This page is here to provide you with information to troubleshoot OBXaMK installation issues. Over the years, we have discovered certain problems that fall into a few specific categories.
The autotune hardware changed during the production life of the OBXa. If you find the OBXa refuses to autotune right after installing the MIDI kit, we have a possible solution.
Hold down the AUTO button while turning on the power. This will change a mode in the firmware. If this works, you never have to think about it again.
You can do this multiple times, but it repeats after 4 attempts. If none of these modes work, contact us...you might have a revision we haven't seen yet.
Glitchy audio or dropped voices
This comes from a variety of problems. The fundamental issue is that the OBXa originally ran at 2.5MHz, while the OBXaMK runs at 4MHz.
We have helped several customers debug these issues, and this is now documented here to help your troubleshooting efforts.
One problem is incorrect parts used during repairs over the last 35+ years. Digital logic comes in many varieties, and speed is an important parameter.
With stock hardware, an improper substitution may work fine. It only becomes evident when the system is running faster, as is the case when the OBXaMK is installed.
We have seen U128 (a 7442) replaced with a 74C42, and that appeared to break the instrument after the MIDI kit was installed.
We have also seen U160 (a 7442) replaced with a 74C42, and that also appeared to break the instrument after the MIDI kit was installed.
We cannot magically know what was done to your instrument before you purchased our kit, so you need to look for things like this.
Later models were often not socketed, and it's a dead giveaway if you find one socketed part. This part should be checked against the schematic to insure it is correct.
In other cases, it's simply a matter of timing tolerance that causes problems. We have recognized a group of parts that tend to cause this trouble.
These problems often will show up as killing 4 voices at once. Either the lower 4 or the upper 4. That is a sure sign you have a timing problem.
The parts to suspect are:
All of these parts should be replaced with LS or HC or HCT equivalents. These parts are still pretty easy to find online in small quantities.
If you don't want to replace those parts, another possible solution is to slow down the clock.
In 2014, we have allowed the MIDI kit to generate a 3.2MHz clock which seems to alleviate the glitchy behavior.
However, this requires you pull the CPU from the socket, lift pin 6 (the clock pin), and solder a small wire to the pad near the Xilinx part as shown.
Remember, the stock OBXa runs at 2.5MHz, and running the clock as fast as possible will help MIDI response. So the best solution would be to replace the slower parts on the boards, but this wire mod should also work ok.
Add this wire for a quick solution
Cassette loading trouble
The cassette has always been a source of trouble, and as the years pass by, the tape degrades more and more.
We've seen tapes that were such poor quality, we could not recover the data from them. But, if you have a good quality tape, it's important the signal be 'hot'.
Low voltage line outputs rarely work. What seems to work best is a headphone jack from something turn up 'fairly loud.'
Another cause of trouble is the actual processor. We have recently discovered that a Z80 processor we use has trouble with cassette loading. For you tech heads out there, this seems impossible, but we solved it by code modification.
So, if you have our midi it and everything is working except cassette loading, you might want to try downloading the latest firmware. One is specifically for a processor labeled "Z80 RAM80", so if your CPU doesn't have that text, don't download that version.
Very quiet audio
Early versions of the OBXa had mute transistors on the voice motherboards. There is an instruction for dealing with this in the installation manual, but a picture here might help.
Instead of completely removing U117, it can be inserted back into its socket if pin 3 and pin 11 are bent up out of the way first. They should not be touching anything else!
(If your U117 is not socketed, it would easy just to clip off these two pins and leave the chip soldered to the board.