Sequential circuits samplers

Hello,
I just bought a Prophet 2002 locally that doesn't power on. The fuse
was blown and every new fuse I put in also blew so I opened it
up to try to find the short and I found this, but I'm not sure if it
is causing the problem, although it seems the most likely candidate. It
looks as though someone tried to "fix" something and got a little
carried away. Any suggestions?

Pictures:
http://cs.1729.googlepages.com/pcb_topdamage2.jpg

http://cs.1729.googlepages.com/pcb_btmdamage.jpg

Also, if anyone has a Prophet 2002 (or even a prophet 2000) and would
like to help out, then I would appreciate it greatly if someone would
take some pictures of the inside for me. Specifically the board that
is attached to the lid you opened to get inside around the same area
that I took a picture of and also a picture of the board on the
bottom.  I just need to see what the area of a functioning 2002 looks
like.

Also, any source for a download on a service manual?

Thanks.

>>Pictures:
> http://cs.1729.googlepages.com/pcb_topdamage2.jpg
> 
> http://cs.1729.googlepages.com/pcb_btmdamage.jpg<<

the first picture is, I think, where one of the front-panel components
attaches to the board; is it the data pot or the balance control
maybe? the second pic is just ground wiring, & both look more alarming
than they actually are. there should be a connector in that first picture.

I have schemos somewhere, & a functional 2002 I can take pics of, + a
dead 2000 which needs a new keyboard controller IC (WC haven't got any).

I will try to find time this week to "pop the trunk", as they say.

your shorting problem is more likely related to the power supply- does
the thing function at all? can you (or a tech, if you aren't one)
check the power supply basics? it's 12s & 5s, as you'd expect, &
pretty generic.. you should be able to isolate the power supply from
the rest of the machine & see if it works off-load, then start looking
for dead capacitors, op-amps, maybe the floppy-drive...

duncan (london, UK)

Yes, I unplugged the connector for a better look in the picture.  In
both photos it is the same wire it just goes from the connector to the
main board and I couldn't tell if it was supposed to be there or if
someone had added it for some reason.

It does not function at all.  I bought it knowing this.  I went to
replace the fuse, thinking I would get lucky and it instantly blew and
keeps blowing all fuses I put it with no sign of life.  However, if I
put a slow blow fuse in, then I see some life from the floppy drive,
but again the fuse just blows.

I was hoping that it was not the power supply as I do not know how to
test those components (I suppose that it wouldn't be hard and I could
do it if I knew exactly which components to test, but I do not have a
service manual or schematics so it is difficult).

Thank you very much for your time and I would very much appreciate the
photos if you can make time.

Thanks.

--- In prophet2000@yahoogroups.com, "duncan" <ferrograph@...> wrote:
>
> >>Pictures:
> > http://cs.1729.googlepages.com/pcb_topdamage2.jpg
> > 
> > http://cs.1729.googlepages.com/pcb_btmdamage.jpg<<
> 
> the first picture is, I think, where one of the front-panel components
> attaches to the board; is it the data pot or the balance control
> maybe? the second pic is just ground wiring, & both look more alarming
> than they actually are. there should be a connector in that first
picture.

> 
> I have schemos somewhere, & a functional 2002 I can take pics of, + a
> dead 2000 which needs a new keyboard controller IC (WC haven't got any).
> 
> I will try to find time this week to "pop the trunk", as they say.
> 
> your shorting problem is more likely related to the power supply- does
> the thing function at all? can you (or a tech, if you aren't one)
> check the power supply basics? it's 12s & 5s, as you'd expect, &
> pretty generic.. you should be able to isolate the power supply from
> the rest of the machine & see if it works off-load, then start looking
> for dead capacitors, op-amps, maybe the floppy-drive...
> 
> duncan (london, UK)
>

First point of call in a PSU is to replace the caps. They are almost always the first thing to go - especially on older gear. Should be easy to spot and

I just replaced the capacitors in the PSU and the problem persists.  I
am going to put the old capacitors back in for now because although I
replaced them in a functional manner, it is a sloppy job and I am
convinced that they are not the issue.  Until I can find more fitting
capacitors, the old ones will have to do.

Any other suggestions?

>>Any other suggestions?<<

yeah- the series regulators that provide the 5V & 12V rails into the
rest of the instrument. 78xx & 79xx devices- these are the things you
see fastened to the back of the case inside, probably with what looks
like toothpaste (thermally conductive paste, in fact) around them.
these things (78xx, 79xx) frequently fail in older gear.

you need to get a tech in there, with a voltmeter. your p2k may be
perfectly alright once these are replaced & working again. but you've
reached, with the greatest of respect, the limit of what you can do
without a voltmeter & some component-replacement chops.

hth-
duncan.

I went to Radio Shack today and was able to get a few of the ICs to
replace 7812 & 2x 7805.  There seem to be quite a number of variations
of this chip (I think), but it looks as though they all do the same in
the end and the ampere rating is well above the 500 mA that the
Prophet uses.  So out of the 4 on the bottom board, I replaced 3
(because I couldn't find a 7912) and the prophet still does the same
thing.  

I also check the boards for continuity since there appeared to be some
lifted traces but I was wrong.  My knowledge in electronics repair is
limited so I have no further ideas of how to fix this.  I ordered a
7912, but it seems very unlikely that the one IC that I couldn't find
to replace would be the one causing the problem.

