Philips PM3305 Oscilloscope noise

[MichR]

Hello,
I've got a Philips PM3305 oscilloscope. This is an analog oscilloscope with digital memory mode. It presents a problem whether I select a microseconds time-base and the MEMORY ON button is pressed. If the time-base is higher than microseconds (ms, s), the waveform is correct:


If the time-base is set in microseconds, the waveform changes:


As you can see, there's like an electrical noise over the entire waveform.
This "noise" remains even though the channel is zeroed:


Since the service manual of this oscilloscope doesn't provide any schematics of the unit, I opened the oscilloscope and I looked for missing/broken components. I found out two leads connecting nothing on the side board (the logic unit):


By surfing the net I found this thread with all the photos of the boards. By comparing them with mine I found out the component was a capacitor:


Anyway I don't know neither value nor the kind of the capacitor. It probably is a 6.8uF 16V Philips tantalum capacitor since they are all over the logic board. It connects pin 17 (Vref) of AD7541JN (digital-to-analog converter) to ground.
I tried to replace it with another 6.8uF 16V capacitor. Now when the oscilloscope is switched on memory mode with microseconds as time-base, the signal is always at 0V.

I hope someone who has this oscilloscope can help me.
Thanks in advance 

[Deathwish]

What is its designator on the board, Cxxx ?

[MichR]

There isn't any reference designator on these boards.

[oldway]

May be a bad capacitor...did you tried another one ?
Are you sure that the polarities of the capacitor are correct ?
Measure pin 17 with and without capacitor...voltage must be the same.

[MichR]

The capacitor is good, I've bought 10 of them and they are brand new Philips 128SAL 6.8uF 16V.
As for the polarity, I followed the picture with the comparison: every capacitor on the board has a plastic ring around the positive lead. If you pay attention in the picture you can see it on the capacitor I've replaced. I'm going to check the voltages.

EDIT: checked the voltage with the capacitor: it's -4.6V (from Vref to GND). Later I'm going to desolder the capacitor and check the voltage again.

[MarkL]

Perhaps 6.8uF is not the right value?  Maybe it's something much less because it's not a fixed voltage?  10nF? 150pF?  Just looking at the parts list.

One side of the cap you've said is connected to AD7541 pin 17.  What else is it connected to?  Vref has to be coming from somewhere.

Look carefully in corners or shake the chassis.  Perhaps the capacitor that broke off is still around somewhere.

Have you tried a PM to "jancumps", the OP of the other pm3305 thread?

[Deathwish]

I pm'd jancumps to find out just that but no reply.

[MichR]

Perhaps 6.8uF is not the right value?  Maybe it's something much less because it's not a fixed voltage?  10nF? 150pF?  Just looking at the parts list.

One side of the cap you've said is connected to AD7541 pin 17.  What else is it connected to?  Vref has to be coming from somewhere.

Look carefully in corners or shake the chassis.  Perhaps the capacitor that broke off is still around somewhere.

Have you tried a PM to "jancumps", the OP of the other pm3305 thread?

You could be right since without any schematics I've just put the most probable capacitor. I looked for the broken capacitor in the chassis but I couldn't find it. However, not so long ago I took the oscilloscope apart and if I remember correctly, I found a ceramic capacitor half opened.

Vref is connected to a 100 ohm resistor, connected to the output of one LM358N op amp:

I haven't had the time to finish the circuit, I hope tomorrow I will be able to go on with it.

I pm'd jancumps to find out just that but no reply.

Just like you I haven't received any message back from him.

Edit: I've just desoldered the capacitor I put and surprisingly memory function now works, but I can't understand why.. 

[jancumps]

I pm'd jancumps to find out just that but no reply.

I don't have the scope or service manual anymore. I sold the package last year.
I had ordered my service manual for 20$ from http://www.jetecnet.com/
It was a high quality scan with all the schematics.

[jancumps]

...
Have you tried a PM to "jancumps", the OP of the other pm3305 thread?

...

I pm'd jancumps to find out just that but no reply.

Just like you I haven't received any message back from him.
...

I don't react on personal help requests. I play along on the public forum though .

[Deathwish]

I pm'd jancumps to find out just that but no reply.

I don't have the scope or service manual anymore. I sold the package last year.
I had ordered my service manual for 20$ from http://www.jetecnet.com/
It was a high quality scan with all the schematics.

Manual no longer available looking at the fact it is non clickable now.

[MarkL]

At the risk of sounding like a hack, you could try substituting successively larger values for the capacitor until the signal looks ok.  Assuming, that is, the missing capacitor is the problem.  We haven't proven that yet.


It would appear from your partial schematic Vref is not a simple fixed reference.  And there should be something more than capacitors connected to the "+" input on the second LM358 (but at the moment it's not critical to know).

I'm trying to pin down which AD7541 we're dealing with to see if there's a snippet of a schematic in the Theory section.  I found the AD7541 can be either D2071 or D2009.  The NE5008 can be D2126.  And the LM358 can be D2131 or D2506.  Since the manual is not OCR'd, I could have missed other designations in the parts list.

