Electronics > Repair

roland 808 drum machine u.s power conversion

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cod65:

--- Quote from: squadchannel on January 07, 2025, 09:54:02 am ---
Toroid are overkill and a waste of money.

--- End quote ---

yes! the only reason I was considering them was becasue nothing else seems to fit 1.7" high  if I had to find a 20-0-20 at 750ma+750ma :o

cod65:
oh wait, no i need 20-0-20 so that would be 40Vct, or thereabouts.

So I dunnoo .... 187B36 or 187C36


https://www.hammfg.com/part/187C36

the 808 manual on page 2 seems to suggest the dual regulator TA7179P will work between VIN(18-30v), so I assume after rectification my 18vac secondary should be what .... about 24vdc ?

squadchannel:
oops, sorry yes needs 187B36. :-+

Swake:
VA and watt is indeed not entirely the same, but in this case it is going to be negligible.

Practice and theory are 2 different things. They're close but the difference is 'experience' and 'wrong information'. Let me explain with this example: The schematic says that between yellow and yellow there is 10 VDC, well good luck with that, if a transformer has DC on its coils then it is more a magnet than a transformer.

The given is that the 2 existing 2 x 3VA transformers make it work. Chances are this is enough power. If you want to make sure then you have to measure. I think this is a good exercise for you because it will give you certitude and explain the theory in practice and build some experience ;)

What do you measure:
First
A/ AC Voltage of each secondary winding in operation, with the synth powered on and preferably making music.
B/ DC Voltage of each secondary winding. Obviously this should be very near to 0, but just in case there is new physics out there or a very big issue that needs repair >:D
C/ AC Voltage of each secondary winding 'open' (= not connected to the synth, so you have to disconnect the wires)
By comparing this you can see how much of a difference the load the machine gives on the transformer. If the voltages in A are dropping heavily compared to C then the transformer is not big enough. I suspect this will not happen as there are no complaints now.

Second
- AC current on each winding while playing full blown music.
Now you can multiply this current with the above voltage in A and you know how much it really needs in VA. You can size the transformers accordingly. Here you indeed take some margin . In this case 'the biggest you can fit 'comfortably' in the case. Else as this is a small power supply, 150% is just a tad overkill but why not. If it was a very big power supply then maybe 110% is already enough.

cod65:
hmmm wouldnt 187b36 be only 170/2 = 85ma per side ? do i need to go to 187C36 ?

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