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.

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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.