Zeners in series...
in [Repair]
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From: whit3rd on 12 Mar 2010 14:15 On Mar 12, 5:30 am, s...(a)repairfaq.org (Samuel M. Goldwasser) wrote: > Sansui Samari <jimjam1...(a)gmail.com> writes: > > On Mar 12, 2:30 am, "Nox" <a...(a)def.ghi...ecc...ecc...ecc...> wrote: > > > Hey there, could somebody take a quick look at this circuit and try to > > > explain me what's the purpose of the 2 zeners near the interrogative point ? > > Maybe they found two zeners that would split the voltage fine,... > A varactor diode there makes even less sense. They probably needed a 270 V > zener as protection against excessive voltage at that location and there > was no single zener that was 270 and adequate power. C603 and the other > component upstream of it limit the current. If the little blobs at crossing points are to be trusted, C603 isn't in series there, and it doesn't limit the current. Winding resistance and rectifier series resistance are all I see on this winding leg. It appears that the whole rectifier/zener assembly is there to limit AC excursions of that leg of the transformer winding (the OTHER leg of the transformer winding has the high impedance).
From: swohlfarth on 13 Mar 2010 11:39 On Mar 12, 1:15 pm, whit3rd <whit...(a)gmail.com> wrote: > On Mar 12, 5:30 am, s...(a)repairfaq.org (Samuel M. Goldwasser) wrote: > > > Sansui Samari <jimjam1...(a)gmail.com> writes: > > > On Mar 12, 2:30 am, "Nox" <a...(a)def.ghi...ecc...ecc...ecc...> wrote: > > > > Hey there, could somebody take a quick look at this circuit and try to > > > > explain me what's the purpose of the 2 zeners near the interrogative point ? > > > Maybe they found two zeners that would split the voltage fine,... > > A varactor diode there makes even less sense. They probably needed a 270 V > > zener as protection against excessive voltage at that location and there > > was no single zener that was 270 and adequate power. C603 and the other > > component upstream of it limit the current. > > If the little blobs at crossing points are to be trusted, C603 isn't > in series there, > and it doesn't limit the current. Winding resistance and rectifier > series > resistance are all I see on this winding leg. > > It appears that the whole rectifier/zener assembly is there to limit > AC excursions > of that leg of the transformer winding (the OTHER leg of the > transformer > winding has the high impedance). The two zeners in series are there to protect TR607 and C604 in case the HV regulator circuit fails (or line voltage is excessive) and TR607 goes into cutoff where HV DC would appear across its C-E terminals. They clamp the DC to 270V. Two diodes were probably used since a single 270V device is harder to find or to split up the power dissipation. Under normal operation, they do not carry any current. TR607 is probably a 300-400V device. The high voltage for the CRT cathode (-1kV) is sensed through R631 and fed to error amplifier TR608. TR608 then controls TR607 inside the DC loop of bridge rectifier MR3. This allows the transistor to control (and regulate) the AC voltage delivered to rectifier strings D616-D618 and D621-623. The +3kV is then cross-regulated since it derived from the same AC voltage as the -1kV. This allows brightness and focus to be stable under varying AC line voltage. -Scott
From: Nox on 13 Mar 2010 15:41
<swohlfarth(a)gmail.com> ha scritto nel messaggio news:0b1f6556-cc35-43cd-832a-ab0555732ffc(a)z11g2000yqz.googlegroups.com... > >The high voltage for the CRT cathode (-1kV) is sensed through R631 and >fed to error amplifier TR608. TR608 then controls TR607 inside the DC >loop of bridge rectifier MR3. This allows the transistor to control >(and regulate) the AC voltage delivered to rectifier strings D616-D618 >and D621-623. The +3kV is then cross-regulated since it derived from >the same AC voltage as the -1kV. This allows brightness and focus to >be stable under varying AC line voltage. Thanks everyone for the kind explanations! :D |