Yamaha MLA8 Power Transformer Rewind

[CJ]

people seem to like this Yamaha 8 ch preamp,  bunch of 4558's with all the right switches and things,

had a dead one come in, had a bad pwr xfmr, un did it, we find a random wound coil with a thermal switch,

1 U racker>

 

[CJ]

was gonna use 50 EI since we have a space restriction, but we would need major turns to keep the core happy,

so we try and reuse the old bobbin with these snap in lams from Mars>

 

[ruffrecords]

Thermal fuses rarely blow for no reason. If there was not a fault in the transformer, it might be wise to check the rest of the circuit before trying out the rewound one.

Cheers

Ian

 

[CJ]

there was a broken lead going to the switch, but we will bring it up slow to make sure everything is ok, thanks for the tip!

speed winding going on offshore,
and we find it distressing to overlap thin wire and then connect it it to 120 VAC

so we wind nice layers and use thin poly tape between layers,  why not go for a 20 KV hi-pot winding structure by inserting some 20 mil Nomex between pri/sec coils, then we can ditch the pri thermal switch because who is going to break this coil, Dr Frankenstein?

snap in the old lams and maybe add a little metal clip to keep the bent up tongues together wile we bake and dip so we don't wake up the neighborhood with 60 cycle hum,

ready for the Cactus Club drummer to mic everything and clear the room, 

 

[CJ]

here is the winding info

since we used extra insulation, there was a bit less room for copper, so the numbers in red are the rewind turns counts etc,

flux goes up a hair but we are still well below 18 KG which is the saturation point for steel lams,

these might even be non grain, if so we are good for 21 KG

 

[CJ]

you can build a spreadsheet for calculating design values,

we use the free Kingsoft program to save a little hard drive space as Excel is now a slow loader,

next we can check pri Henries to see how the lams liked being bent and hammered back,

 

[CJ]

excitation at 60 hz turns out to be about 22 ma, at 24 volts (HP 200CD limit)

so reactance is 24/.022 = 1091 ohms

so Henries be 1091/6.28 * 60 Hz = 2.9 H

at 24 vac we have 3  K k Gauss so the core is still asleep, H will go up to maybe 4 Henries at 120.


seems a bit low for 1326 turns but wait, these lams have a built in gap, no wonder,

why the gap, were they expecting DC?  no,

would it make a quieter core as saturation would be reduced so the lams rattle less? perhaps,

would it cut down on the radiated field? maybe, they use those lams in the V72, remember, this is a mic pre and the pwr supply has to be quiet,

bake at 200 and put it in the tank, varnish keeps better in a jar so you don't have to scrape a big layer off the top every time you open the can, 

 

[CJ]

how long do we leave it submerged? like me when i was baptized, wait until the bubbles stop then call 911 and pray for a slow ambulance,

or long enuff to humiliate russian grand master Petya with that weird A4 opening that gets him off the books from the git go,

wow, somebody is a 3200, must be cheating with Houdini but that might be my inner voice talking, 

 

[CJ]

let that sucker drip for a few minutes and pour the varnish back in the jar,

put the toaster oven out back (unless you are a masochist) and bake at 200 for about 10 to 12 hours,

serve warm on a bed of lettuce,

if the neighbors complain about the smell, tell them it's the meth lab down the street,

pay no attention to that sock sponge,  ;D

 

[Gene Pink]

how long do we leave it submerged?

Have you ever messed around with pulling a vacuum on this sort of thing?

Gene

 

[CJ]

vacuum pumps are used on a lot of hi voltage stuff, it dries the pressboard insulation and sucks the varnish into the crevices when the vacuum is released,,

it is not really needed for low voltage stuff and can be dangerous when using varnish,

it is safe with mineral and silica oils, but not varnish,