Casio PT-50 small keyboard with nice analogue rhythm, accompaniment & ROM-Pack

This instrument from 1983 was the only Casio keyboard with ROM-Pack slot but no key lighting; instead it can load the ROM-Pack musics into an internal sequencer, and with the optional Casio TA-1 module it can even save them on audio cassettes. Even a RAM-Pack cartridge Casio RA-1 was made for this thing to save sequencer data - I never saw any other Casio keyboard designed for using it.

Despite this keyboard looks almost like a twin of the Casio PT-30, there are many small differences. 3 of the 8 preset sounds are changed, the chords and bass timbre is different and also the rhythm set is completely different and includes fill-in. Even the LCD has different segments and shows e.g. a "J" for major chords. Also the sequencer of this thing was changed and is horribly awkward like in Casio MT-70, because it apparently neither can record note lengths in realtime nor melody and chord together, thus everything has to be entered in different modes step by step. (I can only imagine that Casio crippled the sequencer by limited ROM space to make the ROM-Pack features fit.)

The PT-50 was seen (but not heard!) in the Gremlins movie from 1984; that movie scene replaced its tone with a thin plain squarewave beep (likely from a primitive piezo speaker toy keyboard or some kind of transistor tooter).

Due to general similarities I only describe here the differences to PT-30.

different main features:
serial F015914

eastereggs:

modifications:

notes:

Like the Casio MT-800, this instrument came with the ROM-Pack RO-201 and has to load ROM-Pack musics into its internal RAM (takes a few seconds) before it can play them. This is also valid for the demo melody, which doesn't play with ROM-Pack removed (shows error "E -") despite the melody keeps playing when the ROM-Pack is removed while the demo is playing.

caution: The PT-50 may crash during battery insertion and not respond to controls anymore. If this happens, switch it on and press the tiny "P" (reset) button at the case bottom with a pointed object to clear memory (erases sequencer contents). To avoid the crash, if the instrument was left without batteries for over a minute (which corrupts SRAM contents) always slide the power switch into "on" position before inserting batteries or connecting a power supply.

Some preset sounds differ from Casio PT-30. The 'organ' here has neither vibrato nor sustain, the 'violin' and 'flute' here lack sustain also. The sounds 'horn', 'fantasy', 'mellow' of the PT-30 were replaced with "clarinet" (has no vibrato), 'trumpet' (like "horn" without sustain) and 'celesta' (like 'harpsichord' with sustain and 1 octave higher). Most stupid is that even the nice 'violin' multipulse (known from VL-1) has been replaced with a boring 7:1 squarewave.

The manual organ chord voice lacks here the dull organ bass component and instead everything plays an octave lower, which sounds cheapish, because the multipulse bass waveform of PT-30 was replaced with a simple 1:3 squarewave bass that has no separate output pin anymore. But in opposite to PT-30, here you can trill around on different chord type buttons while holding a chord note key. (The PT-30 ignores further chord type button presses, thus you have to also release and press again the chord note key before you can choose another chord type, which was bad for comparing how different chords sound.) With rhythm, the bass voice has here a low, sonorously buzzing, decaying squarewave timbre with sustain, that resembles an e-bass but is also a little brassy. The rhythms set is very different from PT-30 and unfortunately lacks the nice arpeggio styles. But instead the rhythms now have each a fill-in pattern with accompaniment. Annoying is that the rhythms don't start immediately anymore after selecting them, but you have to press the "start/ fill-in" button, which always begins with a 4 step snare lead-in before starting the rhythm itself. Also the pattern restart trick of the PT-30 can not be used here by the lack of "chord change" buttons.
 

Casio RAM-Pack RA-1

I finally found on eBay the mysterious RAM-Pack cartridge for the Casio PT-50. It is named Casio RA-1 and can store apparently only one song. You can even save any ROM-Pack song from the internal PT-50 sequencer memory to it, which is correctly loaded back including the obligato voice.

