HP 8568b spectrum analyzer ?????? - Page 1
Forum section: electronics / repair

author: Oliver
author_rank: Newbie
timestamp: September 12, 2016, 07:52:44 pm
content: I just got a HP 8568b yesterday , the display worked ok , no issues , i left the unit plugged in overnight just to let it equalize, turned on unit and no display at all completely black . I took off the the cover for the display and noticed one of the led indicators that says DS1 red , is this the problem if so how can i start to fix .I am new to this side of things but i am pretty capable of doing things , please help if you can , thanks.

author: singapol
author_rank: Frequent Contributor
timestamp: September 13, 2016, 02:54:13 am
content: Probably cracked solder joints in the display unit? Do be careful...high voltages inside and switching power supply.Performance Tests and Adjustments Manual HP 8568Bwww.keysight.com/upload/cmc_upload/All/08568-90118.pdfHP 8568B Spectrum Analyzer Installation and Verification ... - Keysightliterature.cdn.keysight.com/litweb/pdf/08568-90119.pdf?id=814567

author: Oliver
author_rank: Newbie
timestamp: September 13, 2016, 03:08:07 am
content: I checked some of the points as listed in service guide ,DS1Tp1 is 6.6v where it should be 6vTp2 is 25.6 v instead of 23v Tp3 is 14.9 v instead of 15v DS2 Tp2 is 5.2v instead of 5v Tp3 is 4.5v instead of 100v Tp4 -14.9v instead of 15 so Iam guessing ssomething wrong in this area , anyone have the same issue??.

author: cncjerry
author_rank: Unknown
timestamp: September 13, 2016, 03:51:54 am
content: Join and post on the HP Agilent yahoo groups. Some of the same people are on here. Good luck, it is a great analyzer, I have both the 8568b and 8566b.

author: singapol
author_rank: Frequent Contributor
timestamp: September 13, 2016, 06:22:30 am
content: Quote from: Oliver on September 13, 2016, 03:08:07 amI would not worry too much about voltages of Tp1 ,2, 4 as they are close, you can adjust their respective trim pot if you know where. PCB A1A7 and the relevant schematic on pg.6 & 7.You have a already found the problem in Tp3 which should be 100V. ( the raw incoming AC is 118 Vac.)Look at the 100V fuse on this pcb, is it blown/open? You can pull out the pcb sitting in it's connector? Check fuse with your multimeter in buzzer mode or visually. Make sure your lighting is bright enough for you to seeand work properly. Again high voltages are present be alert!PS 100V supplies the vertical and horizontal deflection circuits of crt. Also know that this instrument is 32 yrs. old so expect voltages and component tolerances to drift some .Be aware that after powering off the high voltage capacitors will still hold the voltage so discharge the cap with a 5W or 10W ceramic power resistor say 1K ohm or higher for slower discharge. You have been warned.before diving in with your hands. Alternatively wait awhile but there will still be voltage on the cap.

author: Oliver
author_rank: Newbie
timestamp: September 13, 2016, 05:28:41 pm
content: I have checked both fuses there and they are good , is there a particular component that I need to look at.

author: Oliver
author_rank: Newbie
timestamp: September 15, 2016, 03:35:59 pm
content: I have checked tp3 which is 4.5v instead of 100v Tp 4 unreg 130v , checked c2 up to spec's 255uf tested checked fuses all good , could it be one of the transistorI am all out of ideas , also tested diodes in circuit seem fine as well . anyone some help please???

author: singapol
author_rank: Frequent Contributor
timestamp: September 17, 2016, 07:16:44 pm
content: Quote from: Oliver on September 15, 2016, 03:35:59 pmRead chapter A1A6/A1A7 power supply and A1A7 schematic. I hope you know how to test for open /short for transistors . Also check which version is your pcb,see attached notes.

author: Oliver
author_rank: Newbie
timestamp: September 18, 2016, 01:49:17 am
content: I end up changing the Q5 transistor , the replacement is a sk3562 , the display now shows but not a full image it's smaller that it should be . Voltage at Tp 3 is now 35.5 instead of 100v can someone give me some insight please .I was told that the sk3562 is direct replacement for 2n4240 so it may or may not be the issue??

author: singapol
author_rank: Frequent Contributor
timestamp: September 18, 2016, 04:34:28 am
content: Quote from: Oliver on September 18, 2016, 01:49:17 amDear friend, you do not know the difference between mosfet and bipolar transistor (2N4240)? Did you substitute 2SK3562 (mosfet) or 2N3562 (SCR)? The component package is different so is pin outs from 2N4240. Granted 2N4240 is obsolete you must use a close or best equivalent available. Please remove this immediately as I'm surprised it did not kill your high voltage regulator! Even the electronic symbols for mosfet ,bipolar and scr are different.2N4240 is high voltage depends on which terminals (see datasheet links) and 2A continuous. The best equivalent I can find is ST13005 by ST microelectronics, whose continous current is 4A. If space allows use a small heat sink with proper insulation like mica sheet for TO220 transistor and screws.You can get such kits.BTW 2N4240 is the driver for the power transistor. Next time verify the specs before committing.Also check Q7 which is also a power transistor and it's associated circuits (opamp and such).Finally please make sure you orientate and fit in the correct pins of new device into the original foot print of 2N4240.http://products.semelab-tt.com/pdf/bipolar/2N4240.pdf2N4240_ more details:http://www.microsemi.com/document-portal/doc_view/10095-msc1058-pdf2SK3562-http://www.farnell.com/datasheets/55580.pdf2N3562-http://html.alldatasheet.com/html-pdf/458560/NJSEMI/2N3562/215/1/2N3562.htmlST13005 -http://docs-europe.electrocomponents.com/webdocs/0473/0900766b804732b5.pdf