PA Chime Generator
Upon replacement of the audio system in 2020, some individuals remarked that they missed the four-tone chime of the prior system.
A (for-fun) project has been kicked off to see if we can bring this element of nostalgia to the new system!
The prior PA unit was an Inkel PA-9100RC Public Address Cassette Receiver, produced by the Korean Inter-M corporation. Unfortunately, as the unit was in disrepair, it was discarded after an extended effort to sell it on eBay. In hindsight, the chime circuit could have probably been recovered.
The schematic in the PA-9100RC/9200RC Service Manual shows that the chime tone is generated by IC101, a BXA4240. No datasheet could be found for this part. Fortunately, a datasheet for a pin-compatible equivalent (the IK Semiconductor BT8040N) has been found here.
The BT8040 is a "melody generator with accompaniment". Some key observations:
- The resistor-capacitor (RC) circuits attached to ENV1 and ENV2 likely provide audible decay (envelope) of the the tones. In the PA-9100RC, each consists of a 4.7 uF (50 V) electrolytic capacitor and 100 kohm resistor. These same values will be used in the new design.
- The SEL1 pin (#6) is pulled HIGH (to VDD) while SEL2 (#5) is left floating (LOW?). This appears to select the "do-mi-sol-do" melody, according to the BT8040N datasheet. This is consistent with a spectral/audible analysis of the PA-9100RC.
- It's not clear whether the musical key is affected by the 1 Megaohm resistor between OSC1 and OSC2, or if this just sets the speed (tempo). According to the BT8040N datasheet, this resistor can span from 500 kohm to 1.5 Megaohm. 1 Megaohm will be used to match the PA-9100RC.
- The MT pin (#4) is used to trigger the tone sequence.
- The driver circuit (P1, OUT1 and OUT2) is not used.
- Audio is provided via the MO pin (#12). In the PA-9100RC circuit, there is AC coupling (C1), an application of DC bias (R1/R2?). The purpose of D1 is not yet understood. This circuit will require further analysis.
As of February 2021, a small quantity of the BXA8240 has been sourced from a surplus dealer in China. When (or if) these parts ever arrive is "TBD". Once they do, experiments will be carried out to see if the tones can be replicated on the bench.
A schematic is being developed. The chime circuit will nearly duplicate that of the PA-9100RC. A mixer circuit, along with a mic pre-amp, will also be included. Details to follow.