Penrod AJ-430 digital squarewave keyboard with C64 sounds & accompaniment

This squarewave keyboard seems to be a technically simplified successor of the MC-3 hardware. Its sounds and features are basically very similar, but the rhythms have now 2 sets of accompaniment patterns, those are also the most interesting new feature of this instrument, because they contain very unique, glitch- like bursts of short, blippy notes, like when a freaked- out arpeggio would play only during the pauses between normal chord notes.

The Penrod AJ-430 also has an additional primitive sequencer (monophonic record & playback). Unfortunately the separate chord volume slider and the manual bass mode were omitted. This instrument was also released as Satellite AJ-430 (and possibly others). The AJ-430 resembles very much the Letron MC-3, thus I only describe here the differences.

different main features:

eastereggs:

modifications:

notes:

pcb label: EK - 430, GQC7. 820.853The PCB of this instrument is labelled "EK-430" and thus might be a far relative of the Hing Hon EK-001. There are many Engrish misspellings on the control panel; a bit odd is also that there is a LED for the single finger, but not for the fingered chord mode.

The OBS preset sounds of the Penrod AJ-430 sound very much like on MC-3, although they are in different order and partly misspelled. The "violin" seems to be identical with "saxophone". Unlike MC-3, the OBS preset sound buttons here respond a little slow and thus are less suited for realtime play tricks.

Also the percussion sounds like MC-3, but the preset rhythm patterns are different. The "rock 'n' roll" pattern seems to be rather a mislabelled tango. With my specimen the maximum rhythm volume was a bit low. Like the MC-3, the AJ-430 has nice accompaniment made from squarewave organ + e-bass(?) tones, but unlike the MC-3  it has a variation button (misspelled "viriation") to select a 2nd set of more complex accompaniment patterns. These patterns sound nicely bizarre and somehow broken because they contain between the normal chord notes bursts of short blipping notes, like when a freaked- out arpeggio would play only during the pauses between normal notes. The only other keyboard I heard with similar styles is the bizarre Ramasio 892.

The monophonic record/ playback sequencer can not be combined with rhythm or chords and has a quite low timing resolution. (But unlike My Music Center it at least doesn't erase its contents when other functions are selected.) The 2 demo melodies repeat each in a loop and switch only by pressing the demo button again. The "500 Miles Away From Home" tune resembles the well known MC-3 one, but contains an additional pattern that alternates with the normal one. The 2nd melody is a short monoto that sounds between a western movie theme and something Chinese.
 

circuit bending details

The hardware of the Penrod AJ-430 appears to be a cheaper version of the MC-3 hardware, because despite very similar features it has now the sound chip integrated into the "CIL-51" CPU. But it has still separate sound outputs for drums, cymbal/ snare, bass, chord and 2x main voice.

CPU CIL-51

When I bough this instrument (used, from eBay), it hummed very badly at low volume setting and distorted at higher volume. I found out that it hummed due to bad GND wiring; I fixed this by soldering a thick wire from GND at the AC adapter jack to the GND pin of the power amplifier IC. When I had set the AC adapter volume to 9 or 12 volt (like indicated on the jack), the amp worked well, but the sound envelopes now started to fluctuate and got stuck at full volume as continuous tones. The instrument freaked out at the 7.5V setting of my unregulated AC adapter because the +5V voltage of the CPU was badly stabilized by a 5.6V zener diode and a transistor, and thus ascended to >5.1V when input voltage was set high enough to make the amp work properly. Possibly someone damaged it by wrong polarity, because instead of a protection diode there was only a wire bridge soldered in. To fix this, I soldered a 47 kOhm trimmer in series to the (slightly burnt looking) 500 Ohm resistor that was wired parallel to the zener diode. By adjusting the trimmer, I found out that the CPU even works well with only 3.5V, but the envelope control of mine freaks out as soon the voltage rises only slightly above 5V (about 5.1V). My theory is that the volume envelope of each main voice channel is generated somehow semi- analogue by an internal capacitor inside the CPU, which is charged or discharged by short current pulses (similar like the external capacitors in the Hing Hon EK-001), and the capacitor turns leaky by zener effects at too high input voltage. Another evidence for the existence of such internal capacitors is that pressing the OBS preset sound buttons during sustaining notes does not stop the notes but only switches their timbre, which is the typical behaviour of capacitor envelopes (although the same phenomenon could be also achieved in a fully digital system when it e.g. keeps its actual numerical volume values in down counting memory registers when sounds are switched). Possibly also the DSG-MC-3 2191 sound chip of the original MC-3 hardware contains switched envelope capacitors, because both instruments sound very similar.

