Another Amplifone "help" thread..... Is this tube toast?

kiphartzell

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Another Amplifone "help" thread..... Is this tube toast?

I have been working on a 25" Amplifone set up for my SW and seem to be having a high voltage problem. Both the Deflection and HV cards have been rebuilt/capped using Michael's FAQ. The HV board is Biege with a Penn Tran HVT (with proper Diodes and 50ohm resistors). The BU406D, voltage regulators and caps are new and the Wx was replaced with wire. The Deflection seems to be working as I hear the typical sounds. The problem is no HV (or at least when the anode cup is attached to the tube. The Penn Tran rings fine with a LOBT blue tester. A couple of observations, the "heater" color is purple/lavender in the tube neck. Is this normal? I may have a blue gun shorted as I only see 2.3ohm between pin 10&11 (I get a 1 ohm reading between pins 9 & 10). Besides this potential gun short. Here is the real odd thing. I get no HV when the anode cup is attached to the tube (I do have +/-24v), but I get 19+kv when I pull the anode cup off the tube and measure it with the HV probe. So the HVT is generating the HV, but something is shorted that does allow the tube to work.

Is my tube toast? Will a rejuvenater fix this type of issue? I just picked up a B&K 490 (still waiting for it to arrive) to attempt to fix the blue gun, but have doubts that I can get this tube working with this other HV anode issue.
 
Thanks ArcRevival for PM with some suggestions. I'll check the connectivity of the DAG spring. It is plugged in, but may be getting a poor connection. The over protection LED is not lit, but I'll check that the LED is not bad or something. Being a Penn Tran HVT from what I read, it would be unlikely that the over-protection is triggering. Also, I have a second HV card with one of Chad's HVTs that blows the +24v VR in a few seconds when connected to this tube.

Anyone know if a B&K Restoring/Rejuvenater checks for a Focus-HV short in the tube? Again, I am looking for reasons why the HV will not energize when the Anode cup is connected to the tube. Thanks.
 
Personally... my number one concern would be the tube itself.
Namely because of the purple/lavender color in the neck tube you described. I've only seen that on any tube when it has been cracked/lost vacuum.

Does it appear to be "arcing" at all in the neck or is it just a stable solid color ?

When you get your crt tester that'll help check lots of things.

And just to clarify -
I get no HV when the anode cup is attached to the tube (I do have +/-24v), but I get 19+kv when I pull the anode cup off the tube and measure it with the HV probe.

So you've actually stuck your HV probe under the anode cap while it's running and attached to the tube?
(didn't know if you meant "no HV" as in it doesn't fire up/static, etc)
 
Yes, I have probed the Anode Cap when it was attached to the tube and got roughly 0.630v (it was registering a small/minimal voltage). Probe was zero with power off. I was using a fluke 80k-40 with a fluke 77.

Also, the purple color could be arcing but is constant in nature.

With all this said, I talked to Scotty at Hawk-Eye Picture Tube Company (nice guy) and he seemed to think the tube was busted.

Novice question; how do you tell if the tube is still under vacuum or not?

Also, my B&K 490 should arrive tomorrow to confirm things.
 
Hey Kip. If the tube is physically cracked, moisture inevitably finds its way into the tube and appears as light (or dark) spots very much like watermarks on the phosphorous. If it is recently cracked, it may take a while to appear. This will be completely different from the dark tiny spots from HV burn.
 
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