Prophet 5 Rev 3 All Lights Illuminated No Sound

[dr_ew]

When I power on my Prophet every light turns on immediately instead of going into the Tune routine.

I'm trying to get a scope to test the CPU and clock, but i'm wondering if there is anything to try in the meantime to bring my Prophet back to life.

From the service manual it says this about troubleshooting:

"The general procedure is to first remove socketed PCB 3 devices: CPU, EPROM, SPAD RAM, and NV RAM-WHICH WILL CLEAR THE PROGRAM MEMORY!"

Does this mean to power off, pull a device, reinstall, power on?

 

[NoisyIndividual]

You need to start with checking all the Power Supplies first.

 

[capacitorless]

+1. I'm pretty certain they do mean reseat the socketed devices with the power off, like you said.

Edit: welcome to the forum. Best around!

 

[dr_ew]

You need to start with checking all the Power Supplies first.

Power supply seems fine. I'm getting +15, -15, and +5

 

[dr_ew]

+1. I'm pretty certain they do mean reseat the socketed devices with the power off, like you said.

Edit: welcome to the forum. Best around!

Yes they definitely mean to remove and reinstall with the power off, but i was just wondering if somewhere in the procedure the power is turned on with the device removed.

 

[capacitorless]

Not 100% on this particular synth, but I've wrenched on a bunch (biggest project in terms of time was a Memorymoog) and I've not yet run across that. I would suspect not, but I understand how that could be intimated from the text

Edit: it could very well be remove the devices and then test for shorts. Hopefully someone will chime in who's done this in the past.

 

[NoisyIndividual]

Power supply seems fine. I'm getting +15, -15, and +5

There are more than that.

 

[dr_ew]

You need to start with checking all the Power Supplies first.

There are more than that.

You are correct and I was mistaken.

I'm seeing incorrect voltages for -5v and +12v

 

[NoisyIndividual]

Some of the logic on the mainboard needs +12v, the VCO's need -5v.

Not a lot will happen without these.

 

[dr_ew]

Some of the logic on the mainboard needs +12v, the VCO's need -5v.

Not a lot will happen without these.

The Red numbers are what i'm seeing with no load attached to the power supply. +15 and -15 are fine.

+8 instead of +5
-28 instead of -5
+28 instead of +12

 

[Khron]

https://elektrotanya.com/sequential_prophet-5_sm.pdf/download.html#dl

Page 134 there seems to have the pretty straightforward schematic of the power supply (a handful of linear regulators).

+5V being 8 (instead of 5.6V or so, -5 being -28(!!!) and -12 being 28 points to some shorted-through 7805 / 7905 / 78(M)12.

There's no saying whether the circuitry they were powering, managed to survive such overvoltages though...

 

[dr_ew]

https://elektrotanya.com/sequential_prophet-5_sm.pdf/download.html#dl

Page 134 there seems to have the pretty straightforward schematic of the power supply (a handful of linear regulators).

+5V being 8 (instead of 5.6V or so, -5 being -28(!!!) and -12 being 28 points to some shorted-through 7805 / 7905 / 78(M)12.

There's no saying whether the circuitry they were powering, managed to survive such overvoltages though...

Any idea what could cause all of the voltage regulators on the power supply to short and stay shorted?

 

[Khron]

Any idea what could cause all of the voltage regulators on the power supply to short and stay shorted?

It's not "all the regulators" - as you've established, the +/-15V are fine, right? And, well, semiconductors tend to stay shorted, once they burn / get zapped, so i'm not too sure why one might expect them to "come back" for any reason...

78xx / LM340 seem to have a max input voltage of 35V; the 317/337 may well be a bit more rugged in that respect. The unit's mains input might've been switched to 115V and plugged into 230V mains, perhaps?

What's the history of this device? Did it just randomly suddenly conk out one day, seemingly out of the blue?

 

[NoisyIndividual]

Is it possible a human messed up?