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 Precedent model 459 EHT problem
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 Return to top of page · Post #: 31 · Written at 4:51:42 PM on 6 December 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2013

If there is no input signal, there is no negative voltage at the video output G1. Hence, the anode WILL go down to 20v because the 6CK6 or 6DX8 is being turned full on. That's why the anode load is getting so hot.

Flashovers in CRTs are not at all unusual, especially if the tube has been moved around with the neck down. Stuff left behind in the big jug finds its way into the neck.

The original CRT in that Precedent would have been a 21CBP4A with NO ion trap.
The 21ALP4(A) CRT is an older design that has an ion trap type gun. These were originally intended for non-aluminised screens.

The -A suffix on a CRT normally (but not always) means an aluminised screen.

How to test the CRT? Here's a post I made a while back. No special gear needed!!


 Return to top of page · Post #: 32 · Written at 12:31:23 PM on 10 December 2019.
Arcadus's avatar
 Location: Nildottie, SA
 Member since 7 April 2018
 Member #: 2236
 Postcount: 43

Both CRTs were tested with a Sencore tester, TOK.
Tried testing voltage using DMM at anode 2nd IF amp. but that kills the signal.
"Video Test" point ( input to video amp) looks ok but is high at 20V.
Perhaps I should check all components passively in this area.

 Return to top of page · Post #: 33 · Written at 4:59:21 PM on 19 December 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2013

Are you putting a valid signal into the set?

Voltages around the IF and video are for a strong signal input.

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