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Cheetah MS6 power supply Dynamic Microphones
Old 24th October 2013
  #1
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Cheetah MS6 power supply

Hello everyone,

I have recently had an issue with a Cheetah MS6. I was replacing one of the CEM3396 chips when one of the voltage regulators bent and snapped without me realising. When I turned it on - 'pop' - and a terrible smell from one of the transformers.

After replacing the voltage regulator, still no luck. I figure I've blown a transformer (they are infamous in the MS6 for going wrong).

The MS6 has two - one to bring it from mains to 15V, and one to bring it to 7V. From here, the rectified signal goes to two separate diode-capacitor-voltage-regulator routes to give +/- 12V and +/- 5V.

Looking at the schematic I think it should be easy just to bypass the transformer and provide 12V AC to the circuit where the signals from the transformers would have come out. The diode-capacitor-voltage-regulator circuits should give the correct voltage output with a 12V AC supply to each; with a bit more heat coming out of the 5V regulators.

I've attached a PDF of what I mean.

Do you reckon this would work? Any feedback would be very helpful.

Tim
Attached Files
File Type: pdf ms6_power_184.pdf (519.0 KB, 261 views)
Old 24th October 2013
  #2
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Hi
No, get a proper transformer for a couple of reasons.
First, the supplies are 'centre tapped' so, without adding some extra circuitry to make them 'split' it won't work properly and you will probably damage other bits.
Second, analogue and digital supplies must remain separate for noise reasons.
Matt S
Old 24th October 2013
  #3
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Hey,

Thanks for the speedy reply.

When you say it will probably damage other bits, I don't think it would. The diodes as they are are 1N4001 which take up to 35V RMS, or 50V peak; so with the 12VAC signal shouldn't be a problem. A similar supply I've made myself uses 1N4004 diodes; which can handle 280V RMS and 400V peak, which seems a bit excessive. Anyway, I could just replace the diodes if it's reasonable to expect +/-25V peaks coming out of the supply.

Then past the diodes the signal would be identical - recitifed AC voltages, now both 12V rather than 15V and 9V to the analogue and digital bits, respectively. So the only difference would be less heat produced by the L7812 and 7912, and more from the L7805 and 7905. But a decent heatsink should deal with that.

The capacitors must be OK for the ripple voltage because they were dealing with an unregulated rectified AC input before, unless they held the input voltages a few volts higher than the V-regulators intentionally to cope with more ripple.

The separate supplies makes sense, but I think it's a bit early on in the circuit for that to make a difference - the digital and analogue grounds aren't even separate yet.

I'm happy to buy some transformers, it would just be neater to plug a 12V AC supply straight into the back, and I don't see a problem with what's there at the moment.

Again, thanks a lot for the speedy reply!
Tim
Old 24th October 2013
  #4
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Richard Crowley's Avatar
 

I agree it seems unlikely to actually toast anything (assuming you disconnect the transformers completely, ALL primary and secondary connections.

HOWEVER, it turns the circuit(s) into half-wave rectifiers, and those input filter capacitors seem pretty wimpy even for full-wave. I would fully expect significantly more hum from the supply if you attempt this. And, as Mr. Syson says, your scheme would completely eliminate the isolation between the digital and analog supply parts of the circuit. Not recommended.
Old 24th October 2013
  #5
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Yup - thanks. The capacitors do look very wimpy. I'll have to get some clunky transformers. Thanks a lot for the input. Thought I may have found an easy solution, but I've only made pure-analogue circuits in the past. I've had some issues with cheap digital single-board synths with zipper sounds when you sent MIDI data, and that drives me crazy. So I don't want to introduce anything like that.

Thanks a lot for the help!!

Tim
Old 24th October 2013
  #6
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I also feel as if it's worth mentioning that the voltage ratings on the two capacitors it says should be 10V max in the schematic are actually 16V (for the 4700uF) and 25V (for the 1000uF) so would be able to handle the voltage.

Tim
Old 24th October 2013
  #7
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So, onto the transformers...

I've been buying things from ESR in general.

Transformers - Mains Chassis

Do you think these two sound ok?

311-312 12 - 0 - 12 250mA 6VA Chassis Transformer

and

311-512 12 - 0 - 12 500mA 12VA Chassis Transformer

I'll put them in a separate box outside and feed the inputs straight into where I was going to put the 12V AC input.

Tim
Old 25th October 2013
  #8
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mjrippe's Avatar
 

If you are paying attention to Matt Syson's coment about the center tap, then yes a 12-0-12 transformer is OK, though it will generate some extra heat from wasted energy. Your original diagram with just 12vac and ground will not work (as Matt already mentioned).
Old 25th October 2013
  #9
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I would think something like a 9-0-9 VAC transformer would be a better choice for the section which creates the +/- 5 VDC rails. The lower voltage will result in lower heat dissipation in the two regulators.

