Philips PM 3305 oscilloscope: issues with time base linearity

[jancumps]

My Philips PM 3305 memory scope has an issue with the time sweep linearity in the analog mode.
The beginning of the x deflection is compressed when using the time/div functionality (the internal sweep generator).
In digital mode the linearity is spot on. When I use an external voltage for the horizontal axis it is also reacting perfectly linear.

I've made a video to show the effect.



The service manual has a block diagram, and instructions on how to adjust horizontal deflection and sweep linearity (both attached as pictures).
The most relevent part where it says: "Check that the trace is linear. If neccessary readjust c1409" is not changing the error.

I have the original user manual with simplified block diagram and a pdf service manual from the web with a more detailed block diagram. I do not have the schematic (I asked Fluke, they took over this branch of Philips ).

I know it's a lucky shot: does anyone have experience with a similar situation?

[jancumps]

i could not find the fault with the block diagram and service manual instructions.

It seems that all service manuals that are for sale on the internet are from a single source.
They are all missing the schematic pages (I contacted sellers to check for those missing pages, they were missing in the PDFs they are selling too).

I asked Fluke Netherlands if they had it in their archive (Fluke took over Philips labo equipment division). They were so kind to reply, but unfortunately didn't have the schemas.

If any of you have a source for the schematic, that would be great.
I'm particularly interested in the analog timebase generator signal the horizontal sweep).

[PA0PBZ]

Look at this, worth a try?

http://www.radiomuseum.org/forum/schaltplan_fuer_oszi_pm3305_gesuchthilfe.html

[jancumps]

I've re-contacted them for confirmation that the schema is there.. I'll report back if the manual has schematic.

[jancumps]

Just like the previous time when I contacted them, I did not receive info if the manual is complete, but an invoice...

[jancumps]

They just replied that their manual is complete and in colour. Checking out the payment methods.
(I have no personal credit card or PayPal  )

[PA0PBZ]

(I have no personal credit card or PayPal  )

I have both, so if you can think of a way to get your money to me I can help if you can't find another way.

[jancumps]

(I have no personal credit card or PayPal  )

I have both, so if you can think of a way to get your money to me I can help if you can't find another way.

Thanks for the kind offer!
I have bought a payment card at our post office, linked that to PayPal, and ordered the manual.
I think I'm sorted...

[jancumps]

Hooray!

I've got the manual, and it includes the schematic  .
So now I'll spend some time studying the sweep generator and checking how to measure it.

[PA0PBZ]

 

[jancumps]

I've asked another forum member to come over with his scope to help measure the sweep module.
In the µRuler video Dave scrolled through the excel file with pledgers and there I spotted someone from my neighbourhood. So I contacted him.

That will help. I can't probe inside my scope with itself (meta-probing ).

[PA0PBZ]

That's why they say 'you can never have enough scopes' 
I remember having to solder a connection in my soldering iron a long time ago, that motivated me to have at least 2 of them.

[jancumps]

Progress!

EEVblogger andete came over to the JanCumpsLab with his scope.
We probed the whole triggering and sweep generation circuit. Nothing seemed to be off.
We measured the caps, diodes and some resistors related to sweep. Nothing awfully off.

So we called it a day. Only then it came to mind that we might not have found something wrong because that circuit is not defect.
So it was time to look at other parts of the scope that might interfere with the sweep gen.
And yes, as soon as you disconnect the point where the sweep sawtooth is diverted to the digital board, the analog part is ok (earlier I had already disconnected the part where the digital board injects the horizontal signal back into the X amplifier to no avail).

This means that the digital board injects DC into the analog part's sweep circuit.

And that is where I need some help.

The sawtooth is routed into a comparator input (NE521N).
What can cause a dc voltage to appear on that comparator input, except for a defect chip (I've measured the feedback resistor and cap, they are ok)?
I've attached the  relevant part of the schematic.

[edavid]

What can cause a dc voltage to appear on that comparator input, except for a defect chip (I've measured the feedback resistor and cap, they are ok)?

NE521 supply voltage not present?
Another pin shorted to something?

[PA0PBZ]

What is the voltage on pin 11?

[jancumps]

I have measured all pins and attached the results, but your question helped me to get further.
There is 7v on that pin 11. If I press with one finger on the area of the chip that is feeding that pin 11, my scope behaves normal. I'm going to remove the logic board once again, look at solder joints and traces, and maybe touch some of the connections there....

[jancumps]

Visual inspection did not reveal any flaws in solder joints or pcb traces. Anyway I took my iron and touched up the pins of the two press-sensitive ICs and surrounding components.

The scope is now working flawlessly. 1 evening help of a fellow forum member helped to achieve something that I was struggling with since May (actualy since I owned the scope but I started my fix attempt in May).

A proper service manual with schematics and component layout turned out to be invaluable during the exercise.