Repairing a Metrix OX 803 Oscilloscope

[lynyx77]

<Mod edit: This was split off from the following thread: https://www.electro-tech-online.com/threads/repairing-a-metrix-ox-8050-digital-oscilloscope.142398
Please do not hijack someone else's thread to ask your own questions>

Hello,

I've a strange issue with an old metrixOX803, the pole relay going mad, I will post a video, it s not easy to explain you.
I'm hoping you can help me.

 

[itsthatidiotagain]

First rule of troubleshooting, "Always check your voltages" Odd behaviour is almost always down to PSU problems, but post the video and we can take it from there.

Also take a look at https://www.badcaps.net/forum/showthread.php?t=42768" (Currently down for maintenance, but no doubt will be back up later today).

Google cached copy here... **broken link removed**

 

[lynyx77]

Ok I will check Voltage on PSU and revert back to you.

Below video of issue

 

[itsthatidiotagain]

My suspicion is that the TDA4605 is not setting up correctly in the switch mode PSU, if it doesn't settle, then it restarts. Check all the switch mode PSU (SMPSU) caps and resistors, particularly the ones related to the TDA4605 - replace any caps that look bulged, leaky or whatever. Also take care with the SMPSU board, it is a cheap single sided board, and the tracks lift very easily if you are soldering on it. I damaged one when I repaired my 803B, and had to add a bodge wire. The TDA4605 reference manual will assist in trouble shooting the SMPSU.

Datasheet for TDA4605 ->
https://www.digchip.com/datasheets/...?id=960742&part-number=TDA4605-15&show=inline

... and do remember there are lethal voltages at play on the SMPSU and the CRT

 

[itsthatidiotagain]

Also suspect the +12V and particularly the -12V smoothing caps, from re-reading this thread, it seems they have an influence on the feedback to the switching circuit on the high voltage side. If you can borrow another 'scope that would help too.

The OP was seeing a similar shimmery, glitchy effect, and I said at the time, "The dodgy -12V might be sufficient on its own to cause the issue you are seeing, since it, along with the +12V is probably used as a reference rail for all of the op-amps and low voltage analog side. I would think that it would cause a multitude of problems."

As I said... always check your voltages

 

[lynyx77]

Update:

Since I've opened the Box and touched the Green collector the Oscilloscope is Ok now.
The issue seem to be on the green connector, probably bad soldering on the main Card and not the PSU
I'm currently uploading a video in order to show you .

I've checked the voltage on the green connector, please find below
All voltages measured with multimeter

C Power connector (J1)

Pin 1) -12V Measured -11.98V
Pin 2) -5V Measured -5.07V
Pin 3) +12V Measured +12.2V
Pin 4) +5V Measured +3.04V (Less than you strange)
Pin 5) +5V Measured +4.97V
Pin 6) GND
Pin 7) GND
Pin 8) Looks like 0V : Measured 0V
Pin 9) +120V DC Measured at 114.7V on multimeter
Pin 10) +180V DC Measured at 173V on multimeter
Pin 11) GND
Pin 12) +5V regulated ->Measured 5V.

So in order to fix the bad contacts on the green connector I really need to separate all cards and make new soldering on theses Pins (12)

 

[itsthatidiotagain]

I would also clean the plug that mates with the pin header with electrical contact cleaner, and check to see if any of the pins look as though they are burned. The most likely cause is dry solder on the the board however. Glad to hear you are making progress.

"Pin 4) +5V Measured +3.04V (Less than you strange)" << This may be a heater voltage or something other than 5V, but it could also be the source of the issue.

If the solder joint on that pin is bad, it will act like a resistor, and that might account for the lower voltage. Check the power board, and see where that pin goes. It could also be some sort of feedback pin for the switch mode PSU (to the opto-isolator)... so it may be a signal rather than a fixed voltage. This is where a 2nd oscilloscope would come in handy.

 

[lynyx77]

This is the video

 

[itsthatidiotagain]

So when you prod the end that runs to the main board, then it misbehaves... check the solder on the pins to that header. Also clean the contacts on the pins and the cable.

I would be a little careful poking that with your finger, a plastic implement would seem a bit safer as, although some of the voltages on the connector are relatively low, there are some CRT signals on the board nearby, so you might touch something *interesting* if you are not careful.

There are also the ....

Pin 9) +120V DC Measured at 114.7V on multimeter
Pin 10) +180V DC Measured at 173V on multimeter

... voltages on that connector that may be more than a little dangerous.

Also the focus and intensity pots on some oscilloscopes can have high voltages on them too, so be aware of that when your hand is near the back of the front control board.

 

[itsthatidiotagain]

When taking out that bottom board, be aware that there may be a couple of wires soldered between it and back of the front panel, there may also be some electrical hazard down towards the left hand side on the same edge as the power connector we are interested in. I don't think you need to remove the CRT to inspect that board, but if you do, discharge the anode cap first, and be aware that there may be a few charged capacitors about on the PSU and CRT boards, so proceed with caution.

