TEK465 - 10 MHz ripple - Page 1
Forum section: electronics / repair

author: francozappa
author_rank: Contributor
timestamp: May 11, 2020, 07:49:49 pm
content: Good evening everyone. I introduce myself. My name is Franco, I live in Italy and I just signed up for this forum.I own a used TEK465 oscilloscope for repair.The problem is the presence of a ripple at around 10 MHz which modulates the beam. The oscillation comes from the low voltage supply voltages. I have tried in various ways to eliminate it, without success.If anyone has had the same problem or has any idea of the cause, thank you in advance. I have the schematics and the service manual.Hi everyone.FZ

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2020, 09:27:16 pm
content: Quote from: francozappa on May 11, 2020, 07:49:49 pmWhat exactly did you tried? Did you replaced all the old electrolytic capacitors already?

author: Whales
author_rank: Super Contributor
timestamp: May 12, 2020, 12:42:42 am
content: How big is the 10MHz ripple? Can you provide a photo + info on what vertical settings you are using?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 12, 2020, 06:35:10 am
content: Do you have this problem with all the metal shields in place and the scope within its aluminium case?

author: francozappa
author_rank: Contributor
timestamp: May 17, 2020, 07:50:13 pm
content: Ciao !!I changed ALL the tantalum capacitors of the scope and electrolytics C1534, C1404 and C1220.

author: francozappa
author_rank: Contributor
timestamp: May 17, 2020, 07:56:00 pm
content: Ciao !!TIME/DIV is 5microsec/DIV and VOLT/DIV is 5 mV/DIV.I attach the photos of the single and double track.Thanks for the tips.

author: francozappa
author_rank: Contributor
timestamp: May 17, 2020, 07:57:26 pm
content: The problem is the same both with the oscilloscope in its metal container and outside.Ciao !!

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 17, 2020, 10:07:11 pm
content: Ciao Franco!First, get yourself some near-field-probes. These are great for hinting down gremlins like that. You can order them cheap from China.For now, do this:If you have access to another scope (you need a scope to fix a scope, yes), connect the ground crocodile clamp to the tip. Coil the lead into two windings, fix it with a rubber band. Now move the coil around the PCBs to pick up the source of the noise.The Signal seems to strong to be a harmonic. It also is around 1,2 MHz, not 10MHz. (6 peaks per 5microseconds = 1/(5microseconds/6).The only thing that generates such high frequencies in an analog scope should be the timebase circuit, so start there.

author: francozappa
author_rank: Contributor
timestamp: May 18, 2020, 06:31:56 am
content: Sorry, I was wrong, the time base was set at 0.05 microsec/DIV.I put photos of the actual situation.Sorry again.

author: francozappa
author_rank: Contributor
timestamp: May 18, 2020, 06:35:13 am
content: The VOLT / DIV knob does not count, the inputs are grounded.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2020, 07:17:14 am
content: Quote from: BreakingOhmsLaw on May 17, 2020, 10:07:11 pmNot always. If you have one working and one faulty channel, sometimes you can use the workign channel to debug the faulty channel.QuoteI'll bet that you could use that technique with the scope, and notice when the "noise" increased. Yes, you would have to mentally subtract the existing noise from the noise detected by the probe, but it would give an indication of the area.QuoteIf poorly setup internally, oscillations can occur. There are many many trimpots and trimcaps in a 465. It may be worth locating the area, then following the manual to twiddle things. I recommend putting a red mark on a trimmer before moving it, so that it can be moved back to the original position.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 18, 2020, 10:02:58 am
content: Try turning your internet router off. Turn off all your ethernet items.I hope that you (or your neighbour) does not have one of those room to room 'internet over the mains powerline' devices.

author: francozappa
author_rank: Contributor
timestamp: May 18, 2020, 10:17:10 am
content: 279/5000Thanks for the advice.I don't have internet devices on the electrical network, only wifi.I add that in the laboratory, attached to the same electrical network, there are two other oscilloscopes in addition to the 465 / Ripple: another identical 465 and a 466 and neither of them presents the problem.Ciao!

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 18, 2020, 08:29:31 pm
content: Ciao Franco,that gives a a noise signal of 120MHz.Please change the timebase and check if the noise is always 120MHz regardless of the setting of the timebase. Post results. We want to know, if changing the timebase affects the noise or the noise is contantly at 120MHz. This will help determine if it is caused by the horizontal deflection circuit or is coming from something else.

author: francozappa
author_rank: Contributor
timestamp: May 18, 2020, 08:58:41 pm
content: Hello and thanks for the reply.If I change the TIME/DIV time base the displayed tracks change in the sense that by decreasing the TIME/DIV the undulations are no longer distinguishable. However, I attach the photos of the signal detected with another oscilloscope on the TP+5V, TP+15V and TP-8V.Changing the time base TIME/DIV does not change the disturbanceFoto1: TP+5/1 microSEC/DIV; FOTO2: TP+5/0,05 microSEC/DIV; FOTO3: TP-8/0,05 microSEC/DIV. VOLT/DIV = 10 mV/DIV.Ciao !!FZ

