Metrix 803B Oscilloscope power supply issue

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

I figured 1W sounded right too. I don't have anything suitable, and its now pretty late here, so I think this will have to be patient and wait for another day.

I ordered a pack of 6 x 220K 1W 1% resistors from ebay, for pennies so I may chance just replacing that first to see if it works, since if it pops instantly I'll have four more to play with.

I noticed that all of the resistors appear to be 1% tolerance, so I went for 1%, but I suspect 5% would work fine. All of the other TDA4605 circuits I have looked at online dont seem too fussed about this, and I suspect it isn't critical. I doubt if it would affect the accuracy of the scope in any way since all of the DC side is regulated.

[budm]

Re: Metrix 803B Oscilloscope power supply issue

They use 1% because the value will drift a lot less with temperature than typical 5%, it is use for sensing the main B+ voltage and shutdown the circuit if the B+ is too low.
You can see the application circuit in the spec sheet on how pin 3 is used.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Progress at last. I finally got an opportunity to take another look at this, and decided in light of the fact that the parts were dirt cheap to just go for the shotgun approach.

I changed all of the chief suspects except the optoisolators.
The 220K resistor was open (confirmed once I had extracted it).
The power mosfet (P4NB80FP) transistor and TDA4605 I replaced on the assumption that if the resistor had cooked, then it was likely that it had done so because of excess load cause by the failure of those other two devices, and I replaced the 22uF 50V 85C cap with a 22uF 63V 100C just bacause it looked iffy. Total cost for repair... around £5.00 Total cost of 'scope including repair £35.00

Cost new... £758.00 ( http://uk.farnell.com/metrix/ox0803b...03b/dp/3430753 )

The PCB material is a little fragile (i.e. if you look at it sideways the tracks tend to lift), so I had to add one small mod wire to cover up my over enthusiastic desoldering.

A quick clean and inspection, then on with the juice... worked first time. It is now sitting doing a burn in test before I check its callibration.

Thanks for all the advice. I am now grinning from ear to ear.

[CapLeaker]

Re: Metrix 803B Oscilloscope power supply issue

great to hear your success! Thanks for the follow up!

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

In summary.

Fault: System dead, wont power up, no low voltages present on interconnect between PSU and main board.

Fix: Replace the following components and check for any other damaged or discoloured items.

C5 - 100uF 50V - Checked - replaced with 100uF 100V
C15 - 22uF 50V - http://www.ebay.co.uk/itm/ECA1HM220-...item2ed0cb867d

Z1 - TDA4605 - Control IC for SMPSU

https://cdn.badcaps-static.com/pdfs/...731590de00.pdf

Q1 - P4NB80FP - N-CHANNEL 800V - 3Ω - 4A TO-220/TO-220FP PowerMESH MOSFET (Equiv = BUZ80FI?) -

R4 - 220K 1/2W 1% (5 band Red Red Black Orange Brown)

Result: Scope works perfectly.

I left it powered on all afternoon, then checked the calibration with a few known sources and its own internal 1kHz 2.5V signal source. I compared with the office 100MHz Rigol, and there was no noticable difference.

The only adjustment needed was to calibrate my new cheap'n cheerful chinese test probe (see pic below of the less than square signal - pre calibration) and order a couple more probes.

My slightly dodgy soldering job (below, pre-cleanup) shows the problem with the tracks on this PSU which lift very easily. This was the main reason I didn't also swap the optoisolators (well that and the fact they looked OK on my diode check).

Next on the agenda is to probe the expansion connector on the main board to see if I can spot the serial port. If I get anywhere with that, I'll report back.

Running the linux "strings" command over the driver files I found on the web suggests it will be configured for 9600 baud, 8 data bits 1 stop bit, so that sounds like a pretty easy thing to track down. I expect the signal is probably ttl level, but now I have a nice new scope I can use it to probe itself

[stj]

Re: Metrix 803B Oscilloscope power supply issue

you can use one of these for ttl to usb!
http://www.dx.com/p/ft232rl-usb-to-t...-boards-310801

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

you can use one of these for ttl to usb!
http://www.dx.com/p/ft232rl-usb-to-t...-boards-310801

Indeed you can. Thanks for the link, that particular one also has DTR broken out on the connector, not all of them do.

