Kurzweil K250 with burnt out Power Supply Board

 

 

 

The K250 was a truly advanced instrument for its time. Stevie Wonder had a hand in designing it; and the original sticker price back in the 1980's was over 10,000 dollars... more than a brand new car.

 

A look at one of the four boards in the rack mount version will show you the incredible amount of circuitry and sheer number of microchips employed in making a true landmark instrument:

 

 

 

This particular unit came to us with a malfunctioning power supply. On it is a type of switching power supply, with several transformers, even some surface mount work mixed in... in short it seemed that the fastest point to a successful restoration would involve just replacing the power supply PCB rather than repairing it on the component level. Although I rarely replace circuit boards, the RT65-C available from JAMECO provided all the required voltages, and enough amps to power this beast. Additionally, it provides overload and thermal protection. After installing this little beauty in there and tapping the voltages to the right wires, it worked very well. I used the original screw type connector terminals mounted on a PCB, which I covered in tape to avoid any unwanted connections. I secured the replacement power supply with screws in the same place as the original, and also added a copper strap for extra security, and  a little bit of heat sinking. I tested it for several hours and it held up well. I put a bit of hot glue over the solder points on the back of the PCB, (the pcb which contains the terminals) to protect against the likely hood of the solder points breaking through the black electrical tape. Although perhaps better aesthetics could have been achieved in this mod, it is effective and secure.

 

 



Mackie Pro FX 8 Mixer repair: weird noise in some channels, blown channels

This Mackie mixer came to me with a terrible and pronounced distortion on the outputs at all times, and also with several dead channels. On disassembly, I was able to find blown surface mount op amps, bulging coupling capacitors, and several dead transistors on the two dead channels.  Once these were repaired, the channels came back and the unit sounded fine...including the dead channels...

Until I put it back together completely (ie: placed the cover on the bottom and screwed it in)... as soon as I did that, the noise was right back!

Thinking maybe I had a loose connection or solder joint, I pulled the cover off and turned it on again. Now, the sound was clean as a whistle. I poked around on the circuit boards, trying to bend things, trying to find a failed connection, but everything was rock solid again.

But as soon as the cover was replaced and screwed in, the thing sounded terrible, and the same two channels failed.

I realized, after some thought, that the ribbon cable, shown below, which is encased in a black rubber sleeve, and comes from the power supply, presses down on some header pins on the FX circuit board (also shown below, header pins are in the blue square). The pressure is pretty intense when the cover is installed on the back (not a lot of extra romm in there). After time, those sharp pins had worked their way through the rubber sleeve and into the cable, and were making contact with the voltages coming from the power supply. They sent this voltage into various points in the signal path for the channels. This had made the noise, and eventually blown out those components on those channels.

This is clearly a design issue.

I cut those header pins down (obviously they are made for a connection which will never be used in this mixer), and then placed some hot glue over them, so that there is nothing sharp left to press into the ribbon cable.

This instantly fixed all issues, and fortunately none of the new components were destroyed in the brief time that I had the unit powered on in this condition.

The bottom line is, if you have a PRO FX8 which is starting to get noise all around, or in a few channels, this is very likely the culprit!

Ensonic Post #2: keyboard calibration error on Ensonic VFX

Since my previous post is also dealing with Ensonics; I would mention here that the VFX keyboard will sometimes say "calibration error" while trying to turn on. In addition to the obvious checking of the cabling to the keybed, it may also be necessary to remove the keys in the middle of the keybed, and bypass the socketed connection between the two circuit boards beneath with jumper wires as shown below.

THis keybend uses metal plates moving on top of  sensors, creating an inductance, to measure velocity. It's a pretty neat scheme, unlike mostly all other touch sensitive keybeds which use rubber contacts. They are not as susceptible to dirt and dust.

On this particular keybed, some of those metal plates had to be glued back on to their respective keys; the customer had noticed that pressing hard on the keybed fixed the "calibration error" issue. However, over time and repeated pressings, he had dislodged the metal plates from the keys. Now that the jumper wires are soldered in place, he will no longer have to press on his keybed to get it to work!

Ensonic EPS disk drive Replacement with HXC floppy emulator, no sound output on EPS

This post will deal with the replacement of 2 floppy disk drives with SD card "floppy emulators"; devices which allow the substitution of an SD card (or USB memory stick) for a floppy disk

.Among the many models out there, I used the floppy drive emulators from HXC. These come with excellent documentation and support, unlike some other emulators I have purchased form EBAY. I was also able to ascertain beforehand that they work on the EPS.

To save on cost, I used the slim model (less bells and whistles), and ordered simply the circuit board, not the enclosure. This made it necessary to fabricate a mounting scheme from wood and a brass plate... the result was actually appealing and kept the job under 200.00 per keyboard, parts and labor included. For a better aesthetic appearance, the supplier also sells a fabricated enclosure which probably would make the repair look less like a mod. But in this case, we wanted to keep costs down.

 

 

 

 

 

 

Incidentally, I have also purchased standard floppy drives and retrofitted them to the EPS and other Ensonic Models; I documented that on a previous blog post . These units used the older "shugart" interface, and require some pins redirected when attaching a PC style floppy drive.

