PolyMoog Repair

This polymoog was one of the most challenging units I ever worked on. The circuit design is complicated and not at all intuitive or familiar. There are no circuit descriptions, and the flow chart could be pondered and studied for months, before the depths of its complexity could be plumbed .

 I never really believed those stories about technology being provided to us by alien beings, but the polymoog made me reconsider.

There were issues with stuck notes. This turned out to be relatively simple, the Polycom IC's, which are installed on little cards, were at fault in this instance. This was relatively easy to repair.This unit came to me with little repair cards, as shown below, so I was able to plug them in and fix this issue somewhat quickly.

These cards contain the polycom IC's, one per note. They create an envelope and mix the waveforms. The resistor values for the resistor shown in yellow change for various octaves up and down the piano.

Another issue was that all of the Bb's (except for the highest 2) of a certain waveshape (pulse or saw, I can't remember) were dead.

This was the result of a dead divider IC, shown below. Since these dividers divide down from the top note, and then go down from there, if one is dead everything below it will be dead as well.

This IC and it's neighbors divide down each of the notes and provide them to the polycom IC's. When one section of one IC goes bad, all the notes below it go bad too. Eg: all but the top 2 Bb's were silent on this keyboard.

Another issue was with the CD4007 IC's. These are used to control the preset voltages going to the VCF, resonators, and pretty much all of the various "top" boards. They are used to rout the particular CV's (generated through resistor networks) to those boards when the unit is in preset mode, or they rout the output of the various faders to ththe top boards when the unit is in variable mode. When they have burnt out, either the Variable mode of a control doesn't work, or the preset mode doesn't work. They are readily available from digikey.com and socketed so as to be easy to replaced. Note that each some are oriented with pin one facing upwards, and some are reversed!

Finally, issues with the VCF came down to bad transistors at the VCF current source (q11 and q12,) as well as some bad CA3080's throughout the VCF section.

When testing these CA3080s, remember that they have a very tiny range of input voltages (a volt and a half or so at max), and fairly low output voltage swings as well, so set your scope appropriately!

Zip tie repairs (!) : on a VK7 with crooked keys AND and an SY55

I don't frequently use zip ties in repairs, however I do believe they can be excellent and versatile tools. There are situations where an original part simply is not available, or cost effective. In those cases, with the customers approval, I have sometimes employed the zip tie with great success.

In this situation, I used a zip tie to secure the keys on an SY55: drilled a tiny hole through the key and used a zip tie to hold it to the chassis. The repair is permanent, sturdy, and the key behaves exactly the same as its neighbors.

notice the zip tie just visible on the Bb key

the other end of the zip tie is in a washer

On the VK7, the bushings tend to wear down, and the keys slide off. Buying new keys doesn't help much, and a new keybed can be pretty expensive. Zip ties between the keys work flawlessly at straightening the keys out, without adding any interference or resistance, and keep the keys on the bushings.. Plus, the metal casing around the keybed makes it impossible for the zip ties to fall off or move around. This is a great, permanent, and effective repair!

Juno 106 noisy chorus repair

I was able to repair crackling on the Juno 106 chorus by replacing all the tiny transistors (npn and pnp) on the jack/chorus board, as well as recalibrating the chorus section. The crackling occurred when bass heavy tones (especially chords) were played.

Some of the replacement transistors, although excellent substitutions electrically, needed to have their legs twisted since the pinouts were different.

K2000 Repair, LCD contrast capacitor added; similar to XP50 contrast issue

This Kurzweil primarily had an issue with the 220 uF capacitor right near the fet transistor  which regulated the 5 volt rail. That was causing the fet to heat up. Once that capacitor was replaced, there were still issues with the LCD contrast. Using the old trick of holding enter and turning the alpha wheel didn't really help; what ultimately fixed the issue was adding a 10 uF electrolytic capacitor between the LCD contrast voltage and ground (see photo). I found that this greatly stabilized the LCD and made the writing legible. Although there is a capacitor that is supposed to filter this voltage already, I couldn't easily find it, and adding another capacitor fixed the issue.
This was a situation where the customer did not want to spend too many resources on repairing the issue; so although this repair is not one" right out of the textbook" , it did fix the problem and the unit is working.

In my opinion, the idea of using the microcontroller and PWM to create a voltage for the LCD is a scheme which is prone to failure. When the user data containing the contrast parameter is lost or corrupted, or the PWM circuit for the LCD contrast fails (as it did here), the user is left with a blank screen. I think it is far more sensible to use a good old potentiometer to provide the LCD contrast voltage.