The company I work for provides TV integration solutions for 5 star hotels. One of the things we do quite a lot is connect the strange serial devices built on to TVs to our custom set top boxes. As a result I have a few of these kind of gadgets lying around. (I have a few of my own at home that I use to program microcontrollers, but that is another story).

If/when I figure out what is what on the internal connector I'll see if I have something suitable. There is of course the possibility that the internal serial port is at RS232 levels in which case I might just pop a 25 Way D type on the back of the scope (there is a cutout for one) and use a null model cable or whatever attached to the PC.

[stj]

Re: Metrix 803B Oscilloscope power supply issue

you know they arent going to solder a max232 or similar in there for nothing!

[rievax_60]

Re: Metrix 803B Oscilloscope power supply issue

Replacement resistors for high voltage need to be carefully chosen. I have been using WELWYN MH37 series with good results and are available on eBay mow.
I find that carbon film resistors to be unreliable no matter what their specs say.
The resistor that connects to pin 2 of the TDA4605 sets the current limiting for the MOSFET. If this resistor drifts high, the power supply usually fails with a blown MOSFET.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Replacement resistors for high voltage need to be carefully chosen. I have been using WELWYN MH37 series with good results and are available on eBay mow.
I find that carbon film resistors to be unreliable no matter what their specs say.
The resistor that connects to pin 2 of the TDA4605 sets the current limiting for the MOSFET. If this resistor drifts high, the power supply usually fails with a blown MOSFET.

Useful information, I see those Welwyn series resistors are good to 3.5KV, so pretty robust. The ones I purchased are metal film, but only rated 500V (Dielectric Strength:: 1kV), however I suspect that should be adequate for this particular fix. I'll bear the Welwyn ones on mind in future, they dont cost significantly more.

When the current limiting resistor fails, does the blown MOSFET take out the TDA4605, or does that usually survive? Or do you just replace the TDA4605 anyway as a matter of course?

[stj]

Re: Metrix 803B Oscilloscope power supply issue

i would say it depends if mosfet shorts the gate to one of the other pins.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

i would say it depends if mosfet shorts the gate to one of the other pins.

Hmm... good point, it would rather depend on the failure mode of the MOSFET. In that case I suspect changing all of the compnents I listed probably is the minimum necessary for this fix.

You might get luck and not need to change the TDA4605, but equally it might be blown, and thus take out your replacement MOSFET, and given the fragility of the board, you probably dont want to have to rework the MOSFET twice.

On the subject of the serial port, the labview drivers give access to the following over serial..

Panel name : Initialize
Panel name : Instrument Setup
Panel name : Channel settings
Panel name : Display settings
Panel name : Time base
Panel name : Trigger

.. so I suspect if I can find the serial port, a little reverse engineering should give me access to that little lot from my laptop. There is a bigger brother of the same scope the OX 8027 that is almost completely physically identical, but has digital storage capabilities, a few more buttons and LEDs and various on screen cursors.

Although the digital hardware is probaly missing, there might be the possibility of doing some other clever stuff with this thing. Given how cheap microcontrollers are these days... I might just be able to hack a few more tricks in to it.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

I had a quick prod around inside the 'scope this afternoon looking for anything that was obviously an RS232 port, and in the process discovered something which might actually be more useful than the serial port.

The OX 803B differs from the higher spec OX 863B in a number of important respects. One of which is that the 863B substitutes on screen cursors and a couple of TV line mode buttons for the "Component Tester" of the 803B

The difference looks to be exactly that, the 803B has a couple of wires going to the connector on the main PCB for the component checker (which looks to be a 1v 1kHz signal generator, but I haven't actually played with it yet). The 863B has in its stead, 7 buttons on a PCB.

Compare..

http://www.ghvtrading.cz/data/imgs/1066b-ox863b.jpg

.. and ..

http://sigma.octopart.com/24577920/i...cs-OX-803B.jpg

.. to see what I mean.

On the front panel PCB of my 806B (below) there is an unpopulated header (highlighted) that looks like exactly the right kind of thing to attach a row of 7 buttons on a ribbon cable to.

I think I *have* to prod that connector with a 10K resistor and see what happens. If I am right, then I should be able to add the features of the 863B

There are three other "missing" buttons beside the "Horizontal" rotary control on the front panel that might also be simply needing to be populated to add those additional features.

Who knows, the jump from 40 to 100MHz may be simply the addition or removal of a jumper/solder bridge. This little toy is turning out to be a lot of fun.