I first hoped that it would be possible to compose own musics on the PT-50 (or even PC through the TA-1 tape interface) and play them on other ROM-Pack compatible Casio keyboards. However unfortunately the RA-1 seems to be incompatible with normal ROM-Pack keyboards; they ignore it like when the ROM-Pack slot was left empty.
The originally packaged RAM-Pack came with plastic tray, 2 blank paper stickers and instruction sheet. The RA-1 contains a 3V lithium button cell "BR-2016"; the manual claims it would last only 1 year, but longer with RA-1 inserted into the keyboard. (This can make only sense so far the keyboard contains batteries.) However in PC mainboards the same kind of "CMOS" memory backup batteries last about 10 years. For battery replacement you have to open the cartridge; first remove the slider, then unscrew the 2 tiny screws underneath.
To analyze the hardware closer, I needed to take out the PCB itself. But the only screw holding it was jammed in so tightly, that its flat soft iron head refused to be moved and only crumbled apart by any serious attempts of loosening it. Thus I had to drill it out; unfortunately the thin drill I used suddenly bent away by too much force, which made my heavy household drill machine violently crash down into the PCB, resulting in a PCB crack that damaged a lot of traces - arrg! Fortunately by my Casio KX-101 nightmare repairs I was more than experienced enough to patch them together again.
The hardware is made of 2 identical 1 KByte SRAM ICs "HD61914B". Interesting is the pin assignment; the left side of the 1st RAM exactly corresponds to the pinout of the ROM-Pack slot, the traces are 1:1 connected with its top side pins and with the left side of the 2nd RAM. To top row pins of RAM 1 are all wired to +Vs. Almost all pins of the down side of both ICs are shorted with each other. I confirmed by PT-50 service manual that the top pin row of the 61914 IC is NC (internally not connected) to makes it easier on the single sided PCB to route traces from other pins underneath. The VDD2 line is not only connected to both SRAM pin 4 but additionally to pin 25 (V4) of SRAM 1 and pin 28 (V1) of SRAM2, so they likely select the address range. Casio used the same IC as internal SRAM in the PT-50 and some PT-30, and a similar looking HD61914C in KX-101.

The ROM-Pack implementation of this keyboard is very different from all my other Casios and a little unobvious. To select a song, press the "R/MT" button once ("R" appears in the LCD), then type the 2 digit song number with the rightmost keys and press "play", which copies the song into the internal RAM (takes some seconds) and plays it. (Pressing "R/MT" twice displays "MT" and apparently tries to load the music file from a cassette through the optional Casio TA-1 module.) Unlike a ROM-Pack, the RAM-Pack RA-1 ignores the selected song number, because it apparently holds only 1 song. The "demonstration" button plays all musics from the inserted ROM-Pack in a sequence, starting with the first. The 2nd melody voice (obligato) of ROM-Pack musics plays a bit too quiet on my PT-50. (Warning: Playing any ROM-Pack musics overwrites and thus deletes the actual sequencer contents.)

Unfortunately I have no manual for this thing, but this is what I found out. Press "chord" and then "play" to play only the chord track. Press "melody" and then "play" to play only the main voice. To delete the sequencer contents hold "record" and press "clear". To record your own melody into the sequencer, hold "record" and press "melody" to enter melody recording mode. Play the melody note by note (press "reset" to quit). Afterwards the note lengths must be entered separately, and also chords can not be recorded together with the melody. :-[  (No joke! The PT-30 had no trouble with that.) To enter the note lengths, press "play" in melody recording mode and step with the right tempo through the melody using the "one key play" buttons until the melody ends (or press "reset" to finish). When no note lengths are entered, the melody plays with tremendous speed. To record chords works similar, but hold "record" and press "chord" for the chord recording mode. To enter chord lengths (not necessary when you played right), press play in this mode and step forward with the "one key chord" button. To edit the sequencer contents, enter the melody or chord recording mode again and step through your track with the "for" button. Any new played notes or chords are inserted at the current position. The "delete" button deletes the last played note/ chord from the track. After edit press play in that recording mode and correct the note lengths with the "one key play" and "one key chord" buttons again. (Note: ROM-Pack musics contain beside melody and chord voice a 2nd main voice called "obligato"; unlike melody and chord, this additional voice can apparently not be edited in the sequencer.) To save the actual sequencer contents on a RAM-Pack, press "R/MT" once and then "save".
 
Like the Casio PT-30, the PT-50 has an expansion cartridge slot for the Casio TA-1 tape memory interface to save the sequencer contents, thus it might be possible to save with this thing ROM-Pack music data on cassettes or upload them to a PC for further analysis or even sound emulation. Unfortunately you can not play them through the RAM-Pack RA-1 on other ROM-Pack keyboards, because the RA-1 is incompatible with them.
 

hardware details

The Casio PT-50 is built around the CPU "NEC D1868G 004" with 2 additional special SRAMs "Hitachi HD61914B". Like its predecessor PT-30 it employs plenty of additional analogue circuitry for percusion and main voice filtering.

The instrument concept of the PT-50 (with ROM-Pack data format) is thoroughly described in the US patent 4624171 and (later) 33607. The internal data format of the sequencer differs between PT-30 and PT-50, so recordings saved on cassettes can not be exchanged. Despite many similarities, the software and many hardware details differ.

