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.
I should begin by saying that working on this turntable presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for almost 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.
Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also I only use parts that are suitable as replacements in various sections of the turntable, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.
I have found out that working with a temperature of exactly 290 degrees Celsius is sane for these vintage Denon printed circuit boards. I have never lifted a pad with this temperature. But it is also true that I never wait more than three seconds with the soldering tip on a pad.
Flux fumes are extremely toxic and should be avoided at all costs. Especially toxic are the fumes released while working on these old Japanese electronic boards.
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.