RCA 46" TV P46725SK won't turn on
in [Repair]
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From: Leonard Caillouet on 9 Jul 2005 19:28 What is the problem in the power supply that causes the HV divider to fail? I have fixed many of these and while we routinely go through the PS I have not ben aware of the PS causing the divider to fail. Leonard <dkuhajda(a)locl.net> wrote in message news:1120944451.316954.211540(a)z14g2000cwz.googlegroups.com... > And repair the main smps power supply which is the typical cause of the > hv block failure. > Do not forget the hv splitter rubber insulation boots that are on all > the wires. >
From: dkuhajda on 9 Jul 2005 21:09 High B+ regulation in standby. The margin for error is very small on the hv in the splitter. As the capacitors fail in the power supply, the standby voltage will drift over time up to as high as 165V, versus the 140~145V that is suppose to be there. Since that is a 15% increase in initial B+ at start up, which is beyond the 10% margin for the insulation in the hv block, the hv is too high initially at start up and this starts internal arcing at start up. Over time these carbon tracks finally cascade and the hard carbon arc short is started. The problem arises in that the regulator kicks into run mode in about one second after turn on. The regulator is then running off the horizontal feedback pulses for regulation. Since this happens so fast and is only slightly above the x-ray trip voltage, the x-ray circuit never triggers. Each turn on with standby B+ running high will cause an arc over. The big problem we found was that if the smps was not repaired and was the cause of the failure, it most times would take 6 to 9 months before we would get a call back on the set being dead. So shop policy had to be verify the smps secondary voltages on ALL ctc169 sets, cr4118 and cr4116 must be in proper range and clean looking on the o'scope. Extra 5 minutes of work to guarantee no future problems. The cr4118 failure can also cause the B+ to run high as it starts to go leaky and fail. However I have never seen it fail to the point that the B+ will run over 150V before the 15V line was too low for the set to attempt to start up. It is mainly the 2.2uF capacitor on the standby feedback line of the smps that goes open that causes this problem. It is a simple matter to measure the standby B+ voltage on the cathode of cr4116 with the tv turned off to verify that there is not a problem.
From: Leonard Caillouet on 10 Jul 2005 06:35 Interesting. I never noticed the standby B+ going that high. I'll have to check more closely. We have several routine checks on the 169 chassis on any set that comes in that includes the caps in the power supply so the get changed if they are bad. I often don't check the standby B+ until after the repair is done so I had not noticed that. We do, however, change lots of HV dividers in sets where the PS caps are fine. In those cases the B+ is checked and I have not seen it out of range. I guess the small margin of safety on the divider is the problem. I have also changed quite a few that were dne by other shops previously and the grommets were missing or the lead was not inserted correctly. So the OP should be careful to install it correctly or be prepared to change it again before long. You wold think that they would not start the horizontal osc. until B+ is regulated. But then, you would think a critical part like a high voltage splitter would be built with a greater safety factor. Thanks, David. Leonard <dkuhajda(a)locl.net> wrote in message news:1120957767.594663.21340(a)g14g2000cwa.googlegroups.com... > High B+ regulation in standby. > The margin for error is very small on the hv in the splitter. > As the capacitors fail in the power supply, the standby voltage will > drift over time up to as high as 165V, versus the 140~145V that is > suppose to be there. > Since that is a 15% increase in initial B+ at start up, which is beyond > the 10% margin for the insulation in the hv block, the hv is too high > initially at start up and this starts internal arcing at start up. > Over time these carbon tracks finally cascade and the hard carbon arc > short is started. The problem arises in that the regulator kicks into > run mode in about one second after turn on. The regulator is then > running off the horizontal feedback pulses for regulation. Since this > happens so fast and is only slightly above the x-ray trip voltage, the > x-ray circuit never triggers. Each turn on with standby B+ running > high will cause an arc over. The big problem we found was that if the > smps was not repaired and was the cause of the failure, it most times > would take 6 to 9 months before we would get a call back on the set > being dead. So shop policy had to be verify the smps secondary > voltages on ALL ctc169 sets, cr4118 and cr4116 must be in proper range > and clean looking on the o'scope. Extra 5 minutes of work to guarantee > no future problems. > > The cr4118 failure can also cause the B+ to run high as it starts to go > leaky and fail. However I have never seen it fail to the point that > the B+ will run over 150V before the 15V line was too low for the set > to attempt to start up. > > It is mainly the 2.2uF capacitor on the standby feedback line of the > smps that goes open that causes this problem. It is a simple matter to > measure the standby B+ voltage on the cathode of cr4116 with the tv > turned off to verify that there is not a problem. >
