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Hi everyone - Just starting a new thread as the older one hit a dead end; not sure if people are actively checking this forum.
Thankfully, my DX7 with E! is no longer dead. I have things partially working - and I think the answer was that the E! needed to be reseated and the contacts cleaned. But now, something else is going on - as the battery replacement seems to have zero effect.
When it's been sitting, when I power it up, I get the?"RAM is Corrupt!" error message from E! -- and I've lost all the settings. Plus, E! seems to behave a bit erratically with different symptoms each time (no MIDI, missing internal banks 8-10, etc.).?
If I keep it on and wait for the?LED digit to change to "01" (maybe 4-5 seconds?), I can shut it off and turn it on again a second time - and then I won't get the same error message. But then,?of course I've lost all the settings as I've said.
So I'm thinking that the RAM and E! board may actually be fine, but somehow the first power-up doesn't have enough voltage to get started...and the second power-up then has enough voltage to?get things to get rolling...but by then?either the backup battery isn't applying enough voltage in there to keep everything stored in RAM - or it's losing the current elsewhere. BTW, the backup battery voltage appeared to be about 4.5V if I remember (as measured by E!), and the CR2032 battery was 3.5V.?
So I am wondering if the E! board may be fine, but the drop in voltage is screwing up E! with strange results upon rebooting.
@aerobaticant - Are you saying that you think the diode may be the reason why it's losing the voltage upon power-down?
Thanks to anyone who has any suggestion as to what to test or replace next!
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The diode is a candidate for causing that problem in a stock DX7, so I don't see why not.? Diodes are easy to replace, and you can try literally any physically small diode.
After that, look for capacitors to replace.? Start with the larger can shaped ones, especially any with swollen ends, or goo around them on the PCB.
But, yes, quite likely it's the diode.
- Andy
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That's great, Andy - and I really appreciate the reply. That's a huge help and I'll have to order one as I don't have any spares on-hand.?
Just bummed out that Radio Shack stores aren't around anymore (at least here in Boston), as I'd just run over there and pick up whatever I need.?
I'll also check the caps - I didn't see anything leaky or anything the last time I checked, but the larger cans - I wouldn't be surprised if they're lurking.
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Hi @smallbluetag,
If the zener diode has failed it can result in spurious writes to the RAM as the power is turned off. It won't necessarily matter how long the power is off since the corruption can occur right at power off, even if the supply voltage supplied to the RAM is still good.
On my DX7 it was causing the names and sounds of presets to become corrupted.
While powered up in normal operation, the RAM voltage is supplied from 5V via Tr1 (main board).
It's unlikely to be a supply voltage issue, but you can check. If you attach a DVM to the RAM power pins (24/12) you can see if the voltage is dropping too low when powered off. If the diode from the battery (D4) is good you should still have around 3V there. Note: the data retention voltage for the original TC5518CP RAMs is 2.0V.
As a slight aside, the 5V regulator on my DX7 was running really hot so I replaced it with a pin-compatible switching regulator (OKI-78SR-5/1.5).? :-)
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It's a small germanium diode, IIRC.
- Andy
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In my original post I was referring to the Zener diode ZD1 on the power board. This is a 6.2V 500mW Zener.
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I see.? I thought we were talking about the small diode that protects the backup battery from the 5V supply, and supplies power to the RAM chips when the supply it turned off - that one is a known culprit for losing the contents of the RAM, on a stock DX7.
- Andy
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Hi guys,
Which diode goes where? I just ordered a set of diodes that should arrive tomorrow:
"Zener Diode kit 500mW +/- 5"?
(1) aerobaticant - I figured replace ZD1 (6.2V 500mW Zener) per your suggestion.
(2) andy2no - Can I use any of the above diodes to replace the one you're talking about? If so, what and where?
Thanks, guys!
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No, zener diodes are a special type of diode, that's just made for a basic kind of voltage regulation.
Any small signal diode would do, for the 3V battery one, but since you're ordering one, it was originally a germanium diode - an OA95, according to the schematic, but any small germanium diode would work if you can't get one of those.
