We had a couple of early 1980's oscilloscopes at the museum which used the Mostek MKB36000 ROMs which are known to deteriorate over time (e.g. FROMs - Forgetful Read Only Memories). One was a Tektronix 468 Digital Storage Oscilloscope which used a pair of the ROMs. There was room to retrofit an adapter board to convert the 24 pin ROM socket to a 28 pin EPROM socket and we copied the binary contents of a pair of good ROMs to a pair of 2764 EPROMs. We made our own adapter boards but there are inexpensive ones on the market such as these. An adapter board is a bit wider and longer than the standard 24 pin outline but there is ample room in the 468. Replacement eproms are available on our webstore.
The Tektronix 7854 Oscilloscope with Digital Storage proved more of a challenge. There are two pairs of ROMs stacked very close together so there was insufficient room for four adapter boards and the board to board spacing and the PCB ejectors limited the vertical height. We could fit a adapters at U200 and U210 so we copied a good set of ROMs and combined them into a pair of larger 27128 EPROMs. We had to solder individual pins on the adapter to minimize the height (e.g. not a strip of pins) and use a low profile socket for the EPROM (although it could be soldered directly to the adapter). We flew a wire to add A14 to the 27128 and combined the two chip select signals into one. The oscilloscope will not operate without the patch EPROMs and FPLA which we did not have but we found binary files on the web which have since been removed. Programming up a pair of 27128s and a matching pair of 2716 patch EPROMs brought this oscilloscope back to life.
These are the modifications for the 7854 (revised 9/13/16)
- Remove U100 and U200. Program a 27128 EPROM adapter for U200 with MSB data and use U200 location for clearance.
- Remove U110 and U210. Program a 27128 EPROM adapter for U210 LSB data and use U210 location for clearance.
- Wire A13 by connecting both 27128 pin 26 to U425 pin 1.
- Wire new CS. Start by disconnecting the existing CS from U200 and U210 pin 20. We did this by simply not connecting pin 22 in the EPROM adapter (which connects to pin 20 on the PCB) and then flying a new CS wire directly to pin 22 of the EPROM.
- Option 1: Add an unused gate by connecting both 27128 U200 and U210 pin 22 to U225 pin 11. Connect U320 pin 13 to U225 pins 12 and 13.
- Option 2: (We have not implemented this so consider it unverified) Cut and lift U420 pins 9 and 13. They will float high or you can connect these pins to +5V.
- Remove FPLA U120.
- Reprogram a 2716 patch EPROM for U400 with MSB patch data. Reprogram a 2716 patch EPROM for U410 with LSB patch data.
We do not have these EPROMS available on our webstore but can program and sell them on demand. Contact us by email for details.