And what makes matters worse is that Philips does not seem to have any method to their part numbering.  Maybe I'm overlooking something.  I was hoping we could find the chip designations and then find what capacitors were in the same range.

Can you post a high-res photo of the board so we can read the chip numbers?

[jancumps]

I pm'd jancumps to find out just that but no reply.

I don't have the scope or service manual anymore. I sold the package last year.
I had ordered my service manual for 20$ from http://www.jetecnet.com/
It was a high quality scan with all the schematics.

Manual no longer available looking at the fact it is non clickable now.

I ordered it by clicking on the "Email : Click Here  to Order Now" link on the bottom of the page.

[MichR]

Yesterday I desoldered the capacitor and surprisingly the memory function (in us) started working. I can't understand why it does since without the capacitor I would have expected to see noise just like before I had soldered the capacitor. Could it have been an oxidized contact?

Voltage on Vref is still the same both with and without the capacitor. I also tried to add/remove the capacitor while the oscilloscope was running: I didn't notice any difference in the waveform.

At the risk of sounding like a hack, you could try substituting successively larger values for the capacitor until the signal looks ok.  Assuming, that is, the missing capacitor is the problem.  We haven't proven that yet.


It would appear from your partial schematic Vref is not a simple fixed reference.  And there should be something more than capacitors connected to the "+" input on the second LM358 (but at the moment it's not critical to know).

I'm trying to pin down which AD7541 we're dealing with to see if there's a snippet of a schematic in the Theory section.  I found the AD7541 can be either D2071 or D2009.  The NE5008 can be D2126.  And the LM358 can be D2131 or D2506.  Since the manual is not OCR'd, I could have missed other designations in the parts list.

And what makes matters worse is that Philips does not seem to have any method to their part numbering.  Maybe I'm overlooking something.  I was hoping we could find the chip designations and then find what capacitors were in the same range.

Can you post a high-res photo of the board so we can read the chip numbers?

I'm sorry I couldn't take high resolution pictures today. I hope I will have more time tomorrow to do that.

[MarkL]

So, everything is working now?  Displayed signal levels are ok too?

Then obviously it's not the missing capacitor.  A bad/oxidized connector contact is a good theory.  You could try wiggling some of the connectors while watching the screen.

Looking at the partial schematic again, it may be the "+" input on the second LM358 is connected to ground on an internal layer, which you wouldn't be able to see.  This makes more sense given that the COMP cap on the NE5008 is usually connected to ground too.  It's not the whole picture, but the Vref may indeed be fixed, and the trimmer on the left adjusts it.

It still doesn't tell us the missing value, but I guess we're back to including 6.8uF as a possibility.

[MichR]

So, everything is working now?  Displayed signal levels are ok too?

Then obviously it's not the missing capacitor.  A bad/oxidized connector contact is a good theory.  You could try wiggling some of the connectors while watching the screen.

Looking at the partial schematic again, it may be the "+" input on the second LM358 is connected to ground on an internal layer, which you wouldn't be able to see.  This makes more sense given that the COMP cap on the NE5008 is usually connected to ground too.  It's not the whole picture, but the Vref may indeed be fixed, and the trimmer on the left adjusts it.

It still doesn't tell us the missing value, but I guess we're back to including 6.8uF as a possibility.

It seems to work. For the moment, here you can find the high resolution picture you asked me to take.

Unfortunately I don't have much time today to try wiggling cables/connectors to find the bad contact.

[jancumps]

...

It seems to work. For the moment, here you can find the high resolution picture you asked me to take.

Unfortunately I don't have much time today to try wiggling cables/connectors to find the bad contact.

On my scope, this digital board was causing issues too. It turned out that the mounting screws shorted some signals related to triggering and memory. I detected it because issues were disappearing intermittent when pressing that pcb. I first thought it was bad contacts and refreshed solder joints in the area where pressing changed behavior. Later it turned out it were the mounting screws that seem to make false contact somewhere.
Carefully looking at the CRT while re-mounting the PCB and fastening the screws took care of the issue.

[MichR]

Thanks for your help guys, it was definitely a bad contact. As jancumps said the mounting screws shorted some pcb tracks. I've put some pieces of paper between the pcb and the screws to isolate them and now the oscilloscope works just fine.

[Chris56000]

Hi!

Can SOMEBODY who's got the Philips PM3305 schematic diagrams and board layouts please upload them? it's not very helpful to other Forum Members who are struggling with faults on this model to say "I've got all the schematics and diagrams" without saying where they were obtained from!

On at least one occasion on this Forum I have personally purchased a manual where there's a number of Members struggling with unreadable, incomplete or simply non–exsistent schematics, and if I find a good diagram or manual in the course of answering a fault–finding post, I always try and provide a link to download it in my answer!

Chris Williams

[jancumps]

link in this post: https://www.eevblog.com/forum/repair/philips-pm3305-oscilloscope-noise/msg801783/#msg801783

[Chris56000]

 Hi!

A complete copy of this PM3305 S.M. will be on KO4BB once they've verified & checked it – I uploaded one (purchased!) today!

Chris Williams

[jancumps]

If you purchased it from the source above, it’s complete with high-definition schematic scans.