I recently bought one of these units and ran into this exact problem.
The service manual was quite easy to find on the net and I noticed that the LCD seemed to be Hitachi HD44780 compatible. So, I bought a standard LCD 2x16 characters and hooked up but it did not work, the same blank display.
Information on the internet is quite sparse about this problem, but I found a post in Spanish which I figured out indicated a problem with the contrast voltage, see schematics below. After changing the termistor/resistor voltage divider to a 2k trimpot, see image below, the leftmost marked rectangle, the hooked up display worked like a charm. This proved that the electronics controlling the main parts of the electronic circuitry worked. Changing back to the original display, it did not work, so I realized that there must be another problem. Also, there was no way to fit the hooked up standard display with its larger footprint into the crammed space of the unit. Back to the drawing board to try to solve the problem with the original display.
Another post about this problem, buried within a review on Harmony Central indicated that there might also be a problem with the capacitors connected to the LCD controller. I had noticed these capacitors, and thought that they might be the problem. Electrolytes have a tendency to fail over time and when I removed and checked them, there are three of them, all seemed to have failed. The value of these capacitors is 4u7 16V and I replaced them with compatible Tantalum capacitors. See image, the rightmost rectangle. I fired up the unit and the display worked again!
To solve this problem, first try to change the resistor/thermistor voltage divider. The only thing needed is to lift the lid of the unit. If this does not work, change the capacitors, which need much more disassembly of the unit, mainly including removal of a metal plate that covers the front panel from behind.
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Now THIS is interesting. Thank you a million, I also have an EMP700 which I didn't use a lot but ran into the same problem. I'll tackle this after taking my GEM S3 back to work. Thanks a lot!
ReplyDeleteThe service manual is available online if you search. It is helpful to disassemble the unit, which can be a bit tricky. Just post if you run into trouble and I will try to help.
ReplyDeleteWhere did you get the termistor/resistor voltage divider? I have an EMP700 with the same issue, but I'm a newbie to electronic innards.\
ReplyDeleteDo you mean where did I get the components, or do you mean how I calculated the component values?
DeleteThe component for replacing the thermistor is a simple potentiometer, available in any local electronics shop. Or online, like this: https://eu.mouser.com/ProductDetail/Bourns/3345P-1-202T?qs=sGAEpiMZZMs26uXuX7MLMc0m%252bsRfP6nT
The value 2k Ohm is taken from the components that I replaced, to get corresponding behaviour as the thermistor.
I meant kind of both, so your response was great. Thanks!
DeleteI have a unit with the same problem. I will try your fix and see if I can get the display working again.
ReplyDeleteThank You
Harry
Hi,
ReplyDeletefor me the capacitor replacement didn't work. I cannot read anything on my LCD. How to replace the thermistor. Can I take a NTC 1k thermistor ? Why did you take a variable poti instead of the thermistor and how did you connect it with 3 pins if there are only 2 pins ?
O also notice that I have a resistor of 24 ohm on the backside of the LCD mounted (R1). In your screenshot there is no resistor. To replace the screen would be complicated because the pins are really small and connected via PCB
Hi
ReplyDeleteAre you sure you got the capacitor polarity right?
Yes, you can of course replace with a thermistor. I choose a trimpot because that was what was lying around. Just connect two of the pins together so that you get a variable resistor, i.e. the slider+one of the end terminals.
/Mats
Hi, yes the polarity is right. I did this a lot of times with old sega game hear consol with same problem. I will replace the thermistor too. Otherwise I will have a big problem :(
ReplyDeleteI am wanting to repair one of these for a friend. I would start by replacing the thermistor, but I am not sure how to source a new one, because I don't know how to read the code on the existing one.It is a grey disk, much like a ceramic cap, with brown, black, and red stripes top to bottom. Can you help with identifying type and value?
ReplyDeleteor
If I want to use the trimpot fix, I need more info as to how you wired it in. Did you just replace the thermistor with the trimpot wired as a rheostat, or did you connect it as a potentiometor,completely replacing R331 and R332 with the wiper connected to the junction between the two?
Then how did you set the trimpot? just at the midpoint of the sweep (assuming linear taper), or by measuring the resistances with a vom?
IF that works, it seems that I could just replace R332 with a normal 1K resistor, no? I am not sure why they would have used a thremistor rather than just a resistor, but there must be a reason.
Any help would be appreciated! Thank you.
My direct email address is groover at wgn dot net
It would be much simpler for me to communicate that way, I don't actively use my gmail account. I will check it if I don't hear from you after a week or so.
Thanks John McDuffie
I now see that my questions were partially answered in your response to Tiger Claw above - using the trimpot as a rheostat and leaving R331 connected as is - is that correct. And this again begs the question could I just use a garden variety 1k resistor instead?
ReplyDeleteYes, thats true, wire as a rhesostat, i.e. mid pin wired to one of the end-pins and solder it as a normal resistor.
ReplyDeleteI guess a 1k resistor should work just fine. It depends on the viewing angle you want. I dont really remember but I think when I put the trimmer about mid value, it gave me a more or less horizontal viewing angle, I placed the unit in a rack at that height.
If you buy e.g three fixed 1k you can try som parallel/serial combinations until you find the correct viewing angle. Check with your friend and then test angle before putting the lid back on.
Hope that helps
Thank you! This makes me wonder why they would use a thermistor rather than a fixed resistor, and whether the thermistor somehow enables the viewing angle to be more reactive in some way?
ReplyDeleteYes, good question. I thought about that also. Normally you would have a 'contrast'-setting/knob. Also, the thermistor is placed quite far into the unit, probably to compensate for heat when put in a rack, from itself and adjoining equipment. I guess they want to have viewing angle more or less horizontal, but by some reason detected angle to change with heat.
ReplyDeleteshould I assume the thermistor is bad and replace it before I determine whether to replace the caps, or replace the caps first? any idea?
ReplyDeleteI should go for the thermistor, the capacitors are trickier to get to.
ReplyDeleteActually, try to save the thermistor, be careful when desolder. If the problem is with the capacitors, you can then try to switch back the thermistor.
thanks! I'm going to try a 1K resistor now!
ReplyDeleteReally it will be the capacitors, they are terrible quality on the display. See my comment below.
DeleteHope you get it working again.
I did a little fix it guide here this week....
ReplyDeletehttps://supersynthprojects.com/projects/yamaha-emp700-display-audio-repair/
I have purchased an used EMP 700 from Japan.After plugging it to my socket here in India it produced some strange vibrating sound.The display and all other indicators lit up properly though, but I realised something was wrong and finally learned that I blew it up due to voltage mismatch.In Japan it is 110v but here in India it is 240v.
ReplyDeleteI just want to know if I am unable to find the original transformer which is about 15-20 yrs old can I replace it with any modern transformer that is easily available to fix it?
Thanks
Ok, sorry to hear about that! I took a quick look at the schematics but it doesn't say anything about either brand or secondary voltage of the transformer. Also, the 220 and 110V versions seems to be a bit different on the secondary side of the circuit, so probably just replacing the transformer does'nt help anyway. After the transformer there is a pretty straightforward linear regulation with a 7812FA, 7912FA and 7805FA providing +12, -12 and +5V respectively. I guess your best shot is to try to get hold of the service manual, I do not want to link to it from here but it should be pretty easy to find on the Internet. Then disassembly and check if there is any marking on the transformer that can guide you in finding a replacement and check if it is possible to replace the components that are different.
DeleteAn alternate solution is to use an external power supply, e.g. from an old PC since they always have +/- 12V and 5V.
However, there is of course quite a risk that the electronics are damaged... Good luck!