The PT-50 service manual lists interesting differences to PT-30. So PT-50 has VDD -4.5V vs. -4V, 2 instead of 3 melody filters, bass & chord is common because of additional obligato circuit, clock frequency is 450 kHz instead of 600 kHz. The CPU is claimed to be"µPD1868G" instead of "µPD1868G-22" however my actual PT-50 has software number 004 while PT-30 has 001. The optional ROM-Pack could contain IC µPD-883G-B (RO-201), µPD-883G-E (RO-301) or µPD-883G-F (RO-302), which hints that PT-50 in different countries likely was shipped with different ROM-Pack. 

The small case is even more crowded than PT-30; between panel and analogue PCB it contains even a 3rd intermediate PCB as the digital mainboard. But fortunately this daughter board at least makes the CPU behaviour a bit easier to examine since you don't need to take out the panel PCB (with a hand full of loose buttons flying around) to access it. Also the LCD here is held by a sheet metal bracket that didn't exist in the PT-30, and the LCD segment layout (particularly chord letter segments) was changed.

I first feared to ruin the LCD foil cable by examining the PT-50, but later I cheaply bought on eBay a shabby 2nd specimen with severe battery leak damage that I dared to analyze further. Under its chord keys all connection between carbon traces and copper were corroded away, so I fixed them with conductive silver paint and replaced some dissolved traces with copper wire. After I got it to work (took weeks), I managed to write down the mainboard wiring.

The digital mainboard is a small daughterboard that contains the CPU, 2 SRAMs and a logic IC "Toshiba TC40H00P" (4x NOR) that is wired as 2x OR to switch 2 multiplexed bus pins on the ROM-Pack port, apparently to disable ROM-Pack during auto-power-off.

rom pack pin 2 = pin 32 OR pin 73
rom pack pin 6 = pin 28 OR pin 73

This is the wiring of both SRAMs HD61914B.
 
SRAM pin I wired to II wired to
1 +Vs "
2 CE1 (cpu 32) "
3 VDD1 (ground 0V) "
4 VDD2 (cpu 30) "
5 ø1 (cpu 29) "
6 ø2 (cpu 28) "
7 OP (cpu 27) "
8 D1 (cpu 26, resistor)  "
9 D2 (cpu 25, resistor)  "
10 D3 (cpu 24, resistor)  "
11 D4 (cpu 23, resistor) "
12 INT (cpu 22, resistor)  "
28 +Vs VDD2 (cpu 30)
24..27,
29..33		 +Vs "
34 1 11
35 2 10
36 3 9
37 4 8
38 5 7
39 6 5
40 7  
41 8  
42    
43 10  
44   2

The +4.5V supply voltage for the CPU is weak and will make the pitch go down when loaded with additional components.

The MT port for the digital cassette interface is wired like in PT-30.

audio channels

The individual sound channels and control lines are accessible on the rear grey 15 pin ribbon cable. It has pin 1 wired to CPU pin 21 (reset), and pins 2..15 wired to CPU pins 80..67 (white noise pin 72 through a 100k resistor).

multipulse squarewave & timbre filter

The main voice is based on the same multipulse squarewave sound synthesis like in its predecessor PT-30, but some preset sounds differ and sound less interesting. Likely to standardize the sound set for ROM-Pack support, e.g. the 'violin' waveform has been replaced with a boring 7:1 squarewave, 'mellow' (with strange tremolo envelope) became a 'celesta' and the bass voice was worsened and has no separate output pins anymore (those became the ROM-Pack obligato voice).

The main voice is routed through 2 fixed timbre filters, those are controlled by CPU outputs pins 68 O4 (highpass) and 67 O5 (lowpass) for 3 timbre settings (both hi = unfiltered).
 
preset sound:
 multipulse pattern
filter O4
filter O5
piano 1111100000000000
 
H
harpsichord = piano
H
 
organ 1110101011001010
 
H
violin 11111110
H
H
flute 11110000
 
H
clarinet 1111011100001000
 
H
trumpet 1000000000000000
 
H
celesta = flute
H
 

The accompaniment bass voice seems to be a 3:1 squarewave that is alternatingly output with the chord and muffled by a filter. In PT-30 it had a unique multipulse and own pins. The percussion works like in PT-30 and uses the same CPU pins.

keyboard matrix

Despite similar control panel, the keyboard matrix is totally different from PT-30 and way less messy. So the keys are here grouped by 6 and not cluttered everywhere across the matrix. I had analyzed this matrix by myself and later go the service manual (which refers output pins KC instead of KO). The only interesting easteregg I found is a locking fill-in.

