Omega Owners Forum
Chat Area => General Discussion Area => Topic started by: Pitchfork on 27 January 2009, 20:26:31
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Can anyone suggest a company that can repair a mains transformer?
It's from an brand new amplifier & the primary winding is open circuit.
The output windings are 27-0-27 & 17-0-17 but I cannot find a suitable replacement anywhere with dual secondaries.
BTW its a conventional chassis type type not torroidal.
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My sons transformers just turn back into trucks and repair themselves :y
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My sons transformers just turn back into trucks and repair themselves :y
I was about to say ask Optimus Prime.... :D
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transfrormers... robots in disguise :D
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Unlikely to be repeirable at all as mass produced units have welded E/I sections.
Whats it off?
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If the amp is Brand New then surely it will be under warranty ?
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I had a transformer fail in a mixer. Like yours it had dual secondaries. Problem was there was no markings on it so I had to so it the hard way of counting the windings. But eventually I worked out the voltages.
The main problem I had was finding a replacement. I ended up having to use two separate transformers to provide each of the outputs the original dual secondaries provided. One was toroidal, the other chassis.
In the end I got it working.
For a like for like replacement you might have to go to a dedicated audio spares place, but like previously said check the warranty first.
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It was bought as B grade therefore no warranty
An 'own brand' made in china for the retailer, no idea of who the actual manufacturer was & the retailer will not tell me
Looks like I'll need to put 2 separate transformers in to overcome the dual secondaries problem
I may try to dismantle in case the open circuit primary fault is near the beginning of the winding. Shortening the length a little would not make a difference to the performance.
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Problem is, the primary will invariably be the innermost of the windings making access difficult.
Depends how neat a job it needs to be. First thoughts would be to replace with 2 transformers, as you say. The likelihood is that most of the power will come from one of the windings, meaning the second set of voltages can be provided by a very small transformer (likely feeding preamp or driver stages).
If you need to do a much neater job two thoughts come to mind:
1) There are companies who will wind one-off transformers to your specifications for not too much money, but obviously more than an off-the-shelf one.
2) Someone (RS?) used to sell bare transformers with just a primary winding so you could wind a secondary to your specifications.
I just bought a couple of toroidals from Rapid electronics for a Hi-Fi DAC I've just built and they have a very wide range, reasonable prices and they look to be nice quality transformers.
Kevin
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Correct the 27-0-27 is rated at 1.4A for the power amp
The 17-0-17 for the preamp @100mA so would be a lot smaller if I go for the 2 transformer solution (which is most probable)
I could even simply drop the higher rails down once DC with some wire-wound resistors & bypass the preamp rectifiers (That's what every other manufacturer does anyway e.g. Marshall, Carlsbro even my beloved Sessions!)
BTW never mention RS in front of me! I work for Premier Farnell ;)
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BTW never mention RS in front of me! I work for Premier Farnell ;)
LOL ;D
Kevin
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Correct the 27-0-27 is rated at 1.4A for the power amp
The 17-0-17 for the preamp @100mA so would be a lot smaller if I go for the 2 transformer solution (which is most probable)
I could even simply drop the higher rails down once DC with some wire-wound resistors & bypass the preamp rectifiers (That's what every other manufacturer does anyway e.g. Marshall, Carlsbro even my beloved Sessions!)
BTW never mention RS in front of me! I work for Premier Farnell ;)
Ok, so we are talking about a 100VA ish transfomer.....so wind your own!
I would not drop the rails using resistors and particularly not wire wounds!
Marshall, Carlsbro and all other session amps are, to be quite frank, piss poor hifi type setups......
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Correct the 27-0-27 is rated at 1.4A for the power amp
The 17-0-17 for the preamp @100mA so would be a lot smaller if I go for the 2 transformer solution (which is most probable)
I could even simply drop the higher rails down once DC with some wire-wound resistors & bypass the preamp rectifiers (That's what every other manufacturer does anyway e.g. Marshall, Carlsbro even my beloved Sessions!)
BTW never mention RS in front of me! I work for Premier Farnell ;)
Ok, so we are talking about a 100VA ish transfomer.....so wind your own!
I would not drop the rails using resistors and particularly not wire wounds!
Marshall, Carlsbro and all other session amps are, to be quite frank, piss poor hifi type setups......
Who said anything about Hi-Fi?
I'm a musician with a 5K PA Rig!
