OBERHEIM SYNTH

Hi All.Have a OB-x that worked great the last time I used it two yrs. 
ago.Now when turned on,random lights come up.I have the  service 
manual.Power supply voltages ok,Clk,address and data busses are up and 
running.I am analyzing the schematics today.Any ideas out there?
               Thanks 
                Jim

Hi All.Have a OB-x that worked great the last time I used it two yrs. 
ago.Now when turned on,random lights come up.I have the  service 
manual.Power supply voltages ok,Clk,address and data busses are up and 
running.I am analyzing the schematics today.Any ideas out there?
               Thanks 
                Jim

Don't need stories about other peoples unrelated products please,just some useful tips on servicing this beast of a OB-X.I don't want to come off as being rude,but...
                          Jim

I have an OB-X and have gotten it back to life many times.

First, your memory battery likely went dead. If the board still works
(you can program it by turning knobs/button pressing) you will likely
need to change the battery. With the battery out, check the resitance
and make sure the battery is seeing M-ohms.  If not, you have a
capacitor that shorted out, and you will need to track it down or the
replacement battery will go dead in a few weeks.


If the board does not work 
1)Double check to make sure all the voltages on the power supply are ok.
2) Remove all voice cards and power it up. See if it looks like it is
working (i.e. light turn on and off correctly/it will let you store
patches.
3) If step 2 is ok, power down,insert a single voice card and see if
it works.  Put the board in Unison so the voice triggers each time you
hit a key.  If that voice works, power down, take it out and put it in
the "good" stack.
4) If all voices work, or none of the voices work it is guess work
time.  I have gone through and replaced just about all of the blue
Tant capacitors on every voice and on all the monther boards.  If you
still have these, it is only a matter of time, they will fail and they
can cause very big problems. I strongly suggest you change out all of
them on power supply, and on each voice board there are several right
next to the voltage regulators.  At a minimum, change these out.

Good luck and let me know what happens. 


--- In oberheim@yahoogroups.com, "jahg88" <jahg88@...> wrote:

>
> Hi All.Have a OB-x that worked great the last time I used it two yrs. 
> ago.Now when turned on,random lights come up.I have the  service 
> manual.Power supply voltages ok,Clk,address and data busses are up and 
> running.I am analyzing the schematics today.Any ideas out there?
>                Thanks 
>                 Jim
>

Did some work over the weekend.Here's what I found.The "D3" line is loaded down to about 2 volts.All the other Data lines are reading 4.8- 5 volts.Pulled the PROMS, didn't make any difference.How many, and what I.C.s can I remove in what sequence without damaging the components to isolate the problem?

Don't pull out ICs - pull out the voice cards.  If you have an 8 voice 
machine, on the right half of the keyboard (high notes) there will be a 
hinged shelve with 4 cards on it.  You take off two or three nuts from 
the far right side of this shelve, and then you swing the shelve up and 
you will see 4 more voice cards below it.  You have to remove all of 
these cards.  They are held in by about 5 screws each.  Make sure you 
get these all out first. 

Please take this as a constructive comment, not an insult, but if you 
don't know what the voice cards are, this project is likely way too 
complicated for you to attempt to do yourself.  An OB-X is worth big 
$$$( at least $1k) and it would be better to simply sell it on e-bat as 
is, than to start pulling things out and doing damage that cannot be 
repaired. 

I have not had any CPU related problems with my board, it has always 
been power supply, voice/mother board capacitor problems, 4053 gate 
problems, and some other mic ICs.  It sounds like you understand data 
transfer and CPUs, but this is much more about capacitors and voltage 
regulators...are you sure you are willing to put this board at risk???



-
-- In oberheim@yahoogroups.com, jim finch <jahg88@...> wrote:
>
> Did some work over the weekend.Here's what I found.The "D3" line is 
loaded down to about 2 volts.All the other Data lines are reading 4.8- 
5 volts.Pulled the PROMS, didn't make any difference.How many, and what 
I.C.s can I remove in what sequence without damaging the components to 
isolate the problem?
>

At 11:14 AM 2/17/09, you wrote:

>Did some work over the weekend.Here's what I found.The "D3" line is 
>loaded down to about 2 volts.All the other Data lines are reading 
>4.8- 5 volts.Pulled the PROMS, didn't make any difference.How many, 
>and what I.C.s can I remove in what sequence without damaging the 
>components to isolate the problem?

Remove any and all ICs that have a connection to D3 one at a time 
until the loading goes away.  You shouldn't damage anything if you 
turn off the power between each part removal.  You've already tried 
the EPROMs -- try removing the RAM chips, latches and buffers 
connected to D3 (A29 through A34, A102 through A105, A110 through 
A115 and as a last resort, try replacing the Z80.

Removing voice cards won't help, as this signal doesn't go directly 
to the any voices.