In the attached picture, you see it labelled as D4 - just above the middle, on the left.? The orientation is important - diodes have two different ends; an anode and a cathode.? They often have a stripe across the cathode end - which is shown pointing to the right, in the picture - pointing away from the positive terminal of the 3V battery.
An OA95 is a clear glass bead, with a red or dark coloured end, indicating the cathode - see attached example.
If you have a multimeter / test meter, you should be able to check continuity between the positive terminal of the 3V battery, and the clear end of your existing diode (the anode).
I don't have a picture of the physical location, but I would guess it's quite close to the battery.
- Andy
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Thanks so much, guys. I figure ask twice and solder once before removing the DX7’s motherboard!?
The 6.2v Zener diode for location ZD1 has arrived (in a kit), but I don’t have the other?OA95?diode yet for location D4 (or a spare kicking around, unfortunately).
1) Can I test either diode on the motherboard with my rudimentary multimedia without removing or unsoldering anything first? Also, I’m assuming it would be tested while it’s powered on? It would be great to be able to successfully test it. Plus, if the one that’s still in there is good, I can just return this kit.
2) Which would you check/replace first? The Zener diode at ZD1, or the OA95 diode at D1?
3) As you said: ?"It's unlikely to be a supply voltage issue, but you can check. If you attach a DVM to the RAM power pins (24/12) you can see if the voltage is dropping too low when powered off. If the diode from the battery (D4) is good you should still have around 3V there.”?
I’ll definitely check this…you’re saying that if the voltage drops when it’s off - that Zener diode is shot? I remember briefly testing the voltages and things seemed a bit squirrelly...
4) BTW - Maybe I’m missing something, but I couldn't see the ?attached photo from the last message. As I’ve worked with diodes before, thankfully I’m good per orientation - as that’s really important. :)
Thanks again! - Seth
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I don't know what happened with the attachment.? Yahoo messages can be very mysterious. I'd just googled for images of an OA95, and added one.? The chunk of schematic is the main bit.
You can check the OA95 (D4) by measuring some voltages with the power off, and with it on.
1)
With the power on, the 3V battery voltage shouldn't be significantly higher than with the power off.? If it is higher, the OA95 may be leaking.? You should see 5V at the cathode, and 3V or so at the anode - the end that connects to the positive terminal of the 3V battery, if it's not leaking.
2)
With the power off, you should see a voltage drop of 0.3V or a bit more, from the anode of the OA95 (battery end) to the cathode (RAM chip end).? Alternatively, measure the voltage at the supply pins of the RAM chips, and across the 3V battery.? If it's over 2V at the RAM chip supply pins, but a bit less than the 3V battery voltage, the OA95 is fine.
- I'm assuming you have a good battery.? If not, there could be a bigger difference in the battery voltage with the power off and with it on, which could confuse matters a little, but measuring the voltage from one end of the diode to the other, in each case, is still a good test. ? With the power off, it should be reverse biased and isolating the battery (cathode at a higher voltage than the anode).? With the power on, it should be forward biased, supplying power to the RAM chips (anode at a higher voltage than the cathode, probably by quite close to 0.3V).
The zener diode is a separate issue.? If the OA95 seems to be bad, replace it and see what happens then.
- Andy
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I concur with Andy regarding the OA95.
The OA95 diode's job is to supply RAM power from the battery when the main power is off.
The ZD1 circuit's job is to detect the power being turned off and put the RAMs into a safe state so that other parts of the circuit don't cause spurious writes of bad data as they get powered down.
Even if your battery is good, and D4 is working correctly, and the RAMs stay powered up (above 2V needed for data retention), if the Zener circuit isn't working then a write of random data could happen as the power is turned off. This would appear as RAM corruption when you next turn on.
I had this exact problem on my DX7 which I tracked down after changing the battery, D4, and several RAM chips, with help from this forum!
For interest, if you didn't already know:
A standard diode only allows current to pass in one direction, from anode to cathode, with a small voltage drop. This forward direction is the direction of the the triangle in its symbol. When the voltage at the cathode is higher than the anode (reverse biased) the current is blocked.