By the way, I know the transformer and everything before it is good
because if I disconnect the transformer from the main board and turn
the machine on, then it does not blow the fuse.

Thanks,
Chase 


--- In prophet2000@yahoogroups.com, "duncan" <ferrograph@...> wrote:

>
> >>Any other suggestions?<<
> 
> yeah- the series regulators that provide the 5V & 12V rails into the
> rest of the instrument. 78xx & 79xx devices- these are the things you
> see fastened to the back of the case inside, probably with what looks
> like toothpaste (thermally conductive paste, in fact) around them.
> these things (78xx, 79xx) frequently fail in older gear.
> 
> you need to get a tech in there, with a voltmeter. your p2k may be
> perfectly alright once these are replaced & working again. but you've
> reached, with the greatest of respect, the limit of what you can do
> without a voltmeter & some component-replacement chops.
> 
> hth-
> duncan.
>

>>By the way, I know the transformer and everything before it is good
> because if I disconnect the transformer from the main board and turn
> the machine on, then it does not blow the fuse.<<

then it might be the bridge rectifier(s) between the output of the
transformer & the regulators you just changed. it could still be the
remaining regulator, but as you say this is now unlikely- the -12V
rail it supplies is quite a low draw. also look at the smoothing
capacitors & any tantalum bead capacitors around the power supply.
can you be sure that the main boards themselves aren't the problem?
can you disconnect them from the power supply? I will get the lid off
my 2k2 this weekend & have a look to remind myself.

d.

I recently had what sounds to be a similar problem with my P2K and I
took it in to be serviced.
The problem was traced to a defect in the power supply.
I dont have my prophet opened up right now to take pictures but the
service invoice says that a Diode (part # 1N4007) was replaced.
And now it works fine. Apparently the diode is only $5 (Im in Canada)
but of course the service cost $90 in the end...go figure.Either way
Im happy to have that problem out of the way.
I remember the diode being to the right of the power supple on the
upper left part of the main board...I may be wrong but thats the area
its in Im pretty certain.
Hope that helps!





--- In prophet2000@yahoogroups.com, "duncan" <ferrograph@...> wrote:

>
> >>By the way, I know the transformer and everything before it is good
> > because if I disconnect the transformer from the main board and turn
> > the machine on, then it does not blow the fuse.<<
> 
> then it might be the bridge rectifier(s) between the output of the
> transformer & the regulators you just changed. it could still be the
> remaining regulator, but as you say this is now unlikely- the -12V
> rail it supplies is quite a low draw. also look at the smoothing
> capacitors & any tantalum bead capacitors around the power supply.
> can you be sure that the main boards themselves aren't the problem?
> can you disconnect them from the power supply? I will get the lid off
> my 2k2 this weekend & have a look to remind myself.
> 
> d.
>

Thank you very much for the tip.  I checked all of the diodes by the
PSU and found one that was shorted (1N5401).  I'm going out to try and
track one down now.  I will post back with the results some time tonight.

Thanks,
Chase

Thanks, it worked.  I replaced the 1N5401 diode with a 1N5402 and now
it turns on and seems to function as normal.  Now to upgrade the OS,
memory, and add outputs (I have serial number 80). I have a friend who
has a memory expansion board and OS IC (mine is 3.0) and I think I'm
going to try to get him to trade for my s900 then I can install the 8
outputs.  Anyone looking to sell anything?  I would hate to have to
deal with Wine Country.  

By the way, I can not get the Prophet to format disks.  I suspect
because I don't have single sided disks (I am giving it DSDD) and it
keeps saying nD (no disk).  Any source on single sided disks or any
suggestions on drive upgrade?  Can it be done, what drives, etc?

Thanks,
Chase

So, I had the floppy cable in upside down, but now I get a Lh (Locked
Head) message.  I'm buying the Teac 235HF drive and hopefully that
will be that.  Also, I'm hoping that I may be able to add individual
outs on this ala the tr-606 with the switching jacks.  I read
something about this on a search, but saw no post revealing the
results.  Has anyone successfully done this?  I don't want to mess
with the traces on the PCB so this seems like the only way I'll get
individual outs.  I will also save money.  I could really use the
schematics as well if anyone has them.

Glad to help. I suggest buying the schematics from Winecountry as the
ones on the net are not complete or clear.
They're only about $25 and they are VERY useful and detailed...plus
you will be helping them stay in business and I find they usually are
alot better about replying to your questions. (I love this group and
all but people don't always want to share their secrets :)
Of course if someone out there figures out how to add more memory to
the P2K for next to nothing($$)...then I'm there because I simply love
these things...exactly why I have two of them.


--- In prophet2000@yahoogroups.com, "plus_321" <plus_321@...> wrote:

>
> So, I had the floppy cable in upside down, but now I get a Lh (Locked
> Head) message.  I'm buying the Teac 235HF drive and hopefully that
> will be that.  Also, I'm hoping that I may be able to add individual
> outs on this ala the tr-606 with the switching jacks.  I read
> something about this on a search, but saw no post revealing the
> results.  Has anyone successfully done this?  I don't want to mess
> with the traces on the PCB so this seems like the only way I'll get
> individual outs.  I will also save money.  I could really use the
> schematics as well if anyone has them.
>