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 Precedent model 459 EHT problem
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 Return to top of page · Post #: 16 · Written at 7:48:00 AM on 13 June 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Thanks for that. I had checked the amplitude before but not the frequency or bias. I will go over again more closely.


 
 Return to top of page · Post #: 17 · Written at 8:34:56 AM on 13 June 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 406

I used to find "Trigon" brand valves to be very hit and miss.
6CM5's and 6AL3's used to commonly go up in a shower of sparks from new.


 
 Return to top of page · Post #: 18 · Written at 5:49:47 PM on 8 July 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Ok, replaced CRT and tried a different EHT winding with no difference. Finally realised that with a very faint crosshatch visible, some of the LOPT must be working. Bridged the 47K resistor between Brightness and Contrast (already a variant from the diag,), a marked difference but still way too bright.
Booster voltage was too low. Disconnecting CRT grid 2 restored booster voltage and EHT
The diag shows a pot but this had a 4.7Meg to the 270V rail and a 2.2Meg to the booster rail.
Disconnecting the Booster rail fixed EHT problem. While trying to measure G2 voltage, the DMM made the picture dark almost perfect.
Connecting a 3.3 Meg to Eth (not Booster), picture is perfect with full control of Brightness and Contrast.
I know it's not right but it's working...with no smoke. 63V now on G2.

This was worrying me but then I found I had neglected to fit a brass collar with a small magnet ( ion trap?) .onto the neck. Doing that , I lost brightness.
Not sure if needed and return everything back and try again.


 
 Return to top of page · Post #: 19 · Written at 12:51:34 PM on 9 July 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 406

That CRT clearly doesn't need an ION trap. If it did it wouldn't have produced a raster without one.
You can usually tell just by looking at the gun assembly.
The one's that need an ION trap have an diagonal offset between two of the elements of the gun.


 
 Return to top of page · Post #: 20 · Written at 4:46:32 PM on 9 July 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Oh ok. The original tube's gun looked as though it had been damaged, gun at an angle etc. I will post a pic.
I replaced the CRT with one from an Astor HSQ chassis.
The collar is a brass clamp with one small magnet.
Anyway putting everything back they way it was and fitting the collar resulted in no raster, etc.
Putting the mods back, now ok.
Next problem, O/P from tuner but no further. ,( was injecting signal into video amp. to test before).


 
 Return to top of page · Post #: 21 · Written at 8:52:37 AM on 10 July 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Photos here of the gun from the original Precedent CRT and the clamp from around the base.
This isn't the original tube or gun. When I bought this the back was bowing out from the neck.
I tried adjusting CRT as close as possible to the front but still didn't fit.
The Astor CRT fits.

Precedent television screen
Precedent television screen


 
 Return to top of page · Post #: 22 · Written at 6:21:40 PM on 12 July 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6574

Photos uploaded to Post 21.


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

 
 Return to top of page · Post #: 23 · Written at 3:38:23 PM on 13 July 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 406

You can clearly see the offset on the gun assembly. The ION trap bends the electron beam back but the ION's being less affected by the ION trap magnet miss the phosphor altogether. The later phosphors were aluminized and were unaffected by ION's thus the gun assembly is straight.


 
 Return to top of page · Post #: 24 · Written at 7:34:59 PM on 13 July 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Thanks for that. I will keep this CRT aside for the future.


 
 Return to top of page · Post #: 25 · Written at 9:10:57 AM on 18 July 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Now with a very good clear picture although I don't know why the screen voltage has to be as low as 63V. I am running the input directly into the grid of the video amplifier.


 
 Return to top of page · Post #: 26 · Written at 9:41:44 PM on 20 July 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2013

Yes that is VERY low for the G2. The CRTs beam spot size will be much larger than optimum and beam will spread on highlights, degrading your picture. Try measuring it through a low pass filter of 100k in series with .1μF across the meter. That is likely to be more accurate, G2 might be carrying blanking pulses which will confuse your meter.

What is the voltage between G1 and K? It's possible that the issue is your direct connection to the video amp G1. The G1 voltage will not have the approx -3 volts from the vision detector and so the video amp will be biassed with the anode voltage much lower, which turns the CRT on harder. Maybe that's your problem.

You might be able to fix this by capacitively coupling the video to G1 and connecting a diode (e.g. 1N4148) from G1 to ground to act as a clamp. K (stripe) of the diode to GND. But a correctly designed video buffer amp is needed to do it properly. Best fix the IF and use a modulator IMHO.


 
 Return to top of page · Post #: 27 · Written at 1:10:57 PM on 2 December 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Brought this out again to test a recently purchased Dick Smith LOPT tester kit K7205. All coils test ok except the EHT reading bad. I don't know how accurate this would be.
Still a problem with the tuner. I can tune in to a point but no picture and low volume. Just going over the tuner now.
Tuner ok now. O/C 820K resistor to the grid of RF amp.


 
 Return to top of page · Post #: 28 · Written at 10:38:59 AM on 3 December 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 406

If an Line O/P transformer has a shorted turn you will get no EHT at all.
I'm sure many LOPT's get changed which aren't faulty and the fault is eventually found elsewhere.
Just to prove the point, on a working set, try wrapping a clip lead around the ferrite core of the LOPT and then connect the ends together.
EHT disappears completely and the clip lead usually starts to smoke.
You cant really have a partial shorted turn. It's either there or not.
What type of CRT did the set originally have?
Is the pinout on the non original CRT the same as the original?
Just a thought.


 
 Return to top of page · Post #: 29 · Written at 9:58:02 AM on 4 December 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

The cct. diagram shows the CRT as 21CBP4.
The substituted Astor shows as 21CBP4A.
I don't know what the A signifies.
The CRT it came with is not original or regunned as the neck is too long forcing the back of the cabinet out. It also required an ion trap.
Now the tuner is repaired the appears to be no difference between injected picture and tuned picture.
A s Ian Robertson said, it does have trouble with blooming on highllights.
The anode of the video output shows as 125V. I read 20V.
The resistor feed on the cct shows as 4K but there is three resistors in parallel testing at 12K.
I substituted 5K (2x10K parallel) but they get hot and no brightness.
It's getting a little beyond me.


 
 Return to top of page · Post #: 30 · Written at 10:38:38 AM on 4 December 2019.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 406

What happens to the voltage on the anode of the video o/p tube if the CRT socket is unplugged?
Maybe the CRT has a grid to cathode short or leakage or some other issues.
Do you have access to a CRT tester?
20V is way to low for the anode of the video o/p tube.
I once had a computer monitor CRT which had anode to g2 leakage which caused all sorts of brightness and focus anomalies.
Since I had nothing to lose I removed the CRT socket, shorted g2 to ground and switched on.
There was a fireworks display in the neck which lasted a few seconds.
Switched off and restored everything to normal.
Fired up again and the short was completely cleared never to return.


 
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