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Old 1st February 2013, 12:29 PM   #11
M Gregg is offline M Gregg  United Kingdom
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The other thing could be,
The transformer is creating a shorted turn via one pin through the laminations and the paint is no longer isolating the end bells from the chassis. ie a shorted turn.

This is assuming that the Tx is not primary or secondary shorted to the laminations (it would be strange but not impossible if the former has melted).
So assuming the power tx is not electrically live, test by isolating the power Tx from chassis and test (AC voltage) with a meter from earth to transformer end bells and laminations. If its dead then the screws through the Tx laminations should be isolated from the end bells with fibre washers or plastic insulators...

Where is the chassis main Earth attached?

As already mentiond take care the Tx laminations or end bells could be live!

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M. Gregg
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Last edited by M Gregg; 1st February 2013 at 12:39 PM.
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Old 1st February 2013, 04:41 PM   #12
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Ok so I took a 10k resistor and placed a .02uf cap (i didn't have a .01) in parallel. I then lifted the safety ground from chassis and placed the 10k/.02uf in series with the chassis and the safety ground. With the amp on I placed one test lead on the chassis and my meter will read anywhere from 12VAC-20VAC. When I placed the other test lead to the earth side of the 10k/.02uf I get 30mVAC.
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Old 1st February 2013, 05:04 PM   #13
Tajzmaj is offline Tajzmaj  Slovenia
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One more thing. If this amplifier has been working before and than started to buzz than probably something kicked the bucket. That something is from your description probably transformer. If this is some diy or vintage, long not working machine, than or transformer isn't for right primary voltage or is destroyed somehow. Again. I suggest test with variac, A and V meter. Could you tell us more about this amp? Brand? Maybe some pictures? Are there any signs of overheating or oxidation? Please tell us more and maybe someone could help you.
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Old 1st February 2013, 05:18 PM   #14
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Quote:
Originally Posted by Tajzmaj View Post
One more thing. If this amplifier has been working before and than started to buzz than probably something kicked the bucket. That something is from your description probably transformer. If this is some diy or vintage, long not working machine, than or transformer isn't for right primary voltage or is destroyed somehow. Again. I suggest test with variac, A and V meter. Could you tell us more about this amp? Brand? Maybe some pictures? Are there any signs of overheating or oxidation? Please tell us more and maybe someone could help you.
Cheers
I don't have a variac, but I do have a transformer for an old American Flyer Train, I think it goes from 5-15VAC. If I was pulling half and amp at 120v than maybe there will be too much current for the train transformer to use for testing.

The amp is a Vox AC100 and I didn't add that information for the sake of forum nazi's moving the thread to "musical instruments". No offense to the guys over in the "musical instruments" section but I like this sub forum better. Anyway the amp gets used and hasn't been sitting for a long time.

I can post pictures but not now for my camera is being borrowed and my phone camera kind of stinks.

Last edited by famousmockingbird; 1st February 2013 at 05:22 PM.
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Old 1st February 2013, 05:32 PM   #15
Tajzmaj is offline Tajzmaj  Slovenia
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15vac won't be enough.
I suspect that idle current of this amp was too high for longer period probably because tubes were worn out. It cause secondary to overheat and than make internal short. Such case make very high transformer idle current. Of course it can be also destroyed by water.....beer.... If you can try to borrow variac.
Cheers
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Old 1st February 2013, 06:21 PM   #16
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I will try and get my hands on a variac for further testing. Also why am I reading 20Vac on the chassis with just one lead? Usually I see dead transformers and not ones that are in between.


Thanks for your help Tajzmaj and everyone else who is helping me too thank you all.
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Old 1st February 2013, 06:53 PM   #17
Tajzmaj is offline Tajzmaj  Slovenia
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You are welcome. Be good.
Chees
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Old 1st February 2013, 07:07 PM   #18
JMFahey is offline JMFahey  Argentina
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If transformer hums while on chassis but not away from it, complete the test:
1) place a piece of wood on the chassis and sit the transformer over it, so there's, say, from 1/2" to 1" separation.
Hum??
2) now replace wood with something still insulating, but physically thin, such as a glossy magazine cover.
Hum??
3) now with the transformer still floating, touch its metallic body (laminations or cover) with a piece of wire, other end grounded to chassis to a point close to where the original transformer sits.
Be careful, there might be a voltage difference between transformer frame and chassis.
Hum??
4) same as 3) , but now measure AC voltage between frame and chassis, obviously without the grounding wire.
Voltage?
5) disconnect *all* secondaries, still 0.5A primary current?
6) leave it on, still secondaries floating, check temperature for a couple hours.
Warm is fine; unbearably hot is not.
Post results.
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Old 1st February 2013, 07:30 PM   #19
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"If transformer hums while on chassis but not away from it, complete the test:"

I haven't removed the power transformer yet but as stated earlier lifting the output transformer from the chassis removes the noise.

"5) disconnect *all* secondaries, still 0.5A primary current?"

I have done this and yes there is still .5A primary current.


I will remove the power transformer and report back my findings.
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Old 1st February 2013, 09:47 PM   #20
M Gregg is offline M Gregg  United Kingdom
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Post #11,

Do you have a shorted turn? This is a magnetic shorted turn, not a shorted turn on the windings?

Regards
M. Gregg
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