Vintage Synth Repair

Anybody have any experience with Prophet 600 repair?  I'm working on one
for a friend.  When you turn it on, power is supplied in the correct
voltages to the right places, but no sound is produced and the LCD
screen doesn't even light up.  Every once in a while when you turn it
on, you will get a random synthesized howling from all oscillators, but
this sound cannot be affected in any way by any of the knobs, switches
or other controls - not even the pitch control or volume control!  I'm
trying to see if I can cadge a Z-80 microprocessor chip off of someone I
know, but that's pretty much pure guesswork...

BaM

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http://www.rocketparkmusic.com/

Brian Andrew Marek wrote:

> Anybody have any experience with Prophet 600 repair?  I'm working on one
> for a friend.  When you turn it on, power is supplied in the correct
> voltages to the right places, but no sound is produced and the LCD
> screen doesn't even light up.  Every once in a while when you turn it
> on, you will get a random synthesized howling from all oscillators, but
> this sound cannot be affected in any way by any of the knobs, switches
> or other controls - not even the pitch control or volume control!  I'm
> trying to see if I can cadge a Z-80 microprocessor chip off of someone I
> know, but that's pretty much pure guesswork...

Of course I meant LED, not LCD, screen... :)

BaM

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Find out more about Rocket Park at our web page:
http://www.rocketparkmusic.com/

On Sun, 17 Mar 2002, Brian Andrew Marek wrote:

> Anybody have any experience with Prophet 600 repair?  I'm working on one
> for a friend.  When you turn it on, power is supplied in the correct
> voltages to the right places, but no sound is produced and the LCD
> screen doesn't even light up.  Every once in a while when you turn it
> on, you will get a random synthesized howling from all oscillators, but
> this sound cannot be affected in any way by any of the knobs, switches
> or other controls - not even the pitch control or volume control!  I'm
> trying to see if I can cadge a Z-80 microprocessor chip off of someone I
> know, but that's pretty much pure guesswork...

  I have repaired Prophet-600s in the past.  It certainly sounds like the 
Z80 microcomputer sytem is not running.  I would suspect the program ROM 
first, then the RAM and CPU.  Entirely possible a glue logic IC failed as 
well.  What software version is the program ROM labeled?

Crow
/**/

Actually, check the CPU reset circuit first.

Crow
/**/

The Old Crow wrote:

>   Actually, check the CPU reset circuit first.

That had occured to me upon glancing at the schematic, in fact.  What would I
be looking for (voltages in various place under certain conditions, et
cetera), and how could I manually reset it for testing purposes?

BaM

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Find out more about Rocket Park at our web page:
http://www.rocketparkmusic.com/

The Old Crow wrote:

>   I have repaired Prophet-600s in the past.  It certainly sounds like the
> Z80 microcomputer sytem is not running.  I would suspect the program ROM
> first, then the RAM and CPU.  Entirely possible a glue logic IC failed as
> well.  What software version is the program ROM labeled?

The label that fell off the EPROM says "0-7".

BaM

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Find out more about Rocket Park at our web page:
http://www.rocketparkmusic.com/