MIDI keyboard fault

[ITAM805]

Hi Kevin, in a word, no. I've used a Korg k/b as a module but only after I'd done a factory reset, this apparently clears the fault for a certain amount of time.

Historically, my friend had problems with it not sending note off etc, somebody tried to fix it but without success. So then he passed it on to me, I did some maintenance as per post #39 and did a factory reset and all seemed well, so I believed I'd cured it, so sold it on. But within a few day the buyer brought it back complaining that it was acting strangely, and here we are

I did another factory reset last night and the fault has yet again disappeared.....

[SiriusHardware]

It's now sounding possible that whatever retains the programmed / variable parameters is not working in the long term, so that when the unit is powered up after a lengthy power down, the memory locations holding the stored parameters are essentially scrambled. Often these scrambled values will be not only different from those originally stored but outside the bounds of the values expected by the firmware, potentially causing some very odd effects.

Not knowing what's inside the unit and in the absence of a diagram, you are probably looking for a memory device or chip which is either backed up by an external battery or has one built in.

It's also just about possible (given its age) that it uses an electrically erasable or flash based non-volatile memory device, in which case the question about a battery is irrelevant, but otherwise:-

If the battery theory is correct then the unit should behave correctly for as long as you leave it powered up after you have defaulted it but then, if you power it off for a while (say overnight) the problem may return.

Have a look and see if there is a memory retention battery, which could be a Lithium coin cell like a CR2032, or a PCB mounted NiMH rechargeable battery, or there could be a 'thick' memory chip from the Dallas 'DS.....' series or ST 'MK....' series which includes an integral memory retention battery.

If there is a separate PCB mounted or holder mounted battery, measure the voltage, is it anywhere near the marked voltage?

If you still have the unit apart or if it is easy to get apart, could you please post some images, taking particular care to get the device numbers readable even if it means splitting the mainboard up across three or four images?

[ITAM805]

Hi SH, my first thought was "memory battery" but alas there isn't one. Only the processor and eeprom as you can see. So it must be as you say. a flash type memory? Device numbers microcontroller (ST9036C1) EEPROM (93C46)

[SiriusHardware]

Ah, it's the 93C46. It's possible that has gone faulty, so it would be worth trying a replacement. The firmware won't be in the 93C46, as the 93C46 contains only 128 bytes or 64 'word' locations, nothing like enough to contain any significant firmware.

The firmware, and the RAM which the firmware uses as scratchpad RAM, will be in the processor chip, with the 93C46 being used only to hold info which you want to be able to change, but not lose when the power is turned off.

Running back, I think you said you had been unable to source a replacement 93C46? I'll have a rummage and see what I have (I know I have a 93... something somewhere, just not sure it's that exact type).

Also, those PLCC sockets (like the one the CPU is sitting in) have a bad reputation for going intermittent. We have equipment we made 10-15 years ago which originally went out with ICs in sockets like that, but when they are returned for service now we remove the sockets and solder the device directly onto the pads originally occupied by the socket. You need not do anything so drastic but what you could do is extract the IC and gently bend each chip pin slightly away from the chip body so that when it is reinserted, each pin exerts firmer pressure on the corresponding socket contact.

Edit: Unfortunately I don't have one. But I had no trouble finding one for sale in the usual place?

[ITAM805]

SH, there are several 93C46's available on ebay, is it a case of just dropping it in?

[SiriusHardware]

Yes, I think so.

A brand new device will be either blank or randomised, and a 'pull' (second hand, taken from some other equipment) will contain nonsense information as far as the keyboard is concerned, therefore the first thing you would need to do after installation is default the unit again, in order to load the new EEPROM with 'sane' values.

There are quite a few package variations, so obviously you'll need to be careful to order the DIP version. I saw one of those for about £1 from a UK supplier. It's not much to gamble on a possible fix.

It's possible that the unit maintains both an EEPROM copy and a RAM copy of the parameters, writing to both whenever you load defaults or make changes, but only reading from the EEPROM (to copy it into the RAM) as the unit initialises after power-up.

Under those circumstances it would be possible for a faulty write to the EEPROM to have no effect for as long as the unit remained powered up following a default, but then if the unit was powered off and then on again, the data, including the 'bad' data stored in the EEPROM, would be loaded into the RAM.

[McMurdo]

Farnell have 35 different makes of 93C46 in stock!

I can vouch for strange things when they are failing, for example corruption of certain parameters but not others. I have to change them regularly in a particular type of lab instrument, which might accept a full calibration only to find the next day it's changed a parameter that should be, say '100' to something like '--Dh' or other nonsense.

These chips have a limited number of write cycles before they go intermittent.

+1 on the plcc socket too. I eject the chip and lightly sand the chip pins with fine wet & dry before replacing. It's cured a few weird faults.

http://uk.farnell.com/search?st=*93c46*

[SiriusHardware]

I use one of those fibre glass brush 'pens' for the same kind of thing, polishes IC pins up nicely.

One reason those sockets are such a menace is that the IC spends its whole life exerting significant pressure on the insides of the socket but there is nothing exerting equal but opposite pressure on the outside of the socket, so, over a very long time, the sides of the socket tend to bow gently outwards, away from the IC, especially in the middle of each side.

[ITAM805]

Thanks guys, new chip ordered. I had removed the ST9036C1 and cleaned it but I'll pull it again and a give the once over with a fibre brush.

I'll get back with the results as soon as

[ITAM805]

Hi guys

I fitted a new 93C46 chip on Friday and have been testing the k/b several times a day since and (touches wood) all seems well

Many thanks for your collective help

[SiriusHardware]

Hopefully an inexpensive fix which I'm glad does seem to have made a difference, but the trouble is, as with all formerly intermittent faults you'll probably have to use it for quite a while before you can feel confident that the fault really has been knocked on the head.

[ITAM805]

Thanks SH, yes intermittent faults eh?