Diagnosing a dead LOPT? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Chris55000 · 21st Apr 2020, 10:22 pm

Hi!

Regarding valve issues in this set, failure of the PL504 line output valve tends to be either o/c heaters due to heater/cathode shorts further down the heater chain, inadequate width that can't be brought out enough to fill the sides of CRT, or running into grid current as a result of being run too long with little or no drive!

The PY88 boost diode can suffer o/c heaters, again from faults further down the heater chain, internal flashovers due to overheating (again, lack of drive) or simply a long time to reach full emission, showing up as inadequate width that gradually fills the screen when it warms up.

Faults in the PCL805/85 are usually associated with the pentode output section, the triode tends to cause wandering field–hold problems in field TB oscillator circuits!

LOPT failure is usually one of:-

a) Very weak or no spark at the DY802 anode, PL504 anode and PY88 cathode top–caps with little or no added boost h.t. volts, often 300V or less;

b) Short–circuits between windings causing the PY88 to overheat very badly as a result of it being shunted across the main h.t. line by the short – this sometimes only occurs after the PY88 warms up!

Don't forget the 33n boost reservoir capacitor C615 between the sections of the LOPT can fail, either open– or short–circuit, open circuits cause lack of width and e.h.t. with "ballooning" – picture expands very rapidly and disappears when the brightness control is advanced, a short–circuit one will badly overheat the PY88! If you need to replace the boost reservoir capacitor use one rated at 1500V d.c. working!

The two 2M2 resistors R865 & R872 from the boost h.t. point to the width preset no. 9 can go high in value, causing lack of width with the preset at maximum available setting!

Hope these fault finding hints help you get your set going again!

Chris Williams

Chris55000 · 22nd Apr 2020, 12:58 pm

PS!

The DY802 can also cause "ballooning" and fading out of the picture as the brightness control is advanced, failure to light due to the free end of the heater shorting to the cathode/internal anti–corona shield, loss of vacuum/air ingress causing internal fireworks or a blue haze inside as soon as the PY88 warms up and the stage starts to operate, and sometimes arcing spots/Corona spots (random bursts of white spots across the screen) caused by imperfect welded connections that arc across internally, and sometimes total loss of e.h.t. with the valve apparently glowing normally – all of these faults can be proved by simply fitting a replacement!

The DY802 is externally treated to reduce Corona discharge across the glass envelope, you can use a DY86/DY87 if your home environment is reasonably warm, dry and damp–free!

Chris Williams

Kyle__B · 22nd Apr 2020, 10:13 pm

Chris, thanks, that's a bible you've written there, i'll save a copy for the future.

The BF195 came in the post today and it was the cure. It's working perfectly again and the suspect valves are in the glass recycling box.

The only minor thing is that to replace this transistor I've had the chassis out, all valves unplugged so that I could flip it upside down and solder it. Putting it all back together, there's crackling sounds coming from somewhere around B407 (another PCL805) for a couple of seconds when it's first turned on.

This doesn't change the behavior of the telly in any way, so I think I probably just disturbed an oxidized contact. It'd make sense, since it stops while it's warming up.
Thanks all

Kyle__B · 22nd Apr 2020, 11:44 pm

Actually one last thing, when you say overheating PY88, are they normally supposed to be this bright?

Because it works and I think it's always been a very bright tube the whole time I've owned this set but I don't really know. I've never owned another to compare with.

Kyle__B · 23rd Apr 2020, 7:10 pm

Ho Hum talking to myself again I suppose

Kyle__B · 27th Apr 2020, 12:56 am

Should anyone read this/feel like replying

I just folded up the chassis into it's normal position, put the back panel on... and the humming from the speaker in my first thread came back.

Took the back off, swung the chassis down, humming went away.

tried it again. Put it back up, hum, down, no hum. Up, hum, down, no hum.

figured alright, this is a constant volume hum that isn't effected by the signal source nor is it effected by volume control, so the amplifier section must be picking up something when it's in the upright position.