Remember glass CRTs are fragile, and expensive.

This is how *not* to discharge an oscilloscope CRT

This is a way better video..

If you do need to remove the CRT, proceed slowly, and assume things are charged up every time you power the thing back up. 6KV hurts.. I can save you the trouble of finding out. Believe me even when powered off, the discharge from a CRT is very unpleasant and potentially fatal.

 

[lynyx77]

I've cleaned the pin header with electrical contact cleaner and now it seems ok and stable , I can't reproduce the issue.
I will monitor fews days if the issue come back again.
Regarding discharge of CRT, I've a Fluke Probe 40KV and I've already done on several CRT, but you right we need to be careful with these HT.
Many Thanks again for your support, it was really professional and useful.
I will update in few days the post.

 

[itsthatidiotagain]

I've cleaned the pin header with electrical contact cleaner and now it seems ok and stable , I can't reproduce the issue.
I will monitor fews days if the issue come back again.

Good news, sounds like a little oxidation of the contacts, or some damp or condensation may have been the cause.

Regarding discharge of CRT, I've a Fluke Probe 40KV and I've already done on several CRT, but you right we need to be careful with these HT.

I used to repair a lot of CRT based devices a good few years ago, and even now, I'm now extremely cautious around CRTs and switch mode power supplies. Anything that bites even when switched off and unplugged demands respect.

Many Thanks again for your support, it was really professional and useful.
I will update in few days the post.

My pleasure, and thanks to you too for taking the time to report your success.

Do you mind if I ask how you came by the 'scope. My 803B was an ebay score, which I got for next to nothing. I was pretty chuffed to get it working. I have another Metrix awaiting a donor analog board. It is an OX 2000, one of the top of the range models, but they seem to be pretty rare beasts, so I think I may have to wait a long time to get the parts to fix it. I like these older scopes, they may be a little on the large side, but they have some advantages over digital ones. Besides they are also repairable, which a lot of the modern scopes simply aren't.

 

[lynyx77]

Do you mind if I ask how you came by the 'scope. My 803B was an ebay score, which I got for next to nothing. I was pretty chuffed to get it working. I have another Metrix awaiting a donor analog board. It is an OX 2000, one of the top of the range models, but they seem to be pretty rare beasts, so I think I may have to wait a long time to get the parts to fix it. I like these older scopes, they may be a little on the large side, but they have some advantages over digital ones. Besides they are also repairable, which a lot of the modern scopes simply aren't.

In fact I'm trying to repair some CRT from retrogaming boxes and some Power supply. Currently I'm trying to Fix an issue on a Sony CRT PVM 2950QM (High end TV model in the 90's ) The blank is Pink ...
So the use of the oscilloscope was evident and I decided to find a scope.
I've bought the metrix OX803 on the website "Leboncoin" in France , the price was cheaper and thinking Metrix is a good value, but probably tektronix or Hameg is better...
I'm a bit old school but I prefer like you the analogic oscilloscope, I prefer to have more buttons and less menu and I don't like the see the noise of the signals with digital ones.
However Digital one have some advantages and many functions, for example they can memorise waves forms , take many measurement in mode auto.
I think the best way is to have the 2 models.
Yes the OX 2000 is really rare, I cannot find any model on the net, If I see some available I will ping you.

 

[itsthatidiotagain]

In fact I'm trying to repair some CRT from retrogaming boxes and some Power supply. Currently I'm trying to Fix an issue on a Sony CRT PVM 2950QM (High end TV model in the 90's ) The blank is Pink ...

Have you got a picture of the pinkness, is it uniform, or does it only affect part of the picture? Have you tried degausing the tube?

So the use of the oscilloscope was evident and I decided to find a scope.
I've bought the metrix OX803 on the website "Leboncoin" in France , the price was cheaper and thinking Metrix is a good value, but probably tektronix or Hameg is better...

Interesting, I've not looked at Leboncoin, but I did purchase the OX 2000 from an ebay seller in France. I think the Metrix 803B is a nice little scope. I used to have a Hameg, it was pretty good too, but I had to hand it back when I changed jobs a good few years back, and have managed to get by, borrowing the Tektronix from the office recently. I spotted the OX 803B on ebay and put in a low offer, and was lucky enough to get it. Fixing it was half the fun of purchasing it.

I'm a bit old school but I prefer like you the analogic oscilloscope, I prefer to have more buttons and less menu and I don't like the see the noise of the signals with digital ones.
However Digital one have some advantages and many functions, for example they can memorise waves forms , take many measurement in mode auto.
I think the best way is to have the 2 models.
Yes the OX 2000 is really rare, I cannot find any model on the net, If I see some available I will ping you.

I don't have a big budget for repairing the OX 2000, but if you spot one "spares or repair" let me know. I think I am only missing the analog board, but to be honest, since there are no service manuals on line for it, there may be other bits missing. It is a half way house between an analog and digital scope, with 150MHz bandwidth so far as I recall, so it would be useful to have working. **broken link removed**