Bought a Prophet T8... how alone am I?

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Bought a Prophet T8... how alone am I?

Postby WhinyLittleRunt » Mon Sep 04, 2017 4:01 am

Well now...

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Just picked this up yesterday at Main Drag in Brooklyn. Been there a while, kept eyeballing it. I had known about the T8 because of Joe Zawinul, and I really like the fact that it's weighted and has aftertouch, obviously amongst other cool features. I traded them my Jupiter 6 and some other stuff. I have another thread about the JP6 buyers remorse and honestly, I really just couldn't get on with it. I keep going back to my OB-8. The Jupiter has a crazy arpeggiator (Europa-enabled) but it also didn't play well with my analog drum machines. Kinda ruined it for me. It was also a grief-spending/impulse buy at the time when I had some inheritance, and it just ended up sitting most of the time. So, that out of the way...

I haven't "fallen in love" with the T8 yet but it's going to be way more beneficial in the long run for sure given my playing style is more 'piano' than 'synth'. Having some experience with the Rhodes Chroma I can honestly say it reminds me of it quite a bit, not sure which really sounds better or if I can even compare; I like that it's hands-on more so than the Chroma if you consider the Chroma as-is without any controller/Enabler. Anyway the reason I thought the T8 could be a good idea was because I could get Prophet 5-like sound with 8 voices and weighted keys, allowing me to play a bit more comfortably. The Prophet 5 will always be on my list but with 5 voices, I still don't know if I'd adapt a different playing style....

So, here are my questions:

1 - who works on these and actually knows them (aside from Wine Country)?

2 - does anyone have a reliable source for the factory patches in WAV format?

3 - I'm getting what I feel is a light overall distortion ... I've only been messing with it since yesterday but it definitely feels like there is something there that I didn't really pick up on until I was home in the quiet with headphones on.

Hopefully someone here can point me in the general direction... worst case I will email Wine Country...
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: Bought a Prophet T8... how alone am I?

Postby synthRodriguez » Mon Sep 04, 2017 12:29 pm

Congrats on the T8 purchase, my favorite action. Pretty much my only 'seller's remorse' synth.

The black knobs indicate those parameters which are stored when saving programs. To that end, the silver knob on the Prog Volume (programmable volume) should be swapped with the Sequencer Rate knob which is not stored in patches. You see these swapped frequently in photos which is odd, it's clear in the manual which is which.

I have a good wav file for the factory patches, PM me your e-mail address and I'll send it over. The T8 likes a hot signal and be sure to use a high-quality cable as well; I recall finding only one in my gear that would consistently transfer data. Clean the jacks with contact cleaner on a plug before trying it.

Regarding the distortion, is it coming out both the Left Mono and the Right Mono outs, or just the summed Mono Out jack? The Left/Right are summed into the Mono Out through an op amp, isolating it to either the Left or Right side would go a long way toward figuring out the problem. If it's not distorted out the Left and Right but just the summed out, you've pretty much nailed the problem as the summer op amp. Likewise if it's only distorted out one side or the other, it could be the op amp for that side before it hits the summer amp. Are you 100% sure it's not just a dirty jack or bad cord?

Not sure who in your area works on them. I do, but I'm in Ohio, a long ways away.
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Re: Bought a Prophet T8... how alone am I?

Postby WhinyLittleRunt » Mon Sep 04, 2017 3:28 pm

synthRodriguez wrote:Regarding the distortion, is it coming out both the Left Mono and the Right Mono outs, or just the summed Mono Out jack? The Left/Right are summed into the Mono Out through an op amp, isolating it to either the Left or Right side would go a long way toward figuring out the problem. If it's not distorted out the Left and Right but just the summed out, you've pretty much nailed the problem as the summer op amp. Likewise if it's only distorted out one side or the other, it could be the op amp for that side before it hits the summer amp. Are you 100% sure it's not just a dirty jack or bad cord?


So the distortion is the same on all 3 jacks. I should probably clarify, it's not really a "distortion" per se, but it just feels like there is something there that's noticeable, if that makes any sense... almost wonder if it's just the filter that's grainy and i'm hearing it as distortion? I thought about reloading the patches just because one of their techs said he loaded all the factory patches but 2 of them were corrupt. I don't know if it's only 2, but I do know that on a few programs, especially when you link the left and right, say I choose 65, and if I type in 65, the left channel will end up 37... that happens on a few of them, but the rest all link the program correctly between the two channels. I also don't know how or where they got the factory sounds from. I'm not saying they couldn't have already had them but I don't think they would have bought the cassette from Wine Country...

I also don't know if the power supply has been recapped or worked on. I don't really want to start opening a can of worms on a very expensive instrument that works, but I almost can't help myself but to start tearing into this stuff preparing it for the future...
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: Bought a Prophet T8... how alone am I?

