Cheetah MD16 MIDI Ballsup

cyllall · January 28, 2011, 06:45:44 PM

Hi everyone,
Seeing as nowhere apart from here even mentions the Cheetah MD16 drum machine, I thought I would ask the knowledgeable fellows here.
On my MD16 it seems that the Midi ports are back to front. Midi thru is actually Midi in and Midi thru doesn't seem to even work, even though there is a socket there with components behind it I can't get anything to transmit from Midi thru. So it seems the machine has only Midi in and Midi out with incorrectly labelled sockets.
Anyone had the same experience as this?
Thanks
Martin

Circuitbenders · January 28, 2011, 07:35:04 PM

I think the Midi thru is wired directly to the Midi in on a MD16, so they might as well be the same socket. You can certainly use the thru socket as a Midi in if your Midi in has been eaten by battery leakage. If you think about it all you actually need from a Midi thru is an exact copy of whats at the Midi in anyway.

You see a lot of MD16's with non functioning Midi as Cheetah kindly stuck the incredibly leaky battery right by the Midi circuitry. I'm assuming your machine has had a battery leak at some point?

cyllall · January 28, 2011, 07:56:21 PM

Hi,
Thanks for the quickie.
No battery leakage. I'm no expert but it looks like they've got pin 1 and 2 of the PC900 type device(can't find a sheet for the one in the MD16) the wrong way round(comparing it with The MS6 Schem). Midi in seems to do nothing. Can't get anything in or out. Could try a logic probe to check for some pulses on Midi in and see if they get stuck somewhere?.Never probed Midi before.Is it safe?
Cheers
Martin

Circuitbenders · January 29, 2011, 07:42:52 PM

i'm not entirely sure from what you say if you're actually getting any Midi at all?

It seems very unlikely that the opto-isolator is in the wrong way round as it would never have made it out of the factory, unless someone has tried to do a repair and replaced it wrong. I've got a spare MD16 board somewhere so if you want to post a photo i'll compare it with mine.

It should be perfectly safe to put a scope on the midi section to see if you are actually getting anything at the input and output of the opto-isolator.

Are you absolutely sure there has never been a battery leak? Maybe the battery has been changed and any leak has been cleaned up. Theres a circuit trace for the midi running from the big chip on the bottom left, right under the battery to the midi in socket. Fairly often this has been destroyed by battery leakage which disables the midi.

cyllall · January 29, 2011, 10:22:41 PM

Hi,
I had an orientation issue with the Midi in socket. It is wired correctly and like you said it wouldn't have left the factory like that.
There's a photo online at cykong.com of the internals of an MD16RP. His board says 07/142 mine says 07/177.I Have version 2.06 eprom. My Midi configuration looks the same from above at least.
I put a new battery in 2 days ago. Didn't notice anything bad. I know what these nicads can do as one made a mess of a Roland CSQ-600 I have.
But just in case I missed anything I have just de-soldered the battery and the optocoupler.
The traces look fine.
I presume the trace you are referring to is the one under the optocoupler that goes to pin 4 of the CPU? This also seems to join a transistor which looks like it goes to midi thru.
My midi in does not work but midi thru works as a midi in.
Is it possible the optoisolator has gone, can I change the 220 ohm resistor on midi in to something else?
Cheers
Martin

cyllall · January 30, 2011, 03:39:37 PM

If it's the optocoupler that's the problem, can anyone suggest a replacement. I'm guessing it's more like a 4N25 than a PC900v?
Cheers

cyllall · January 30, 2011, 05:08:56 PM

I don't know what the GE P0438 Component is, short of being a optocoupler and yet pin 6 doesn't seem to be connected to anything. It doesn't make sense.

Circuitbenders · January 30, 2011, 10:04:48 PM

the GE P0438 is definately an optocoupler / optoisolator but i've never seen that precise IC on an MD16 before. In fact i don't think i've seen one of those anywhere!

