A vintage (1985) Digital Delay, DD 1000, from Ibanez, bought “used” with one delay channel not functioning. The unit bought by a friend to be used (at least the working part of the unit) with an electric guitar.
After a conversation I had with him I decided to give it a try to see what is malfunctioning and if it is repairable to proceed with a repair.
I searched the net for any info, especially a service manual or schematic but nothing found free, except at MusicParts.com (http://www.musicparts.com/products.asp?Company=Ibanez) for $20.
Under the case of the unit there are two identical boards with different component population. It is a classic Japanese design and implementation circa mid eighties with discrete through-hole components and op-amps (a good sign for repairability of the unit).
But under the board (solder layer) there is an SMT chip (IC28, 60 pin QFP) the MC4101F. I didn’t find any datasheet for this IC and I think it is a multifunction audio signal processor including A/D, D/A conversion, effects and memory management function for storing a digitized audio signal to an external memory and reading it back delayed to produce delay effects. It seems to be the most critical part of the unit so I had to verify its proper operation.
At http://mirosol.kapsi.fi/2014/03/ibanez-dfl-digital-flanger/ I found info about another unit the Ibanez DFL Digital Flanger that is built around the same chip. I downloaded (free) its schematic from http://www.schematicsunlimited.com/i/ibanez to study the function of MC4101F.
I feed at the input of the unit (back panel connector) a signal (sine 440Hz/700mVpp) and traced it on the board. FET Q1 seems to be an electronic switch that injects the delayed signal with effects to mixed output path when “Effect” key is activated in the front panel.
INPUT signal (440Hz, 700mVpp)
Effect switch active. Output at Q1 drain pin. No signal – only noise
I feed at the input of the unit, again, a signal (sine 440Hz/700mVrms) and traced it on the board until it enters MC4101F (IC28 pins 41, 42, 45). At pin 16, IC28 outputs a signal (in some form of pulse modulation) to write it in the external RAM (IC27 uPD4164, pin 2). This signal driven to an audio amplifier can be heard through a connected loudspeaker and the 440Hz tone can be recognized. Doing the same at the output of the RAM (IC27 uPD4164, pin 14) nothing heard (only some low level noise). MC4101F is heart-beating but ram chip seems malfunctioning.
uPD4164 is a DRAM chip organized as 64k x 1bit now obsolete but very popular at early home computing days (ZX spectrum, ORIC, IBM PC etc). I found an IBM PC compatible having enough 4164s, all installed in ic sockets. So I used it as a donor. In the DD1000, IC27 was direct soldered with no socket, so I cut its pins and desoldered by hand all individual pins with a soldering iron and a sucker. Then soldered a 16pin DIL socket. (CAUTION) Silk screen shows DRAM chips placement with wrong orientation. I installed the socket and the new IC according to the orientation of the removed chip. A good practice for this is to take a photo before any repair attempt and refer to it when in doubt.
Power up and voila! Digital Delay channel “A” works again.
Effect switch active. Output at Q1 drain pin. Good signal
Input signal vs datastream at IC27 p.2 DRAM input
Details of datastream in relation with the input signal (yellow line). It seems to be “DELTA” modulated pulses
Misc info:
main components pinouts: ibanez-dd1000
Ibanez DFL schematic with notes regarding DD1000: ibanez-dfl_dd
