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Eliminate Batteries In Old Synths And Effects
By Sid of Stone Marmot
Jan. 28, 2019
Many old (pre-2000) synthesizers and effects had small lithium primary batteries in them to maintain the power to the memories that remembered the settings between power ups. If the circuits were properly designed, these batteries would last 25 to 35 years before needing to be replaced, though the batteries could fail in less than 5 years in not so well designed circuits.
The problem is that many of these synths and effects are now 25 or more years old. Battery failures are common for these old devices. To make matters worse, many used archaic and obsolete means of storing their settings externally, such as floppy drives or tape recorders, if the data could even be stored and reloaded external to the devices. It would be nice if we didn't have to deal with these batteries.
Fortunately, the technology has changed significantly since the 1980's and 1990's. Non-volatile read/write memories that can store data with no power applied are now readily available. Examples can be found in solid state drives (SSDs), USB thumb drives, SD cards, etc. The problem is that most of these devices use flash memories, which have special timing requirements and aren't in packaging that can be easily plugged in place of the existing memories in old devices.
An exception is the ferroelectric nonvolatile RAM, or FRAM, which was developed and originally brought to market by Ramtron, who I understand has since been bought by Cypress. These memories have timing requirements compatible with most of the similar sized common static RAMs, EPROMs, and EEPROMs available in the '80s and '90s. These FRAMs also have pinouts compatible with the JEDEC standards for these similar sized static RAMs, EPROMs, and EEPROMs.
The most common and readily available of these FRAMs is the 8kx8 version, which has a part number of FM1608 or similar (I think the most recent version is FM16W08). These devices were available in a standard 28 pin 0.6 inch wide DIP when first produced, as shown in Figure 1. This 28 pin DIP could often be installed directly in place of an 8kx8 static RAM, such as the HM6264 or TC5565, with no changes. This 28 pin DIP is no longer produced, though some may still be found in existing distributor stocks.
Figure 1 - 8kx8 FRAM in 28 pin 0.6 inch wide DIP
The FM16W08 is now readily available as a 28 pin SOIC with the same pinout as the DIP. There is a big enough market in other businesses, such as the maintenance of old video arcade machines, that you can readily find this device already mounted on a carrier board that is compatible with the standard 28 pin 0.6 inch wide DIP, as shown in Figure 2. I actually prefer this version as I can often make many, if not all, the mods necessary to adapt this memory to the device I'm installing it in on this carrier board.
Figure 2 - 8kx8 FRAM SOIC mounted on a 28 pin 0.6 inch wide DIP compatible carrier board
Many of the old synths and effects used 2kx8 static RAMs, such as the HM6116 or TC5517, for their battery backed memory. You can find vendors who sell the FM16W08 28 pin SOIC mounted on a plug in compatible 24 pin 0.6 inch wide carrier board already permanently wired in a 2kx8 configuration. But these devices, like most semiconductors, are much cheaper in quantity and I have a lot of devices that I'd like to install these FRAMs in, so I buy the 28 pin 8kx8 versions in quantity and modify them myself as needed to the 2kx8 configuration.
Figure 3 shows the 8kx8 version modified to be 2kx8 and installed in a JL Cooper MSB Plus Rev 2. Note that all the mods are on the FRAM carrier board, as discussed in this article.
Figure 3 - 8kx8 FRAM SOIC mounted on carrier board modified to replace 2kx8 RAM
Figure 4 shows the pinout of the FM16W08 FRAM compared to the pinout of a typical JEDEC compliant 8kx8 static RAM. Note that the only difference is that the 8kx8 RAM has two chip enables, one active low (CE1L) on pin 20 and one active high (CE2H) on pin 26, while the FM16W08 has only one active low chip enable (CEL) on pin 20, with pin 26 having no internal connection (NC). This usually is not an issue as many circuits only use CE1L, with CE2H being tied permanently high. But there are exceptions, such as the Yamaha FB-01, which has two 8kx8 RAM chips and uses CE2H to select between them, with the CE1L enables tied together and routed to the power supply dropout protection circuit. So you do have to examine each circuit you plan to install these FRAMs in.
Figure 4 - Comparing pinouts of 8kx8 FRAM and 8kx8 static RAM
All memories from different vendors have slight timing differences and you do have to consider these when installing these FRAMs in your devices. The timing requirement most likely to cause problems is that the FRAM internally latches the address bus on the falling edge of the chip enable (CE low) signal. That means that you need an active CE signal with a falling edge during the valid address for each read and write to the FRAM. This wasn't a problem for most of the devices I've installed the FRAMs in to date, such as multiple Sequential Circuits Six-Traks, a JL Cooper MSB Plus Rev 2, and even an old Rockwell AIM65 computer. But it was a concern for a Roland JX3P I modified. The processor in the JX3P multiplexes the data and part of the address bus to save I/O pins on the chip package. So extra gates were needed to assure the CE enable falling edge to the FRAM occurred when the address was on the bus (to be discussed in a future article).
Another concern is wearing out some memory locations in the FRAM. The original FRAMs were specified to allow at least 1 trillion read/writes to a row in memory before a failure would occur. More recent devices, such as the FM16W08, are specified for at least 100 trillion cycles before failure. If you leave your devices on 24 hours a day and the device accesses some locations frequently, this could be a problem. For example, the battery backed up memories are the only read/write memories in Sequential Circuits Six-Trak. The most frequently accessed portion of these memories that I found is the section that holds the interrupt return pointers, which are accessed about 10,000 times a second. If you left your Six-Trak on 24 hours a day (never turned it off), you could wear out the older FM1608 in a little over 3 years and the newer FM16W08 in a little over 30 years. But if you are like most people and only use your device an average of an hour a day or less, this is not a concern. This is also not a concern for devices like the Roland JX3P which have a dedicated scratchpad memory for frequently accessed stuff that is in a different memory chip than the nonvolatile read/write memory.
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