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Old 24th March 2014, 09:25 PM   #1
jblmar is offline jblmar  United States
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Default 6x5 tube

I'm not sure if this is the right forum, so Mods feel free to move this thread.
I know nothing about tubes, so bear with me
I have an Eico 221 VTVM. The transformer runs hot after only a few minutes on as well as giving off a strong odor. The 6x5 tube glows at the top and gets warm fast. Looking at the schematic, the tube is directly after the transformer winding. I'm not sure if the transformer or the tube is at fault.
Any suggestions.
Is it possible to check the tube without a tube tester?
I can post the schematic if it helps.
Thanks for looking.
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Old 24th March 2014, 09:59 PM   #2
DF96 is offline DF96  England
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Something drawing far too much current? The transformer and rectifier tubes may be innocent victims.
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Old 24th March 2014, 10:23 PM   #3
jblmar is offline jblmar  United States
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Thanks, DF96. Should I pull the tubes one by one and power up to start eliminating components in the circuit or start checking components in the circuit?
Is it normal for the 6x5 tube to light up on the top (apex) and heat up fast after a minute on or is this an indication that it's drawing too much current?
How can I check if the tube has an internal short w/o a tube tester? (if possible)
Shows how much (nothing) I know about tubes. I'll spend the evening Goggling tubes and how they work.
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Old 24th March 2014, 10:59 PM   #4
jblmar is offline jblmar  United States
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I pulled the 6x5 tube with the same results. The transformer heats up and makes a boiling sound as well as giving off a terrible odor. I'm beginning to think the transformer insulation has boiled off causing the windings to short. I may remove the transformer and check DCR as well as ac voltages.
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Old 25th March 2014, 12:40 AM   #5
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Before doing anything else, disconnect all the transformer's secondary leads from the sockets. Then wire up a standard 40 watt incandescent lamp in series with the transformer's primary. Upon power, the lamp should not light. If it does, the tranny is bad. If you still hear a smoldering sound, it's bad. DC resistance measurments may not tell you much unless you have previous numbers to compare to plus a special ohmmeter capable of making accurate very low resistance measurments. But a boiling & pungent smelling transformer is never a good sign.
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Old 25th March 2014, 01:19 AM   #6
jblmar is offline jblmar  United States
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Thanks, HollowState.
My guess is the transformer too. If it weren't, something would have smoked.

Here's the schematic to make things easier.
Attached Images
File Type: jpg eico221-3.jpg (54.7 KB, 43 views)
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Old 25th March 2014, 10:22 AM   #7
DF96 is offline DF96  England
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Shorted turns can't usually be detected by a DC resistance check. Do a lamp test as HollowState suggests.
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Old 25th March 2014, 12:58 PM   #8
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Some 6X5s are known to short H-K. Many Zenith's power transformers have died because of them.
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Old 25th March 2014, 01:12 PM   #9
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While the diagnosis of power trafo failure is probably correct, that may be a manifestation of an underlying problem. Electrolytic caps. in the PSU filter go bad, with age. It is inevitable. Old 'lytics may have taken the rectifier and power trafo down.

If the instrument is to be restored, make certain to replace ALL electrolytic caps., in addition to correcting the flaws detected. Don't send good money after bad.
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Old 26th March 2014, 12:53 AM   #10
jblmar is offline jblmar  United States
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Not worth the trouble or expense to repair this one.
I opened the transformer. The typical burned Smell.
Did the transformer fail or did it have help?
Like you say Eli Duttman, no need to spend $ on this one.
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