Vermona DRM

Vermona DRM overview

The Vermona DRM was manufactured in East Germany to the beginning of the 80ies, it’s nearly 40 years old now. 11 Instruments (SD in two flavours) and an Accent can be programmed in realtime or manual step programming (TR-808 like). There are 10 (!) storage places. Quite cool: 32 steps per rhythm. The rhythms can be chained to songs. The 16 presets are not that spectacular imho. The DRM offers MIDI for triggering the sounds and Clock-sync as well.

The documentation (manual as well as service manual) are still available on Vermona’s website, brilliant!

 

Technical details

The sound generation is completely analogue on two PCBs. Many many pots to tweak. Cymbal and Hihat are based on the typical 6-Oscillator ringing-noise. The post-filter of HH/CY are quite sharp and high frequent. Toms and Snares are built similar to the ER-9’s, simple but effective circuits around 1 or 2 transistors with t-filters designed to swing when getting a trigger. The handclap is missing a reverb but still quite unique and nice somehow. Bassdrum and Cowbell even use OpAmps, wow.

 

After removing the bottom plate you get a sight onto the ‘Elektronengehirn’, labeled ZERH. Although the chip’s names like e.g. UB880, UB857 or C530 do not sound familiar, they are more or less replacements for the western parts like Z80 CPU, Z80 CTC etc. There is also an ADC for reading the TEMPO pot and a lot of logic chips.

The imho biggest weakness of the DRM: the cabeling. Many thick, drilled cable trees are leading to hardly trustable connectors, sometimes many cables from different locations leading to one connector pin. The whole machine can not be opened completely (without removing connectors from PCBs), only half wide – then the inner cables start to hold against.

Cleaning

Apart from cabeling the DRM is mechanically a brilliantly built machine. Metal top with welded adapters for LED display and buttonpanel. The plastic is high quality, even after 40 years nothing is worn out or brittle. The pushbuttons can be disassembled completely without destructing anything. Most of them still worked after nearly 40 years.

The plastic parts came into ultrasonic bath. The contacts were cleaned with Caig Deoxit and the PCB got rubbed with alcohol.

Everything clean again.

 

Hardware issue after cleaning

May the following lines help the one or other despaired DRM owner:

The first test after reassembling the cleaned unit was negative. Although TEMP worked (CPU running) not any of the pushbuttons showed a reaction, LED of BEAT 1 was on. All 8 send signals (for the buttonmatrix reading) perfectly were sent from the digital board – all pressed buttons were sent back to the digital board.

After about 2 hours of searching and the unnecessary replacement of a CTC – whose timer outputs looked suspicously inactive – i spotted 4 loose cables around the backside jacks. Two grey ones, two in yellow. They were ripped off from two footswitch jacks on opening the DRM. The footswitches engage functions (Solo, Fill In etc.) when conducting to GROUND. Each footswitch jack has 2 cables of identical color. One is ‘sending’ kind of signal (from the functional view it’s like a pullup resistor), the other cable is going into the digital part. If these two cables are NOT connected (no matter if with or without footswitch jack) the CPU interprets this as “footswitch down”. According to the manual the DRM only reacts to the very first button-down (footswitches are handled as buttons). The following simultaneous presses are ignored. So the CPU ignored all buttons because the loose cables made it think the footswitches are down. After soldering the cables back to the footswitch jacks the DRM worked perfect again.

 
 

Trigger Inverter

The 30 pin connector jack on the backside of the DRM offers individual audio as well as trigger outputs for all instruments. For whatever reason these trigger outputs are negative (OFF=5V, Trigger=GND). I did not like that as it does not fit to the rest of my setup. I added zwo 4069 Oktal-Inverters to invert all trigger signals going to the jack. Now they are working as desired: Off=GND, Trigger=5V. This can be made even without removing the mainboard, just remove the 100 Ohm resistors in the upper right corner.

 

Breakout box

Individual outputs as well as trigger outputs are fine. But i did not want to build a breakout box with 26 jacks! I decided for a pragmatic solution with female flatcable connectors – 10pin for Audio and 16 pin for the trigger outputs. Another 10pin goes to the jacks. Using breadboard patchcables it’s now possible to send any audio or trigger signal to one of the 10 mono jacks. The flipswitches are conduction Digital Ground (Trigger) or Analog Ground (Audio) to the row of 5 jacks beside them.

 

3D Print for missing knobs

My DRM was missing the original knobs for Tempo and Volume. I gave a 3D print another try. After the 4th attempt they fitted quite nice. The white dots are simply painted on.

 

Demo

coming