OTOH, 12-0-12 VAC for the 12 VDC rails is marginally low.

Totally unknown are the current requirements for the various rails, and hence how big of a transformer will be required. The 1N4001 diodes tell me the current will be below 1 Amp, but how much below 1 Amp?

Bri
Old 25th October 2013
  #10
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Ah I see. What was confusing me was the volt-amps. I was just reading that as volts. There was a 250mA fuse inside from the mains input, but I'm not sure what the current draw is at the low voltage range. I don't think the 7809 etc. can take more than about an amp either.

So I am thinking of getting a 6-0-6 1.2VA; but I'm confused about the 9-0-9 in the schematic - how can you get +/- 12 V from a +/- 9V transformer? Is that a mistake?

Tim
Old 25th October 2013
  #11
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Hi
It looks like some background fundamental reading is necessary. Simple supply circuits mainly.
The grounds are separated for a good reason and as it cost money to do it originally it was not for 'fun'.
A 1.2VA transformer seems pretty low power although I don't know what the unit is.
Brian's comments about voltage choices are in line with my thoughts, there is not much margin for possible 'low' mains on the original ratings, if this is actually what they provide.
Matt S
Old 25th October 2013
  #12
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My background is not in electronics repair. I'm fairly proficient at soldering so I was replacing a voice chip, which is easy to do. And then when I plugged it in it went 'pop'. I would like more background in the area, but right now I don't have the time to read up too much. I will in the future.

As far as I can see the grounds are not separated in the original schematic. They both go to a common ground after the centre-tap transformers, which seems odd. The 5V digital section splits into digital and analogue ground after the voltage regulators. So that's what I'm talking about. I know it's very sensible to separate analogue and digital ground, but it doesn't look like the original engineers have.

So for the transformers, should I use something higher - maybe 5VA? The only thing I can go by is the initial fuse on the mains, which was 250mA, so the combined current drain of the two transformers cannot have been over about 200mA. So something like a 6VA transformer, rated at 250mA, would have blown the fuse, and so is likely overkill.

It's a 19" rackmount unit, and I guess it doesn't draw much power. ESR sells 100mA 2.4VA 12-0-12 transformers, and 100mA 1.2VA 9-0-9. That brings it up to around the 200mA the fuse would be able to handle. Do you suppose these would be reasonable to buy? Or should I get something with higher volt-amps? And is it best to use a transformer with higher voltage than the v-regulators in case the ripple is high? Or am I way off?

Unfortunately the transformers that came with it do not have any specifications on at all. They are just labelled "1" and "2" and look identical.
Old 25th October 2013
  #13
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Sorry - spotted a mistake in the 4th paragraph. I meant 100mA 1.2VA 6-0-6 transformer, not 9-0-9.
Old 25th October 2013
  #14
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Ok - so I've read up on AC to DC conversion - I didn't realise the AC voltage was given in RMS. So The peak voltage of a 9V AC supply is 12.7V, so is enough for +/-12V DC. So I suppose this means I could use a minimum of ~4V AC (so 5.7V) for the digital supply. Is this right?

Also, thanks a bunch everyone for all the help.
Old 25th October 2013
  #15
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I would be aiming for at least 500 mA from the secondary of each of the transformers. A 250 mA mains fuse means the fuse can tolerate a total primary load of 60 VA.

Bri
Old 25th October 2013
  #16
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Amazing. Thank you.

I've been looking at Farnell's website and found some PCB-mounting ones with the same dimensions as those I removed. But the pins don't seem to be compatible. So I'll just make a separate external box and feed the 6 leads in through the old mains-connector hole.

Do these look ok?

VTX-120-4206-206 - VIGORTRONIX - TRANSFORMER, 6VA, 2 X 6V | Farnell United Kingdom

and

VTX-120-4206-209 - VIGORTRONIX - TRANSFORMER, 6VA, 2 X 9V | Farnell United Kingdom

Again, thanks very much.

Tim
Old 25th October 2013
  #17
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Or do you reckon this would be better for the 9V supply?

VTX-120-5412-209 - VIGORTRONIX - TRANSFORMER, 12VA, 2 X 9V | Farnell United Kingdom

(667mA rather than 333)
Old 25th October 2013
  #18
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brianroth's Avatar
 

I have a more fundamental question. Have you checked to see if either/both transformers are producing AC output? If only one is bad, you should be able to leave the good one in place.

