we'll attempt to repair a scope that was sent to me with an unknown failure.

So let's see what's happening.... (turning on)

Some things seem to happen... We can hear relays clicking.

But clearly, there is nothing on the screen, even after pushing the beam intensity..

I've short-circuited the input... Manifestly there's othing on the screen... taht's the failure.

The first thing that can be said is, the standard power supply, trigger levels and input electronics seem to work.

So I would think there is an issue with high voltage powering the electron tube.

as seemingly, the rest of electronics seems to work correctly.

I'll open it to see how it's made, and we'll try to detect some anomalies.

And we'll use this high voltage probe for the task, as this kind of tool is required for cathodic tubes.

It's out of the question to hook up a multimeter directly on it with a few kV.

This will allow us to see how to do high voltage measurements, what precautions to take, because it is dangerous

There are reflexes to learn, things to do and mainly things NOT to do.

So now I'll crack open this scope in order to examine its inside.

So this is the inside... extremely rational, a bit like Tektronix.

The layout is tidy, airy, well shielded around the tube

It looks very well designed.

Here is the box. Very well done, only two screws to open, very practical for repair.

There are the rotative commuters, which are completely proprietary

so if it breaks, we're usually in a bit of a trouble to find new parts.

Same sort of boards at the bottom.

This one is a board that must have been added as an afterthought

In fact, it sends calibration signals to calibrate the oscilloscope.

It's made in a different manner, but it's very practical for checks.

Here, we have a board... I believe for the horizontal and vertical amplifiers.

A look at the manual would help.

The round stuff here are transistors with a cooler

This is pervasive in amplifying stages for horizontal and vertical plates.

Next... it's very rational, very airy. Simple and effective.

And of course, the fully shielded power supply

and there, you don't want to put your fingers is behind the cathodic tube.

There is a plastic shield, with a sign that says

Danger: 2000V

Unfortunately, it's where we'll have to look closer. So its where you need to be careful.

This shield has clearly been opened before. Screws are missing.

I'm going to remove the shield to see...

and I'll print the schematics of the service manual.

It can be found on internet.... here it is, complete.

Here there is everything needed for localizing the zones of the apparatus.

The design is very modular, so it's easy to find what we are interested in.

I see two interesting things:

the schema of the small board behind the cathodic tube, here on the left

There is here a connector to the power supply board.

and here another one that goes to the XY deviation plates.

Given there is no beam in the tube, I'm interested in the high power.

and it's this connector that goes to the PS board which is the next one.

Here is the power supply schematics.It's the circuit I'm going to check now.

So let's print these 5 pages: cathodic tube and power supply.

This will give us measurement points and voltage values that are indicated in the schematics.

This allows us to compare measurement values to the indications and localize a potential difference.

I'll try to move the device to allow filming

First, off we'll work on high potentials, so the absolute required precautions to take are :

1) no metallic tool anywhere, plastic screwdrivers like this one.

Of course, the power cord is unplugged, always.

In this probe, there is a single contact.

The ground wire is separate. We'll hook it to the chassis.

And from now on, we work with a single hand.

And that's all.

In case of a manual error, this prevents the current to go through the body and to the other hand

It's still better to touch with two fingers. It will sting but won't be lethal.

With two hands, the current goes through the whole body, which is far more dangerous.

So I repeat: it's better to have a non working oscilloscope rather than a failed operator.