Ensoniq EPS-16+ Repair (stuck in test mode)

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belzrebuth
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Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Wed Mar 04, 2020 1:03 pm

Hi all,
I got an Ensoniq EPS 16+ RACK in need of repair.
I must say that I've already have a working unit so that may be of great help in fixing it.

The faulty EPS16+ is stuck on test mode when booting up.
Test mode confirms that the panel is indeed okay (all the characters on the screen are showing as the service manual indicates).
Sadly I don't have the schematics for the machine only the service manual.

Voltages are also okay and no significant ripple has been observed.
CPU has some signs of activity and I can see clocks etc on other chips too, cpu reset is fine too.

One thing that's suspicious is that the UART chip (MC68681P) is maybe preventing connections since it shows the following :
(in order of Rasputin's post concerning ensoniq VFX troubleshooting)

pin 20 [gnd] -> ok
pin 40 [vcc]-> ok
pin 32 [clk] -> little messed but has a waveform (see picture)
pin 34 [res] -> high
pin 35 [cs ] -> alternating between high and the waveform in the picture (bad?)
pin 10/11 [serial rx/tx] -> 10 high / 11 almost low
pin 30/31 [midi tx/rx] -> both hi
pins 16-19/ 22-25 [DO-D7] -> pulsing [ok]
pin 8 [r/w] -> mostly high with negative blips [ok]

Here's a link with the waveform images:
https://www.dropbox.com/sh/pgyh8oydyr1d ... C67Ha?dl=0

The CS pin on the UART chip comes straight from the custom es5700 chip.
It's mostly high but turns to the picture waveform and then returns to high state.
Each state lasts less than a minute but I think "high" state lasts longer.
I raised the CS chip pin and the pad underneath has the same behavior so it's not the chip that's affecting it somehow.
I got a new UART chip which is on the way but I don't really think that that's the culprit.
Another thing I forgot to mention is that when the machine boots up instrument leds #6 and #8 briefly light up and then all the instrument led remain lit.
I don't know if this an error code of some sort as I haven't seen anything describing led behavior in the service manual.
Any suggestions would be really welcome!

Rasputin
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by Rasputin » Fri Mar 06, 2020 3:13 am

belzrebuth wrote:
Wed Mar 04, 2020 1:03 pm
The faulty EPS16+ is stuck on test mode when booting up.
Sadly I don't have the schematics for the machine only the service manual.

One thing that's suspicious is that the UART chip (MC68681P) is maybe preventing connections.
For starters, get the EPS16 schematics: http://zine.r-massive.com/ensoniq-techn ... chematics/

Secondly, UART is absolutely critical, so if serial TX from UART is not sending anything meaningful then there's no way the display will ever come out of self-test.

I'll try to keep my eye of this thread and to think about it some more.

belzrebuth
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Fri Mar 06, 2020 3:27 pm

Thanks Rasputin.
I'm sure your valuable help will help me isolate this issue.
I'm concerned about that CS pin on the UART chip since it's coming from the custom 5700 ensoniq chip directly and it's mostly stuck high.

I've probed the 5700 chip and took a photo of my findings.
Here's a dropbox link:
https://www.dropbox.com/sh/9g3cq6dve4vd ... rfTCa?dl=0
You could check it out when you have time and maybe point out anything unusual.

I've also probed the NE556 chip and it's output (pin 9) is high.
Shouldn't it be oscillating?!

I know that at some point I should open my working EPS 16+ and start comparing the two units but I try to avoid it for the time being since It's all racked up and I'm using it almost daily.

Rasputin
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by Rasputin » Fri Mar 06, 2020 6:34 pm

belzrebuth wrote:
Fri Mar 06, 2020 3:27 pm
I'm concerned about that CS pin on the UART chip since it's coming from the custom 5700 ensoniq chip directly and it's mostly stuck high.
Don't make the mistake of thinking that I know everything there is to know about the EPS, VFX, or anything else. Some or all of my musings could be erroneous. It's something I'm interested in, but I'm no expert. :|

OSRAM and OSROM chip select are also served by the 5700 and on the same data bus as the DUART. The chip select is active low. My point being that the chip select for all those devices should primarily be in a high state, except for a brief period of time when they are on the data bus, right?

So do the OSRAM, OSROM, and DUART selects on 5700 pins 25, 38 and 41 all share the same behavior? According to your notes, they are simply "Hi", but if they are blipping low on occasion then that says to me they are having bursts of traffic.

