The Story of Serial Number 44:

Cosmetically, my Spirit is in very nice shape, however there were several electrical / functional problems when it arrived, and I'll keep a log of my repairs and restoration below:

Transformer:

The mains transformer in the Spirit is rather weak and prone to breaking. I had read of this problem on the internet, and the transformer in my Spirit also broke during shipping. If your Spirit is still working and has the original transformer, I'd recommend trying to reinforce things a little bit.

The original transformer is attached to the power supply with small solder tabs at the two mains taps and the three secondary taps. These solder tabs tore out of the plastic body of the transformer, and ripped out the primary-side windings. (photo 1 / photo 2) Luckily I was able to re-insert the metal tabs into the transformer and re-solder the primary wires. I put some hot glue under the corners of the transformer and strapped it down to the PCB with wire ties. Hopefully the stronger connection will keep the problem from reoccurring (photo 1 / photo 2).

Tuning:

The Spirit was also pretty far out of tune when it arrived. The previous owner hadn't used it in years, and I think years of dust/moisture are the biggest culprits. Not only was the scaling off, but the pitch glitches wildly when adjusting any of the octave, detune or waveform select switches. The oscillator pitches were also drifting around a bit (randomly - not synced to LFO, etc).

At first, I removed and reseated the connections to the VCO board, and things got a little better - particularly the random drifting. Next, I noticed that all but two of the trimmers were open frame. I wiggled them slightly with a screw driver, and they seem to make better contact now, but they will definitely need replacing.

Here's a photo of the VCO board. Oscillator A is on the left and Oscillator B is on the right. Those trimmers are pretty rusty and grungy looking up close. I noodled around with the VCO scaling, and was able to get Oscillator B working fairly well. Oscillator A was still a problem - the scale is too 'wide' (an octave spans 14 keys), and I couldn't adjust it back into range.

I removed the VCO board from the Spirit and cleaned and relubricated the trimmers. While the board was out of the synth I removed the 1nF timing capacitors from each VCO and replaced them with polystyrene capacitors for a little better performance. I'm sure the benefit is very slight, but I happened to have some in stock. I also took the time to use contact cleaner on the 3 headers that connect the VCO to the wiring harness. I also took the time to remove the CEM3340s from their sockets and gently cleaned the pins before reseating them.

After reinstalling the VCO board I was able to get both oscillators roughly scaled and tuned (I wasn't going to do a real thourough calibration, since I still intend to replace all trimmers). For the first time, my Spirit is starting to sound really nice; I can see why so many people like them. I have made a PDF diagram of the trimmer and timing cap locations that makes tuning a little easier.

My next order of business will be pulling the front panel circuit board and inspecting, cleaning and lubricating the pots and switches to get rid of the glitching. I've noticed that the more I play the Spirit, the less glitchy it becomes, so I think using the controls is starting to wear some of the tarnish / crud off of the contacts. I'm pretty hopeful that a squirt of cleaner / luricant will bring them right back to life.

When I'm satisfied with the operation of the octave, detune and waveform switches, I'll replace the trimmers and try a VCO calibration. I really need to verify that the keyboard scanner is working correctly, too (outputting the proper CV voltages). It would be a shame to calibrate the VCOs to a keyboard that it is out whack (A mistake I once made with a different synth). Check back for an update as I get further along.

Filter Envelope:

The filter envelope is currently dead. This fix isn't real high on my list yet - I want to get the oscillators working right first. It's built around half of a 556 timer and some 4066's - fairly standard stuff, so hopefully fairly easy to diagnose. It also may be as simple as a bad envelope level pot The level pot looks like a 100KB with a center tap (schematic is fuzzy) - which will be a bit difficult to source.