Fatar SL-880 Repair - replacing crystal with oscillator

This keyboard would not turn on reliably. I did at first attempt a hard reset, pressing UP/DOWN/ENTER while turning on. This had some effect, but it still frequently failed. I also tried the instructions referenced here, which possibly helped but not completely.

I did find that when it was locked up there was no clock pulse around the crystal... however replacing the crystal had no effect. After downloading the datasheet for the micro controller: ST9036, I found that the crystal could be replaced with an oscillator, a device which creates a pulse on its own, eliminating the need for much external circuitry.

I had a 24mhz oscillator around, and connected it to ground and to the 5 volt regulator. I removed the old crystal and the small value caps that were around it (they are not needed with an oscillator) and made sure that the output of the oscillator went to the "osc in" pin on the microcontroller.

Somewhat surprisingly, this worked perfectly!


I glued the oscillator to the board, and the unit has worked fine ever since. It is not a pretty repair, aesthetically, but it does work!

I tested the idea with a breadboard first, connecting the osc output to the microcontroller with alligator clips

The oscillator is similar to the crystal, but needs to be powered with 5 volts, and doesn't need to have capacitors around it

The fixed board has an oscillator glued to it where a crystal once was.

ARP 2600 Repair: Blown Tantalum CAPACITOR, dead 15v rail!

This nice instrument came to me completely dead. One of the trimmers on the PS board was cracked... but even when replaced one of the rails (-15v) was dead.
The 2600 is actually fairly easy to troubleshoot...the issue boiled down to a shorted tantalum capacitor, visible in the second photo.
Tantalum caps are easily burnt out... I am not sure if the broken trimmer caused a spike which burnt out the capacitor, or if the shorted cap caused the trimmer to heat up and crack.

The burnt out cap is shown below.

Farfisa Mini Compact Distorted Divider

This Farfisa came to me with distortion on particular notes: the D's, except for the highest, were distorted.

The first step is dis-assembly. To get inside one of these, you remove the plastic cap on the top first (the red plastic thing). That is easy enough... just  a few screws.

Then you get underneath it (have it standing on its legs) and remove all the thumb screws (5) that are hanging down. You can then lift the whole chassis up and out of the wooden case(but you can't lift it very far, just high enough to sit i on top of the wooden case, as shown below.

Oscillator boards, which are accessible through the back, when chassis is lifted from the wooden case...

Looking at the distortion with a scope revealed that the 'divider' circuits were producing a modulated oscillation, they were not cleanly dividing the incoming wave... they were erratically triggering and including erratic harmonics which didn't belong.

This was a difficult one to troubleshoot, because the problem was somewhat intermittent.

I made some progress by replacing the tuned caps for the effected divider, C3a and C3b, especially when I used appropriate polystyrene capacitors (good and stable for oscillator circuits). But this just made it a bit less intermittent, but did not solve the problem.

I replaced the germanium transistors for that divider, thinking they might be noisy but this did nothing.I have seen them fail too, but when they do they cause silence, or they cause the divider to output its input tone and not divide.

Ditto for replacing the 1uF coupling caps which output the divided signal to the key contacts (circled in yellow below). I have seen these fail as well, but they either cause the divider output to go silent, or distorted in a different way: they don't seem to interfere with the frequency of the divider's output when they fail, and the distortion I was witnessing was a modulated frequency.

I did notice that putting a high ohm resistor across the caps (okay I will admit it was actually my thumb) that couple each divider to the next one downstream would cause the distortion to go away. Ditto for a high ohm resistor coupling one divider output to another! I realized at that point that the caps which couple one divider to the next were the culprits.

Those I have circled in red in the schematic below:

You will notice that the schematic calls for different values for these caps, between the different dividers (470pf, 1nf).

 I have on several occasions found these boards with ONLY 1nF caps installed as coupling caps between all the dividers, and they work fine. I am not sure if this is something Farfisa changed as time went on.

Anyway, adding a 1nf Polystyrene cap between the dividers fixed the problem.

Perhaps that original coupling cap, between the dividers, drifted over time and with heat... and did not feed the next divider downstream a good steady waveform to divide.

Anyway, I hope this helps anyone working on this issue!

Another blog by another guy who deals with this type of stuff and seems to have maintained his sanity is here.

Elka Synthex Repair

This Synthex came to me for two reasons... primarily for the replacement of some very bad potentiometers, which was pretty straightforward after I found some adequate replacement ones from Digikey, and secondarily due to some issues with the Square Wave output from the LFO.