Guitar & PA amp manufactureres all use ceramic WW resistors to lower the voltages for pre-amp stages as they all use BiFet op amps somewhere in the circuits that run at +- 15v :)
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Yes, which means two things
1) The opamp supply rails will follow and be affected by the music transients.
2) The opamp supply rails now have significant inductance (due to the use of wire wounds) which will affect the dynamics!
It just highlights how cheap the setups are in relaity (and mega over priced to you and me!)
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Yes, which means two things
1) The opamp supply rails will follow and be affected by the music transients.
2) The opamp supply rails now have significant inductance (due to the use of wire wounds) which will affect the dynamics!
It just highlights how cheap the setups are in relaity (and mega over priced to you and me!)
Therefore the original design (which I admit is good, as is the build quality- apart from the transformer obviously) with 2 secondaries is the way to go.
Would the inductance of the transformer secondary also have a detrimental effect?
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I've just dismantled the transformer & there is an open circuit thermal fuse rated at 135 deg C in series with primary. I suspect that it may have been faulty as the amplifier had not been used.
Question: Do all transformers have a similar device in them? If not I'll just remove it, see if I can reassemble the transformer & try it in circuit monitoring the temperature etc.
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Many transformers do have thermal fuses built in, especially small "mains adaptor" types which can be seriously overloaded within their fuse (if any) rating. Worth a try removing it. With luck that'll be the problem. I'm sure you could get a new one from Farnell if you want to do a proper job of it. ;)
Kevin
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Yes, which means two things
1) The opamp supply rails will follow and be affected by the music transients.
2) The opamp supply rails now have significant inductance (due to the use of wire wounds) which will affect the dynamics!
It just highlights how cheap the setups are in relaity (and mega over priced to you and me!)
Therefore the original design (which I admit is good, as is the build quality- apart from the transformer obviously) with 2 secondaries is the way to go.
Would the inductance of the transformer secondary also have a detrimental effect?
Yep......its why you can never achieve the perfect power supply.
As for the thermal fuse, these are often used on cheap units where the amount of iron in the core is right on the edge of working and hence there can be a risk of saturation......they fit the thermal fuse to prevent runaway!
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Yes, which means two things
1) The opamp supply rails will follow and be affected by the music transients.
2) The opamp supply rails now have significant inductance (due to the use of wire wounds) which will affect the dynamics!
It just highlights how cheap the setups are in relaity (and mega over priced to you and me!)
Therefore the original design (which I admit is good, as is the build quality- apart from the transformer obviously) with 2 secondaries is the way to go.
Would the inductance of the transformer secondary also have a detrimental effect?
Yep......its why you can never achieve the perfect power supply.
As for the thermal fuse, these are often used on cheap units where the amount of iron in the core is right on the edge of working and hence there can be a risk of saturation......they fit the thermal fuse to prevent runaway!
Looking at the spec of thermal fuses it states that they must not be soldered in but clamped or in a terminal block etc. This one was soldered in very close to the body & I imagine this may have weakened the device.
BTW when reassembling the transformer, should I clean off the lacquer in case it's crept onto the lamination faces slightly?
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Difficult one really......Idealy not is the answer!
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Many thanks for yr inputs
If ever you want a Barn-Dance............ ;)
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We just need a barn.....for a cambelt meet!
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We just need a barn.....for a cambelt meet!
I'll leave that thought with you!! :)
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Ive just rewound a Microwave transformer to build a fast battery charger! That was all welded together, and was a bit of a pain to rewind. May have helped ive I used the correct wire though I guess! :-X
Its VERY VERY crude, but has brought a car battery back from been VERY VERY dead!
Down side is the transformer gets very hot after been on around 5-10 mins! And its only running 50% duty too! (Only got one big diode on the secondary) :-/
It seems to work well though! I tested it with a battery that was left on a car for a good 7-8 months (and not started! So it was flatter than a flat fart! ;D) When I removed it and put a multimeter across the terminals it showed less than 2 volts! :o I was going to skip it but decided to try and recover it!
I put my normal battery charger on it, and it wouldn’t take any charge, even after been on charge for 30 mins, it still read under 7v! Rigged it up to microwave oven transformer, and within 2 mins it was pulling so much current it was starting to melt the wires! :-X
Good new is that it managed to start my mates Astra the other day, and his battery was that flat that the interior light only glowed a dim orange! ;D (Wouldn’t trust the battery though, as its probably sulphated up like crazy inside! Will be used for a backup battery though!)
