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[Pitchfork]

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.

[Kevin Wood]

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

[Marks DTM Calib]

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!

[Pitchfork]

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?

[Marks DTM Calib]

Difficult one really......Idealy not is the answer!

[Pitchfork]

Many thanks for yr inputs
If ever you want a Barn-Dance............

[Marks DTM Calib]

We just need a barn.....for a cambelt meet!

[Pitchfork]

We just need a barn.....for a cambelt meet!

I'll leave that thought with you!!

[Ian_D]

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!

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!  ) When I removed it and put a multimeter across the terminals it showed less than 2 volts!   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!  

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!   (Wouldn’t trust the battery though, as its probably sulphated up like crazy inside! Will be used for a backup battery though!)

[Pitchfork]

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

[Kevin Wood]

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!

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!  ) When I removed it and put a multimeter across the terminals it showed less than 2 volts!   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!  

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!   (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.

Kevin

[Ken T]

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

[Pitchfork]

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

[Ken T]

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

[Ian_D]

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!

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!  ) When I removed it and put a multimeter across the terminals it showed less than 2 volts!   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!  

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!   (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.

Kevin

Use with valve amps.... Hmmm never thought about that one  

How do you mean exactly? As an output transformer?  :-?