This latter issue required a bit of diagnosis, an endeavor which was impeded by the strange (Italian) design of the Synthex. I came to find out that the Synthex is particularly hard to work on because 1) the schematic lacks any labels or circuit descriptions, and 2) the circuit boards are essentially piled on top of one another, so you cannot insert a test probe onto any circuit boards except for those on the very top.

As far as dealing with the schematic, I was able to find the LFO output on the 5850 circuit board, after searching for familiar patterns on the schematic. I found that the various LFO output shapes (sawtooth, sq wave, etc) are routed through a few 4066 IC's on that board, on their way to the oscillators or other point they can effect.

 As far as dealing with the inaccessible circuit boards,  I was able to do some testing by soldering tiny wires onto various points of the board, running the wires out through the tiny spaces between the boards, and then connecting my scope probe to those wires. This is an arduous process, but it does work.

Ultimately I found that the square wave LFO was getting into, but not out of, IC 2E, which is a 4066 switching IC. .I found that the reason was that IC 2A, a 40174, had a dead output, and was not activating the switch, and not connecting pins 8 and 9 of 2e as it should. Replacing IC 2A solved the problem.

I now understand why Synthexs' have a reputation of being difficult to fix!

The circuit boards, except for the top two, are impossible to access with a probe... so I had to pick some test points and solder small wires to them, and then reassemble and test.

The 5850 Circuit board, where the problem was... IC 2E has been removed here.

These Digikey Potentiometers were pretty close replacements... Linear 5k... however the shaft was a bit long and also the diameter of the busing slightly small.. but the shaft was easily cut, and the pots fit securely when tightened up.

The square wave was detected going into IC 2E right here... but not coming out the other side! The problem was a dead output on IC 2A.

The problem ultimately was not the with IC2E, but with the flip-flop IC 2A which was controlling it.

Suzuki Keyman Portable Piano

This keyboard wouldn't turn on. The external power supply provided the correct voltages (+-24), the power inlet jack was good, there were no blown fuses.. the power switch was good... this was a perplexing mystery. Although I didn't have a schematic, I was able to figure out that there was a regulated 12 volt rail that was always turned on, and when the power switch was closed a relay was connected to this rail,  and the relay turned on the rest of the unit.

The output of the 7812 voltage regulator, which powered this rail, showed up as a paltry 6 volts DC, not the 12 it is supposed to supply..  An ample voltage of  24 volts was supplied to it.

 I verified that the 12 volt rail was not shorted or drawing too much current, so I replaced the voltage regulator... and still found the output to be 6 volts! This was very perplexing, until I decided to check that 12volt rail with a scope, and found that there was massive oscillation coming from the 12 volt regulator.
In other words, the capacitor charged with filtering that 12volt rail had failed, and this is a necessary component when using the 7800 series regulators.

I was able to replace the capacitor and the rail came instantly back up to 12 volts... and the keyboard turned on. A homemade schematic is shown below... notice the diode, which is in place to filter out the inductive discharge which comes from the relay coil when the relay is turned off! This diode protects the rest of the circuit.

sh-09 disassembly to fix key contacts

The SH09 is a neat little keyboard, described on vintage synth explorer right here.

The sh-09 uses an unusual key contact system that I haven't seen in many other Roland synths. I believe it was probably some type of passing phase between j-wires and rubberized contacts.

This one came to me with bent contacts which caused some keys not to sound. I also noticed that the bent contacts could also cause mis-triggering of notes: if the envelope was left open with ADSR settings, keys with bent contacts would trigger other notes when they were released. This resolved to be an issue if the lower key contact (there are 2 per note) was bent away from the lower rail.

This was a somewhat difficult synth to dis-assemble. There are a large number of screws visible from underneath, and if you just remove them all you will wind up with a big jumble of parts (transformers, circuit boards, etc) spilling out like entrails when you open it up.Nobody wants that.  For that reason, I posted a few pictures of the necessary screws to remove when taking the synth apart. I count 13 all together, to remove the top cover from the bottom (metal) plate. I think I got them all here, if I missed any let me know.

sh-09 key contacts: this is an unusual setup.

On the back panel there are 3 screws to be removed- I marked these with a white box in this photo.

There are 10 screws here to be removed. I think I managed to list them all here.

Korg MONOTRIBE Mods: individual outputs and BD, SD, HH, SD noise DECAY

On this Korg Monotribe I added the switches and pots to control the Snare Drum Decay, Bass Drum Decay, High Hat Decay, and Snare Drum Noise decay. I also added the 4 individual outputs.

