IWATSU SS-5711 Oscilloscope Tear Down/Repair - Help me repair this thing please! - Page 1
Forum section: electronics / repair

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:28:17 pm
content: I got this scope about 2 years ago for £30, and it’s always been in need of a repair. The trace on the screen is quite faint, and only just about visible in a darkened room. The contacts/buttons on the front definitely need some cleaning as there is noticeable signal noise as buttons are being pressed or potentiometers being turned. When a signal in connected to an input, the TRIG'D LED is turned on (after massaging a few buttons to make contact).I’m now at a point where I believe I have the time to fix it, suitable tools to aid in diagnose and the need of an analogue scope. I have a bit of experience with mains level circuits, but no experience with the high voltages of a CRT based device. I would love to get this device back on its feet and added to the tools of my workshop.So this thread is to show some pictures of the tear down/repair of this scope as I go along. Hopefully I can eventually repair this thing. Any suggestions/ideas/help is very welcome.The service manual is too big (141MB) to be attached here. The Instruction manual found herehttps://archive.org/details/5711a/page/n147/mode/2upand the schematics can be found from page 110 onwardsI've also tested the DC supply voltage. The following rails read as the below:Enhance area of the Power supply PCB-12v:-12.16+12v:12.15+5v:4.986+39v:39.57+129v:130.8All look to be ok.Also the calibration signal generator (1KHz, 0.3v square wave) is working.My next step would be trying to locate and follow the troubleshooting flow chart to test everything else and report back here.

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:29:03 pm
content: images

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:30:10 pm
content: images batch 2

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:30:51 pm
content: images batch 3

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:31:50 pm
content: images batch 4

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:32:46 pm
content: images batch 5

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:33:50 pm
content: images batch 6

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:34:28 pm
content: images batch 7

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:35:01 pm
content: images batch 8

author: alextrical
author_rank: Contributor
timestamp: March 24, 2025, 08:38:15 pm
content: images batch 9.1.Sorry accidental duplication of image upload. I cant replace the image in the previous post due to 8mb file limit

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 24, 2025, 09:43:40 pm
content: HiIf it is a typical Iwatsu scope it will have the high value resistors in the CRT grid – cathode circuitry having gone high in value, BUT be aware there is some 3KV present on the grid and cathode.I have not repaired a SS-5705 but I have repaired a number of SS-5712 scopes and they ALL had problems with the high value resistors going high in value resulting in either a gradually fading trace or a very faint one to start with. You MUST use high voltage rated resistors such as the Vishay VR series as replacements.G Edmonds

author: metrologist
author_rank: Super Contributor
timestamp: March 25, 2025, 12:13:08 am
content: Cool! I really liked my Iwatsu SS-5711D. The flyback developed a leak so I sold it for cheap after failing to repair.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 25, 2025, 08:23:11 am
content: Hi againHaving now had a little time to look at the problem a number of questions come to mind.You say it is a SS-5705, I can only find a manual for a SS-5705A, the link to a service manual in your posting is to a SS-5711A, not the same. Most of the IWATSU SS scopes are somewhat similar around the CRT, BUT not similar enough to use the wrong service manual.When you have a faint trace can you extinguish it with the front panel Intensity controls?What does using the beam finder do?G Edmonds

author: alextrical
author_rank: Contributor
timestamp: March 25, 2025, 09:38:00 pm
content: Hi GeorgeThank you so much for your help.The Title was incorrectly typed, the unit is indeed a SS-5711.The control for Intensity can indeed make the trace fainter, to the point where its not visible.Beam finder shifts the beam to a dot on the screen if i remember correctly from testing, earlier. I will try and grab a photo of the behaviour in the morningThe high value resistors going out of specification sounds like a great place to start, can help track down the equivalent circuit on the SS-5711, presumably it would be page 142/the image attached.According to the manual R12 is 15M, R42 is 10M and R36 is 2.2M. Are these a good starting point to test (I assume they have to be out of circuit to attempt to test)

author: alextrical
author_rank: Contributor
timestamp: March 26, 2025, 09:29:50 am
content: In regards to the trace Intensity control, I would say that at minimum it is barely visible, and when turned to 20% its at its full brightness, any further turning has no effect on the brightness.(images taken with unit on its Right hand side)

author: alextrical
author_rank: Contributor
timestamp: March 26, 2025, 09:34:55 am
content: Quote from: George Edmonds on March 25, 2025, 08:23:11 amThe beam finder looks to be functional, the trace is condensed inside of the window, as per the images below. This is my first alanloge/real scope, so im not particularly familiar with how it should operate yet, please forgive my ignorance.I will get the 3 Mega ohm resistors tested and replacment Vishay VR resistors on order. While im making an order from a reputable source (CPC, Farnell, RS, Digikey or Mouser), is it woth replacing the electrolytic and other capacitors as a preventative issue at this stage while its on the workbench? If so any guidance on the main ones to focus on would be greatly appreciated.Ben J

author: ondraN
author_rank: Regular Contributor
timestamp: March 26, 2025, 09:50:42 am
content: This looks more like a CRT at the end of its life.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 26, 2025, 09:59:16 am
content: HiLet me think about your last postings, but it is almost looking like the CRT may have reached the end of it’s life. Have you any idea how much use this scope has had?G Edmonds

author: alextrical
author_rank: Contributor
timestamp: March 26, 2025, 12:03:39 pm
content: Unfortunatly I dont know the history of the machine, and dont have contact with the person i bought it from anymore.It is quite likeley that it has had a lot of use. I hadn't considered that the CRT was a part that could wear out (other than the filament burning out)Thats a shame if thats the most likeley culprit, What are the odds of me being able to source a replacement CRT? Is it even worth it?

author: ondraN
author_rank: Regular Contributor
timestamp: March 26, 2025, 01:19:45 pm
content: Perharps helphttps://groups.io/g/TekScopes/topic/restoring_crt_emission/81119509

author: alextrical
author_rank: Contributor
timestamp: March 26, 2025, 02:55:39 pm
content: Interesting concept. So essentially they apply a 1.5X times normal running voltage to the heater, and then apply an increasing voltage to the grid until it conducts at 40mA. If it works anywhere near like the links below imply, its worth a try. I dont have a variac, so would have to try another method to generate the 1kV DC, maybeone of thesehttps://www.aliexpress.com/item/32904763494.html?The question would also be how to read the High voltage and Low current without destroying my Multimeters, rated for ~600V. I guess a voltage divider would work as long as the resistors are correctly rated to handle the voltagehttps://boginjr.com/electronics/hv/crt-regen/http://www.ke5fx.com/crt.html

author: ondraN
author_rank: Regular Contributor
timestamp: March 27, 2025, 06:30:49 am
content: To work in the HV section of a CRT oscilloscope, an HV probe for a multimeter (usually 1:1000) is necessary. But it is not a quite cheap thing.

author: alextrical
author_rank: Contributor
timestamp: March 27, 2025, 09:41:43 am
content: As its only going to be used once or twice, This may be the method i try to go with to make a DIY probe.https://www.instructables.com/High-Voltage-Probes/

author: ondraN
author_rank: Regular Contributor
timestamp: March 27, 2025, 10:13:42 am
content: That is also a possible solution. I solved it by buying an old HV probe (it was very cheap) for another ancient voltmeter at a local bazaar and adapting a divider resistor for my multimeter.