Author Archives: StefanHuebner

New hope for Morley

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As green is said to be the color of hope, those two oldies should now have lots of it.

They came to me with the classic light bulb, one missing the photo resistor, and both featuring a nice 115VAC transformer and a power cord good for an average home.
The task was to identify a suitable photo resistor (bad, bad CdS stuff that the EU does not want us to use anymore. Maybe they think all the cadmium is urgently needed for cheap chinese power tools that find an early end in the dustbin due to their quality anyway?), remove the circuitry and design a driver circuit for a LED – a green one, matching with the sensitivity maximum of the photo resistor. A switch mode converter was used to allow the longest possible operation from 9 volts battery. For this purpose, a battery compartment has been mounted into the base plate. As soon as the battery voltage drops below 7.5 volts, the red indicator LED on the front starts flashing slowly.

Here’s the new circuitry:

Update for the Wave 2 modern display firmware

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Lately LCD modules showed up that do not comply with the unwritten standard that returning the cursor to home position on a non-shifted display will not take longer than 40 microseconds. The datasheet of the HD44780 controller chip clearly says 40µs up to 1.64ms, with the latter being common for un-shifting shifted displays. A display taking longer than approx 53µs for executing the home command, erratic behaviour will occur.

As the controller originally used in the Wave 2 does not know about shifting, the problem was solved by using a command to explicitely set the cursor position to 0 (zero) instead of the home command.

Here’s the result:

 

A big one: Arp 2600

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This Arp is here for several reasons. Although a basic repair has already been done before it came in (S&H replaced and some other typical tasks for the 2600), but there is a lot of work to be done. Here’s a first impression of the Arp

There are may modifications, some not yet fully understood. The main problem are several broken potentiometer levers – original parts are not available, nor aren’t rebuilds of similar quality. Therefore we are evaluating levers and knobs made using 3D printing techniques. This work is carried out by drucke3d.de and the parts are now waiting for installation.

Here are some photos of the major modifications and additions:

This is a simple 2 transistor astable multivibrator which is actually not connected to any parts of the Arp.
The output signals are routed to separate jacks and may be used for modulation purposes.

 

A 2nd order RC filter with switchable corner frequency. The filters are passive with an buffering op amp in between.

 

One of the two transpose circuits connected to each of the two VCOs. Two switches expand the transpose possibilites by fourths and fifths.

During repair, all modifications will be reverse engineered and documented. For now, here’s an overview of the modules in the Arp:

Another 360A leaves the lab

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The next PPG 360A has just left the workshop. It seems that all 360A that have been sitting around for years or maybe decades will need several hours of troubleshooting and repair and a bunch of 15..20 ICs.
Fortunately most of the parts are easily available, and the next one is already in work.

As you can see, two boards are missing – between IO and TONRAM there is one of my RAMPROM replacement boards hiding. Less ICs to fail, less power, less heat.

Korg Trident – Intermittent Problems

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After restoring the volume potentiometer from contact cleaner abuse and replacing a bad SSM2044, the Trident still showed some intermittent problems.
One time, the attack on one voice has gone, the next time another voice was lacking release. After some measuring and finally swapping the double transistors, the fault eventually disappeared, but new problems came up.
It turned out that the solder joints of the rivets used as vias on the paper laminate board showed microsopic cracks. Resoldering brought all ADSR features back, needless to say that all rivets of this board need to be reworked.

An example of the affected vias – resolder all of them, there are probably 50+ on this board good for all kind of intermittent trouble.

A side note: this synth uses quite interesting ADSR generators, a 1µF electrolytic is charged and discharged by controlled current sources built around double transistors which are selected by the MOSFETs in a CD4007 package.
The generated envelope controls a VCA built from two selected standard transistors – not always easy to troubleshoot and repair, but the parts are much easier and cheaper to obtain than the typical CEM3310/3360 combo.

