The Zombie Synths

A strange month April has been, a lot of DIgital DSP based stuff has been through the workshop lately, it’s not what we really do, not being Vintage, Analogue or a Synth. The time consuming part of all these kinds of things is that the online forums have to be read and the owners manuals likewise, to see if anything is actually wrong or if it is pilot error.
The manufacturers will tell you nothing of any issues with their products, and their technical support is not just second to none, it is just none.

Nevertheless there have been a couple of Analogue nasties this month…

We got an Ebay special Juno 60 which was bought from a region of what used to be Yugoslavia.
Ostensibily it was in perfect working order but they all say that from thousands of miles away.

The new owner was someone who had switched from a very digital Waldorf Blofeld to his first analogue synth. He brought it to us to ascertain whether this was how a Juno 60 was meant to be, and we had to break the bad news that no, this is not how a Juno 60 is meant to be.

The presets mostly worked and the keyboard seemed perfect (apart from 3 broken keys, the contacts still worked fine) , but most of the buttons didn’t work, a lot of the sliders didn’t work, it was strange.

You could play it if you did not need any programming facilities, but this is not why you buy an analogue synth.

The Voice card was quite dirty and corroded, all the trimmers have a green patina from where the metal plating has been eaten through and the copper underneath has oxidised.
This was one of the first synths we have ever had where the option “to scrap for parts” was seriously considered.
The first and most important thing for us to do first was to check the functionality of the rare and expensive envelope generator chips and the filter IC’s, as if any or many of these were faulty, then it was game over.
Despite their very fetching green legs, all of these seemed fully functional.
The left side front control panel board had lots of wire links tacked onto the back, and lots of the connectors had been touched up with solder, and as none of the switches worked properly and the sliders were either not working or just hideously scratchy, this pcb had to come out.
Taking out the 60’s front panel is a tricky job, there are a lot of screws and a number of cable ties to be cut.
To avoid straining the connectors, the right hand side pcb has to come out as well.
With the left side control panel board on the bench and in good light it was time to see what the damage was…

It was nasty, in the past someone or something had punched through the Arpeggiator on/off switch and had cracked the pcb and broken several tracks, not least the ones on the switch itself.
The half-assed soldering on the connectors had lifted many tracks and actually broken some, so continuity checks on all connectors had to be done after repairs were made.
A google and ebay search fairly well confirmed that new sliders were not available, neither were the push switches to select waveforms, so they all had to be stripped and rebuilt.
Actually the Juno’s waveform select switches have a tiny conductive rubber button at their heart, and unlike most tact switches in synths, these are capable of being dismantled and cleaned and rebuilt, so this was done.

I guess at this point in time, We should make some reference to the heading of this month’s blog.

At what point can a synth be said to be really alive?
How many faults should a synth have before it’s one of the living dead?
When should you just shoot it through the head? (or scrap it for parts)

This Juno is one of those, as was the Polysix from last months blog, the number of hours required to successfully rebuild this exceeded its value many times.

Of course it is not just synths that can be the living dead.
Boss BF1 flanger

We get a steady stream of guitar effect units through the shop, most of them are analogue and most are
vintage and most of them have relatively simple faults.

Most effects pedals have only a handful of IC’s and a few transistors in them, so most are relatively quick and inexpensive to repair. The most common issues are related to the power supply section (if an external power supply can be used), battery connectors, jack socket issues or broken wires.
But not always…

This particular pedal was another internet buy and was sold as not working, so no surprise that it wasn’t.
On opening up the pedal, there was an obvious black scorch mark near where the power supply regulator lives, and signs of some rather bodged repair attempts, and the replacement of 2 diodes, one of which was the wrong type, and the other was put in backwards. There was no power to the pedal until this was rectified.
According to the label on the bottom of the pedal itself, power is to come from a 9 Volt centre pin negative similar to the type used by all later small Boss pedals, but the schematic and the BF1’s onboard power regulators both indicate a power supply voltage of at least 12 and a half volts.
After buying new components for the power supply, and new capacitors for the whole pedal, and having installed them, it was time to power it up from a 12.5 Volt power supply.
The pedals power supply was now putting out a nice clean 11 and a bit Volts at the regulator but a lot less at the important circuitry. While poking around with a multimeter, there was a lot of heat coming from something, and that heat smell from components that never bodes well.
The 2 CMOS chips that provide the clock pulses to the BBD delay chip were too hot to touch, something that CMOS IC’s should never normally do, so they had to be replaced, and good quality IC sockets put in to take the new ones.
Once this was done, the power was correct across the entire pedal’s circuit board but it still didn’t work.

There were clock pulses for the BBD now, and the manual control changed their frequency, but the modulation oscillator didn’t change the frequency at all, and the essence of a flanger (or chorus) unit is the up and down sweep of the BBD clock frequency.
Guess what, the LFO chip was dead. Another nice socket and new IC and we had the clocks doing something similar to what the service manual said they should, but still no flanging.

Input signal went into the BBD chip and suitable clocks also did, but there was no output from the BBD at all. A lot of testing was done to make sure that everything else was OK and that the BBD itself was the next culprit. It seemed to be and in time another rare and expensive SAD1024 chip was obtained at a cost of nearly €50 .
Another nice socket went in and the replacement chip was unpacked to go into it.
Easy and job done… like heck it was.
The new chip looked liked it had been stepped on with work boots, all the pins were crushed and bent into the underside of the chip body. It took a lot of very careful manoeuvering with precision pliers and tweezers to even line the chip legs up to go into the socket, and when it did the flanger still didn’t work.
In the vain hope that the new chip might be good, another chip in the post BBD section was replaced.
Still no joy, 5 out of 7 of the chips in the pedal have been replaced, and no one is going to invest in another SAD1024 BBD to double check if the so called new BBD was actually faulty.

It is on hold until either satan skates to work or another pedal comes in using the same BBD IC so that we can confirm one way or the other.

Zombie equipment, landfill was too good for this one.


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