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Old 1st February 2013, 11:43 PM   #21
JMFahey is offline JMFahey  Argentina
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Sorry but your wording is confusing.
We have been talking about the Power transformer and you agreed, now it's about the OT?
Ih well.
Don't mix both in the same phratse.
All tests , including measuring primary current, disconnecting secondaries rever to the PT.

So, forget everything and let's re-start from square 1.
You say you hear hum with amp powered but standby off?
You say you lift the OT from the chassis and the hum stops?
You hear it through the speakers or is it some mechanical chassis vibration?
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Old 2nd February 2013, 12:11 AM   #22
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I'm not real sure what you said on post # 12. The valid current leakage test is, one lead of the AC voltmeter to chassis (presumably same voltage as transformer frame, but depends on the washers) the other lead of the AC voltmeter to safety ground (wall plug round pin in USA). This puts the meter in parallel with the 10k resistor and capacitor. If you're reading 30 mvAC, then you don't have a current leakage problem- yet. A shorted turn will eventually cause one. If I want to test a power transformer for shorted turn, I load it down with power resistors sufficient to sink the rated current. This is with the transformer disconnected from any amplifier load. Then I leave it like that for a while. If the voltage across the resistors is proper for the designed current , and the transformer doesn't get too hot after an hour or two, it doesn't have a shorted turn. I did this with some surplus transformers I bought; some were okay, some not. Got the load resistors at a surplus house, too , conveniently. Have the resistor logs on brackets of a steel piece that holds them above the coffee table. Had the transformer under test in an aluminum pie pan on top of a piece of wood, on the coffee table.
If I want to test the whole amp, including output transformers, I take the 8 ohm 225 watt resistors, put them on the amp, insert music to get output up to rated AC voltage, leave it that way for an hour or two. This proves the power transformer, rectifier tube, output tubes, and output transformers. Bad output transformer will be low voltage out on load resistor with proper b+ voltage going into the output tubes; plus good AC swing going into the transformer. Never had one, it was always the first 3 things wrong on the ST70. Warning, most DVM's don't measure music AC worth a ****. My Simpson 266 VOM seems to reflect music performance quite accurately. 17.5 VAC on 8 ohms is my ST70 rating.
I've got some 5 ohm 200 watt resistors to move my load test procedure into the transistor age. $5 each surplus.
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Last edited by indianajo; 2nd February 2013 at 12:20 AM.
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Old 2nd February 2013, 09:03 PM   #23
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Maybe I worded some things oddly but I did do the current leakage test correctly by placing the meter in parallel with the 10k/.02uf tester. I was just stating that when I had placed only one lead of the meter on the chassis my meter would read 20VAC, which I thought was strange.

"So, forget everything and let's re-start from square 1.
You say you hear hum with amp powered but standby off?
You say you lift the OT from the chassis and the hum stops?
You hear it through the speakers or is it some mechanical chassis vibration?"

I hear a hum through the speaker while the amp is IN Standby mode (no B+)

When the output transformer is lifted from chassis the noise through the speaker goes away.

I hear it through the speakers yes, and you can also hear the mechanical noise coming from the power transformer too. The power transformer causes the chassis to vibrate.

Last edited by famousmockingbird; 2nd February 2013 at 09:33 PM.
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Old 2nd February 2013, 11:22 PM   #24
Tajzmaj is offline Tajzmaj  Slovenia
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Just few thoughts. This amp is good factory amp. It means it was properly working without any hum anything. All theories how transformer should be isolated from the chassis and so on don't make any sense. Of course not for testing purposes.
What I think happened is :
There has been some old tubes installed and idle current was way too high. Probably there were all anodes red hot....
Than transformer overheated. My guess would be primary. Some turns maybe even whole layer come in to short circuit. That cause 2 things. First one huge current draw even at idle and second core saturation (lower number of primary turns because of short). Because of core saturation there is huge stress magnetic field present and part of it is than received by output transformer and result is hum which is 60Hz. If you place power transformer closer to the output one you will probably hear stronger hum. We can discuss now how that nasty magnetic field travels around more through air or more through steel chassis....maybe someone would even suggest to insulate output transformer with piece of wood and place it in mumetal shield and even what Ni/Fe ratio is best for magnetic shield.
But I think your power transformer is fried. You need new one or someone to rewind it. And I also suggest check output tubes and negative grid bias circuit. At least some -35V should be present. Of course if there is such idle regulation. Than inspect all coupling capacitors from phase splitter to first grids. I think this amp has 4 output tubes so there should be 4 capacitors. Also inspect psu. Rectifier circuit and smoothing capacitors. All those things could cause power transformer to fail. I've probably forgotten something but I'm sure other guys will give some other possible reasons...
Cheers
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Old 3rd February 2013, 12:30 AM   #25
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Quote:
Originally Posted by M Gregg View Post
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!

Regards
M. Gregg

I didn't remove the Power transformer yet but it is grounded to the chassis. So I lifted the safety ground from chassis and placed my meter in series, I measured 20VAC.