I used the schematic provided here at skatona-electrix , to set the individual decay controls,  as well as the information provided by monomodder , specifically his recommendation to use 3 k resistors in series with the individual outputs.All of this worked great.

One thing I noticed was that the SD noise mod had no effect. Then I realized that there was no SD noise output on the whole unit! I did some checking, first of all my own work, and then Korg's, and went on to find that C51 was loose from one of its pads. See picture 3. I think this was a factory defect, specific to this unit, but it turned a 2 hour job into a 4 hour job!

Anyway, we were happy with the mod, and should anyone be interested I would perform this procedure for 150.00 parts and labor.

Jupiter 8 vcf BEND fader issues

On this nice JP-8, which was here for a re-capping,  there were issues with the VCF bend fader on the bender board. The fader had no effect; it was pinned at "0" no matter what. Consequently the bender had no effect on the VCF. My first thought was to make sure the VCF pedal jack had no issues; the jacks are one of the most vulnerable areas on a keyboard, so it was worth testing that jack to make sure there were no shorts there, especially since it is so easy to test. There were none; and in retrospect it is possible that this would have an effect on the JP VCF bender because the microprocessor would probably ignore the jack input if it was unchanging.

The next thing I checked was the fader; faders and controls are also a very vulnerable part of all keyboards. In this instance I noticed that pins 1 and 3 were shorted (no resistance). Looking at the schematic, (figure 1), I could see that there was no way that pin 3 would be connected to ground unless (1) the fader was shorted (how could that possible be?), (2) the op amp, IC2, was somehow shorting pins 6 and 7 to ground, or (3) there was a piece of solder or a solder bridge touching the traces or some other type of mechanical issue.

I investigated for number 3 first, a solder bridge or piece of debris, since it seemed most likely, but to no avail. I then investigated for number 2, the possible "op amp short". There were some connections from that op amp to ground, so it was possible that it could be the culprit, but after removing it, the issue remained.

I couldn't imagine how a fader could short out, but I removed it anyway. When I did, I realized that it was soldered in wrong... the orientation was upside down, and consequently pins 2 and 4 (shorted by design) were in the holes designed for 1 and 3.

This is the second time I've encountered faders soldered in wrong, recently. The last time was on an EMU. I turned the fader the other way, soldered it back in, and all works fine!

As technicians, we look for failed components; but there are occasions where the components were never installed correctly in the first place, and sometimes  the problem was ignored for years and years.

Prophet 10

This is a beautiful instrument that suffered from intermittent lockups. There are a number of possible causes for this, but it's easiest to start with mechanical problems... by that I mean loose molex connections between boards, cracked solder around voltage regulators and filter caps, anything around heat sinks and all solder around electromechanical devices like the power switch, etc.  Cracks in circuit board are also prime suspects, as are socketed IC's where pins can work loose over time.

Start by using the highly advanced technique of poking around with a stick, (a non conductive poking device) while the unit is turned on, to see if you can induce the lock up. Poke around all those areas I mentioned. That's what worked here... see the highlighted portion of the photo... some IC's needed to be reseated and re tensioned. I found them by poking on them with the eraser of a pencil.

If that hadn't worked, I would have removed boards, starting with the power board, and then the microprocessor boards, and looked for cracked solder, especially around the heavy caps and the heat-sunk components.

If the visual inspection completely failed, I would have also checked the voltage rails with a scope to make sure the device was getting clean power.

Sometimes thermal issues can be induced by blowing some hot air (not too hot!) around various areas of circuit boards. Seriously, don't use too much heat, but often you can find a faulty component or other thermal issue by blowing some hot air on various areas to induce a lockup.

If you have a politician handy, you can get some hot air that way... if not you could use a hair dryer or a hot air soldering station with the heat turned down.

The integrated circuits in the highlighted area had worked loose. They needed to be carefully removed, the pins were cleaned with solvent and re tensioned. Notice that they are right above the Power Supply area... they were constantly being heated up. This surely contributed to their coming loose.

Laptop Overheating Issues: fan cleaning on an HP laptop

It has been my experience that 90% of laptop overheating issues are caused by dust clogging the air ducts leading to and around the cooling fan.



In all the laptops I have worked on, the procedure for cleaning the fan has involved extensive dis-assembly to get to the fan and the air ducts. Although each laptop is somewhat different, the dis-assembly procedures seem to be remarkably similar, so here is a generic set of instructions for taking apart laptops and cleaning the fan.

