HP 1220A-Help with repair - Page 1
Forum section: electronics / repair

author: robimarko
author_rank: Regular Contributor
timestamp: May 16, 2014, 10:03:54 pm
content: Hi everyone this is my first topic on eevblog so I will maybe make some mistakes.I need help with repairing my first oscilloscope that good guy in my country Croatia gave me for free.He had it lying around and he wasnt using it in years.But when he tested it he noticed that you cant focus beam on the screen.Here are 2 picturesBut i still wanted the scope and he sent me it for free.It is not in perfect shape but it is good for mine first scope.Another issues is that I still dont have the probes,I will order P6100 from China tommorow because in Croatia even cheaper chinese probes cost over 20 USD,and it is too much for a teenager.Now today i started repairing it and I have purchased a scan of manual which has all of the schematics and information that you can ever need.First i checked low voltage power supply and all of the voltages except +12V were within range specified by manual.But then i adjusted +12V with variable resistor(Which is what manual says to do) to be exactly 12.00V according to my UT39A DMM.Manual specifies that +12V is spouse to be within +-0.05V.Before that adjustment focus knob had a very little to extremely little effect on beam and now it doesnt have effect at all.What can be the problem?I am suspicious about high voltage power supply.I really need help since this is my first scope,so i hope someone will be able to help meI am attaching manual that i purchased,so anyone can freely use it without charge.Manual:https://www.mediafire.com/?645hp951kd2sjwbHere are the remaining pictures:

author: robimarko
author_rank: Regular Contributor
timestamp: May 17, 2014, 12:51:10 am
content: All pictures have been made in X-Y mode,but here is a video i madehttps://www.youtube.com/watch?v=GjG22PlNRiU&feature=youtu.beTommorow i will further inspect focus issue,but i need help from more experienced people

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: May 17, 2014, 11:23:45 pm
content: Ok,look at the focus circuit,resistor of adjustment,caps,other resistors and diodes.If you have any way to measure the EHT try it,but remember it's dangerous!!

author: robimarko
author_rank: Regular Contributor
timestamp: May 18, 2014, 12:22:54 am
content: Guy who gave me scope suggested also to test focus circuit and without removing CRT i was able to measure resistance in the focus circuit which is spouse to be around 3.2Mohm but in my case it was over 30Mohm,so i tested one resistor(R22) that i could without removing CRT and it is spouse to be 500K resistor but my UT39A DMM measured around 34Mohm resistance on it so i will replace it definitively.I will test other resistors too when I remove CRT

author: robimarko
author_rank: Regular Contributor
timestamp: May 19, 2014, 05:23:01 pm
content: I have sucessfully fixed my scope.Problem was that one of 5 500K resistor that are connected to focus regulation was faulty so i was able to solder 2 1Mohm resistors in parallel on exsting faulty resistor and it work,focus i really good now.I just have to wait for probes to arrive from China,

author: GreyWoolfe
author_rank: Unknown
timestamp: May 20, 2014, 06:30:55 pm
content: on the repair. Enjoy the scope.