Focus problem on  a PM3209 oscilloscope  - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 15, 2015, 01:10:05 pm
content: Dear All,I hope you will be able to help me to repair a breakdown that is most entirely my fault.I was finishing to tune a PM3209 oscilloscope and was checking voltages after replacing a cap,when my DMM probe occasioned some flash (spark ? ) in the vicinity of the neon bulb.I must say it was quick and I do not know if it was a spark or a flash from the neon.The result is that the focus was immediately destroyed and the trace of the screen becomes a bigfuzzy spot.The focus setting of the front panel of the oscillo has now no effect on the setting.Nevertheless, I was able to restore the focus to reasonable setting by using the focuspot on the board that is visible in the picture (VR2011 on the schematics), but it is not perfect,and the tuning from the front panel does not work.The schematics for the 3209 is difficult to find on the WEB (I could not find it ), but I got the PM3208 servicemanual which looks exactly the same, maybe with some minor changes.http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/pm3208.pdfI attach here the part of the schematics.My problem is that I really do not understand how the focus work, and what is the effect of thefront panel focus pot with respect to the one on the board (VR2011).Any hint on where to search for a blown component would be useful.PS : I still do not know how to insert a picture in the text. If anybody can tell me ....Edit : I think I found the way to do it. Using the link of the image that has been already downloaded to the eevblog website.Then blanks after "img " needs to be removed.Quote

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 15, 2015, 01:10:52 pm
content: Here is the schematics

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 20, 2015, 02:21:38 pm
content: I dod not make any progress, but at least I know now how to include pictures in the text.Quote from: JacquesBBB on March 15, 2015, 01:10:05 pm

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 20, 2015, 03:56:43 pm
content: I understand that not many of you know about the focus circuit in the oscilloscope.Nevertheless someone may be able to answer this simple question :Is there a way to test on board the Neon NE-2 bulbs ?Or do I have to remove them to test ?Thanks

author: SeanB
author_rank: Super Contributor
timestamp: March 20, 2015, 04:57:28 pm
content: The neon lamps only light for arcing in the tube, normally they will be off. You must measure each and every resistor there in the focus system, as high value resistors generally fail by going high in value. As well check all those high voltage ceramic capacitors for leakage. The nice thing is the first check for resistance can be done in circuit, if the measured value is equal to the actual printed value then it is likely the resistor is fine. if it is higher it is easy to remove the resistor and replace it, then after doing all that try again, it is likely that your problem will be fixed.Check C3017, C3016, C3015, D3013, D3022, D3021, R3021 R3022, D3015, R3017, R3016, preferably be removing and checking for leakage. Caps if the resistance is under 20M and diodes the same they likely are fine.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 22, 2015, 02:00:47 pm
content: Thanks Sean for the hint.I look to all these components and more in the vicinity but did not find any suspect componentapart from a small 10uF cap that I changed, but do not think he was the culprit.I have also changed the big caps of the power supply (C1001, C1010, C1012).Unfortunately, I must have done something wrong as I do not have horizontal sweep now.This has been a bad day as this is the second apparatus I broke yesterday, trying to repair it.This is the A test square waveAnd the B square wave

author: sdg
author_rank: Regular Contributor
timestamp: March 22, 2015, 06:50:13 pm
content: The first photo looks like X-Y mode to me...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 22, 2015, 06:57:21 pm
content: Quote from: sdg on March 22, 2015, 06:50:13 pmI am not sure, I have to look back.The thing that is sure is that I do not have any time base any more.I can trigger the x direction by using an external signal from a function generator for example,but apart from that, I have nothing.The remaining functions seem to be OK, and still no focus on the front panel.