Bri
Old 25th October 2013
  #19
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Hi
If your original story is correct, snapping some pins on a regulator is more likely to have disconnected the transformer's power contribution so that would be fine, but leaving one of the supply rails unpowered. This can damage ICs as some go into a 'crowbar' state with one rail absent. Transformers do not tend to go 'pop' but rather heat up and smoulder gently over time. Very low power transformers can have 'short circuit proof' windings where their internal resistance is sufficient for self protection.
The peak voltage from a full wave rectified AC is around 1.4 times the RMS value (minus rectifier losses). When you load the circuit the voltage will fall and 'ripple' will appear. The DC available before the regulator must be around 3 volts more than the required regulated output otherwise the regulator will 'drop out' and ripple will appear on the output.
There are also influences due to the resistance of the transformer windings which muddy the waters further, hence Brian's suggestion of a 9.0.9 transformer for the 5 Volt supply and at least 12.0.12 for the 12 Volt supply. Rectifier losses are more pronounced at low supply voltages. You also need to consider what happens when the mains is 'low' by a permissable 10 percent.
The mains fuse rating is a very poor indicator of actual requirements as they often need to be over rated to withstand inrush current.
Matt S
Old 26th October 2013
  #20
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Hey,

Great. Thanks very much. I've been talking to a guy in Scotland who's said exactly the same thing - 12 0 12 for the 15V and 9 0 9 for the 5V. He's dealt with the Cheetah MS6 before and knows a company that makes pin-compatible transformers. He also said the schematics are generally wrong for the Cheetah. I'll get back to you once these arrive (may be a couple of weeks to a month).

In response to Brian, it is the 9 0 9 transformer that doesn't seem to work. The 7 0 7 one was still functioning. But as these transformers were apparently very cheap and are prone to internal shorts, I thought it would be best to replace both for the longevity of the synthesizer.

(I hope I haven't busted any ICs...)

Thank you everybody!

Tim
Old 14th August 2015
  #21
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Hi Tim,

can you please provide the solution for your problem. I have bought a Cheetah MS6 via ebay and on both transformers the pins are broken on one Side. that means 4 of the 8 Pins are nok on each transformer. Can you or somebody in the Forum hall where i can order 2 new transformers.

Thanks in advance
Elvir from Frankfurt Germany
Old 14th August 2015
  #22
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Hi Elvir,

Unfortunately I never managed to fix it! The transformers weren't pin compatible at all, and I just had to give up. A real shame. I also bought mine off eBay and it was dodgy from the day I got it. One of the voices had no resonance at all, and it was just a mess inside. It looked like many people had tried to patch up initially bad soldering, and done a bad job of it. A real shame, but I've had to commit it to the dusty box of things I intend to have another look at, but don't think I ever will.

For a piece of advice, I saw other people had taken the approach of making an external power box with the two transformers in it, and taking the wires straight from that to the back of the MS6 through a DIN plug. That might be the best approach, as I think you will have a tough time finding pin-compatible transformers (unless anyone else has any ideas!)

Sorry for the bad news! I hope you have better luck.

All the best,
Tim
Old 15th August 2015
  #23
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Hello Tim,
Thanks for sharing. I thought allready that there will not be a quick solution but if i find one and i will Share
it with you.

All The best
Elvir
Old 15th August 2015
  #24
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Hello Tim,

i had contact with Kristofer from maad.net and he said that there was i guy with the same problem and he will get in contact him. I will provide the answer when it reaches me. Also i will contact a friend who is an expert in repairing synths. I am still optimistic :-)

Elvir
Old 17th August 2015
  #25
Gear Head
 

Hi Elvir,
The other guy would be me. I'm currently awaiting the arrival of 2 more capacitators to replace. If installing them won't solve my problems then i too will have to find the transformer(s)..
My problems are described more indepth in this topic, which has a wealth of Cheetah trouble shooting tips btw:
https://www.gearslutz.com/board/elec...eetah-ms6.html
Old 17th August 2015
  #26
Gear Head
 

Btw i just read this within the description for an MS6 that sold earlier this year:
"The are two brand new transformers to the original specification mounted inside the unit, this should give good reliable service for years to come."
Which might indicate that MS6-type transformers do exist

link:
Cheetah MS6 power supplyhttp://www.ebay.co.uk/itm/CHEETAH-MS...p2047675.l2557

(photo nr. 4 shows the area of the transformers and it seems as if the lid is really squeezed over them)
Old 20th August 2015
  #27
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Hi all,

Thanks Bolobeat for the informations. I searched the internet 2 days for an MS6 Type transformer but nothing :-(

i just finished the repair of the transformers and everything works fine.

I was very lucky at the end
Old 26th August 2015
  #28
Gear Head
 

Right, so at the moment there are more and more people (desperately) trying to source the right power transformators to bring their Cheetah's back into full bloom. People mailing, brainstorming, following (obsolete) old links etc. If you read this and have any shred of info as to where replacements can be found that can be fitted in, then i'm sure you'd make quite a few people happy
Old 1 week ago
  #29
Gear Head
 

Just a lil' bump for all the Cheetah enthousiasts
Still wondering how to get this bad boy to 100% working order
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