Feb 25, 2015
Autotune trouble
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.
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 recongnized 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:
U142: 74C174
U143: 74C42
U38: 74C174
U39: 74C42
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.
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 recongnized 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:
U142: 74C174
U143: 74C42
U38: 74C174
U39: 74C42
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.
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Encore Electronics
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.
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From: "richtrix@... [oberheim]" <oberheim@yahoogroups.com>
To: oberheim@yahoogroups.com
Sent: Wednesday, February 25, 2015 1:54 AM
Subject: Re: [oberheim] Finding Clock frequency of my OBXa [1 Attachment]
[
Attachment(s)
from richtrix@... [oberheim] included below]
Thanks Bob,
This EXACTLY what I was looking for. I suppose I should explain. About 9 months ago I got an Encore MIDI kit from Tony Karavidas. There was an issue with voices 5-8 though blipping and popping. His CPU clock he supplies is above that of stock. This was causing problems with my rev of the OBXa but apparently not others. He was very helpful and concerned I even mailed him my LCB and UCB at one point and he replaced an IC on it but when I got it back the error was still there. Finally he mailed me a MIDI kit that ran at "Stock" clock speed. This greatly improved things. The blips still persist though less extreme. He has gone kind of incognito on me recently though. Last I heard from him he suggested that now that it is running at "stock" there should be no issue so it must be my console having an error or damage. I've taken the console back to its stock EPROMs and Z80 and it functions fine though (except the memory banks seem to be not working really, even when saving patches from Manual Mode when I try to play them back they sound nothing like the stored setting. Also my pre-MIDI-kit tape dump loads fine without errors but all sounds are completely F*!$#*, I am not too concerned with this though as the goal is to revert to the MIDI kit). Since the stock controller hardware renders all the voices sounding wonderful (in manual mode) I want to test the stock clock speed and then the Encore clock. If there is a discrepancy maybe Tony can adjust it again and that will fix it. If anyone has any other thoughts or ideas please share. I have attached what the blips with the midi kit sound like.
Thanks Bob,
This EXACTLY what I was looking for. I suppose I should explain. About 9 months ago I got an Encore MIDI kit from Tony Karavidas. There was an issue with voices 5-8 though blipping and popping. His CPU clock he supplies is above that of stock. This was causing problems with my rev of the OBXa but apparently not others. He was very helpful and concerned I even mailed him my LCB and UCB at one point and he replaced an IC on it but when I got it back the error was still there. Finally he mailed me a MIDI kit that ran at "Stock" clock speed. This greatly improved things. The blips still persist though less extreme. He has gone kind of incognito on me recently though. Last I heard from him he suggested that now that it is running at "stock" there should be no issue so it must be my console having an error or damage. I've taken the console back to its stock EPROMs and Z80 and it functions fine though (except the memory banks seem to be not working really, even when saving patches from Manual Mode when I try to play them back they sound nothing like the stored setting. Also my pre-MIDI-kit tape dump loads fine without errors but all sounds are completely F*!$#*, I am not too concerned with this though as the goal is to revert to the MIDI kit). Since the stock controller hardware renders all the voices sounding wonderful (in manual mode) I want to test the stock clock speed and then the Encore clock. If there is a discrepancy maybe Tony can adjust it again and that will fix it. If anyone has any other thoughts or ideas please share. I have attached what the blips with the midi kit sound like.