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I've just completed reassembling the transformer(lost count of the number of laminations to 'hammer' back into place) & I get outputs of the correct voltages
It's too cold in the garage to continue tonight but tomorrow I'll put it back into amp & see if It's happy driving the rectifiers :)
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Ive just rewound a Microwave transformer to build a fast battery charger! That was all welded together, and was a bit of a pain to rewind. May have helped ive I used the correct wire though I guess! :-X
Its VERY VERY crude, but has brought a car battery back from been VERY VERY dead!
Down side is the transformer gets very hot after been on around 5-10 mins! And its only running 50% duty too! (Only got one big diode on the secondary) :-/
It seems to work well though! I tested it with a battery that was left on a car for a good 7-8 months (and not started! So it was flatter than a flat fart! ;D) When I removed it and put a multimeter across the terminals it showed less than 2 volts! :o I was going to skip it but decided to try and recover it!
I put my normal battery charger on it, and it wouldn’t take any charge, even after been on charge for 30 mins, it still read under 7v! Rigged it up to microwave oven transformer, and within 2 mins it was pulling so much current it was starting to melt the wires! :-X
Good new is that it managed to start my mates Astra the other day, and his battery was that flat that the interior light only glowed a dim orange! ;D (Wouldn’t trust the battery though, as its probably sulphated up like crazy inside! Will be used for a backup battery though!)
Microwave transformers are a bit of a special case, IIRC. I believe they are designed so that the core saturates until full load is pulled from them which is probably why you are having issues running it with a light load. Shame, cos they'd be great for valve amplifiers otherwise. ;D
Kevin
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Here's one that is pretty close to the outputs you need
http://cgi.ebay.co.uk/TOROIDAL-MAINS-TRANSFORMER_W0QQitemZ330303594897QQihZ014QQcategoryZ294QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Thermal fuses normally reset themselves, don't they ?.
Ken
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Here's one that is pretty close to the outputs you need
http://cgi.ebay.co.uk/TOROIDAL-MAINS-TRANSFORMER_W0QQitemZ330303594897QQihZ014QQcategoryZ294QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Thermal fuses normally reset themselves, don't they ?.
Ken
Seen the e-bay item
Thermal fuses go open circuit when they reach the rated temperature
You're probably thinking about thermostats
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No, I'm thinking about PTC's. These reset themselves when they cool down. I have a 240v to 110v 100W chinese (crap) transformer that runs my 85W soldering iron for about 10mins. It then trips, after 20-30mins it resets and works again, for a further 10mins. A-D also fit them to the camera bases we are checking.
Ken
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Ive just rewound a Microwave transformer to build a fast battery charger! That was all welded together, and was a bit of a pain to rewind. May have helped ive I used the correct wire though I guess! :-X
Its VERY VERY crude, but has brought a car battery back from been VERY VERY dead!
Down side is the transformer gets very hot after been on around 5-10 mins! And its only running 50% duty too! (Only got one big diode on the secondary) :-/
It seems to work well though! I tested it with a battery that was left on a car for a good 7-8 months (and not started! So it was flatter than a flat fart! ;D) When I removed it and put a multimeter across the terminals it showed less than 2 volts! :o I was going to skip it but decided to try and recover it!
I put my normal battery charger on it, and it wouldn’t take any charge, even after been on charge for 30 mins, it still read under 7v! Rigged it up to microwave oven transformer, and within 2 mins it was pulling so much current it was starting to melt the wires! :-X
Good new is that it managed to start my mates Astra the other day, and his battery was that flat that the interior light only glowed a dim orange! ;D (Wouldn’t trust the battery though, as its probably sulphated up like crazy inside! Will be used for a backup battery though!)
Microwave transformers are a bit of a special case, IIRC. I believe they are designed so that the core saturates until full load is pulled from them which is probably why you are having issues running it with a light load. Shame, cos they'd be great for valve amplifiers otherwise. ;D
Kevin
Use with valve amps.... Hmmm never thought about that one ::)
How do you mean exactly? As an output transformer? :-?
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Use with valve amps.... Hmmm never thought about that one ::)
How do you mean exactly? As an output transformer? :-?
I was thinking more along the lines of HT supplies for transmitting amplifiers. A mate of mine made such a device in an old microwave oven case once but found the transformer less than satisfactory. It needed 2.5 kv of HT at about half an amp IIRC. ;D
It has been said that it "comfortably" exceeded the 400w power limit. 'course, he wouldn't have turned it up above that. Oh, no. ::)
Kevin