TV's EHT wants to jump
in [Repair]
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From: frank on 5 Nov 2009 11:10 Hi all, this little TV set (14" by philips, more than 20 years old) started some months ago to hiss, it was mainly a little EHT leakage from the junction between the LOPT and the ultor cable. It was annoying but my parents didn't care at the time (I'm not a TV user). It had no other visible symptoms, so I think the line final transistor (little BU508) was holding well. Since some days, the EHT started to spark violently from the cable to ultor junction and the set was shutting down, so I couldn't ignore the problem anymore, my parents want to watch the TV! I opened the set, cleaned 20 years of dust, redo many solder joints that were just looking suspicious, put an heavy amount of epoxy in the leaking junction on the EHT transformer and some teflon band over the epoxy and did the same on the cable to suction cup junction. I also checked the +125V line (this is primary supply on the output transformer that then goes to the BU508 collector) and it was well in specs, however lowered to 124.6V which is still in specs and gives a little less EHT. Ok, all this effort cured the leakage over the transformer but after a good amount of epoxy, the juntion near the ultor cup was still sparking (this started again after one day from the first "cure"). I re-opened the set and put a good amount of hot glue over the epoxy and it's holding again now. It's normal for 25 kV to jump over some 4-5 mm of epoxy or I'd better check if the EHT is way over its normal value for this set? Yes, I have a 1000:1 probe but I never used it on a TV set, it's just for oscilloscope repairs (where voltages are much less than that). Also I can't really see any other malfunction on the set, so I wonder.... Any expert out here? How do you measure the EHT on a TV set exactly (if this is something doable in the first place)? TIA Francesco IZ8DWF (who watches tv only from the back of the set and not very often)
From: ian field on 5 Nov 2009 13:02 "frank" <frank(a)ottone.venere.it> wrote in message news:4af2f913$0$690$5fc30a8(a)news.tiscali.it... > Hi all, > > this little TV set (14" by philips, more than 20 years old) started some > months ago to hiss, it was mainly a little EHT leakage from the junction > between the LOPT and the ultor cable. It was annoying but my parents > didn't care at the time (I'm not a TV user). It had no other visible > symptoms, so I think the line final transistor (little BU508) was > holding well. Since some days, the EHT started to spark violently from > the cable to ultor junction and the set was shutting down, so I couldn't > ignore the problem anymore, my parents want to watch the TV! > I opened the set, cleaned 20 years of dust, redo many solder joints that > were just looking suspicious, put an heavy amount of epoxy in the > leaking junction on the EHT transformer and some teflon band over the > epoxy and did the same on the cable to suction cup junction. > I also checked the +125V line (this is primary supply on the output > transformer that then goes to the BU508 collector) and it was well in > specs, however lowered to 124.6V which is still in specs and gives a > little less EHT. > Ok, all this effort cured the leakage over the transformer but after a > good amount of epoxy, the juntion near the ultor cup was still sparking > (this started again after one day from the first "cure"). I re-opened > the set and put a good amount of hot glue over the epoxy and it's > holding again now. > It's normal for 25 kV to jump over some 4-5 mm of epoxy or I'd better > check if the EHT is way over its normal value for this set? Yes, I have > a 1000:1 probe but I never used it on a TV set, it's just for > oscilloscope repairs (where voltages are much less than that). Also I > can't really see any other malfunction on the set, so I wonder.... > Any expert out here? How do you measure the EHT on a TV set exactly (if > this is something doable in the first place)? > > TIA > > Francesco IZ8DWF (who watches tv only from the back of the set and not > very often) A failed flyback tuning capacitor can cause excessive peak pulse voltage.
From: hr(bob) hofmann on 6 Nov 2009 15:50 On Nov 5, 10:10 am, frank <fr...(a)ottone.venere.it> wrote: > Hi all, > > this little TV set (14" by philips, more than 20 years old) started some > months ago to hiss, it was mainly a little EHT leakage from the junction > between the LOPT and the ultor cable. It was annoying but my parents > didn't care at the time (I'm not a TV user). It had no other visible > symptoms, so I think the line final transistor (little BU508) was > holding well. Since some days, the EHT started to spark violently from > the cable to ultor junction and the set was shutting down, so I couldn't > ignore the problem anymore, my parents want to watch the TV! > I opened the set, cleaned 20 years of dust, redo many solder joints that > were just looking suspicious, put an heavy amount of epoxy in the > leaking junction on the EHT transformer and some teflon band over the > epoxy and did the same on the cable to suction cup junction. > I also checked the +125V line (this is primary supply on the output > transformer that then goes to the BU508 collector) and it was well in > specs, however lowered to 124.6V which is still in specs and gives a > little less EHT. > Ok, all this effort cured the leakage over the transformer but after a > good amount of epoxy, the juntion near the ultor cup was still sparking > (this started again after one day from the first "cure"). I re-opened > the set and put a good amount of hot glue over the epoxy and it's > holding again now. > It's normal for 25 kV to jump over some 4-5 mm of epoxy or I'd better > check if the EHT is way over its normal value for this set? Yes, I have > a 1000:1 probe but I never used it on a TV set, it's just for > oscilloscope repairs (where voltages are much less than that). Also I > can't really see any other malfunction on the set, so I wonder.... > Any expert out here? How do you measure the EHT on a TV set exactly (if > this is something doable in the first place)? > > TIA > > Francesco IZ8DWF (who watches tv only from the back of the set and not > very often) The 1000;1 probe can be checked on a known OK voltage before you use it on the ??able tv set. If the high voltage - what we call it on the western side of the big pond - were too high, it would accelerate the electron beam too fast and the picture would be reduced in size, the opposite effect of "blooming" when the high voltage is too low.
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