Korg sp-250 repair: no sound on outputs

 

 

This Korg came to me with no output sound. Looking around the around the power amp IC, I could see the standby/mute function (pin 5) was not going high, as it is supposed to. I was able to trace the trace back to Transistor 4, which did not ring out as a transistor should. Suspecting that the transistor was the fault, I searched for a replacement or a cross, and found that the transistor is actually a DTC114ESA, known as a "digital transistor". I had not encountered one of these before, but found that it is actually a regular NPN transistor with the biasing resistors built in. It is called "digital" because it is biased to be used as a switch, as an inverting switch. It saves the designer from having to bias the transistor externally with resistors.

 

From its datasheet, here is a description of the circuit:

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And here is how it appears in the schematic for the SP-250:

 

 

 

And finally, here is where it sits in the circuit board:

 





 

 

It is useful to add that the keyboard works and sounds fine with this transistor removed. By removing the transistor, you cause the mute rail on the power amp IC to go high right away. You may get a bit of a "pop" when turning on the keyboard though, so it is better to have it installed. However, a handy way to test whether you are having a problem with this transistor (or somewhere upstream in the standby circuit) would be to pull it out and see if the keyboard starts working again.

Korg Triton Touch Screen Replacement

This Korg Triton came to me with a broken touch screen. The touch screen, or "digitizer" is attached right to the lcd screen, and sits right above it. On this Triton, the digitizer did not work but the LCD was clear and in good condition. A new screen, available here, costs $250.00. This part includes the LCD as well as the digitizer. Replacement digitizers are available on ebay for 50.00 or so, so we went that rout, intending to attach the digitizer to the existing LCD.

The part that arrived looked a little different, but it was the right size. However the cable was smaller. Additionally, the cable is upside down (the conductive area of the pins is on the opposite side). Further, if you look at the picture of the original part, seen below, the pinout of the cable is slightly different. What would be pin 5 on the new screen was routed to pin 8 on the original part. See the picture below.

I was able to salvage a ribbon connector from a salvaged dvd player, and then go through the mental work of connecting the pins in the correct sequence with jumper wires to the korg circuit board. If you don't have a salvagable ribbon connector, I am sure you can find one on Digikey or Mouser, as long as the spacing is correct for the ribbon cable on the new digitzer.

 Routing the wires from the ribbon connector to the Triton circuit board takes some brain work, but I was able to achieve this, and it works perfectly. I routed pins 1 -4 from the digitzer to the same pins on the Korg Triton circuit board.  Pin 5 from the new digitizer goes to pin 8 on the Triton (that's the first one that crosses over.  Pin 6 from the new digitzer then goes to pin 5 on the internal connector, 7 goes to pin 6, 8 goes to pin 7, and if you did it right pin 8 on the Triton is already attached to pin 5 coming from the new digitizer.

This worked, I would recommend not giving up if you don't get it right. Double check your pinout or see if you need to completely reverse the pinout you tried. Don't physically attach the digitizer to the LCD before you are sure it works right, too.

Why not? In my case, I was scratching my head because the X and Y axises seemed to both be reversed on the new touch screen... if moved my touch more to the left, the touch would register more to the right..Ditto for up and down I realized that, rather than fooling around more with re routing pins, this problem was solved by turning the digitizer around 180 degrees. I was glad I hadn't attached the digitizer o the LCD with the double sided tape yet.

Finally, after that I found that the new screen had to be calibrated. The very top areas of the touch screen didn't work. Worse, I couldn't access the calibration feature in the korg because this requires access to the upper right area of the touch screen to accomplish.

Performing a reset does not reset the calibration (enter+o).

However, entering the internal test mode does erase the calibration data... so I was able to do so by doing that. The test mode is entered by pressing 0 + 4 while turning the power on. That info is available in the service manual, which can be googled and downloaded for free.

Finally, since I had done the reset, I had to reload all the factory sounds. I did this using my  floppy emulator drive. These seem to work with the Triton, but you have to play around with the jumper settings, and for me it only worked if I first let the triton format the drive, then put it in the computer and loaded the factory sounds, then moved it back to the Triton.

SO, if you are in a position where you have a replacement digitizer that has the wrong size cable, don't give up, but be prepared for an ordeal, or send it to here Offbeat Electronics and we will do it for you.

The Original Ribbon Cable rerouts some of the pins. This is not done on the replacement.

The original triton digitzer has a wider cable.

Alesis DMPRO repair

Alesis DMpro

 This unit had no outputs when it came to our shop. The issue with these Alesis drum machines is that the filter capacitors fail, and some of the other electrolytics. I believe it is because they are really small.... that's why I always recap the whole unit with higher voltage capacitors, and if they are a little bigger I move them a little bit as shown in the picture below. A small bit of hot glue holds them in place, and they last much longer than the original ones.

