Roland Juno 6 - Power Supply

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Andy Jackson

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Apr 11, 2006
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Glasgow, Scotland, UK etc...
Hello guys!

I have an old Juno 6 which is in a sorry state. The old girl cosmetically is pretty shook up, but internally (visually) it seems fine. On first measuring the PSU, there was no +15V or -15V on the board. I duly replaced all of the electrolytics, and remeasured. Nowt.

I reckon that the +/- regulator (an old TA7179P) has probably shuffled off this mortal coil. I then remembered that the little MNats PSU (with the LM317/LM337 combination) could output the + or - 15V required.

Looking at the schematics of the Juno 60 which has an identical circuit for this part of the PSU (attached), it seems that a supply capable of supplying 1A max. should do the trick here - and I'm sure I read somewheres that the MNats Little PSU is capable of safely supplying up to 1A.

I connected the secondaries to the MNats PSU board, and powered up, and adjusted the variables so I had 15V at the outs. All good so far. However, the regulators are running EXTREMELY hot.

I then measured the secondaries directly with no load which gave 22V on both. Am I correct in assuming that we're looking at 30V in to the regs in the PSU after rectification? Which means that they have to dissipate a LOT of heat (around the 26W mark) when regulating down to +/-15V? I was thinking of the series resistor option (something like a 5.1 ohm / 50W job) to try and get the regulators not working as hard, and also to get the temperatures down a tad...along with a heatsink of sorts...

I suppose I just need a critical eye cast over my thoughts, which would be much appreciated. I'm really hoping to get the old girl up and running again.

Best wishes & thanks in advance!


Andy.
 

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RuudNL

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Apr 26, 2009
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The transformer is rated 300 mA, so this will be the worst case current.
18 V * 1.41 = 25.4 V. (DC)
25.4 - 15 = 10.4 V (This is the voltage over the regulator)
10.4 V * 0.3 A = 3.1 W per regulator (worst case)
This is not an alarming situation!
However: the regulator should have a heatsink to stay cool.
 

Whoops

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It's normal that a transformer rated at a certain output voltage will actually put out 15% to 20% loaded or unloaded.

So 18x1.2= 21.6 VAC
after full wave rectification

21.6VAC x 1.414 = 30.54 VDC

Are you using Heatsinks like Ruud said?

You have to use good heatsinks with thermal paste compound on the regulators.


 

Andy Jackson

Active member
Joined
Apr 11, 2006
Messages
41
Location
Glasgow, Scotland, UK etc...
Hi Whoops!

Thanks for the reply! I am indeed using heatsinks and thermal paste. They do seem hot, but they also seem to be pretty stable. The synth is back to life now, and putting out sound.

Would be great if there was a +5VDC, +/-15VDC custom PCB somewheres for the supply...hmmmm...now there's a thought...might get on that...  :)

Edit: There seems to be one BA662A chip out, as there's no output from voice three at TP4. Bugger. Ach well - now to buy a clone BA662!
 

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