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 18, 2020, 09:11:24 pm
content: Quote from: francozappa on May 18, 2020, 08:58:41 pmuse the X10 MAG button

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 18, 2020, 09:25:50 pm
content: Wait, if you can detect the noise on the power rails, it's easy to narrow it down. Disconnect the variuous boards from the power supply (start with A7 timing board) and see which one causes the oscillation.

author: francozappa
author_rank: Contributor
timestamp: May 19, 2020, 06:07:20 am
content: Quote from: BreakingOhmsLaw on May 18, 2020, 09:25:50 pmI get it.So I disconnected the -8V power supply from the A7/TIMING BOARD board (removed the dummy resistor near J2) and the noise disappears. What can we deduce from you?

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 20, 2020, 05:54:18 am
content: Well, something on that board is injection noise into the power rails. Try again if any button affects the noise. And i mean ALL, including all the VAR settings on the timebase, magnification buttons. Try everything. Probe around on that board to see which part of the circuit is causing it. Check around the high speed timing caps C6015 and C6048.

author: francozappa
author_rank: Contributor
timestamp: May 20, 2020, 07:47:41 am
content: Quote from: BreakingOhmsLaw on May 20, 2020, 05:54:18 amThank you so much for the advice.Today I can do the tests as soon as I can, then I'll let you know.Ciao !!

author: francozappa
author_rank: Contributor
timestamp: May 20, 2020, 08:48:12 am
content: Quote from: BreakingOhmsLaw on May 20, 2020, 05:54:18 amThe abbreviations of the capacitors you have indicated to me are not in the TIMING BOARD. I believe we are looking at different versions of the scheme. My scope has S/N 704952 .My manual is "465 Oscilloscope with options SERVICE S/N 250000 & UP". So far it seems to me that it corresponds to the instrument. However, I attach the photos of the TIMING BOARD wiring diagram and the wiring.

author: francozappa
author_rank: Contributor
timestamp: May 20, 2020, 10:11:45 pm
content: Trials done tonight: the noise changes only by pressing MIX+AUTO.With MIX+AUTO pressed two things happen:1. the noise pulsates in amplitude with the horizontal frequencyThis can be seen on the power rails and in other points of the circuit (eg VR1099 and VR1039).2. the beam is always off the screen horizontally.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 21, 2020, 07:45:20 pm
content: The A and B timebase knob is connected to a long enclosed cam switch on the A7 board, which is very difficult to access.If the A and B timebase switch contacts do not close completely, this leaves the following amplification circuits with an aerial, which can cause parasitic oscillations.Usually this only happens on a few timebase settings (not on all the timebase settings).To test this theory you need to insert a stout plastic ruler behind the A7 PCB and gently force the A7 PCB towards the cam switch.The objective being to bring the cam switch closer to the PCB, on which it is mounted.Hopefully the cam switch contacts will close and the oscillation will disappear? Of course I could be completely wrong.

author: francozappa
author_rank: Contributor
timestamp: May 21, 2020, 08:07:59 pm
content: Thanks for the tip, in my case the noise occurs in all the positions of the TIME/DIV knob, it seems unlikely that all the contacts are open, also because the time base works, in the sense that by rotating the knob the scan speed changes.I think I have to concentrate on the fact that pressing the MIX+AUTO keys the noise pulsate in amplitude with the horizontal frequency and the beam leaves the screen. This means that the potential difference applied at rest on the horizontal plates is different from zero. But this depends on what .....

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 22, 2020, 09:03:46 am
content: Can you show us a screenshot of that behaviour?

author: wolf22
author_rank: Newbie
timestamp: May 22, 2020, 10:46:09 am
content: Quote from: francozappa on May 21, 2020, 08:07:59 pmI had a nearly similar problem with my TEK2465 - I do know, that this is a different device. But the actual failure was surprising: There was one leg of a rather big electrolytic capacitor on the power supply board rotten away at the point, where it came out of the capacitor. So your problem may be similar.regards

author: francozappa
author_rank: Contributor
timestamp: May 22, 2020, 08:48:07 pm
content: Quote from: wolf22 on May 22, 2020, 10:46:09 amThanks for the tip. I will check carefully. We hope to find something.However I try to post a video to show what happens.

author: francozappa
author_rank: Contributor
timestamp: May 22, 2020, 09:06:54 pm
content: Quote from: BreakingOhmsLaw on May 22, 2020, 09:03:46 amCiao.I do not know how to do. I made a short video, but I can't upload it.With a screenshot, nothing is seen.

author: francozappa
author_rank: Contributor
timestamp: May 23, 2020, 07:01:47 am
content: Quote from: BreakingOhmsLaw on May 22, 2020, 09:03:46 amCiao.If you want to give me an email address to send the video I made to, otherwise I wouldn't know how.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: May 23, 2020, 08:39:55 am
content: Dump it on YouTube