I experiment with that. I folded the chassis down again, take the speaker out and, while it's plugged in, wave it about over everything and especially the audio valves. Doesn't pick up any hum. I did the same with the front panel controls, no effect.

I even, as a hail mary, turn off and unplug everything remotely around the TV in case some of it's beaming interference into it which can only be picked up when the chassis is at a certain angle. No dice.

So somehow the chassis being too close to the picture tube/neck board/yoke makes the amplifier pick up 50Hz.

Have any of you ever seen anything like this? I mean, I know I have a "cure" but I don't think leaving the back off and the chassis hanging out forever is a practical long term solution.

I'm almost tempted to put a resistor in series with the speaker, on the reasoning that I could then just turn the volume up a bit and drown it.

Sideband · 27th Apr 2020, 9:18 pm

Check lead dressing....that is to say, make sure there are no long leads draped across the circuit when in the 'up' position. An example on one of my own TV's......this particular set of mine had two main PCB's....timebases on one side, signals on the other side. When the PCB was in the 'up' position a loud buzz was present on sound. The audio from the volume control was coupled with a fairly long lead to the audio output valve and whilst working on the set, the lead had become dislodged from its clips along the top of the chassis and it was draping across video and line circuitry when the chassis was 'up'. Clipping the lead back and checking that other leads were also secure, cured the fault.

Check also that video leads around the CRT base are not draping across the audio stages etc.

Kyle__B · 27th Apr 2020, 10:22 pm

That could be very hard because the CRT neck board goes through a hole in the chassis when it's closed up. The neck wires have no other choice but to rest on the chassis.

I've messed about with all the other wires and they're dressed about as well as they could possibly be. Not any different from any of the pictures of this set online.

21st Apr 2020, 10:22 pm	#21
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,873	Re: Diagnosing a dead LOPT? Hi! Regarding valve issues in this set, failure of the PL504 line output valve tends to be either o/c heaters due to heater/cathode shorts further down the heater chain, inadequate width that can't be brought out enough to fill the sides of CRT, or running into grid current as a result of being run too long with little or no drive! The PY88 boost diode can suffer o/c heaters, again from faults further down the heater chain, internal flashovers due to overheating (again, lack of drive) or simply a long time to reach full emission, showing up as inadequate width that gradually fills the screen when it warms up. Faults in the PCL805/85 are usually associated with the pentode output section, the triode tends to cause wandering field–hold problems in field TB oscillator circuits! LOPT failure is usually one of:- a) Very weak or no spark at the DY802 anode, PL504 anode and PY88 cathode top–caps with little or no added boost h.t. volts, often 300V or less; b) Short–circuits between windings causing the PY88 to overheat very badly as a result of it being shunted across the main h.t. line by the short – this sometimes only occurs after the PY88 warms up! Don't forget the 33n boost reservoir capacitor C615 between the sections of the LOPT can fail, either open– or short–circuit, open circuits cause lack of width and e.h.t. with "ballooning" – picture expands very rapidly and disappears when the brightness control is advanced, a short–circuit one will badly overheat the PY88! If you need to replace the boost reservoir capacitor use one rated at 1500V d.c. working! The two 2M2 resistors R865 & R872 from the boost h.t. point to the width preset no. 9 can go high in value, causing lack of width with the preset at maximum available setting! Hope these fault finding hints help you get your set going again! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 21st Apr 2020 at 10:33 pm.

22nd Apr 2020, 12:58 pm	#22
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,873	Re: Diagnosing a dead LOPT? PS! The DY802 can also cause "ballooning" and fading out of the picture as the brightness control is advanced, failure to light due to the free end of the heater shorting to the cathode/internal anti–corona shield, loss of vacuum/air ingress causing internal fireworks or a blue haze inside as soon as the PY88 warms up and the stage starts to operate, and sometimes arcing spots/Corona spots (random bursts of white spots across the screen) caused by imperfect welded connections that arc across internally, and sometimes total loss of e.h.t. with the valve apparently glowing normally – all of these faults can be proved by simply fitting a replacement! The DY802 is externally treated to reduce Corona discharge across the glass envelope, you can use a DY86/DY87 if your home environment is reasonably warm, dry and damp–free! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