[rievax_60]

Re: Metrix 803B Oscilloscope power supply issue

Useful information, I see those Welwyn series resistors are good to 3.5KV, so pretty robust. The ones I purchased are metal film, but only rated 500V (Dielectric Strength:: 1kV), however I suspect that should be adequate for this particular fix. I'll bear the Welwyn ones on mind in future, they dont cost significantly more.

When the current limiting resistor fails, does the blown MOSFET take out the TDA4605, or does that usually survive? Or do you just replace the TDA4605 anyway as a matter of course?

It depends on how much high voltage appears at the gate while the MOSFET is breaking down.It's best to replace it anyway. Weather the TDA4605 had survived or not, it's important to look for damaged components between pin 5 and the MOSFETs gate. Damage in this area can cause the power supply to seem to run properly but the MOSFET can be running too hot because of poor drive signal to the gate.

[stj]

Re: Metrix 803B Oscilloscope power supply issue

there's a lot of split "option" pads there.
maybe try to find a service-manual/schematics for the better model to see how they are set.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

there's a lot of split "option" pads there.
maybe try to find a service-manual/schematics for the better model to see how they are set.

I had a good hunt round the internet for a service manual for any of this series of scopes, but found nothing. I even emailed the manufacturer, who didn't even bother to respond (I emailed a bunch of email addresses, so I presume it must be company policy to treat their users this way).

It was mainly as a result of the manufacurer's attitude that I decided sod them I'll fix it without their assistance, and then decided to publish my efforts here.

It looks like others have been in this boat before me... for example...
http://www.eevblog.com/forum/testgea...manual-wanted/

If anybody here has a service manual, I would be very interested, as I suspect they are a closely guarded company secret.

Most of the front panel buttons and switches appear to have associated bed of nails test pads on the rear of the board (look closely for the little approx 1mm square pads connected to the tracks scattered around the back of the board). Probing these while I press the front panel buttons should give me a clue whether to pull the button low or high.

The missing LED connections presumably also have associated bed of nails test pads, so it should be possible to wire something up with care from these pads.

All of this assumes the firmware on this scope allows for these functions, which is a big assumption, but given the headache of maintaining different software builds for different models I suspect there is a reasonable chance this will work.

[stj]

Re: Metrix 803B Oscilloscope power supply issue

they should make the manuals public or risk a loss.

as someone who ued to reverse engineer products in the past, i can tell you that a pro does not need long to derive a full schematic from something using double-sided or even hidden-layer boards.

we also have a number of methods for identifying unmarked chips.

so any assholes sanding/grinding or painting the top of chips is just wasting time.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

they should make the manuals public or risk a loss.

I quite agree, if I had purchased a bunch of these scopes for work, and later discovered there were no service manuals available, then that would be one less company on my preferred suppliers list.

As far as reverse engineering goes however, the layout of the scope is fairly obvious, digitlal stuff mainly on the front panel, mixed digital/analog on the main board. I haven't even whipped off the front panel, but from the look of the tracks on the back of the front digital board, there is one, perhaps two large surface mount chips on that board, probably a microcontroller and perhaps some support logic.

There are lots of test points to probe, so I will probably take a look at those next.

I also found some source code for the serial driver for the OX 863, which looks similar to the strings from the compiled driver for the OX 803B, so I should be able to talk "Metrix Speak" if I manage to track down the serial port pins. The protocol looks pretty simple and the different models look sufficiently similar to suggest that variations of the same protocol probably covers a bunch of models (the 803b, 863b and 860 etc).

[stj]

Re: Metrix 803B Oscilloscope power supply issue

you know, something has been bugging me about your board foto's for ages.
i finally realised what it is - it looks like a philips designed/produced pcb!

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

you know, something has been bugging me about your board foto's for ages.
i finally realised what it is - it looks like a philips designed/produced pcb!

That would not surprise me in the slightest. The Metrix onine sales brochure states that the tube is Philips, "All models are equipped with blue screen
filters and, thanks to their Philips tubes..." so the rest of the design being outsourced or based on a Philips design would not be a surprise.

I have also seen the same scopes with an ITT badge and the Metrix designation on them. This is a pretty incestuous industry it seems.

There would be a higher probability perhaps of finding a Philips manual, if they ever produced a similar product under their own banner.