 1) It has usually been necessary to remove all the easily removable components on the back of the laptop: the battery, cd drive, ram, hard disk- generally all of the stuff you can get to from the back of the unit. I then usually remove all the screws visible from the rear- some of them are hidden under things like the hard drive, etc. Sometimes the screws will be hidden under the adhesive round rubber "feet" the laptop sits on. Sometimes, too, you will find a screw hidden under a sticker. If you push on the stickers with the rounded, plastic handle of a screwdriver, you may find an indentation... a telltale sign that a screw is hidden there.

2)After you have removed all the screws and components you can access from the bottom of the laptop, turn it right side up again. You may have to remove the little panel  right above the keyboard, which houses some buttons and lights, usually the power button and other similar buttons. That part is usually snapped in, and possibly screwed in from the rear. Most laptop parts require some force and prying to come apart... but never too much. If you feel the need to pry too hard, check for some screws that you missed, or check to see if  you can take another component off first.

3)The keyboard usually needs to be removed next in order to access more screws which will be hidden under it, which hold the top of the laptop case to the bottom.Be careful removing the ribbon cable to the keyboard.

 The goal is pull the laptop apart (pulling apart the upper and lower portions of the case).You will almost definitely have to do all the things mentioned above, as well as take off some other bezels, cables, screws and wires.

The motherboard may also need to be removed, involving more screws and ribbon cables. Be careful not to bend it too much.  Sometimes a heat sink will have to be removed from the CPU, in order to get to the fan or the compartments where air travels. If that is necessary, make sure to have some heat sink grease around in order to put the heat sink back on... the grease is really important. .

One thing I have noticed is that it is always better to mark and remove all those slender ribbon cables you encounter while you are going in there. Snap a photo or two before you remove them, and use a sharpie to mark where to put them back. It is always better to remove them, rather than leaving a component dangling around connected only with a ribbon cable. If you do that, you will accidentally move the component too far and bust the ribbon cable or the connector, and then you will have a much bigger project on your hands!

In short, extreme care should also be taken in dis-assembly, because those cable break easily. You will most likely have to take all types of things apart in order to get to the fan.

===========

Once you clean the fan and are ready to assemble the unit, make sure you connect all the ribbon cables as you reassemble the unit. Go slowly.

Additionally, if you removed the heat sink screws, be sure to tighten them in the proper order... so that it seats flush on top of the CPU. If the order is not marked numerically on the heat sink, at least try to stagger the screws so that all the screws you tighten are far apart from one another.

When screwing in circuit boards and components, make sure you don't put screws in until you have reached the right point at which to do so. Some screws are made to go through several boards, so you don't want to put them in until you have all the boards assembled. If you put the screw in too early, you will wind up in the end looking at a screw hole with a screw head buried inside of it, and will have to backtrack...or just pretend you didn't see it.

Here are a few photos of the most recent laptop I have disassembled and cleaned. The overheating issues were fixed, and the unit ran much cooler once the dust was removed from the fan duct and cooling fins.

Oberheim OBX-A repair: uneven envelopes

Pin 16 on all of the 4051's should be at 5.6 volts.

This is the circuit used on each board (upper and lower) to create 5.6 volts. If the diode drifts, the voltage supply will also change.

Add caption

the upper control board

one of many suspect mechanical connections

This particular unit came to me with a disparity between the envelopes on the upper and lower sets of voice cards: the envelopes on the 4 upper voices were shorter than those of the lower 4. When running through the voices in rotary mode, you could hear 4 distinct sets of voices 1-4 with a short envelope, and then the next 4(5-8) would have a longer envelope.

It seemed unlikely that calibration was the answer to this problem, at least in terms of the individual voice cards, because there was such a clear distinction between them in terms of groups of four.

Through some testing and looking at the schematic, I was able to determine that the CV's for the sets of four voice cards came from the "upper control board" for the upper four voice cards, and from the "lower control board" for the lower 4 voice cards.  I was able to compare, for example, the decay CV from the lower control board with the decay CV from the upper control board... and I found a disparity of about .05 volts. Although this may not seem to significant, the range in voltages for that particular parameter seems to be .3 volts, so it is clear that .05 volts could have an audible effect. I found that this disparity was involved in all the CV's,  the lower board was consistently .05 volts different from the upper.