Supply voltage and keyboard matrix are on the transparent 22 pin ribbon cable to the left of the PCB. To the right you see the pinout (on PCB counting from bottom to top).

 KC11
KC10
KC9
KC8
KC6
KC7
KC5
KC4
KC3
KC2
KC1
KI8
KI7
KI6
KI5
KI4
KI3
KI2
KI1
GND
+Vs (4.5V)
M1
 2 KI1
3 KI2
4 KI3
5 KI4
6 KI5
7 KI6
8 KI7
9 KI8
 
CPU pin
in 1
in 2
in 3
in 4
in 5
in 6
in 7
in 8
in / out
 
 o
F#1
o
C2
o
F#2
o
C3
o
F#3
o
F1
tempo
-
-
out 1
10 KC1
o
G1
o
C#2
o
G2
o
C#3
o
G3
o
F1
tempo
+
S.
one key 1
out 2
11 KC2
o
G#1
o
D2
o
G#2
o
D3
o
G#3
S.
melody
S.
demo
S.
clear
out 3
12 KC3
o
A1
o
D#2
o
A2
o
D#3
o
A3
S.
chord
transpose
-
S.
delete
out 4
13 KC4
o
A#1
o
E2
o
A#2
o
E3
o
A#3
S.
one key 2
S.
R/MT
S.
play
out 5
14 KC5
o
B1
o
F2
o
B2
o
F3
o
B3
S.
for
transpose
+
S.
reset
out 6
16 KC6
O.
piano
O.
harpsichord
O.
organ
O.
violin
O.
flute
O.
clarinet
O.
trumpet
O.
celesta
out 7
15 KC7
C.
C
C.
D#
C.
F#
C.
A
C.
min
C.
maj7
C.
sus4 
R.
select
out 8
17 KC8
C.
C#
C.
E
C.
G
C.
A#
C.
7th
C.
6th
C.
dim 
R.
start/ fill-in
out 9
18 KC9
C.
D
C.
F
C.
G#
C.
B
C.
min7
C.
min6
C.
one key chord
 R.
start/ fill-in lock
out 10
19 KC10
-
-
-
-
-
-
-
S.
record
out 11
20 KC11

The input lines are active-high, i.e. react on +Vs. Any functions can be triggered by a non- locking switch in series to a diode from one "out" to one "in" pin.
 

legend:

  
"o"
 = keyboard key
R.
 = preset rhythm
O.
 = preset sound ('orchestra')
C.
 = chord
S.
 = sequencer
orange
background 		 = easteregg (unconnected feature)
grey
background		 = unconnected doublet

  • fill-in lock button

    • A button wired at KC10->KI8 behaves like the regular 'start/ fill-in' button but keeps the rhythm fill-in running forever. To end the fill-in, press the 'start/ fill-in' button.

pinout HD61914

The "Hitachi HD61914 x" (44 pin SMD, pins count anticlockwise, x = a letter) is a 4 bit SRAM with built-in "self control circuit" that was used in Casio keyboards with D1868G CPU and programmable calculators (with CPU "Hitachi HD61913 xxx"). The "HD" and letter are printed above the "61914". Said letter can be absent, A, B or C; it is unknown whether it indicates the memory capacity or speed or other special functions (e.g. address decoding scheme); at least the version without letter has 1 KByte capacity (2048 words x 4 bit, seen in Radio Shack PC-4 service manual). According to PT-50 service manual, the HD61914B (also 1 KByte) differs from HD61914, so HD61914 can not be installed into PT-50 instead of HD61914B, but in PT-30 a HD61914B  instead of HD61914 would work.

Most important is that the first 11 pins have the same pinout like a ROM-Pack and so version B was also used in the special RAM-Pack RA-1 for Casio PT-50. In my 1st PT-30 the HD61914B seemed to have 3 additional pins 14, 17, 19 multiplexed with keyboard matrix and datasette port (likely NC to ease PCB layout). My 2nd (likely older) PT-30 contains a HD61914A (not B) and has an additional IC "TC40H000P" (4x NAND).

In PT-50 both SRAMs are basically wired parallel. The only main difference seems pin 28 that is +Vs on the left IC but wired to pin 4 and cpu pin 30 (VDD2) on the right. SRAM pins 1..12 go to the cpu, while 24..27 and 29..33 are on +Vs. Although pins 34..44 are connected with various pins from 1..11 (see here), they are NC (confirmed by PT-50 service manual) and only used to simplify the PCB layout. But since VDD2 is only a standby signal, it may be that there is indeed a deeper meaning behind this (possibly decoded by some odd XOR tests). Pin 12 sends an interrupt to halt the CPU when autoplay has finished. Pins 13..23 are NC (confirmed by PT-50 service manual).

