Once upon a time, a Space Station from the constellation Ursa Major landed right on my table, making awful noise when set to reverb.
The cause was easily found – one of the op amps in U12, a quadruple legend named 4741. Looking around I found some more of these noise generators in disguise, some of which also not working properly. I decided to use OnSemi MC33079 because of good experiences in other circuits. Needless to say that the engineers who wrote the SST-282’s service manual were proved right:
So I had to calculate the right compensation to eliminate the 2MHz oscillation of the op amps in the filter ciruits. The result was an unaltered frequency respone up to 9kHz, the maximum to expect in a time-discrete system sampling with 16kHz.
The RAMs were another area to work on. I found one of the original MK4015 chips to be defective, but unfortunately no single 4015 or 4027 – not to be confused with the CMOS logic CD4015 and CD4027! – was on stock. At first sight, a 4116 should be a perfect replacement, the additional address line of the 16k chip is on the location of the 4015’s chip enable, which is connected to ground in the Space Station. But this would have been too easy – the 4k chips Ursa Major used have one little difference: their data out pin remains latched for at least 10µs after the CAS line has gone high:
Within this period of the, the data is transferred to the D/A converter for output. Finally I’ve found and tested a solution that allows to replace any number of MK4015 / MK4027 in this and probably other circuits with 4116 or even 4164 RAMs. The exact circuit will vary from application to application, but I’m quite confident that no one will have to worry about replacing 4k DRAMs in the future. Feel free to contact me for a custom solution – or to have the DRAMs replaced in your SST-282…