Denon GT-700: Restoration
Servicing a turntable should not be a very complex task. If we're talking about electronics, that is. In terms of mechanical adjustments, the turntable could prove to be insanely difficult to align. Honestly I don't have that precise mechanical skills; thus, I am not going to tackle with tonearm adjustments anytime soon. I'm thankful that mine is in good condition and working properly.
In terms of electronics, if the turntable doesn't have a very complex quartz locked direct drive system, which is not that complex anyway, then servicing could be done by anyone who has basic electronics knowledge. However some turntables have lethal voltages on the PCBs as well. Some have the hi-voltage motor driver circuit airwired and you could easily spot it and avoid touching it. But some, such as this one, have the hi-voltage directly on the motor controller PCB assembly. In multiple points. So capacitor discharging needs to be done before attempting to desolder the old parts. I used a 1 MΩ / 0.5 W resistor connected with insulated leads.
After making sure the hi-voltages are out then it is safe to proceed to actual investigations. Randomly measuring several resistors proved them OK by all means. Thus, electrolytic capacitors is what we're chasing. More on this below.
Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled with electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.
Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I a standard and a precision desoldering pumps and desoldering wick in various widths. To clean the flux, I use isopropyl alcohol and high purity acetone.
Empirically, I found that working with a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.
Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable in specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.
Main Circuit Board Restoration
Here are some before and after pictures of this turntable.
This turntable comes from an era when they made no economies while building audio gear. I really like the nice layout of this unit. It is very simple, well spaced and the various subsystems are well positioned. The mains power transformer has a thick Faraday cage shielding installed on top.
Initial state of the printed circuit board assembly.
Some capacitors already replaced. Originally the bigger capacitors were fixed with some glue to the PCB. I must admit this is the best glue I've ever seen. The capacitors were solidly fixed and the glue removed very easy without the aid of any solvent.
All new capacitors are in place.
Detailed view on the MLCC parts.
I have used some glass beads as spacers for the miniature TDK capacitors.
View from another angle.
There is a slight variations of blue color shade and texture of the BT series sleeves of various proveniences. The 350 V part for instance is made in Japan, according to Mouser. The smaller ones are made in Malaysia.
Final state of the PCB assembly.
Only a handful of old parts have been removed from this turntable. I have measured them and while most of the bigger ones were OKish, the smaller ones had ESR values of up to 70 Ω. I guess this combined with the changing of parameters while reaching nominal working temperature caused the speed deviations.
This concludes the restoration of this unit. This was an easy job after all.