This difference in voltage could be caused by a disparity in the load (perhaps a short or failing component in the voice cards), or a disparity in the supply current (coming from the upper and lower control boards). To test for this I switched a CV from the upper board and directed it to the lower board. The discrepancy in voltages remained the same respective to the control boards, and I was thus able to conclude that the problem originated from the upper and lower control boards, which generated the CV, and not from problems with the voice boards.

Whenever there are boards connected to boards with plugs and wires, it is useful to clean all the connections. This was the first step I took, as well as cleaning all the socketed IC's. This actually improved the problem, but a discrepancy was still present.

Using the schematic, I could see that those cv's came through op amps, the gain of which was set with 1% resistors. I tested the resistors and could find nothing outside of spec. I swapped op amps between the upper and lower boards, but the problem did not move. Tracing the CV further upstream, I saw that it passed through some resistors, and there were some .01 caps to ground to filter out any ac that may leak into it. I checked these components and swapped caps out, as a leaky cap or a resistor that was out of spec could change the CV... everything was fine, the problem was not there.

I then traced the CV back to a 4051, (demultiplexer), and there was a disparity evident there as well, on the output pin. Swapping the 4051 had no effect.... but at this point I noticed that the VCC on the 4051's (all of them) was 5.6 volts. This was generated through the use of a diode and resistor between the 15 and 5 volt rails... the diode voltage drop of .6 volts was used to drop the 15 volts to 5.6 volts.

There was a separate diode on the upper and lower control boards, and the voltage drop had differed by .05 volts between the two of  them... so in essence the lower control board was sending its 4051s a voltage of 5.55 volts and the upper 5.61.

Swapping the diodes (4148's) and the resistors resulted in a much more even VCC for the 4051's on both boards, and resulted in an almost exact match between the CVs.... but the problem was still slightly evident, although the voltage disparity was now .01 volts as opposed to the previous .05 volts. The human ear is a remarkable instrument however, and it was still audible.

Finally, I ran a few soldered jumper wires between the upper and lower control board vcc's... the +/- 15 volts, the 5, the ground, AND the output of the DAC, which supplied the analog voltage to all the 4051's. These actions bypassed all the relevant mechanical connections between boards, made the voltage discrepancy go into the 1000ths instead of 100ths, and resolved the problem! It was only through a combination of these strategies, however, that the problem was repaired. I think the most significant correction came from replacing those drifted diodes with ones I handpicked to have similar voltage drops. The second most effective strategy was soldering jumper wires between the 15, 5, -15 ground and DAC output rails on both boards, bypassing the mechanical connections.

More information about Keyboard and electronic repair can be found at my website, offbeatelectronics.com

Yamaha 03d with DEAD inputs

In this particular mixer I found that the ADC chip, AK5390 VP, was faulty, and causing the channels to not work. I was able to follow the path of the signal to the inputs on the 5390 (pins 3,4, 25,26), and noted that it was arriving clean. However,I listened to the digital outputs of that 5390 and it was full of noise and distortion. Once I removed the circuit board and looked underneath, my suspicion that the 5390's were the culprits was confirmed by the fact that there were burn marks on the circuit board underneath of the suspect chips. I replaced the bad chips with sockets, so that new ones can be put in easily, or the unit can be used as is with only 12 inputs.
I suspect that dead and distorted channels on the 03D can often be attributed to this failure.
CS5390 is supposedly the same chip, and a working substitute for the AK5390-VP.
Hope this helps someone in a similar situation! The adc chips can be seen in this photo... notice the burn marks on the circuit board revealing how they have seen some heat.

Replacing a Backlight in a DYNEX 19L200a12 HD TV set ... similar to many other flat screens

Many times a flat screen tv or monitor will flash on for a second when it is turned on, and then the screen will go black. Most often, this results from a bad backlight bulb. Generally there are two backlight bulbs, one at the top of the screen and one at the bottom. Their purpose is to light up the screen: without them the screen is black.

Backlights are like the fluorescent lights we use in stores and industrial places: they light up as a result of gas being excited by a high voltage alternating current. A device called an inverter generates this ac voltage. Sometimes the inverter fails, but more often it's one of the two bulbs... especially if you get a screen that flashes on and then quickly goes out.

So if the problem is usually one bulb, why does the screen light up and then go completely out? Why doesn't it just stay 1/2 lit up?

The reason is that the inverter is "smart" enough to shut down if it figures out that one bulb is blown out. That's why you get the flash, and then the darkness... the inverter waits a few seconds to see if both bulbs light; if not it shuts the whole unit down.