Midas Verona Soundboard

At Offbeatelectronics.com, we repair soundboards and mixers as well as keyboards and synths. We do on-site work as necessary, and assure the quality of our work with an excellent warranty.

This particular mixer resides at the Jubilee Fellowship Church in South Denver; an excellent place. This repair was performed on site, due to its large size.

Don't forget a torx screwdriver when taking this thing apart! 

The main issues were the necessity for a thorough cleaning, and deoxidizing. Other issues were the result of some loose ribbon cables between boards, and the most serious issues were broken TRS jacks on the outputs and inputs. On many of the insert jacks, the tip connector was broken off, making the inserts appear to be malfunctioning. It seems that the 1/4" jacks are prone to freeze to the tips of the plugs, and the jacks break when the cable is pulled out.

This unit was in excellent condition after we performed the repair.

These jacks were the culprits in many issues with the soundboard

Roland KR650 repair: bad electrolytic capacitors

This particular model suffers from leaky electrolytic capacitors on the main board. By leaky, I don't mean electrically; I mean there is some nasty liquid leaking out of these things! The result is all kinds of distortion in the outputs, low volume, intermittent problems, etc.

I have repaired a few of these. The repair has always been permanent, however on most instances the liquid that leaks from the capacitors burns through traces on the board. On this one, the liquid burnt out the V- supply to one of the op amps near the DAC chips.

So, the repair consisted of removing all the surface mount electrolytics on the main board, cleaning up all the liquid, replacing the caps, and repairing a trace that supplies the V- to one of the op amps near the DACs.  Other traces may be damaged too, so if it still doesn't work when you replace the caps you will have to check the traces that run near them.

I used standard through hole caps instead of surface mount on this job. I bent the leads and attached them to the pads; there is plenty of room for this, and I applied a bit of hot glue on the leads to hold them in place. If my memory serves, they were all 3.3 and 10uF caps.

The instrument sound great now. I also replaced all the tact buttons. There sure are a lot of buttons on that thing!


Finally, the contacts on this one were good. However, I think this was a transitional model, and the key contacts are no longer available. This may be a consideration in judging the cost effectiveness of repairing this model; if the contacts show signs of failing, it would be worth trying to repair them first (perhaps by recoating them) before investing time and money in fixing the other issues.

The highlighted area is where I found destroyed traces, supplying the V- to one of the SMT op amps near the DACS

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Yamaha DX7 repair : no sound, new battery, loading patches

This dx-7 suffered from no output volume. The solution involved replacing the relay which comes just upstream of the outputs (both phones and mains). . It's main purpose is to disconnect the outputs during power up and power down, so that there is no annoying "pop".

For testing purposes I removed the relay and connected the outputs with the little jumper wire you see in the photograph; this enabled me to make sure my diagnosis was correct, and that the keyboard worked with the relay bypassed. It then continued to work once I put a new relay in.

A photograph of the mainboard with the relay removed and bypassed is shown below.

Relays seem to be a common point of failure in older gear, especially audio equipment. I have found bad relays in casios as well.... in fact the muting circuits in many keyboards, primarily in place to eliminate the popping sound present when turning the unit on and off... are in general prone to failure and should be inspected. Other newer keyboards use fets to shunt the outputs to ground, and these fets sometimes go bad and cause distortion or clipped volume.

I recently worked on and RD300 which used fets for this purpose.. they had started to fail and caused clipping and distortion on the outputs, and a considerable loss of volume. I removed them and it sounded like a brand new keyboard.

Anyway, I also put a new battery in the DX7, as well as a socket to make future battery replacement easier (see below).

 Loading the presets is possible through midi, but there are a number of steps you have to go to on the DX7 to get it ready to receive midi sysex messages. The info here is useful.

Another Farfisa Problem : hum in outputs, reverb tank, and bass tones

For those of you wondering how the bass tones are developed on your Farfisa compact (the black notes), they are simply the lowest octave notes sent through a filter. What I mean is, there is not another divider on the oscillator cards for those lowest octaves, those notes are just the same ones as the lowest octave but are routed through a filter before being sent to the outputs.The card with the filter is shown here:

The signal goes into that card from the contacts for the bass notes, and comes out and is routed to the output.

Another note:

 The old high voltage capacitors shown in this picture, below, were completely dead in a unit I recently worked on, and this added a lot of hum to the outputs. These caps need to be high voltage because they filter the power to the tube pre-amp. Apparently the Farfisa used this style preamp because they wanted it to work with their pizo-electric reverb tank, which is discussed on this blog .
This style of tank was used for reasons discussed here.

Since the tank was completely destroyed in the unit I was working on, I did manage to get a more traditional spring reverb tank working in the farfisa, but it required that I add a preamp in the circuit to drive it, not on the recovery side but on the driver side.I also replaced the rectifier, shown here. It is a more modern one and I was ultimately able to attach it to the inside of the chassis for the preamp.