Diode leakage detection ??   - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 11, 2011, 09:03:55 pm
content: It supposed that diodes operating on a high frequency circuitry like for example the horizontal reflection coils on analogue TV sets,that they could start having a leakage, that possibly it could cause excessive ripple.Funny enough this specific circuitry , it can simulate the classic analogue TV set like to was an oscilloscope.Because any damage in this specific circuitry it is visible on the CRT screen as interaction.Is there any special tester or practical way to detect such leakage signs ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 12, 2011, 12:59:35 am
content: I have never heard of a diode failing by increasing recovery time. If it does (in a power circuit), the heat dissipation would likely cause complete failure.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 16, 2011, 04:08:00 am
content: Are you referring to the embedded diodes in the EHT winding of the Horizontal Output transformer,or the single diode/capacitor setup driven from the overwind on this transformer,which produces (around)+150v to supply the vertical & audio output circuits?If there is anything wrong with this latter circuit,you usually either lose,or have reduced,vertical deflection.It also effects the horizontal scans,causing waveform distortion.I have had to replace the diode on a few occasions(It must be replaced with a fast-recovery diode with similar characteristics to the original),but in most cases,it has been a faulty filter capacitor.Again,you must replace the cap with the same type--a low ESR type.If it is a problem with the embedded diodes,it is time for a new Horizontal Output transformer.You should be able to determine what problem you have using your Oscilloscope,especially it you have another good unit to compare with.VK6ZGO

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 16, 2011, 05:26:21 am
content: Well you look expert on the subject and so I have an small quiz for you ..I am knocking my head on the wall for 25 days or so, so to cure the problem on my B&O ,related to geometry changes due the higher and lower brilliance .Actually at very low brilliance the problem looks to move away,but there is no static movies in this world, and so the changes is there all the time.Check the pictures about the effect.What I did so far , tested and replaced some caps on the primary PSU,and so I have stable 150V ( actually 151.8V) .About the fly-back transformer I did replaced most of the capacitors at the 1KV after the BU508.My actual problem was that other than the waveform distortion, the horizontal was larger than what it should be,and I had to use an smaller capacitor at the filter LC after the BU508 so to force it to shrink, originally was there one 300n/250v,and I replaced by a 280n/250V.It did shrink a bit , and came in to one acceptable level.Currently I am totally confused.The diodes that I was speaking , are also after the BU508, and somewhere I read that could be the issue.The diodes in the fly-back transformer could also be the problem, but in this scenario, you have to replace the fly-back.Last but not list, it could be the TDA 1940 oscillator..I had replace all the caps around with new, but I did not see any cure.Check and the schematic and if you have any ideas be my guest.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 16, 2011, 06:33:44 am
content: Well,I haven't done this stuff for quite a few years,& it was mainly on Sonys then!The patterning on your screen is in the same sense as the video signal--in other words,one half cycle makes the screen brighter,& the other half cycle goes towards black,so it looks like it is riding on the video.I suggest you have a look at the video output to your CRT.If you can see the HF patterning on that signal,have a look further back in the video chain.If it is only on the output stage,check for it on the dc supply to that stage.If it appears on previous stages,check their dc supplies.From my reading of your circuit,it looks as if the overwind on the Horiz o/p transformer provides two supplies (+&- 12v),instead of the +150v that Sony used.If these supplies go to the video circuit,they are possible suspects.If the derived suppies are used to supply early stages of the deflection circuits,& they are low in voltage,or poorly regulated,the EHT will fall in voltage,making the beam easier to deflect,hence too wide picture.Hope this helps--as I said,it's been quite a while!VK6ZGO

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 16, 2011, 04:48:02 pm
content: Thanks for the tips.I will inspect this +/- 12V.

author: zaoka
author_rank: Frequent Contributor
timestamp: September 21, 2011, 11:57:27 am
content: You have bad flyback transformer. Its nothing to do with power supply as long as 150V is stable.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 21, 2011, 03:46:43 pm
content: Quote from: zaoka on September 21, 2011, 11:57:27 amTrue , but I had contact the B&O main service in Greece, and they told me that this model was not having issues with the flyback,it was like telling to me that those flyback are immortal ..The positive part of the story are that the specific flyback works also for SABA/ Nordmende.And so its not that hard to be found, the replacement part could be the HR6356.I would wish to be able to find even a used one so to test it, but this is a long shot.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 22, 2011, 02:58:46 am
content: Quote from: zaoka on September 21, 2011, 11:57:27 amYou misread my original comment.In the Sonys I worked on,the +150 volt supply was derived from an overwind on the Horizontal output transformer.If the filter cap is faulty,a DMM may still read it as a slightly low value,but "stable".In this case it affects the vertical scans,& also the EHT regulation.From the circuit shown,the B&O doesn't have the derived +150v supply,but seems to have a +12v & -12V supply derived in the same way.A check with an Oscilloscope at these points will show whether there is a problem.I would not trust a DMM for this test,but if you do use one,& it reads slightly low,try the meter on the AC range,If you get any reading,you probably have a faulty filter cap.As I said before,Most of my experience has been with Sony TVs----B&O may do things differently.VK6ZGO