On every MD16 i've seen theres a H11A1 in that position. I've just checked my MD16RP and a spare board i have and they both have the H11A1. The datasheet is here:

http://www.datasheetcatalog.org/datasheet/motorola/H11A1.pdf

You can get them from Farnell or cricklewood electronic for about 50p or so. I know a PC900 won't work there.

If your midi thru works as a midi in at all i'm guessing the optoisolator is fine though. I'd still be suspecting some kind of hardware problem like a broken track or socket. It'd probably be a good bet to check everything you can with a continuity tester if you've not already done it.

I have the service manual somewhere so if you can't work this out i'll see what i can do about getting my scanner working.

cyllall · January 30, 2011, 11:09:43 PM

Thanks for this information, it's a great help.
I took a shot in the dark and ordered a 4N25. Comparing it to the H11A1, they seem almost identical with only rise/fall times being slightly different.
This is identical to mine http://www.cykong.com/Synths/Cheetah%20MD16RP/Images/MD16RP-Inside-PCB-Large.JPG.
I have used a logic probe and I can follow the pulse from Midi in to pin 2 of the optocoupler. Nothing pulses at pin 4 of the optocoupler.
Midi thru doesn't seem to go anywhere near the optocoupler and just goes through a resistor and straight to pin 4 of the CPU?
I will have a better look tomorrow, but I can find no fault with any passive components. I have followed the circuit with a continuity tester.
Cheers
Martin

Circuitbenders · January 31, 2011, 01:52:59 AM

take a look at the attached scan of the midi section of the schematic............ i stand corrected!

The midi thru is completely independent of the optocoupler, which is labelled as an H11A1 on the schematic. From the look of it if you'd ruled out a hardware fault, the only thing that could possibly be wrong is a dead H11A1, or P0438 in your case

cyllall · January 31, 2011, 09:49:46 PM

Looking at a schematic is so much easier than flipping a board over every 2 seconds because you've just forgotten what you looked at on the other side. I can't thank you enough for that.
I was suspicious of the BC5488, because when I tested it with an old in circuit transistor tester it was showing a fault but after I removed it, it tested fine, so the only thing left was the optocoupler and seeing the schem makes it all the more clear. Also explains how Midi thru works as an in too.
Thanks
Martin

cyllall · February 01, 2011, 06:26:18 PM

I got a 4N25 in the post today, soldered it in and Midi was fully functional again.
Thanks for all your help I can't thank you enough, now I'm going for a sysex dump receive.
Cheers
Martin

01012k7 · April 06, 2012, 10:48:34 AM

I have a cheetah MD 16 Rack unit i have some switches that are not working it was one of the first ones to come out of cheetah and was upgrade from the 48 sounds to 700 sounds as it had a new rom after a few weeks , i was at a demo by cheetah at andertons in guildford many years ago and order the rack unit but switches are not working and are damage any idea on replacements or maybe a new mod with new switches .. have used switch cleaner

Gordonjcp · April 06, 2012, 08:08:16 PM

I wonder if they're just tactile switches? They're probably not too horrible to replace.

There's a guy in Wales who sells photocopies of Cheetah service manuals on eBay, who apparently used to actually work for Cheetah back when they were going. No, I won't give away scans or copies, go and pay the man some money. He's probably still owed a fortune...

Circuitbenders · April 06, 2012, 08:42:21 PM

Quote from: Gordonjcp on April 06, 2012, 08:08:16 PM
There's a guy in Wales who sells photocopies of Cheetah service manuals on eBay, who apparently used to actually work for Cheetah back when they were going.

He's also been selling knackered MD16's spray painted back as being 'refurbished' rare models. Someone sent me one they had bought from him recently and he hadn't even bothered to change the battery or clean up the battery leakage that was slowly killing it. He even generously included a spare battery,which was obviously secondhand and in atrocious condition! They are blatantly rebuilt from parts he has lying around with the bare minimum done to get them working, however briefly. The guy who bought it was understandably pissed of at having been ripped off for £120 or so.

There are no MD16's with 700 sounds. Theres no way they would have put that many on two ROM's in 1990.

The buttons on the rack are some kind of easily available tactile switch if i recall correctly. Take one out and have a look.