A similar issue occurs on the Roland XP50, another synth which uses the microprocessor to store the LCD contrast parameter. When you change the battery on an XP50, you may wind up with a blank screen In that case I used the info kindly listed here, which tells you how to ground the contrast pin in order to bypass the PWM controlled LCD contrast voltage.

Jp8000 Repair: cracked circuit board

Sometimes an apparently tiny crack, like the one on the circuit board of this JP8000, can be a very insidious problem. I repaired this one with regular insulated, stranded, small gauge wires. The hardest part was going through each trace and locating good spots to solder the wires, and keeping everything in order. The board was restored to 100% functionality. It is good to note that on this particular keyboard, when a reset is done, the pitch wheel must be re calibrated: otherwise it will not work and the jp8000 will be out of tune

. .

The repaired circuit board

Polivoks repair: dead outputs, 120 volt conversion, scaling issues

This unit came to me almost completely dead: neither oscillator working, just a power light and a slight hiss.

Bad paper and oil filter capacitors were the ultimate cause of the dead oscillators, but once they were replaced there were serious issues with the keyboard tracking.

At this point it became tricky since the only schematics that seem to be available are in Russian!

I made some headway by replacing the op-amps on the keyboard circuit board. I used a reference table found here,

Russian Ref	Europa/USA Ref	Function
KR140UD8B	uA740 , uA741, LF351,...	Single standart AOP
KR140UD12	uA776, MC1776, LM4250,NTE888	Programmable low power AOP
KT315G	2N3904A	NPN transistor

which states that the russian op amps cross with uA741's, which are super standard monolithic op amps.

This fixed many issues, however, there was a gradual detuning of every note if the sustain was turned up.

With no circuit descriptions (that were in English), I was ultimately able to use the international language of schematics to find the sample and hold circuit on that keyboard circuit board. It consists of a fet, and an op amp and a few capacitors. Essentially, while the key is depressed the voltage flows through the fet and charges the .22 capacitors. Then the key is let up, the fet opens up and the capacitor remains charged to the same voltage, and continues to flow through the op amp (a7 on the schematic below). Since the op amp has super high impedance, the capacitor discharges so slowly that it holds at the same voltage for a long time, and the op amp output stays at the sampled voltage for quite a while. A good description is available here.

The sample and hold circuit thus demands a fet based op amp, so that it doesn't draw too much current from the capacitor and cause the sample voltage to decay too quickly. The 741 op amp worked perfectly everywhere else as a pin for pin substitution for the russian op amp, but not in this sample and hold circuit!  I replaced it with an original russian one, and everything worked fine. (I am sure I could have replaced it with any fet based op amp, but I had russian ones left over since I had replaced quite a few on this circuit board, having use a shotgun approach.)

Interestingly, here is a website which shows a sample and hold circuit using the 741... notice the addition of the 10k resistor, which probably helps the 741 work without drawing too much from the capacitor. 

Here is a picture of the circuit in question

Once all this was settled, the keyboard worked well. I installed the mods located here for external CV and Gate, and I also swapped out the transformer with one that can convert 120 volts down to the 34 volts, center tapped, that the polivoks needs for power (thus you can run it off from regular wall power). I installed an IEC connector and it is all set!

Rheem Mark VII Organ Repair

 

This gorgeous instrument would shut down unpredictably, or sometimes there would be excessive distortion on the outputs. My first suspicions were with broken solder joints, or perhaps failing electrolytic caps in the power section or on a circuit board. Sure enough, over time, I was able to detect low voltages from the power supply, as well as some AC on the DC rails. I replaced the filter capacitors, the rectifier, however the problem still persisted. This was an unregulated power supply,so there wasn't that much to replace or check.

I then disconnected,  board by board, each of the various circuit boards from the power supply, to see which area was loading it down, but there seemed to be no particular issue with any of the different rails.

This was a real mystery, until I decided, finally, to check the resistance across the terminals of the power switch. Although the switch was working mechanically...opening and closing as it should, there was a 30 - 70 ohm resistance across the contacts of the switch even when the unit was turned on.

I was able to disassemble the switch, and spray it with some cleaner and used my new fiberglass cleaning brush to clean the contacts, and all was fine. It has not usually been my experience that AC power switches can fail in that manner, but after 30 years or so of service, I suppose anything can fail.