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.
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.
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.
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.
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.
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:
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.
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
A first impression of a Eurocom II on the emulator
For my work on vintage electronics, I sometimes need a device programmer for (E)PROMs and PLDs my Labtool 48 cannot handle.
2708 EPROMs are one example, they need three supply voltages, 2532 EPROMs are another problem because of different pinouts between manufacturers and so on.
Therefore I bought a Data I/O 29B with an Unipak 2B adaptor some years ago on eBay – some more info and a photo here: http://en.wikipedia.org/wiki/Data_I/O)
The PC software for computer control mode was obviously not written with evolution in sense. I tried it on a Pentium III at 400MHz first, then on a Pentium MMX, but I got lots of transmission errors and timeouts although neither the COM ports of the PCs used nor the system 29B interface was bad.
Now I finally started the project to write a new, portable, GUI-based application to talk with the 29B.
The functions planned so far:
Basic functions are already implemented, a 2708 is the only selectable device at the moment, which can be read and saved to disk. I use the default format MOS Hex (81) for data transfers from and to the 29B, which contains checksums for every record of 16 bytes which are compared with the actual data to make sure no transmission errors have occured. I have not yet checked what needs to be done to allow larger devices (>64k) to be handled with this format as it uses fixed 16 bit addresses, nor how 16 bit devices or even PLDs can be used.
The program is written in C++, using the Qt Creator as IDE and obviously Qt for GUI “artwork” and OS abstraction of serial and file I/O.
Here’s a very first impression, more to follow:
The hex editor on the right shows a file actually written with my program. The contents are from one of the Sequencer EPROMs of a SCI Prophet 10, by the way. Om mani padme hum!
Here are some facts about the drawer – the control panel – of a Mk V Ondes Martenot.
Two swichtes (1, 2) are missing and the touche d’expression is absent as well. No fear, they are kept warm and cosy.
The switches are built from Pertinax (laminated paper) which has suffered over the decades, so that I will drop genuinity in favour of reliability and combine the old nice levers with 2013s switches.
The controls in detail, from right to left:
– switch C/R (rightmost) : selects between clavier (keyboard) and ruban (ribbon)
– three metal knobs : that’s for the circuit benders amongst the ondistes – here your finger becomes part of a series connection with a small capacitor,
directly influencing the oscillator.
– two transparent push buttons left from the C/R switch : transposes down by some amount (only in ribbon mode)
– the big rectangular hole normally hold the touche d’expession, the main volume control button of the Ondes
– switch 1 : adds a LC low pass into the D1 output (red – in, white – bypass)
– switches 2, 3 : some kind of tone control applied to the output amplifier (red – filter, white – off)
– switch 4 : determines the amount of 2&3 (red – full, white – attenuated)
– switch 5 : controls the gain of the pre-amplifier tube – probably into limitation, so a sine will become somewhat rectangular
– switch 6 : adds a filter to the metallic speaker (D2)
– switches D1, D2 & D3 : select the speaker(s) used. D1 is the main speaker, D2 the metallic (gong) speaker, and D3 the palme (with the strings)
– switch 8: adds a bridge rectifier into the signal for speaker D1, thereby doubling the frequency
The wheel numbered I..V : volume control for the main speaker (D1) – to adjust the balance between D1 and D2/D3
Many Polysix’ will be hit by a sudden photocoupler death somewhen.
There are many “facts” around on the internet describing how do determine whether it has failed and how to substitute the unavailable IC.
Fact 1: it is not true that you can check the photocoupler by measuring the voltage drop on the 4k7 resistor inseries with the LED or even the voltage drop across the LED.
LEDs do fail without exhibiting any dramatic change in electrical parameters. You will find LEDs which drop 1.6 volts @ 20 millamps and emitting no light!
(That – and some scientific applications – is why some applications still require incandescent lamps, and that’s why I’ve designed a controller for a 1960s filament winding machine in 2010!)
In this special case, the LDR saturated at 5.4 megohms with full LED current, so either the LED has become dark or the LDR insensitive.
The only way to check the photocoupler is to remove it from the circuit and measure the LDR resistance versus the LED current and compare it to a known working part (see diagram below)
Some background: the photcoupler is used in a feedback loop to stabilize the expo converter which makes up the V/Hz scale for the VCOs from the octave/V control voltage.
To use only one expo, Korg has used multiplexers to feed the 6 VCO CVs to the expo, followed by a calibration voltage and then the CV for standard pitch (only for the new production voice board).
The two latter outputs are routed to two independent controllers; the last one introduces an offset to the expo input to shift the standard pitch to its desired value, while the converted calibration voltage drives a current through the photocoupler’s LED, while its LDR is placed where you would normally expect the 3300ppm tempco resistor in similar circuits.
This leads to an eady way to check whether a tuning problem is related to the photocoupler circuit at all: measure the voltage on pin 7 of IC18. Within regulation it should be somewhat between -2 and -6 volts, -3 are more normal.
When the loop breaks open due to failure of the photocoupler or related parts, it will be stuck on the maximum negative output, something around -13 volts. There must always be a small current through the LED, otherwise the loop does not work and the TUNE HIGH preset has no effect!
Fact 2: the types VTL5C2 and VTL5C3 which are widely discussed as replacments are simply the wrong choice. In order to make the regulation work, the LDR resistance needs to be in the range of below one to a few kiloohms. The maximum LED current is limited by design to slightly over 2 mA. According to the datasheet, a VTL5C2 has 2 kiloohms at 2mA – regulation will never take place! Even if it seems to work, the controller will work close to the margin and operation cannot be guaranteed. Furthermore, resistance variation is quite large for those devices; the VTL5C2 I have tested had about 20kohms at 1mA.
The diagram below shows a known good original part (blue graph) and a VTL5C9 (yellow graph, values from the datasheet)
As you may have expected from my previous text, the red line represent the data sheet values for a VTL5C2.
While the VTL5C9 closely matches the original part in the most interesting region, the VTL5C2 resistance is way too high for any current.
What about RoHS…?
Photocouplers with LDRs are, as any LDRs which are commonly based on CdS (cadmium sulphide), forbidden for new products within the EU for some years now.
There is an exemption for the use in electronic music instruments, because there is no practical solution to control varying ac voltages better with LDRs.
(MOSFETs, for example, become highly nonlinear introducing distortion).
In this special case – a closed control loop and very small voltage variation across the device – a photo-mos device would probably be possible.
But it won’t be a solution for the control of audio signals in many applications. The EU has decided that science will have developed and indutry produces an alternative from december 31, 2013 on.
Science hasn’t, and industry doesn’t – except from Macron, who even try to force the EU to withdraw the exemption. Unfortunately, their devices are now way available at major european distributors by now…
Although it is always a good way to reduce hazardous substances, eurocracy has lost any sense of proportion for this topic. While power toys (sorry, I refuse to claim such crap tools) with short life spans flooding the market at low prices are allowed to contain batteries with large amounts of Cd, small photocouplers containg micrograms of Cd typically used in high-priced gear which will last for decades and serviced in case of a problem are to be banned.