Theoretically the stuck CPU envelopes at 5.1V might be usable as a sound effect (they warble louder and quieter at a threshold voltage), but I don't recommend this because  overvoltage can generally cause chip cancer over time (known e.g. from overclocked Pentium PCs), despite the CPU of my keyboard did not run hot from it. The CPU has a few unused pins those purpose (if any) I didn't figure out yet.

keyboard matrix

21
22
23
24
39
40
41
42
43
44
45
46
 
pin
in 1
in 2
in 3
in 4
in 5
in 6
in 7
in 8
in 9
in 10
in 11
in 12
in/ out
 
o
o
o
o
o
o
 o
o
 o
 o
o
o
out 1
25
o
o
o
o
o
o
 o
o
 o
 o
o
o
out 2
26
o
o
o
o
o
o
o
o
o
 o
o
o
out 3
27
o
o
o
o
o
o
o
o
 o
 o
o
o
out 4
28
o
 
R.
program
R.
play/ space
P.
base
P.
tom
P.
snare
P.
open cymbal
P.
close cymbal
 vibrato
sustain
 
out 5
29
R.
synch
R.
start
R.
stop
 
C.
manual bass
C.
fingered
C.
s. finger
C.
off
transpose
demo
tempo
+
tempo
-
out 6
30
O.
aSound
O.
aSound
O.
aSound
O.
aSound
O.
aSound
O.
aSound
O.
aSound
O.
aSound
 O.
aSound
 O.
aSound
O.
aSound
O.
aSound
out 7
31
R.
rhumba
R.
march
R.
pops
R.
bossanova
R.
slow rock
 R.
disco
R.
16 beat
R.
waltz
R.
rock 'n' roll
 R.
swing
R.
samba
R.
tango
out 8
32
 
 
 
 
 
 
 
R.
variation
 R.
fill-in
 rec
(demo 1?)
 play
(demo 2?)
 vibrato 2
(weaker?)
out 9
33
 
 
 
 
 
 
 
 
 
demo 1 
 demo 2
one key play 
out 10
34

All unknown function names and in/ out numbers in this chart were chosen by me. The additional rhythms were named after those on Superb Sound EK-902 (see below); the "samba" sounds more like beguine, the "tango" more like heavy metal. The input lines are active- low, i.e. react on GND. Any functions can be triggered by a non- locking switch in series to a diode from one "in" to one "out" pin.
 

legend:
"o"
 = keyboard key
R.
 = rhythm
P.
 = drumpad
C.
 = chord
O.
 = orchestra (main voice sound)
orange
background 		 = easteregg (unconnected feature)

The most interesting eastereggs are the 4 additional OBS preset rhythms due to the unusual alternative accompaniment patterns selectable by the variation button. The "fill-in" button plays a rhythm fill-in pattern. The "manual bass" button permits to play to the accompaniment a monophonic manual organ bass instead of accompaniment. The "vibrato 2 " button seems to add a slightly weaker vibrato than the normal one. The "transpose" button pitches the keyboard down in semitone steps (works only with accompaniment?). The "rec" and "play" buttons shown in the matrix sometimes played demo song 1 or 2 (one per button) during my tests, but I am not sure if I just measured wrongly. The "one key play" button plays the sequencer contents note by note by re- pressing it (works even during demo), which mutes the keyboard until the sequence is finished.

separate volume controls

An annoying limitation of the Penrod AJ-430 is that it has no separate accompaniment volume controls. But the CPU still has separate sound outputs {drums, snare/ cymbals, bass, chord, main 1, main 2} at pins 2 to 7, thus separate faders for the individual tracks can be easily added. The main voice seems to be output through 2 different pins depending on the selected preset sound; likely these were intended to be muffled differently by external capacitor filters. With my specimen I yet only soldered a 100 Ohm resistor parallel to R39 (10 kOhm) on the potentiometer PCB to increase the (quite low) rhythm volume.

Another keyboard of this hardware class seems to be the Superb Sound EK-902 (37 midsize keys, case shape resembling Superb Sound EK-905, seen on eBay), which feature lists corresponds to the AJ-430, although it has 12 preset rhythms (additional rhythms: bossanova, swing, samba, tango).
 

 removal of these screws voids warranty...    
WarrantyVoid
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