From: Art on 10 Jul 2005 07:24 I have noticed a few of these chassis having the increased standy B+ and have serviced the chopper, regulator circuit before returning then to service. It is an interesting question tho, did the HV block fail because of the increased B+ or is it just co-incidence? Due to the age of these sets the caps are going bad anyway and need replacing. Good point David! "Leonard Caillouet" <lcaillonospam(a)devoynet.com> wrote in message news:BR6Ae.31654$up5.29048(a)lakeread02... > Interesting. I never noticed the standby B+ going that high. I'll have > to check more closely. We have several routine checks on the 169 chassis > on any set that comes in that includes the caps in the power supply so the > get changed if they are bad. I often don't check the standby B+ until > after the repair is done so I had not noticed that. We do, however, > change lots of HV dividers in sets where the PS caps are fine. In those > cases the B+ is checked and I have not seen it out of range. I guess the > small margin of safety on the divider is the problem. I have also changed > quite a few that were dne by other shops previously and the grommets were > missing or the lead was not inserted correctly. So the OP should be > careful to install it correctly or be prepared to change it again before > long. > > You wold think that they would not start the horizontal osc. until B+ is > regulated. But then, you would think a critical part like a high voltage > splitter would be built with a greater safety factor. > > Thanks, David. > > Leonard > > <dkuhajda(a)locl.net> wrote in message > news:1120957767.594663.21340(a)g14g2000cwa.googlegroups.com... >> High B+ regulation in standby. >> The margin for error is very small on the hv in the splitter. >> As the capacitors fail in the power supply, the standby voltage will >> drift over time up to as high as 165V, versus the 140~145V that is >> suppose to be there. >> Since that is a 15% increase in initial B+ at start up, which is beyond >> the 10% margin for the insulation in the hv block, the hv is too high >> initially at start up and this starts internal arcing at start up. >> Over time these carbon tracks finally cascade and the hard carbon arc >> short is started. The problem arises in that the regulator kicks into >> run mode in about one second after turn on. The regulator is then >> running off the horizontal feedback pulses for regulation. Since this >> happens so fast and is only slightly above the x-ray trip voltage, the >> x-ray circuit never triggers. Each turn on with standby B+ running >> high will cause an arc over. The big problem we found was that if the >> smps was not repaired and was the cause of the failure, it most times >> would take 6 to 9 months before we would get a call back on the set >> being dead. So shop policy had to be verify the smps secondary >> voltages on ALL ctc169 sets, cr4118 and cr4116 must be in proper range >> and clean looking on the o'scope. Extra 5 minutes of work to guarantee >> no future problems. >> >> The cr4118 failure can also cause the B+ to run high as it starts to go >> leaky and fail. However I have never seen it fail to the point that >> the B+ will run over 150V before the 15V line was too low for the set >> to attempt to start up. >> >> It is mainly the 2.2uF capacitor on the standby feedback line of the >> smps that goes open that causes this problem. It is a simple matter to >> measure the standby B+ voltage on the cathode of cr4116 with the tv >> turned off to verify that there is not a problem. >> > >
From: dkuhajda on 10 Jul 2005 12:10
Coincidence due to age? Maybe. What I find mostly is the cap will be marginal. After the set is plugged in a while it all warms up and the B+ seems to be close enough to the ideal 143V. But when the set is cold, the B+ is higher. Typical bad capacitor problem. A 10% increase in B+ on start up is a 10% increase in HV, 28,000 volts is quite a lot less than 30,800 volts. They do have the unit regulating at start up. The standby rough regulator feedback is doing the regulating, when it fails to regulate on the money, there is nothing that will keep the set from coming on, until it goes so high that there is a catastophic failure. So as always, check/replace the capacitors in the smps and cr4118 everytime. |