-- Paul White
http://www.electrongate.com/dmxfiles

I wish you both the best of luck.  Let me know when you decide to 
sell it as parts. 
--- In oberheim@yahoogroups.com, "Paul J. White" <pjwhite@...> wrote:
>
> At 11:14 AM 2/17/09, you wrote:
> 
> >Did some work over the weekend.Here's what I found.The "D3" line 
is 
> >loaded down to about 2 volts.All the other Data lines are reading 
> >4.8- 5 volts.Pulled the PROMS, didn't make any difference.How 
many, 
> >and what I.C.s can I remove in what sequence without damaging the 
> >components to isolate the problem?
> 
> Remove any and all ICs that have a connection to D3 one at a time 
> until the loading goes away.  You shouldn't damage anything if you 
> turn off the power between each part removal.  You've already 
tried 
> the EPROMs -- try removing the RAM chips, latches and buffers 
> connected to D3 (A29 through A34, A102 through A105, A110 through 
> A115 and as a last resort, try replacing the Z80.
> 
> Removing voice cards won't help, as this signal doesn't go 
directly 

> to the any voices.
> 
> -- Paul White
> http://www.electrongate.com/dmxfiles
>

I know that it's not power supply related.I have replaced some of the tantalum caps that fried on one of the voice cards.Yes, I know what the voice cards are :).I have a four voice model.It is the D3 data line thats causing the problem.There's only about seven or eight chips that use the data buss.From experience,usually buss latchs,74244,245 or 373 tend to fail first.There is a tantalum cap used in the reset for the proceesor but reset timing looks good via O-scope.
Yeah ,I know they are worth alot-less than 1000 were made.I do have extensive digital troubleshooing experience,just not with stuff quite this old.I have to dust off what I learned 20 yrs ago to fix this one.
Object is to get it up and running 100%,then sell to the highest bidder.I have an OB-8,along with 14 other keyboards,mostly old analog stuff.Wife says I have to sell some of it.Little does she know I'm going to spend it all on Ebay!

wives never understand this obsession of some of us with old synths.  I tell mine, think of it as the equivelent of your desire to have lots of shoes.  Of course,despite my compelling logic, after that, I spend the rest of the night on the couch with my oberheim......








Jonathan G. Chance, Esq.  
JC Law Offices 

1840 Gateway Dr.
Suite 200
San Mateo, CA 94404
650-378-1383 (office) 
650-921-4696 (cell)
650-429-2048 (fax) 
lawjcrg@yahoo.com
www.jchancelaw.com

--- On Wed, 2/18/09, jim finch <jahg88@...> wrote:

From: jim finch <jahg88@...>
Subject: [oberheim] Re: OB-x Troubleshooting
To: oberheim@yahoogroups.com
Date: Wednesday, February 18, 2009, 10:23 AM






I know that it's not power supply related.I have replaced some of the tantalum caps that fried on one of the voice cards.Yes, I know what the voice cards are :).I have a four voice model.It is the D3 data line thats causing the problem.There' s only about seven or eight chips that use the data buss.From experience,usually buss latchs,74244, 245 or 373 tend to fail first.There is a tantalum cap used in the reset for the proceesor but reset timing looks good via O-scope.
Yeah ,I know they are worth alot-less than 1000 were made.I do have extensive digital troubleshooing experience,just not with stuff quite this old.I have to dust off what I learned 20 yrs ago to fix this one.
Object is to get it up and running 100%,then sell to the highest bidder.I have an OB-8,along with 14 other keyboards,mostly old analog stuff.Wife says I have to sell some of it.Little does she know I'm going to spend it all on Ebay!

Won't be selling it for parts-I will fix-what, no faith in my abilities?

Well,It appears to be the Z-80 CPU.With all the chips associated with the D3 line are pulled,D3 still swings between 0-2V.No shorts to ground,or other D-bits,that D3 line just won't swing full TTL levels.
Question though,all the Z-80 CPUs are either higher clock speed,NMOS/CMOS.Does it matter what Z-80 is used as long as it meets spec?Original CPU is a Mostek.Another question.Have replaced a few of the 6.8uf caps.How critical are the values.Most of the caps. I find are at least 10uf.Acceptable to replace the 6.8 with 10uf.And still another question.Why Tantalums as output coupling caps.?I have seen them in some "British" Eq circuits.Unique distortion?
                   Jim

Another question.What are the 4 switches on the panel board for?

Another question.What are the 4 switches on the panel board for?

At 10:25 AM 2/25/2009, jim finch wrote:

>Well,It appears to be the Z-80 CPU.With all the chips associated with the D3 line are pulled,D3 still swings between 0-2V.No shorts to ground,or other D-bits,that D3 line just won't swing full TTL levels.
>Question though,all the Z-80 CPUs are either higher clock speed,NMOS/CMOS.Does it matter what Z-80 is used as long as it meets spec?Original CPU is a Mostek.

A higher clock speed Z80-CPU should be fine.  I don't think NMOS vs. CMOS matters in this case either.

>Another question.Have replaced a few of the 6.8uf caps.How critical are the values.Most of the caps. I find are at least 10uf.Acceptable to replace the 6.8 with 10uf.

Probably OK, but it may depend on what part of the circuit they're in.

>And still another question.Why Tantalums as output coupling caps.?I have seen them in some "British" Eq circuits.Unique distortion?

Possibly less change in capacitance over time?

-- Paul W.

At 10:25 AM 2/25/2009, jim finch wrote:

>Well,It appears to be the Z-80 CPU.With all the chips associated with the D3 line are pulled,D3 still swings between 0-2V.No shorts to ground,or other D-bits,that D3 line just won't swing full TTL levels.
>Question though,all the Z-80 CPUs are either higher clock speed,NMOS/CMOS.Does it matter what Z-80 is used as long as it meets spec?Original CPU is a Mostek.

A higher clock speed Z80-CPU should be fine.  I don't think NMOS vs. CMOS matters in this case either.

>Another question.Have replaced a few of the 6.8uf caps.How critical are the values.Most of the caps. I find are at least 10uf.Acceptable to replace the 6.8 with 10uf.

Probably OK, but it may depend on what part of the circuit they're in.

>And still another question.Why Tantalums as output coupling caps.?I have seen them in some "British" Eq circuits.Unique distortion?

Possibly less change in capacitance over time?

-- Paul W.