Postby WhinyLittleRunt » Mon Sep 11, 2017 1:14 am

I haven't been able to solve the problem yet but I did do a few things and came to some conclusions...

1. The power supply 15v rail was high by about 1/2 volt. Adjusted that back down to 15v.
2. I replaced the 2 op-amps in the output stage; didn't fix the problem.
3. I am noticing that as the unit warms up, the static 'pop' noise I'm hearing is far more apparent. I'm also thinking that it's coming from the left voice board. I'm not certain about that, but it seems that when I roll the knob to the left, I hear the pop, but when I turn it right, I don't think I hear it. However, the right side seems to be where I'm getting the distortion. At least it seems that way. Mixed together, you'd never be able to tell. The pop also feels like it's part of the filter. I'm tempted to try swapping chips around but I'd like to figure out which voices are actually active when I'm hearing the pop... in the service manual, during the scaling procedure it states that you can probe pin 8 of the oscillator to see which voice is active, and I'm wondering if this will work in regular mode as well?

FWIW:

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I don't know who hot-glued the trimpots on the voices either, maybe the tech at the place I bought it from? But that was super annoying...
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: Bought a Prophet T8... how alone am I?

Postby madtheory » Mon Sep 11, 2017 10:49 pm

The board traces are a work of art.

Beautiful synth inside and out.
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Prophet T8 Service / Repair

Postby WhinyLittleRunt » Sun Sep 17, 2017 4:25 am

So I got a little further on the T8 and narrowing down where I think the issues may be...

To start, I believe I have the 'pop' narrowed down to voice 4. I drop the filter to nothing, no decay, etc... and raise the cutoff just so I can hear the faint idea of a pitch coming through, and then I cycle through the voices (play notes in succession) and when it hits voice 4, it is just a pop/click sound, barely any audio behind it although if you raise the filter decay, etc, it will blend back in. I'm determining it as voice 4 with my scope and monitoring when I see a change on the sawtooth output of the oscillator; that was the only way I could track down the voice, since I can't disable voices. I've switched pretty much all the socketed stuff over to voice 3 and vice versa. No change in the issue after chips were swapped.

-I recapped voice 4. No change. I'm looking at the voice boards and up close I can see a lot of trace repairs etc... not good. For all I know something is shorting. Also, while recapping I noticed that some of the caps on different voices seem to be installed in opposite directions. In other words, you'd think the cap orientation would be consistent across all the voices but it doesn't look like it. The caps do appear to be original though so I'd guess that's fine.

-Regarding the distorted output, believe it or not (genius idea..lol) turning down the oscillators in the mixer section seems to (I think) resolve that. I know at full mix volume they should not distort, so I will have to investigate where those tie into.
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: Bought a Prophet T8... how alone am I?

Postby tommym » Sat May 12, 2018 5:31 pm

RUNT TYPED: I don't know if it's only 2, but I do know that on a few programs, especially when you link the left and right, say I choose 65, and if I type in 65, the left channel will end up 37... that happens on a few of them, but the rest all link the program correctly between the two channels.

That's not a bug- it's a feature. It enables you to (when the LINK button is enabled) to call up a split or double patch made up of the 2nd one on the other side (Left if you're in Right, or Right, if you're in Left) of the instrument. It gets saved with your patch when you save it. But if the LINK button is not lit, when you recall the patch, the Linked (possibly differently numbered) patch on the 'other' side, will NOT be recalled.
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Re: Bought a Prophet T8... how alone am I?

Postby WhinyLittleRunt » Tue May 22, 2018 4:38 pm

tommym wrote:That's not a bug- it's a feature.


Yeah, I think I was able to finally understand that now that I've gotten to play with it a while.

I was actually going to post an update on the issues I am having with it still, though they're not deal-breakers. Just wondering what could be causing a couple things:

-Tuning suddenly became audible. Used to switch off and go mute as it tuned. Now it's full-blast as it runs through its tuning cycle... tried replacing some components that I thought pertained to the tuning but made no change.

-Still getting what I call a 'harmonic distortion' when I play a mellow patch with the filter turned way down. Almost like it's slightly overdriven. I've pretty much confirmed that with the OSC mixer turned up full on both oscillators and set to normal volume I hear a buzzy harmonic distortion. When I turn the mixer down to say, 3, and turn the volume up nice and high, I can confirm the signal sounds nicer and more balanced, really no distortion. Also, I say "harmonic" because it's really noticeable when both oscillators are balanced together or slightly de-tuned, as I would suspect since they're wavering in and out.

Does anyone know what circuit in the T8 is managing the actual OSC mixer section?
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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