A Zener diode is a special type of diode that is designed to 'break down' at a particular reverse bias voltage and will allow current to pass from the cathode to the anode. In the case of ZD1 this voltage is close to 6.2V.
The location of ZD1 in the DX7 PSU allows it to detect if the voltage before the voltage regulator has dropped and ultimately turn off the chip selects on the RAMs before the output of the regulator (with lots of power supply capacitors) drops.
I hope this makes sense!
Ant.
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I think it does make sense! Just been a very busy week and I'm hoping to get back in there to work on the DX7 this weekend when I have a bit more time.
BTW - I picked up a 1N4004 diode for the OA95 replacement, as that was the only available general-purpose diode I could find locally. Would that work? If so, please let me know so I can return it if not before opening the packaging.?
You guys are a huge help! Thanks. :)
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Hmm, it's a bit hard to say, definitely.? It wouldn't be my first,
or even second, choice.
A 1N4004 is a silicon power rectifier diode, designed to conduct up
to 1A of current; while the OA95 is a germanium small-signal diode,
designed to conduct up to 50mA of current.? Since the DX7 only needs
a tiny current through the diode to keep the RAM from losing its
data, the current rating doesn't matter.
What *does* matter is the forward voltage drop of the diode.? This
is voltage lost by the act of conducting current, and is never
zero.? The drop varies with the current flowing through the diode,
but is generally lower for germanium diodes than their silicon
counterparts; and lower for small-signal diodes than their power
equivalents.? Looking at the data sheets, an OA95 is likely to drop
0.2-0.3V at the currents required, while the 1N4004 is likely to
drop 0.6-0.7V.
What does this mean to you?? In the best of circumstances, it will
probably work, but the backup battery is going to read as 'bad' long
before the end of its actual life.? A lithium coin-call battery is
about 3V when new, and it drops to around 2.3-2.4V when it's pretty
well used up.? I don't know the minimum voltage needed by the DX7's
RAM chips, but let's assume that it's 2V.? If so, an OA95 can work
until the battery is 2V + 0.2-0.3V = 2.2-2.3V; the 1N4004 can only
work until the battery is 2V + 0.6-0.7V = 2.6-2.7V.
In the worst of circumstances, some other property of the DX7
circuitry requires a higher voltage than 2V to work properly,
perhaps as high as 2.5V.? In this case, even a nearly-new battery
may not be able to provide enough voltage to meet or exceed this
situation, leading to data corruption.
If an OA95 isn't available, I would recommend (in rough order of
preference)
- Another small-signal germanium diode.
- A Schottky small-signal diode, such as a BAT81S or BAT83S.
- A silicon small-signal diode, such as a 1N914 or 1N4148.
- A higher-power germanium diode.
- A higher-power silicon diode, such as a 1N4001 or 1N4004.
Of course #1 through #4 are not likely to be found at the local
DIY shop; one would have to order them from Mouser, Farnell,
Allied, etc., or find one on eBay.
Just my $0.02USD ...
Regards,
-BW
Bruce Wahler
Ashby Solutions.com?
bw@...
978.597.7008
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?
I think it does make sense! Just been a very busy week
and I'm hoping to get back in there to work on the DX7
this weekend when I have a bit more time.
BTW - I picked up a 1N4004 diode for the OA95
replacement, as that was the only available
general-purpose diode I could find locally. Would that
work? If so, please let me know so I can return it if not
before opening the packaging.?
You guys are a huge help! Thanks. :)
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Bruce - Thank you so much! What a pleasure to meet you; I know about your half-moon switch and coincidentally visited your website last week. (I purchased a used Mojo 61 a few weeks back as both my DX7 and Hammond XB2 died before an important band event; I'll be sure to give you a shout if I go the half-moon direction - I've been a pedal guy for 30 years when it comes to Leslies.)
What's killing me is that not only are Radio Shack stores gone, but that I used to have tons of germanium diodes around as a kid (crystal radios, etc.). Thanks for the heads-up; I'll return the diodes and just order the exact item once and be done with it. Stay tuned and so many thanks.