But floppy drives and disks are getting rarer and harder to find. In general, I avoid installing them nowadays, in favor of the emulators.

If you look up HXC floppy emulators on the internet, you will find all the information you need in terms of ordering one and setting it up. The trick is knowing how to create disk images from your floppy disks, and how to format and put them onto the SD card.

You need to install a special driver on the PC you are using to read from the floppy disk, not the standard windows floppy disk driver. You then need to use software to convert the disk to the HFE format. You then need to copy this HFE image file to your SD card, and rename it DSKA0000.HFE . or DSKA0001.HFE,,,, you can put tons of images on the SD card but they all have to be named sequentially starting at 0 with the convention I just illustrated (DSKAxxxx.hfe), and they all have to have the .hfe extension. You also need to have a .cfg file on the SD card, the right one for the EPS, at all times. You can only put the images on the root of the SD card, along with the cfg file.

But once you jump through all these hoops,  it truly works great in the end!


The only other issues with these EPS's was that the memory expander card needed to be removed, all the contacts cleaned, and then reseated. Issues with this card were causing there to be no sound output whatsoever on the unit.

Flat flexible cables: what to do when you encounter a broken one, or when you loose the little wedge that holds them in their socket:

Although often called a ribbon cable, this type of cable is actually an FFC, a flat flexible cable. Most of these cables are not unique, at least most of the ones I have encountered have an amount of contacts divisible by 10, and have either a .5 mm or 1 mm "pitch", which is the amount of space between the center of one contact to the center of the other.

They can be cut with a nice sharp pair of scissors, useful if you have encountered one where the end is a bit damaged, but may have enough conductive material left on it that you can fix it by just shortening it a bit. They can also be carefully cut vertically, if you have one that has too many wires ( is too wide).

On a few occasions I have received items for repair where the little wedge, that holds the ribbon cable into the socket, has been lost. That's a mistake I just don't make anymore... but many do.

I have had some success buying full replacement sockets from digikey and stealing the wedge (I think it's called a slide lock). The replacement sockets are not hard to find, as long as you have the correct number of contacts on the cable ends, and the right pitch, you may very likely find that exact socket for sale, and then use its wedge.

Of course, counting the number of contacts on the end of a tiny ribbon cable is kind of a pain.

When I have to do something like that, I take a nice digital picture of it, then blow it up on my computer screen, so I can slowly move my mouse over it and count out loud while I do it.

Chroma Polaris Membrane Repair

While repairing a Chroma Polaris, I was forced to disconnect the ribbon connectors (technically FFC connectors) that connect the membrane to the panel PCB's. To my horror, even though I was being painstakingly gentle and slow, one of them tore, almost immediately, as soon as I tried to move it

Additionally,  the other 3 connectors -which didn't break as the first one did- still became cracked and intermittent when they were unbent from the position they had been resting in for 35 years.

In all cases, the cables broke where they had been bent... fairly close to the socket.

To repair the situation, I was able to cut the damaged portion off with scissors, and then grind away the insulating material (from the black side, not the silver side) with a wire brush attached to a dremel.

This may seem barbaric, but it actually worked flawlessly. ( I practice on the discarded portion of the ribbon cable first). You just need to buff it just enough so that you are able to ring through the traces on the ribon cable with your multimeter. If you buff it too much, you will grind through the conductive material, and render it useless. So you have to buff, and test, buff and test, until it rings through. It is naturally a good idea to practice first on the discarded portion of the ribbon cable.


On 3 of the connectors, I didn't have to shorten the cables too much, and they still stretched to their sockets

 

 

. On the 4th, I had to install a few wires and actually move the socket closer to the cable.

Either way, the repair has seemed to be permanent and hopefully the membrane will last another 30 years, especially if it is undisturbed. An additional step may be to recoat the exposed conductive part with conductive epoxy or paint; however it did not seem required in this situation.

One additional note: these flat ribbon cables are actually called FFC's, flat flexible cables. They are not called ribbon cables at all. Ribbon cables, technically, are made up of many insulated wires attached to each other. If these break, it is much easier to repair them. I am going to put up another post discussing FFC cables.

I also recapped the power supply and replaced a few faders.

 

 

AXIOM 49 with dead midi output

I posted this unit on the blog because it's failure was caused by a most unlikely culprit: a failed ceramic capacitor which was used to filter out noise on the midi output. Although I encounter countless failed aluminum capacitors, I rarely (if ever) have encountered a failed ceramic capacitor.

Nevertheless, this was the case here.

The unit came to me because the midi out was failing, it was completely dead. Of course I checked the jack and the connections around it, and all seemed fine. I reset the software.

Just for a lark, I removed the tiny small value capacitor that filtered the midi out line to ground, and found that the midi out came right back.

Although the capacitor did not read as shorted, it's capacitance must have drifted and increased significantly, enough to actually be filtering out the midi signal, rather than just noise.

It was a 220 PF cap, shown below, and the unit worked fine when it was removed, and continued to work fine when a new one was installed. The second picture shows the area where the cap was located... I believe labelled C4 on the silkscreen.