Prophet T8 CPU glitch? Video link included

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Prophet T8 CPU glitch? Video link included

Postby WhinyLittleRunt » Wed Aug 15, 2018 2:49 am

Watch on youtube.com


I've included the issue and also I'm showing what happens when you hit the tune switch. Then just a general overall pan of the CPU and voice card boards.

So recently, another issue cropped up where as the synth warms up, I suddenly only get about one voice and the LEDs and active switches start to blink in a cyclical pattern. Tonight it only took about 20 minutes from a cold boot to this problem. When it first started happening I had left it on overnight (or longer because I forgot) and found the problem when I went to play it. I felt the lid on top above where the CPUs sit and it felt pretty hot.

Tonight I pulled the lid off, powered it off and waited only a few minutes. Turned it back on, seemed normal. Gave it about 3 minutes and it acted up again. I felt the 2nd CPU chip (right-side) and it was pretty hot to the touch. I mean, uncomfortable hot. Does that mean the CPU is bad? Maybe... I was told when this first happened to check the power supply voltages. I haven't done that yet :roll: because I haven't had time to really bust it apart. But, in terms of what has been done, the power supply was completely re-capped months ago and the voltage regulators changed out with new thermal pads etc. I re-capped the voice boards, and re-capped the pot boards with the display and switches. The pot boards were done more recently, and the problem surfaced after this now that I think back. Although, I can't see how that would relate to what is happening...

If anyone has any ideas, let me know... this could be similar to Prophet 5 stuff, I just don't know.
sequential circuits prophet t8 | oberheim ob-8 | moog minimoog model d | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | linndrum | roland cr8000 | korg kr55 | simmons sdsv | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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Re: Prophet T8 CPU glitch? Video link included

Postby Yekuku » Wed Aug 15, 2018 8:31 pm

Hi WhinyLittleRunt,
when the problem appears you should check the power rails for correct voltage and noise free operation.
It is obvious that a component starts to fail when the unit gets warm, unfortunately these kind of problems are the most difficult to solve because it could be anything.
Usually these problems are solved by using a hairdryer and a freeze spray in order to pinpoint the problematic part.
On cold start use a hairdryer to heat parts until the problem occurs, then use freeze spray to cool down parts until the problem goes away. This way you can find what is causing the issue.
Sorry I dont have any specific T8 repair suggestions as I have never seen one in real life.
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Re: Prophet T8 CPU glitch? Video link included

Postby WhinyLittleRunt » Wed Aug 15, 2018 8:50 pm

That's ok... I used Freeze spray on the offending CPU to see if that changed anything but it didn't. I'll keep poking around.
sequential circuits prophet t8 | oberheim ob-8 | moog minimoog model d | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | linndrum | roland cr8000 | korg kr55 | simmons sdsv | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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Re: Prophet T8 CPU glitch? Video link included

Postby Yekuku » Wed Aug 15, 2018 10:31 pm

Hi buddy,
I would try freezing parts like resistors, caps , TTL, Cmos etc
Do not spray on a wide area, try to focus on specific parts.
Before you go on freeze spraying frenzy , you have to find the affected area using the hairdryer trick and a cold start.
Also have in mind that there are stories of vintage components failing due to freeze spraying, so any overuse might do more harm than good.
Are you sure this is not a cold/broken joint?
When the problem appears, slap the T8 a few times and see if it affects operation.

edit :It would be wise to verify that the psu is working right before poking around and freeze spraying .
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Re: Prophet T8 CPU glitch? Video link included

Postby WhinyLittleRunt » Thu Sep 13, 2018 2:18 am

So here's where I am at with the T8 and this problem:

I really think it's the Z8002A processor. I originally checked the power supply to confirm the voltages were correct. Everything is good there. However, I did not check the power rails on the board itself when the problem started; I kept a meter on the power supply to verify there were no weird spikes or anything. I suppose I can check this too but, the CPU is getting really really hot. I used the hair dryer on it and got it piping hot and I feel as if the problem occurred way sooner from the added heat...

The T8 has two processors, a Z80 and then the final CPU is the Z8002A which one if its functions is to send the final CV to the 8 voices.

I'm just a total t00l when it comes to using a scope, not to mention I have an old analog scope that may or may not be on the fritz (my 4th one to date... why haven't I bought a digital scope yet? :roll: )
sequential circuits prophet t8 | oberheim ob-8 | moog minimoog model d | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | linndrum | roland cr8000 | korg kr55 | simmons sdsv | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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Re: Prophet T8 CPU glitch? Video link included

Postby WhinyLittleRunt » Mon Sep 17, 2018 11:49 pm

Well... it wasn't the processor...
sequential circuits prophet t8 | oberheim ob-8 | moog minimoog model d | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | linndrum | roland cr8000 | korg kr55 | simmons sdsv | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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Re: Prophet T8 CPU glitch? Video link included

Postby meatball » Wed Oct 03, 2018 7:43 pm

possibly have a short going through the processor ... do you follow synthchaser on youtube? he recently did a repair (blow by blow) on a prophet five with similar-ish issues ... you will most definitely get a better grasp on your problem watching that
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Re: Prophet T8 CPU glitch? Video link included

Postby WhinyLittleRunt » Wed Oct 03, 2018 11:48 pm

Yes, I follow Synthchaser for sure. We've had some conversations related to this problems actually. I just ordered a replacement Z80 because as a test, I had heated the chip up pretty hot and then powered it on from a cold start and it glitched out immediately, so maybe I'll get lucky and that's the problem. I'll keep posted.
sequential circuits prophet t8 | oberheim ob-8 | moog minimoog model d | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | linndrum | roland cr8000 | korg kr55 | simmons sdsv | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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