Electronics > Repair
roland 808 drum machine u.s power conversion
cod65:
--- Quote from: Swake on January 07, 2025, 10:31:22 am ---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.
--- End quote ---
brilliant. I cant thank you enough for your time..... means a lot to me!
I hear you on 'experience' tip, but I am way too intimidated by this all-metal class 1 chassis to run it like that with "hood open" -the top lid section, which contains the majority of post power supply circuitry, can only lay upside down when it is open - I would have to work controls from underneath somehow- very awkward and scary ! :) I hear your testing plan and I think I understand the theory, but I'm way too intimidated to run this thing open like that - I will be happy if I can disconnect the top circuits isolate the power supply and verify clean 5 volts and 15 volts dc..... if i can do that I'm good!
thank you though.
Swake:
--- Quote from: cod65 on January 07, 2025, 10:24:14 am --- the dual regulator TA7179P will work between VIN(18-30v), so I assume after rectification my 18vac secondary should be what .... about 24vdc ?
--- End quote ---
Yes.
18 x SRQ(2) = 25,4 V
Minus the bridge rectifier. So take 2 diode junctions int consideration = 1 x 0.6 V less -> 24.2
That lower voltage will make it, and Q1/Q2, run somewhat cooler too.
cod65:
yeah, i was weirded out by the secondaries listed in AC also- thought maybe they were talking about after rectification, or peak or something, but i didn't say anything because i didn't want to seem like a dummy ;D
Maybe its just a translation error from Japanese
cod65:
of course another presumption here is that the person who did the mod before me, and put these transformers in place of original, chose the right capacities .
So ... come to think on it further- why isn't my mentor correct- that if i have 500ma fuses and only 150ma-300ma max capacities - why would want to potentially allow my transformers to blow before my fuses? or is it just they are less susceptible to blowing under great surges (transformers are more analogous to 'slow blow' by their physical nature ?)
I read up on this , and it seems that transformers are more robust in their repsonse to big in-rushes, can handle brief peaks much better than glass fuse could - that makes sense why the fuse would be above its capacity- its mainly a matter of how long it can survive an overload current, right ?
Then why would my mentor, a knowledgeable guy, insist that i wanted to have a transformer that was rated a good amount ABOVE the fuse ? I need to understand this.
Swake:
Choosing transformers and fuses are 2 different things. While there will obviously be a relation as both are connected to the same circuit, you will find very quickly that there is not a one rule fits all. The particularities of the circuit to be protected and powered as well as its operating conditions are important to consider when choosing either.
A short insight in how to choose a transformer: https://tameson.com/pages/transformer-calculator.
Fuses: https://www.digikey.com/en/articles/fuse-tutorial
Note that the schematic shows primary side and secondary side fuses. It's is a good thing, actually a must, to have both as these have different goals. Protecting the transformer vs protecting the circuit and of course both are also helping avoid your house burning down which is the often forgotten main reason why they have to be implemented.
The table specifies 250 mA AT fuses. Note the 'T' that comes from 'träge', the German word for 'slow' -> they want you to use a slow blow fuse.
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