Also in PT-30 3 SRAM pin traces are additionally wired to other pins (6 to 19, 7 to 17, 11 to 14), those are likely NC to ease wiring. SRAM pin 6 (clock phase?) is wired through a diode to CPU pin 28. In my 2nd PT-30 SRAM pin 6 is additionally wired through another diode to the output of a driver (2 chained NANDs) from CPU pin 30 and some RC network circuitry to to the power switch "off" contact (likely to prevent data corruption), which hints that SRAM pin 6 is always input.

In PT-50 service manual is the RAM-Pack schematics with 2 SRAMs. The VDD2 line from ROM-Pack port is not only connected to both SRAM pin 4 but additionally to pin 25 (V4) of SRAM 1 and pin 28 (V1) of SRAM2, so they likely select the address range. The remaining pins of 24..33 are wired to pin 1 "GND". Pin 3 VDD1 is also the negative backup battery voltage from 3V button cell (with serial diode and 100 ohm resistor), which positive end is on pin 1. Pins 2, 4, 5, 6 have 1M pulldown resistors.

It is unknown what pins 24..33 exactly do; in the pocket computer Radio Shack PC-4 (HD61914 with no letter) they are all wired to pin 1 and the optional RAM-Pack (pinout like a ROM-Pack) is simply wired parallel to the internal SRAM. That RAM-Pack (only empty PCB layout shown in service manual) apparently contains a single "HD61914 x" with pin 1 wired only to pin 33, pins 2 additionally to 23, pin 3 additionally to 22, pin 4 additionally to 28, pin 27 only to 29, connector pin 1 to IC pin 24. These additional pins likely map the address range for the 2nd RAM at $800h directly above RAM 1, which in the PC-4 starts at $200h above the CPU internal RAM. Additionally the traces to pins 3..11 are wired through 21..13, and pins 34..44 are all interconnected (likely NC to ease wiring). But by their wiring at least pins 22 and 23 seem to be not NC (contrary to their naming in PC-4 schematics). Also in RA-1, PT-30 and PT-50 there are many such connections, of those many pins may be NC to simplify the PCB layout. In the PC-4 the cassette interface port (to an 28 pin DIL IC HD43110 for data compression, 300 baud "Kansas City standard", serial data on D4) is wired parallel to the RAMs and only uses a different ø2 clock line from the CPU, which shows that Casio used this 4 bit bus in many places.

This pinout was based on the service manual of the BASIC programmable pocket computer Radio Shack PC-4 (rebranded Casio PB-100) and the website of Piotr Piatek (thanks to him). The PB-100 was a crippled version of Casio FX-700P, which had an additional function button and the 2nd RAM already built-in (no RAM-Pack slot), marketed in 1982 as scientific calculator. Later I got the PT-50 service manual for verification.

caution: The service manual indicates that this IC uses "negative logic", i.e. technically +5V is its GND while 0V is its -5V supply voltage. So the voltages are not was the pin names suggest. I use the positive voltage naming convention (from 0V to +5V, not -5V to 0V).
 
pin name purpose
1 GND supply voltage (+5.5V, diode 3V from backup battery)
2 CE chip enable
3 VDD1 ground 0V (also from backup battery)
4 VDD2 auto-power-off voltage (hi when off)
5 ø1 clock1
6 ø2 clock2 (phase shifted against ø1)
7 OP data /address select
8 D1 address/data bus
9 D2 address/data bus
10 D3 address/data bus
11 D4 address/data bus
12 INT interrupt
13 NC  
14 NC  
15 NC  
16 NC  
17 NC  
18 NC  
19 NC  
20 NC  
21 NC  
22 NC (device address select?)
23 NC (device address select?)
24 DC (alternative backup battery supply voltage?)
25 V4 device address select
26 V3 device address select
27 V2 device address select
28 V1 device address select A11
29 R/W (device address select?)
30 BL4 (device address select?)
31 BL3 (device address select?)
32 BL2 (device address select?)
33 BL1 (device address select?)
34 NC  
35 NC  
36 NC  
37 NC  
38 NC  
39 NC  
40 NC  
41 NC  
42 NC  
43 NC  
44 NC  

According to Piotr's website the write command is $4 (OP=0), followed by a 4 bit device address, followed by the 11 bit address within the device (MSB is ignored). After this any amount of data can be written (in an OP=1 cycle), which makes the address counter advance after every 4 bit word. The read command is $0 and works similar. A single $0 word regularly appears on the bus when the calculator is idle, which may be a kind of soft reset.
 
 removal of these screws voids warranty...    
WarrantyVoid
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