Quote:
Originally Posted by Tajzmaj View Post
Just few thoughts. This amp is good factory amp. It means it was properly working without any hum anything. All theories how transformer should be isolated from the chassis and so on don't make any sense. Of course not for testing purposes.
What I think happened is :
There has been some old tubes installed and idle current was way too high. Probably there were all anodes red hot....
Than transformer overheated. My guess would be primary. Some turns maybe even whole layer come in to short circuit. That cause 2 things. First one huge current draw even at idle and second core saturation (lower number of primary turns because of short). Because of core saturation there is huge stress magnetic field present and part of it is than received by output transformer and result is hum which is 60Hz. If you place power transformer closer to the output one you will probably hear stronger hum. We can discuss now how that nasty magnetic field travels around more through air or more through steel chassis....maybe someone would even suggest to insulate output transformer with piece of wood and place it in mumetal shield and even what Ni/Fe ratio is best for magnetic shield.
But I think your power transformer is fried. You need new one or someone to rewind it. And I also suggest check output tubes and negative grid bias circuit. At least some -35V should be present. Of course if there is such idle regulation. Than inspect all coupling capacitors from phase splitter to first grids. I think this amp has 4 output tubes so there should be 4 capacitors. Also inspect psu. Rectifier circuit and smoothing capacitors. All those things could cause power transformer to fail. I've probably forgotten something but I'm sure other guys will give some other possible reasons...
Cheers

I think you are right about the magnetic field being injected into the output transformer, the closer I bring the output transformer to power transformer the noise gets louder, and when I move it further away the noise gets quieter.

I talked to my buddy and he just wants me to replace the power transformer. I am going to order a Mercury Magnetic VXP-100 and hopefully it will cure the noise issue.

Thanks for all the wisdom shared
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Old 3rd February 2013, 09:30 AM   #26
Tajzmaj is offline Tajzmaj  Slovenia
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Hi. One more thing. Again. Please take advice and check output tubes and those components which could be the reason for such malfunction. Transformer is expensive thing and it's waist to burn another one. I checked right now and schematic is available. There are only few other really critical components to inspect besides 4 output tubes.
Cheers
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Old 3rd February 2013, 10:15 AM   #27
M Gregg is offline M Gregg  United Kingdom
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Something to try,

With the ouput Tx in its normal position....Remove and replace the pins and nuts through the power transformer end bells /laminations test it with each one removed. See if it makes a difference..

ie shorted turn...if you have a pin/bolt through the laminations that is in contact with both sides of the end bells of the Tx it acts like a piece of wire..if you have a second connection either via the chassis to the end bells it acts like another piece of wire so now you have a circuit(through the laminations to the end bells and back via the chassis..this acts like a single turn secondary winding in the magnetic field...it draws high current and will produce a higher magnetic field.
it can also over heat the power Tx..its worth a try before you spend on a new power Tx. It can also happen with two pins through the laminations providing the short rather than the chassis and one pin..


Regards
M. Gregg
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Old 3rd February 2013, 09:01 PM   #28
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Quote:
Originally Posted by Tajzmaj View Post
Hi. One more thing. Again. Please take advice and check output tubes and those components which could be the reason for such malfunction. Transformer is expensive thing and it's waist to burn another one. I checked right now and schematic is available. There are only few other really critical components to inspect besides 4 output tubes.
Cheers
I thnk one of the tubes went into thermal ruanaway because the 100R 5W resistor for one of the power tubes was burnt open. All the tube sockets were corroded, the power tubes socket pin holders were loose so they got tightened and were cleaned with deoxit. I replaced the 5W 100R resistors with non inductive wire wounds. Power supply caps had been replaced previous but the EL34's were running at 500v and they had a JJ can rated 500v for the power supply reservour cap. I used two 220uf 350v in series with balance resistors to replace the JJ can. The phase inverter power node cap was rated at 450v but running at 470v so that got upgraded to 500v to be safe.

So yes I do believe that when that plate resistor on the EL34 popped due to too much current it probably shorted a turn in the power transformer. I just talked to my buddy again last night and he confirmed that when he was playing it something happened and all of a sudden it sounded worse and had a noise through speaker, that is when he shut it off and called me. I thought it was just a bad tube or whatever but I tested the siemens EL34's on my hickok and they were fine. So then because the way the sockets looked I thought it lost it's bias supply (negative voltage was getting to the sockets but the pin holders were loose and corroded) so I cleaned and tightened them. After all the repairs there was still this hum through speaker on standby and that was were the real fun began

I pulled all tubes and from there have been isolating and testing the transformers and well you guys know the rest.............
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Old 4th February 2013, 09:48 PM   #29
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Ok so no luck borrowing a variac but I did some more testing today after work. I have the output transformer on a chair hooked up to the speaker so I removed the power transformer from the chassis and placed it next to the output transformer on the same chair (made from wood). I wired up the power transformer and left the secondaries taped up floating. When powered up you can still hear the noise through the speaker and when I move the output transformer closer to power transformer the noise gets louder and as I move away it gets quieter.

I also measured the outside of the power transformer to my wall outlet's ground and I get 125VAC. I suspect that this thing is dangerous
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Old 4th February 2013, 09:56 PM   #30
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I also wanted to add that I had the primary side of the power transformer hooked up to a dim bulb tester and there was only 107VAC on the primaries. The reading from chassis of power transformer to earth of 125VAC I am assuming is a HV secondary problem.
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