22nd Apr 2020, 10:13 pm	#23
Kyle__B Banned Join Date: Mar 2020 Location: Nijmegen, Netherlands Posts: 142	Re: Diagnosing a dead LOPT? Chris, thanks, that's a bible you've written there, i'll save a copy for the future. The BF195 came in the post today and it was the cure. It's working perfectly again and the suspect valves are in the glass recycling box. The only minor thing is that to replace this transistor I've had the chassis out, all valves unplugged so that I could flip it upside down and solder it. Putting it all back together, there's crackling sounds coming from somewhere around B407 (another PCL805) for a couple of seconds when it's first turned on. This doesn't change the behavior of the telly in any way, so I think I probably just disturbed an oxidized contact. It'd make sense, since it stops while it's warming up. Thanks all

22nd Apr 2020, 11:44 pm	#24
Kyle__B Banned Join Date: Mar 2020 Location: Nijmegen, Netherlands Posts: 142	Re: Diagnosing a dead LOPT? Actually one last thing, when you say overheating PY88, are they normally supposed to be this bright? Because it works and I think it's always been a very bright tube the whole time I've owned this set but I don't really know. I've never owned another to compare with. Attached Thumbnails

23rd Apr 2020, 7:10 pm	#25
Kyle__B Banned Join Date: Mar 2020 Location: Nijmegen, Netherlands Posts: 142	Re: Diagnosing a dead LOPT? Ho Hum talking to myself again I suppose

27th Apr 2020, 12:56 am	#26
Kyle__B Banned Join Date: Mar 2020 Location: Nijmegen, Netherlands Posts: 142	Re: Diagnosing a dead LOPT? Should anyone read this/feel like replying I just folded up the chassis into it's normal position, put the back panel on... and the humming from the speaker in my first thread came back. Took the back off, swung the chassis down, humming went away. tried it again. Put it back up, hum, down, no hum. Up, hum, down, no hum. figured alright, this is a constant volume hum that isn't effected by the signal source nor is it effected by volume control, so the amplifier section must be picking up something when it's in the upright position. I experiment with that. I folded the chassis down again, take the speaker out and, while it's plugged in, wave it about over everything and especially the audio valves. Doesn't pick up any hum. I did the same with the front panel controls, no effect. I even, as a hail mary, turn off and unplug everything remotely around the TV in case some of it's beaming interference into it which can only be picked up when the chassis is at a certain angle. No dice. So somehow the chassis being too close to the picture tube/neck board/yoke makes the amplifier pick up 50Hz. Have any of you ever seen anything like this? I mean, I know I have a "cure" but I don't think leaving the back off and the chassis hanging out forever is a practical long term solution. I'm almost tempted to put a resistor in series with the speaker, on the reasoning that I could then just turn the volume up a bit and drown it.

27th Apr 2020, 9:18 pm	#27
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,577	Re: Diagnosing a dead LOPT? Check lead dressing....that is to say, make sure there are no long leads draped across the circuit when in the 'up' position. An example on one of my own TV's......this particular set of mine had two main PCB's....timebases on one side, signals on the other side. When the PCB was in the 'up' position a loud buzz was present on sound. The audio from the volume control was coupled with a fairly long lead to the audio output valve and whilst working on the set, the lead had become dislodged from its clips along the top of the chassis and it was draping across video and line circuitry when the chassis was 'up'. Clipping the lead back and checking that other leads were also secure, cured the fault. Check also that video leads around the CRT base are not draping across the audio stages etc. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

27th Apr 2020, 10:22 pm	#28
Kyle__B Banned Join Date: Mar 2020 Location: Nijmegen, Netherlands Posts: 142	Re: Diagnosing a dead LOPT? That could be very hard because the CRT neck board goes through a hole in the chassis when it's closed up. The neck wires have no other choice but to rest on the chassis. I've messed about with all the other wires and they're dressed about as well as they could possibly be. Not any different from any of the pictures of this set online.