Also, can you observe data moving from the 27C512 program ROMs? What I'm trying to get at is that if OSRAM, OSROM, and DUART chip select behaviors are the same or similar, and the data from the program ROMs is apparently being read properly then the chip select behavior for the DUART is probably correct.

If the program ROMs are not being accessed properly then that says more is at fault than just the DUART. The CPU is obviously not going to be able to send out the proper display initialization to the DUART if the CPU cannot even boot up its own program.

However... that's all probably academic because the DUART data lines show traffic. That means something is using the data bus, and it's a fair guess that the CPU is accessing the program ROMs, booting, and is now trying to send init data to the DUART so that the DUART can then init the display controller--which would take it out of self-test.

If DUART pin 11 does not show activity even after the CPU sends init data to the DUART then that means serial TX is broken--either the DUART is bad or something else is preventing it from changing states. Lifting the pin would be a way to rule out the later case.

ABOUT RESET ISSUES:

"I've also probed the NE556 chip and it's output (pin 9) is high. Shouldn't it be oscillating?!"

The 68000 micro, 1772 floppy controller, etc. use an active low reset. When the CPU (or any other IC depending on a power-on reset) sees the RESET line go low for a certain duration then it is forced into reset mode and reinitialized. Once the RESET line goes high then the CPU comes out of reset mode and goes into normal operation mode.

If the NE556 is generating the timing pulse for the RESET line then it would be expected that the NE556 output would be low for a very brief period after power-on (as the voltages stabilize) and then remain high until power-off.

I believe Q4 pulls the RESET line to ground until NE556 pin 9 "flips the switch" and allows RESET to come up to 5V.

belzrebuth
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Fri Mar 06, 2020 10:12 pm

Thanks for the valuable insight Rasputin, maybe you're no expert but you know more than me for sure.
So I feel safer and very grateful doing things knowing that you're here to help.

From what I gather I should do the following:
1) raise pin11 of DUART in case something else is driving it low
2) check for data activity at the 27C512 program ROMs
3) check 5700 pins 25, 38 and 41 (SRAM, OSROM, and DUART selects)

Be right back :D

edit :

1) raised pin 11 duart -> DC (about 1.38 V) maybe low if threshold is about 4V (?) (same thing as when it's connected) (really hope that the DUART is bad as it's already on the road)

2) there is plenty of data activity on the 27c512 EPROMS

3)
pin 25 -> high
pin 38 -> alternates between high and waveform shown in photo (two brief negative bleeps then back high)
pin 41-> little busy waveform shown in photo (squarish in nature/ bits?!)
https://www.dropbox.com/sh/p2pdmwngsoa5 ... wLCya?dl=0

belzrebuth
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Fri Mar 06, 2020 11:15 pm

I zoomed a bit on the pin11 and I can see faint serial data output periodically..
But I'm not sure for the 1.38V DC..I think it should be higher.
https://www.dropbox.com/s/axz9wggbpmzql ... 4.jpg?dl=0

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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by Rasputin » Sat Mar 07, 2020 1:08 am

belzrebuth wrote:
Fri Mar 06, 2020 11:15 pm
I zoomed a bit on the pin11 and I can see faint serial data output periodically..But I'm not sure for the 1.38V DC..I think it should be higher.
I agree. I don't think that is proper behavior. I would say there is a good chance that replacing the DUART is the solution or at least a key part of it.

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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Sat Mar 07, 2020 5:21 pm

I think that we should just wait for the DUART chip to arrive and go from there.
Worst case scenario would be to rule out the DUART
but I see no point to further troubleshoot if there's a key component on the way.
I'll post here when it arrives.

belzrebuth
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Mon Mar 16, 2020 1:59 pm

It was the UART chip after all, replaced it and the unit now boots.
Still need to test for basic functionality but things are looking better than before; that's for sure :P
Thanks for the help Rasputin.
I really appreciate it!

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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by Rasputin » Mon Mar 16, 2020 6:38 pm

belzrebuth wrote:
Mon Mar 16, 2020 1:59 pm
It was the UART chip after all, replaced it and the unit now boots.
Great to hear. Now to solve the SCSI mystery in the other unit...

belzrebuth
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Re: Ensoniq EPS-16+ Repair (stuck in test mode)

Post by belzrebuth » Mon Mar 16, 2020 6:42 pm

Yes, there's definitely something wrong there.
Now that I have two working units I will swap the SCSI board around and report to the other thread.
Hopefully the EPS 16+ saga ends soon :mrgreen:

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