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 Precedent model 459 EHT problem
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 Return to top of page · Post #: 1 · Written at 4:19:20 PM on 31 May 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 31

Recapped this only to find EHT is 7kV.
Picture is blooming but can see scan lines. Full brightness with controls having negligible affect.
The windings aren't O/C .
I assume LOPT is faulty. Would there be anything else to check?. CRT checks OK.

Unknown TV Screen
Unknown TV Screen


 
 Return to top of page · Post #: 2 · Written at 4:46:28 AM on 1 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

Assume you tried the 1S2 EHT rectifier?

If LOPT is bad, the HV secondary winding (that's the honeycomb wound disc) will get hot.

What is the B+Boost voltage? That's at the junction of R131 and C102. Should be about 570 volts.


 
 Return to top of page · Post #: 3 · Written at 3:48:13 PM on 1 June 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 31

I tried another 1S2 and 1S2A, ( don't know the difference), but still the same.
The disc gets warm but not that hot although I haven't run it that long. DC resistance is 250Ohm.
The B+ is a little high at 586V.
The grid on the 6CM5 is supposed to be 97.5V but is 126V. The 6K resistor was burnt and o/c.
I have tried replacing 6CM5 and 6AL3.
I can see evidence of someone else tracking through this. The 1Meg resistor to CRT G4 was disconnected.
R47 220K (brightness) was bridged for 150K.
Incidentally, the brightness cct. differs from the diagram.
The power switches Eth to the wiper of Contrast with a 47K between Contrast and Brightness pot.


 
 Return to top of page · Post #: 4 · Written at 11:35:13 PM on 1 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

EHT winding should not get hot at all. How long did you run it?
Slightly high B+Boost is OK, means LOPT primary side, 6CM5 etc is OK. Probably means LOPTx is OK too.
1M resistor connects the focus. Once you have a picture, connect it to wherever gives the sharpest focus of the scanning lines.
Can you see a glow inside the 1S2? (The -A just means a specially treated glass is used)
Should be a reddish -orange. If not, check the 1.5 ohm resistor in the 1S2 base. It may have gone high and so under-running the filament.


 
 Return to top of page · Post #: 5 · Written at 8:39:27 AM on 2 June 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5500

Photos uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 6 · Written at 4:37:32 PM on 2 June 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 31

Thanks Ian and Brad. I will check that area again. Will be a few days as I am back on shiftwork.


 
 Return to top of page · Post #: 7 · Written at 12:19:44 AM on 3 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

Move those yoke wires well away from the 1S2! That could be your problem.


 
 Return to top of page · Post #: 8 · Written at 10:31:56 AM on 3 June 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 570

Just a thought prompted by Ians remark about EHT leaking.
The EHT assembly looks pretty clean, but how about unbolting the thing and checking for tracking UNDERNEATH?
The 1S2 socket could be dead short (eht wise) to the chassis.
It only takes megohm leakage and poof goes your EHT.
Fred.


 
 Return to top of page · Post #: 9 · Written at 1:59:43 PM on 3 June 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 192

Try having a good look and listen in a darkened room for signs of arcing.
Look for the tell tale smell of Ozone also.
Another possible solution would be to obtain a stick rectifier 1S2 replacement.
That Line O/P transformer looks like a replacement. Is it wired correctly?


 
 Return to top of page · Post #: 10 · Written at 12:07:19 AM on 4 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

That transformer looks like an original Precedent part to me.

I don't like selenium stick rectifiers. They allow the EHT to appear before the rest of the set is ready, 1S2s take a few seconds to warm up and allow the raster to shrink in in a way that is more pleasing to watch.


 
 Return to top of page · Post #: 11 · Written at 11:58:18 AM on 4 June 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 192

Have a look at the soldering on the transformer lugs. It doesn't look original.
As far as the stick rectifier goes I only meant it as a diagnostic tool. I too have an aversion to using them.


 
 Return to top of page · Post #: 12 · Written at 8:39:20 AM on 5 June 2019.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1074

Going through old issues of Television magazine. A case of low EHT was due to an open-circuit boost capacitor.


 
 Return to top of page · Post #: 13 · Written at 3:53:09 PM on 8 June 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 31

Thanks everyone for your suggestions. I have checked them all.
The EHT winding isn't getting warm just radiant heat from surrounding valves.
Not sure which one is the boost capacitor but have replaced all and rechecked values.
Turning H Hold right up raises EHT, turning it right down kills the EHT.
I followed the H osc. adjustment at the bottom of the diagram but shorting pin 1 of V5b resulted in a 3 second lightning show within and destruction of the 6 CM5.
Replacing it and the EHT is slightly higher at 8KV with the B+ dropping about 10V.
That's it for now


 
 Return to top of page · Post #: 14 · Written at 2:21:10 PM on 12 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

6CM5s of a certain brand and vintage were known to do that.
Try getting a valid signal into it and locking the hor osc to the correct frequency.


 
 Return to top of page · Post #: 15 · Written at 5:00:23 AM on 13 June 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 570

I would be scoping the horizontal section, checking the drive to the 6CM5 grid.
Compare to the circuit diagram values or similar models.
That would establish whether the drive amplitude and the waveshape is sufficient to swing the 6CM5 and narrow the fault type down.
That is drive is good/bad or 6CM5 is weak/overloaded by coil fault.
Assuming the 6CM5 HT rails and the heater voltage are normal range.
Fred..


 
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