Backlights are cheap. With a few bucks, and a whole lot of patience, you can replace them and make your unit as good as new.

Now, what you have to do first is take the back of the unit off (all screws visible from the back side). Of course, make sure the thing is unplugged. Also make sure it is sitting on a rug or something soft so you don't scratch up your screen while you are doing the repair.

You will have to gently pry with a thin, thin screwdriver or an exacto knife to CAREFULLY pry the plastic case apart (once you have all the screws out, that is). Work slowly around the edges, prying it apart as you go.It will snap and pop and slowly come off. Don't force it more than you have to; if it seems stuck check to make sure you didn't miss a screw.

Once your inside the case, you should use a spare backlight bulb to test and find out which bulb is bad. There are two bulbs used in most screens (top one and a bottom one), but you can use either one for testing purposes: You can plug a bottom-style one into the top socket or vice versa. But they won't fit right if you try to install them permanently in the wrong spot. For this reason, I would get both a top and a bottom one if you are doing the repair.

BE VERY CAREFUL that the unit is unplugged between testing the top and the bottom... don't try to plug them in hot. You might get a 500 plus volt shock (unpleasant), but most likely they won't work.

Detach the wire from one of the two backlight bulb sockets, and plug your tester bulb into the socket (and leave the other bulb plugged in). Plug the unit in, turn it on, if you see a long lasting blinding light from your tester then you found the bad one. If not, then unplug the unit, plug your tester into the other socket, plug the other bulb in, and try again. You are not permanently installing these bulbs at this time, just plugging them in and gently setting them down to see if they light and stay lit when you power the unit on. (see photos at bottom).

Remember that the inverter will turn off unless both bulbs are plugged in and lit... so don't ever expect to get a light with just one bulb plugged in.

Once you have found which one is bad, you must unplug the wires that go from the screen circuit boards to the boards attached to the black housing This is so you can remove the housing and work on the screen without the housing getting in the way. You can, however, leave the boards attached to the back of screen alone. They won't get in your way.

Now that you have the screen separated from the housing, take off the 4 tiny screws that hold together the metal frame around the screen. Very carefully pull/pry the metal housing of the frame apart. Use a sharpie to mark each piece because they only fit back in the spot they came out from.

Then very carefully start lifting/prying off the black plastic retaining frame. It's like another frame which holds the screen's layers together. Finally, you should be able to lift the screen up, and then the cardboard white flat reflective thing (I am sure there are better technical names for these parts). You will then see the precious backlight. You will have to  Remove it very slowly and carefully, like a surgeon. You have all the time in the world. You will probably see that it is blackened or broken on one end. Also, it contains mercury and it is very sharp... resist the urge to eat it, as well as be careful not to poke yourself with the sharp poison glass. Note how the wires to the bulb are routed... everything has to be routed the exact same way when you put the thing back together.  

Getting it in there takes an extraordinary amount of patience the first time (. Be very careful. Move slowly and carefully). You can bust the delicate bulbs by bending them, or perhaps even by sneezing on them. Never force anything! If it doesn't fit, step back for a moment, and consider what may be a better way of doing it.

Once you have put your tv together enough to test it, turn it on. Do this before you put it completely together, in case  you missed anything. After that, reverse the order of installation to put it back together. Go slow, and very gently. Don't force anything.

Ensonic VFX-SD

Here is a unit which would not turn on, or would turn on intermittently or after a long warmup. Applying the scope onto the diodes, where they go to the filter caps, exposed a 60 cycle wave. It was not the result of a leaky diode, but rather of failed filter capacitors in the power supply.

The power supply appeared clean without a load on it, but when the board was drawing current the wave was visible in the outputs of the voltage regulators.

I also replaced the disk drive. Since the newer PC floppy drive uses a new interface, I had to redirect pin 10 to pin 12, and connect pins 2, and 34 together completely. The drive needs the disks to have the tape put over the hole that marks them as double density.

This is a method I have developed for using PC floppy drives in applications where the older Shugart interface was used. It often works... but sometimes you have to also play around with shorting the "disk ready/ disk change" pins. In a few situations I have had to install a button or switch on this pin, so that the unit would know a new disk was inserted. 

For more info about me and what I do please see My Business:OffbeatElectronics.Com

SH101

Here is one where the hold button seemed to be on all the time, on the sequencer board. This was actually the result of broken solder on the Jack Board. It had to do with the shorting jack that is used for an optional hold pedal... the jack was not shorting out as it is supposed to, making the unit "think" that a pedal was connected when it was not.