- Seth
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A simple 1N4184 does the trick - BTDT ;-)
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Considering the previous post describes in great detail why the 1N4148 is NOT the ideal choice, I would use it only if a germanium is not available. It shouldn't be that hard to find a suitable germanium diode.
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A small silcon diode, like 1N4148 would work fine, but a Germanium diode will allow you to squeeze more life out of a 3V battery, by dropping less voltage.?
If the battery is in a clip, it's less important than it was originally, but Germanium diodes are not anything particularly exotic, so get one of those, yes.
FWIW, I've bought 1N34A Germanium diodes on ebay - I got 20 for ?1, though they took a while to arrive, so you'd probably want to shop closer to home.
To check a diode is a germanium one (Ge), use a meter that has a diode test or put it in series with a 1k resistor and use a 9V battery, then measure across it.? In either case, it should show close to 0.3V.
- Andy
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Hi guys -?Sorry for the very delayed follow-up. Lots of stuff got in the way this past month. I think diode D4 is good, but I figured I'd see if you guys could confirm this.
I finally opened the DX7 back open, and here were my voltages:
(1) With the unit powered OFF:
Battery: 3.31v E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v Diode D4 3.25v
(2) With the unit powered ON:
Battery: 3.31v E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v
Diode D4 4.93v
(3) I also did a quick retest the unit powered OFF (about 24 hours later). The battery was accidentally drained a bit due to my accidentally shorting it for a few minutes during the CR2032 battery holder installation. Hopefully these are the correctly recorded values:
With the unit powered ON:
Battery: 2.83v (a bit drained as I just mentioned) E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v
Diode D4 — INACCESSIBLE under Grey Matter board
NOTE: D4?was measured with the E! board removed, as you can't access it otherwise. Test points were from one end of the diode and the transistor that it leads to (as you can't access the other side of the diode without removing the motherboard).
So does this mean that Diode D4 is actually fine after all??
Again, it loses its memory after 15 minutes or so and still has the same error message/symptoms.?Just trying to figure out the next steps here.?
Thanks again, everyone!
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?
3.25V on U4 with unit OFF sounds suspiciously high. It should be more like
3.00V if the battery is 3.31V. The germanium diode should drop it about 0.2 to
0.3V. It sounds like maybe some capacitor somewhere did not have time to
discharge yet after you turned it off. The most important thing here is the
voltage on U4 after about 15 minutes. Can you turn it off and watch the voltage
there for a few minutes to see where it ends up after 15 minutes?
?
Don Backshall
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From: smallbluetag@... [YamahaDX]
Sent: Friday, April 12, 2019 1:46 PM
To: YamahaDX@...
Subject: [YamahaDX] Re: Not-so-dead DX7 Mark I - The saga
continues...
?
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Hi guys - Sorry for the very delayed follow-up. Lots of stuff got in the way
this past month. I think diode D4 is good, but I figured I'd see if you guys
could confirm this.
?
I finally opened the DX7 back open, and here were my voltages:
?
(1) With the unit powered OFF:
?
Battery: 3.31v
E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v
Diode D4 3.25v
?
(2) With the unit powered ON:
?
Battery: 3.31v
E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v
Diode D4 4.93v
?
(3) I also did a quick retest the unit powered OFF (about 24 hours later).
The battery was accidentally drained a bit due to my accidentally shorting it
for a few minutes during the CR2032 battery holder installation. Hopefully these
are the correctly recorded values:
?
With the unit powered ON:
?
Battery: 2.83v (a bit drained as I just mentioned)
E! Grey Matter RAM chip U4 (pins 14 and 28): 3.25v
Diode D4 — INACCESSIBLE under Grey Matter board
?
NOTE: D4 was measured with the E! board
removed, as you can't access it otherwise. Test points were from one end of the
diode and the transistor that it leads to (as you can't access the other side of
the diode without removing the motherboard).
?
So does this mean that Diode D4 is actually fine after all?
?
Again, it loses its memory after 15 minutes or so and still has the same
error message/symptoms. Just trying to figure
out the next steps here.
?
Thanks again, everyone!
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AndyHornBlower
Michael Whocares