My gear (Synths, Drum machines, String Machines...)
Before being a synth repairer, I am a synth collector.
!
A significative part of my synthesizers was bought with some malfunctions. I'll list it and also frequent problems I deal with.
!
Alesis

Alesis D4

Alesis

Alesis Micron

ARP

ARP Axxe

Common problems:
-Keyboard bushings
ARP

ARP Odyssey

Common problems:
-Keyboard bushings
Arturia

Arturia Origin Desktop

Common Problems:
-Big knob - skipping values
-Power supply - no power

Casio

Casio CZ-1

Casio

Casio CZ-1000

Casio

Casio HT-6000

Casio

Casio VL-1

This was my first synthesizer back in 1980, I was 7 years old, although I consider my first "real" synthesizer the Korg 01/W later in 1992. VL-Tone may look like a toy but was used by many mainstream artists from Human League to Tony Banks. German band Trio hit "Da Da Da" made a lot for the success of VL-1.

The heart of VL-1 (and several other Casio models) is a LSI chip made by Nec, the uPD1867G. Although many people think it is a analog synthesizer, it is in fact not only a digital synthesizer but also the first commercial digital synthesizer. It was released in June 1979.
Casio

Casio XW-P1

Underrated machine. Will never be loved because of the name CASIO.
Common problems:
-None that I know
Cheetah

Cheetah MS6

Common problems:
-Battery leak
Crumar

Crumar Bit 99

Crumar Bit 99 had various incarnations: Bit 99, Lem Bit 99 (the one I've got), Unique DBK in USA, Chase Bit 99 in UK...

Crumar

Crumar DS-2

Although the synthesizer section is monophonic, there's a paraphonic string section.
Crumar

Crumar Multiman -S

Crumar

Crumar Stratus

Crumar

Crumar Synthaphone

This incredible little machine has breath control and a piezo sensor to control vibrato.
I was delighted the first time I played the Synthaphone - it may look like a childrens toy but man, this is an expressive instrument. This instrument was designed in 1982 by engineer Luciano Jura of Crumar, outside his working hours as Crumar was not interested in the project. Synthaphone had a enormous success during its presentation in 1983 but only a hundred copies were sold by Crumar which makes this instrument a rare find. Synthaphone is based on the SGS M110B1 monophonic synthesizer chip and includes a Curtis CEM3320 filter.
Dave Smith Instruments

Dave Smith Instruments Evolver

Davoli

Davolisint

Very likely the first serial produced synthesizer in Europe.
Doepfer

Doepfer A100BS2

Elka

Elka EK22

Elka

Elka EK44

Some say that Elka managed to get Yamaha FM operator YM2203 to their EK44 by pretending to be a Arcade games developer...
There might be some veracity in this story because the Yamaha chips part numbers are sanded and replaced by a simple G1 and D1!
Elka

Elka Rhapsody 490

Eminent

Eminent Solina String Ensemble

I have the last version, SE-IV with stereo output. Some say that previous versions sounded better...
Emu

Emu Emax

Ensoniq

Ensoniq ESQ-1

Common problems:
-Battery
-Rust (metal chassis version). Actually, the rust starts in the screws.
-Display/keypad controller (Rockwell R1121-11)