Missing High Voltage in CRT OSC  - Page 1
Forum section: electronics / repair

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 03:02:49 am
content: Hi allI repair osc for my friend , PSU using 7812 - 7912 for provide +-12V , and HV for CRT Tube using 12V rail . You can see picture attach.And i find out short Diode in voltage multiplier , HD950 HD951 , next i checking C962 C951 and out of tolerance ( >+-20% ) so i replace all HV Cemramic Cap .And next I recap all Electrolytics in area . When power on i have Q950 - 2SD880 very hot quickly . and +12V go to low ( ~9V) , -12V ok ( -11.9V)Next i checking T950 - HV transformer . I have pin 4-1 very low Henry ( 8uH - 128mR - Q=0.2xxx ) . So i think T950 short in pin 4-1 .I checking all active componant in OSC , everything work well . And when i take out U950 ( u741 ) +12V go to back ( +12.01V ).I need everybody review schematics and help me for sure about fail T950 .Thank you so much !

author: singapol
author_rank: Frequent Contributor
timestamp: September 06, 2016, 04:24:16 am
content: Hi , it would be helpful to others who have same scope to put brand and model od scope in your header title.Normally for horizontal deflection it's the power transistor Q950 and high voltage tramsformer that are the likely suspects but in electronics circuits anything can happen. Strange you did not check Q950. Pin 1-4 of T950 will look short because it is a coil winding but then who knows but at this time I will accept it as working condition.You did not check zener diode ZD952 and C953 which connects to U950 (741) positive power supply butsince you removed 714 and +12 is back so maybe C953 is also good. So I think putting in a new 741 is worth a try as no oscillator means no horizotal deflection and also high voltage for crt.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 05:37:31 am
content: Hi SingapolMy friend OSC it's Pintek PS-200 , it's copy from Heath SO-4552 but cutdown some part and replace by other cheaper .I replace all cap, check zenner work ok , Q950 Work OK . I desoldering T950 and check L , R , Q and compare with spec in Heath schematicsOpamp using Pintek it's Ha17741 , and I replace buy u741 and have same problem.

author: singapol
author_rank: Frequent Contributor
timestamp: September 06, 2016, 07:28:48 am
content: Quote from: chihaxinh on September 06, 2016, 05:37:31 amYou got the service manual here?Heatkit SO-4552 scopehttp://www.badcaps.net/forum/showthread.php?t=13252Eventually OP said he asked Pintek and they said T950 is bad and can order it but it's 2016 now?PS OP did not place order as Transformer too expensive + shipping.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2016, 07:33:18 am
content: Sounds like the primary side transistor is turned on continuously and it is not oscillating the current in the primary winding. This can be due to the feedback circuit not working properly. These oscillators normally run in the 20+ KHz range.Also check the resistor divider chain from 2 kV to +12V.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 07:57:32 am
content: QuoteYes i have 4552 service manual .QuoteCan you help me determine feedback circuit , i dont have much exp about smps . Your mean it's i need check in red circle ?

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2016, 08:31:30 am
content: Curiously, in another threadhttps://www.eevblog.com/forum/testgear/floating-an-oscilloscope-for-measurements/msg1020390/#msg1020390I've just writtenQuoteNow clearly that is a different thread, person and problem, but the cautionary advice is relevant here.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 08:45:54 am
content: Quote from: tggzzz on September 06, 2016, 08:31:30 amThanks tggzzzI know , and i never touch to 2kV in that case . So you know i measurement only in primary side of HV Trans . You know i don't want touch to 2kV so i need go to forum and ask somebody.So i need question and hope somebody have answer for learning growth .

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2016, 08:46:22 am
content: Quote from: chihaxinh on September 06, 2016, 07:57:32 amThe resistor divider chain you circled in red is along with the 741 is part of the regulator, resistor values must be correct for it to regulate @ -2kV. I can see no other HV adjustments in the snipped schematic.I think this is a Colpits oscillator.Feedback comes from the Xformer Pin 1, coupled and smoothed by C955, 944 and through R951 to drive the transistor base. Once it starts it is self oscillating and 741 controls the base voltage which controls the Xformer output.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 09:11:37 am
content: Quote from: tautech on September 06, 2016, 08:46:22 amI alway like information basic about circut topology .Let's me check in tonightMany thank tautech !

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 06, 2016, 02:31:31 pm
content: QuoteNo luck , i checking all resistor in divider chain , all ok and in toralance . Q950 again very hot

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 08, 2016, 02:08:52 am
content: You tried swapping out the 741 for another one?Do you get any waveforms on the output of OpAmp, and the base of Q950?Did you remove and test (or replace) Q950, or at least do a quick diode check of it in place?

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 08, 2016, 04:25:03 am
content: Quote from: FlyingHacker on September 08, 2016, 02:08:52 amYes , i repalce opamp, replace Q950 , check all diode in place and no help . I check pin 1 - 4 HV Trans T950 have low Q. it's feedback line to Q950.So i think HV Trans have short copper wire inside .

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 08, 2016, 03:46:21 pm
content: HV transformers do seem to fail a lot... At least certain ones are known for it. Perhaps you get get a junk one, or a parts unit or something.Or perhaps you can get resistance readings off the transformer and compare them to a known good one, if you can find someone else with a known good one.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 09, 2016, 01:38:22 am
content: Quote from: FlyingHacker on September 08, 2016, 03:46:21 pmOSC to old, so my friend decied re-winding HV Trans, so when finish i will update status .I have 3 OSC need repair so much spend time for other !

author: Hamid_tehran62
author_rank: Newbie
timestamp: May 17, 2019, 11:08:03 am
content: I am working on a pintek ps250Its high voltage is corrupted.One resistor is burned ( R950 )I need its shematic or atleast its value .Thanks.

author: Dave.
author_rank: Newbie
timestamp: January 08, 2024, 10:37:30 pm
content: My PS-200 had same issue; no oscillation high current draw.Finally discovered HV rectifiers were faulty, including replacements! This placed too much load on the circuit for oscillation to start. Fixed using HV diodes from a CCFL power supply: very high voltage rating as they have a 5 volts forward drop. UF4007 diodes may work but use two in series to replace one diode.