Friday, 16 January 2015

Bentley Rhythm Ace - FR-8L - Roland TR77 - Mods - Adding clock sync

We (Cobramatic & I) purchased some Bentley Rhythm Ace  & TR-77 drums the other day.
Some we got to mod and some to just preserve in their vintage glory.

The TR77 and the Bentley Rhythm Ace are Roland's very first drums so we feel that they have
a special place in the history of electronic music.
They are both beautiful & warm sounding analog (1972) machines with discrete components.
Thus they are perfect candidates for modding.
One of their drawbacks however, is the lack of a way to sync them to our modular gear.
I guess Roland never envisioned that someone 50 years into the future would want to sync them with a modular synthesizer.

BurnKit 2600 has a mod for the Roland version of the Bentley
http://www.burnkit2600.com/tr-77/
that shows where to add external clock sync.
However, after opening the Bentleys we discovered that their PCBs are slightly different to that of the Roland TR 77.

After a bit of probing we located the revised sync input for the Bentley.


 Here is a closer pic:

A +5V clock pulse will do the trick.


If you are game to open your Bentley PLEASE BE CAREFUL.

The Power Supply section to the right is exposed and will KILL YOU if you aren't careful.
If you are not sure leave the mods to a qualified technician.

 The power supply - be very careful !!


The brown PCB on the left is the voice board.... ripe for individual triggers. (a future mod)
The TR -77 uses a very basic circuit to produce the sounds. The circuit consists of just 3 main components : A resistor, a capacitor & a inductor. Called a RLC circuit or "tuned resonant circuit"
The circuit forms a harmonic oscillator for current and will resonate.

Further reading that you may find useful re resonant circuits:  
Inductor-capacitor "tank" circuit
and
Simple parallel (tank circuit) resonance
and
https://www.youtube.com/watch?v=8vu9WDjBAho


The Voice board apparently needs -5 to -12v spikes to trigger the sounds.
CircuitBenders UK has very informative site re this:
http://www.circuitbenders.co.uk/newsarchive/BRA.html

Another great site with TR 77 mods is here:
http://www.adambaby.com/studiotech_TR-77.html

Bently Rhythm Ace test video
The Roland Drum Machine History

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