[Fixed] and "need repair again"  - Signal Burst after repair oscilloscope - Page 1
Forum section: electronics / repair

author: chihaxinh
author_rank: Regular Contributor
timestamp: August 28, 2016, 02:47:17 pm
content: Hi AllI have a problem when after repair osc . When power on in 5 min so everything ok . Signal clean and i can change focus ok. But after 5 min , signal go to burst like picture.I open case and turning trimmer in HV PSU ( focus trimmer ) not work.Pls help me repair and take equipment back to work.Many thank you !Best RegardsNam Nguyen Hai

author: siggi
author_rank: Regular Contributor
timestamp: August 28, 2016, 06:02:01 pm
content: Hi Nam Nguyen Hai,It would help if you told us the make and model of the oscilloscope, and what you've done to repair it so far.That being said, it looks like you may have a classic case of degraded or bad mains filter capacitors, or possibly output filter capacitors if your oscilloscope has a switch-mode power supply.Try set your oscilloscope to "line" trigger and a time base of 1ms or so, see whether there's a period of 50/60Hz to the "noise"?Siggi

author: singapol
author_rank: Frequent Contributor
timestamp: August 29, 2016, 05:13:22 am
content: I think he's refering to his TDS520 scope.https://www.eevblog.com/forum/repair/need-help-repair-tektronix-tds520b/First I think he needs to check the voltages of the crt circuit and then adjust the focus eg. adjust until a smalldot (trace) appears and then the front panel pot (focus) as they are interactive. Hope this helps.Please refer to the service manual for instructions.

author: chihaxinh
author_rank: Regular Contributor
timestamp: August 29, 2016, 05:57:07 am
content: Hi AllMy osc it's Iwatsu SS-7810 ( 100Mhz ) . I Have 2 scope for measurement tube amplifier . Digital OSC i using for solidstate amp.So one scope i repair LV PSU , and when i swap LV PSU from work scope to fail scope . Problem not gone .I thing HV PSU have problem . But i want make sure about big burst trace ( Noise come from gain or att or only problem on HV PSU - focus or etc...)Many thank you !Best RegardsNam Nguyen Hai

author: singapol
author_rank: Frequent Contributor
timestamp: August 29, 2016, 04:24:42 pm
content: First thing first, get SS7810 crt voltage (focus) working then can you determine if or what noise is present.Can you find service manual?

author: chihaxinh
author_rank: Regular Contributor
timestamp: August 29, 2016, 11:57:01 pm
content: QuoteHi SingapolThis it's link to service manualwww.iti.iwatsu.co.jp/manual/pdf/SS-7810-05-04_ServManl.pdf

author: singapol
author_rank: Frequent Contributor
timestamp: August 30, 2016, 03:33:18 am
content: From Page:2-23 SS7810 service manual:QuoteSS7810 can make adjustments from menu with Jig IE-1066 but you don't have this jig so everything is done manually.For HV schematic see page : 5-27Also see V output AMP schematic page 5-10 for crt power connections.PS: Make sure your DMM can read up to 1000V and have adequate DMM protection quality.Avoid checking R51 (1 Mohm) -625V first.Just check those below 600V and see what happens.If possible wear eye protection goggles.DANGER HIGH VOLTAGE!

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 15, 2016, 07:17:10 am
content: Many thank singapol !I Fixed device, i replace resistor in focus function and equipment back to work very well .Nam Nguyen Hai

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 23, 2016, 03:43:33 am
content: Hi AllAfter finish repair, i shipping equipment to my friends, and shipping service make broken HV PCB. So i fix by epoxy bond and rewire broken track.After repair , when power on i missing all HV voltage , B+ , G123 , Vk , and Heater .I check Q1 2SC3570 ok , Q2 2sc1815 ok . L1 same specs not burn or short .IC1 i repalce new ( 4558 JRC) .I check in Secondary of HV Trans, CP3003 haven't osiclator . and haven't Voltage.I Checking CP3003 not short or burn.Thank so much !Best regardsNam Nguyen Hai

author: singapol
author_rank: Frequent Contributor
timestamp: September 23, 2016, 06:38:23 am
content: How can pcb break when it's inside scope unless you only shipped pcb only and did not put enough packing andprotecting filler in package? It's amazing you are willing to go as far as gluing both pieces together but common sense will tell you that you must add connecting wire links to EVERY BROKEN copper trace which first you scrape off the protecting varnish layer to see bare copper ( use solder flux to help with solder wetting). Have you done this? Very sad to hear this has happened.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 23, 2016, 08:55:28 am
content: Quote from: singapol on September 23, 2016, 06:38:23 amYes , i rewire all broken track . You can see picture , HV PCB Below MCU PCB and isolation with MAIN PCB in other side . Gap within 2 PCB about 3cm . OSC keep in sponge box, but HV PCB not make by FR4 material all weight of HT Trans press to PCB when it's have vibration ==> Bingo, pcb crack to 2 part.

author: singapol
author_rank: Frequent Contributor
timestamp: September 23, 2016, 02:12:39 pm
content: Hi, actually it's easy to troubleshoot by working backwards. From HV schematic L1 needs +50V from main power supply schematic 39. Also -295V and - 325V needs +150V to bias Q7 and Q8.If there is +50V at L1 then Q1 and Q2 will start switching and AC is produced at primary coil 9 & 10 and 11 & 12.Then there will be step up voltage at secondary side of T1 - CP 3003. Do not measure pin 5 & 6 with respect to pin 7 ground as they are at 2Kv.Pin 1 & 2 = heater 6.3V. Pin 7 & 8 is AC voltage multiplied to higher voltage for CRT. If there is AC voltage at pin 7 & 8 it means HV transformer is working.Because of the accident use magifier glass to inspect coil windings of T1 at the pins ( wire break) if +50V is present at L1 and no HV.Good luck and be careful, personally I try not to work at night as one may be tired after day's work.

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 23, 2016, 03:51:02 pm
content: Hi singapolI checked 50V unreg. It's OK , but i check pin 12 - 6.3V heater and 7-8. Don't have any voltage.Do you thing circut don't have oscillator ?

author: singapol
author_rank: Frequent Contributor
timestamp: September 23, 2016, 04:15:59 pm
content: Quote from: chihaxinh on September 23, 2016, 03:51:02 pmCheck Q1 & Q2 if there is switching waveform output with another scope? If not check the caps and other components.

author: rf+tech
author_rank: Frequent Contributor
timestamp: September 23, 2016, 04:18:19 pm
content: Hi chihaxinh,I agree that the circuit does not oscillate.Is -12 Volts present as shown in the partial schematic below?What are the voltages measured on the emitter, base and collector of Q1 and Q2?RF+ Tech

author: chihaxinh
author_rank: Regular Contributor
timestamp: September 24, 2016, 02:31:32 pm
content: Quote from: singapolCheck Q1 & Q2 if there is switching waveform output with another scope? If not check the caps and other components.[/quote