AKG ADR68k

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An ADR68k showed up with multiple problems and a common symptom: no function, except from the remote telling it cannot find the main unit. The first obvious fault were broken ZIF sockets and an EPROM travelling the 19″ case. Replacing the sockets did not help much, as the power supply was starting to develop a high current smell.
The crowbar circuit tried to force down the linear +5V regulator which is good for up to 10 amps, causing the SCR to get very hot until it finally shorted out. The regulator uses a sense circuit with the power and regulator ground being seperately connected to the main board via a connector – this one:

I know those connectors in a similar or worse condition quite well from pinball games. In this case the resistance of the power return has increased, the regulator tried to compensate and finally lost against the crow bar.

A careful rework of the PSU, including replacement of previously installed cheap capacitors, soldering the wires directly to the main board brought the unit back into operation.

But there still was a input level indication without any signal. It turned out that the PCM53-I DAC in the ADC circuit had an offset on the output and needed to be replaced.

 

CS50 – CS for CraftsmanShip

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After all those plastic synths and heavy metal profile construction with no chance to operate in disassembled state this Yamaha CS50 was a real pleasure to work on. Nice condition and just a minor fault: the external modulation input did not work anymore. There is not much in between the input jack and the modulation source switch – just an OP amp and a capacitor, and unfortunately no protection resistor. This means the OP amp is blown when a high level signal is applied to the modulation input with its level pot turned fully clockwise. After replacing the OP amp and adding a series resistor to protect its input the external modulation was back again.

 

Evolve!

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Some trouble evolved within this Evolver – the barrel jacks permanently failing in laptop computers don’t do better in a synth. Once used on stage, a broken jack, plug, cable… is almost certain. A quick repair on the kitchen table brought it back to live, but a serious improvement would require another connector.

Clashing with the style

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Some time it’s the time to make a decision. The rocker switches in the drawer seem to be self-made by M. Martenot and have degraded seriously over the years. Poor contact, some are sluggish, others have too much slackness. For the use of the instrument it seems much more important that the switches operate correctly without causing additional audio noise due to poor contacts than having genuine switches under the hood.

Here’s what I’m planning to use:

New switch for the Ondes Martenot drawer

The Marquardt rocker switches will be mounted in an additional sheet of metal which will also keep the axles for the original transparent levers in place.

 

Testing the ELTEC

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The PPG Waveterm A was built around an industry processor board named EUROCOM II V7 made by ELTEC. Based on a 6809 CPU it contains two 6821 PIAs, a 6850 ACIA with baud rate generator, a 1793 FDC controller, 64k of dynamic RAM and two 4k ROM sockets. One out of three 16k memory pages can be mapped to the integrated discrete monochrom graphics controller.

Due to the complexity it can become rather complicated to trouble shoot those boards. As lots of counters and other TTL ICs are required for the memory to work properly, chances are good that an application’s firmware won’t run due to memory problems.

For the software I’ve created only the CPU, ACIA and baud rate generator need to work. The first routines run completely within the CPU registers and do not require any RAM.
Once started, a welcome message should appear on the connected terminal. If not, basic 6809 system troubleshooting needs to take place.

As soon as the terminal comes to live, the first 4k of RAM will be checked by a AA/55 pattern. If a mismatch occurs, the routine will stop and the original and read bit pattern is displayed.
Assuming the first memory page is in good condition, the stack pointer is initialized and a menu appears.

For now, four tasks are available:
1. Memory test. The whole memory between 1000 and EFFF will be tested, an error will be displayed with the actual address, the written and the read bit patterns. The test will run forever.
2. PIA output. All 8 bit PIA ports show a square wave with a frequency of 19.2kHz on PA0/PB0 decreasing to 150Hz on PA7/PB7.
3. PIA input. Binary patterns of all four 8 bit ports are displayed continuously.
4. Screen test. A test image will be show.

To Do’s:
– add two more test screens, switchable using the page select lines
– add a test for the hardware scrolling functions
– keep away from the FDC, this will have to be troubleshooted in the target system if necessary

ELTEC Eurocom II V7 board under test

A first impression of a Eurocom II on the emulator