Hello.
First I’m electronic newbie.
Second I’m French (poor English).
I’ve bought a Cheetah MS6 on Ebay.
Unfortunately, the 2 PSU transformers are broken.
I need to replace them but I don’t know which ones I have to buy.
I join the schematic of the PSU.
Perhaps someone have an idea what “707v” and “909v” mean.
References are welcome.
I’ve just built my smashTV’s modules and now I need my little Cheetah work (I plan to use Maad’s firmware).
Thank you.
Best regards.
Jérôme.
Hi,
what you need are two center tapped transformers that can drive the 7805/7905 and 7812/7912 voltage regulators
which supply a bipolar voltage to the circuit. They should have enough headroom for fluctuations in the grid and the voltage drops over the diodes and regulators
but shouldnt exceed a certain voltage since the voltage difference at the regulators will be “burned” by the them and dissipated as heat.
Calculation:
Utransformer=(((Uout+Uregulatordrop/20%_Ripple)+Udiodedrop)/(Urms*GridFluctuation_in%)
Utransformer 7805/7905 =(((5+2.5V)/0.8)+2V)/(1.414*0.9)= 8,963V –> common value: 9Volt
Utransformer 7812/7912 =(((12+2.5V)/0.8)+2V)/(1.414*0.9)=15,8V –> common value: 18V
That means a 230V primary / 2x9V secondary transformer for the 7805/7905 and a 230V primary / 2x18V secondary transformer for the 7812/7912 should do the job.
I dont know how much current the cheetah needs but i assume less then 1A.
To be safe i´d look for 10-15VA transformers…
Thank you Phunk for your answer and your (excelent) explanations.
It is not so easy to find center tapper transformers but I’ve finaly found this products (signaltransformer.com):
Dual Primaries 115-230V / 20VA / Series: 20.0 VCT @ 1.0A / Parallel: 10V @ 2.0A
Dual Primaries 115-230V / 20VA / Series: 36.0VCT @ 0.56A / Parallel: 18V @ 1.12A
Is-it possible to confirm this references and the fact I have to connect together Pin 9 and 10 (center tap).
Best regards,
Jérôme.
Thank you Phunk for your answer and your (excelent) explanations.
It is not so easy to find center tapper transformers but I’ve finaly found this products
The reason for this is that they are not sold as “center tapped” transformers but as 2 x 9V or something like this.
Is-it possible to confirm this references and the fact I have to connect together Pin 9 and 10 (center tap).
Yes, the transformers stated should work. At least they have the specifications given. (but do you really want to buy them in the US? Reichelt.de ships to france as well)
And yes, you connect Pin 9 & Pin 10. So you get Pin 12=+VCC , Pin 9/10= GND, Pin 7= -VCC
For the grid side you do the same and connect Pin3 and Pin4.
Thank you phunk.
I’ve finaly made an order on Reichelt even if I don’t speak German (with Google help!).
I hope my MS6 will be rescued after that.
Best regards.
Jérôme.
I was very excited to find this thread today, as my old MS6 has been dead with the same problem for several years (someone else switched it to 115V and plugged it into 230V). After replacing the PSU filter cap, it does turn on, but the voltage to the 7812 is too low so I think the 909V transformer is damaged.
Fairlight, please do post again with your results when you have time; I would love to be able to get mine working again. And thanks to phunk for the advice and the links!
So I’ve been researching transformers online, and one problem I have is with the size. The original units are about 43mm wide x 36mm deep x 33mm tall, they have 4 input pins and 3 output pins. The case lid basically rests on the top of the transformers, so there’s no room for more height. The big filter cap is directly below the 909 transformer. The RAM battery is just to the right of both transformers. The regulator heatsink runs along the left side of both transformers. The possible replacements I’ve found so far are all bigger (eg these, which are 68x57x27.6). Some components could be moved and/or the heatsink changed, but there’s really not much space in there.
The other problem is that all the transformers I found have 4 output pins, so they wouldn’t go straight onto the board. There would need to be some kind of spacer/adapter, which would raise the height, meaning the lid wouldn’t fit. It would be quite easy to drill new holes for the lid and fit it higher but then there would be a gap and it would look bad.
If exact replacements are not available, maybe it would be better to move all the power components to an external PSU and use a 5-pin DIN socket to supply AC 18,18,9,9,GND to the MS6.
How are you doing, Fairlight?
Hi steveeeee.
Sorry for the late answer but in fact it’s not me who try to repair the MS6 but a friend of me because I fell (more exactly I felt at the beginning) myself to newbie in electricity/electronic to touch 230V and I didn’t want to break the MS6 more than it was.
He has few spares times but he should try new transformers before the end of the week.
As you could see, the transformers I bought are too high so we plan to put them outside the case of the MS6.
I’ve also bought another faulty MS6 on ebay which seems to be in best condition than the first one (if we can bring to life the 2 synths…).
I’ll comme back as soon as I have news.
Jérôme.
OK, looks like we’re both building external PSUs then. It might take me some time to get around to doing this, but I will be very interested to see your progress. Good luck!
