Vintage Audio
this section covers some restoration work that I did on my vintage HI-FI gear
Article #1 | 13:40 PM Friday 18/01/2019

Denon GT-700: Introduction

This is my one and only turntable. I have used it extensively during the past few years or so with different cartridges. It is still in very good working condition but some preventive maintenance cannot possibly hurt.

It is a simple direct drive design without any fancy side-options. This is why I like it a lot. The operation is fully manual.

But even with all this simple construction it has developed some speed issues. Once powered up it will reach up "nominal" speed then will slowly drift up a bit. Sufficient to make me rise from the sofa and correct the speed via the appropriate potentiometer. Once I do this, there is no more drifting for the entire audition session. I learned this pattern and once I reached the correct speed I never touch the speed controller again. The next time I power up the turntable I let it spin for 10 minutes or so before I load a vinyl disc. The speed slowly drifts up to the correct value. However this is not a normal behavior.

In these conditions I plan a full restoration on it. I suspect dried out electrolytic capacitors or drifted carbon resistors. I don't know yet and I wasn't able to find a service manual or any schematics for it. This calls for a shotgun repair. You don't know what that means? It's simple: everything that looks suspect must go away.

Let's go then.

Article #2 | 04:30 PM Thursday 26/09/2019

Denon GT-700: Technical Data

This turntable has the following technical characteristics.

TECHNICAL PARAMETERS
ParameterValue
Drive SystemDirect-drive direct-coupled AC motor
Motor Type6-pole outer rotor AC motor, condenser starting
Supported Speeds33-1/3 rpm
45 rpm
Speed Control SystemServo system through AC voltage regulation
Speed AdjustmentIndependent adjustment for 33-1/3 rpm and 45 rpm
Wow and Flutter<0.04% WRMS at 33-1/3 rpm
Starting Time2.1 sec for 0 - 33-1/3 rpm
TurntableAluminum alloy die cast, ⌀ 338 mm, strobe scope patterns
Signal to Noise RatioBetter than 60 dB
Tonearm TypeStatic balanced, S-shaped pipe arm
Tonearm Featuresanti-skating, oil-damped lifter
Tonearm Length235 mm
Tonearm Overhang14 mm
Offset Angle20 °
Vertical Tracking Force0 - 3 g in 0.1 g increments
Cartridge Weight4.5 - 10.5 g
Power Voltage240 V at 50 / 60 Hz
Dimensions / Weight163 mm (H), 484 mm (W), 405 mm (D) / 9 Kg

Article #3 | 10:00 AM Friday 27/09/2019

Denon GT-700: Parts List

I have assembled a parts list for this turntable. Wherever possible I decided to change electrolytic capacitors with MLCC modern parts. These will outlast the turntable in terms of performance and lifespan. For the rest of the capacitors I decided to go with high temperature Nichicon BT series parts. There are no high temperatures inside but reliability is pursued nonetheless.

Descriptions and Parts Listings

The main printed circuit board is located in the bottom-right part of the wooden chassis as you look at it. It implements the motor speed control function. Very easy to service and you don't even need to remove the wires as they are very long. You can service the PCB in place if you are careful enough.

Motor Controller Circuit
IdentifierSchematicRecommendedMouser Number
C1100 uF / 50 V647-UBT1H101MPD
C210 uF / 350 V647-UBT2V100MPD
C547 uF / 50 V647-UBT1H470MPD
C722 uF / 25 V22 uF / 50 V647-UBT1H220MPD
C141 uF / 50 V1 uF / 50 V MLCC810-FK16X5R1H105K
C151 uF / 50 V1 uF / 50 V MLCC810-FK16X5R1H105K
C16220 nF / 16 V220 nF / 50 V MLCC810-FK16X7R2A224K
C17220 nF / 16 V220 nF / 50 V MLCC810-FK16X7R2A224K
C18470 nF / 50 V470 nF / 50 V MLCC810-FK16X7R1H474K
C191 uF / 50 V1 uF / 50 V MLCC810-FK16X5R1H105K

The construction of this turntable is straightforward and poses no problems for the restoration.

As always, let me throw a piece of advice. Do not hurry, take your time and do the job once. And do it well.

Article #4 | 09:24 PM Tuesday 08/10/2019

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.

General Considerations

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.

Aftermath

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.

Article #5 | 09:27 PM Tuesday 08/10/2019

Denon GT-700: Impressions

So how does it sound? Same as before. But there are no more speed drifts. There isn't much to say here. The unit is working properly now.

This turntable has seen various headshells, cartridges, and styli. In the end I have settled up to only two cartridges that sound good enough and came at an affordable price tag. Thus I have an old Audio-Technica AT-20SLa and an AT-150Ea/G. The first one has a Shibata stylus while the other one has an elliptical stylus. While ATN20 styli are difficult to find and ATN150Ea ones are even harder to get in Europe, I decided to use the AT-20SLa cartridge only with new discs, every once in a while. I am mainly using the AT-150Ea/G nowadays. Once the stylus will wear down I will use VMN50SH styli which are compatible with this cartridge. The AT-20SLa will suffer from obsolescence even though it is one of the best cartridges I have ever heard.

Now back to spinning discs.

Article #6 | 10:06 AM Monday 14/10/2019

Denon GT-700: Cleaning the Styli

For over twenty years I have cleaned the styli only with a soft brush. It wasn't even a dedicated brush but a standard painter's fine brush. While looking for replacement styli on a local Internet site, I have spotted the Audio-Technica AT607 leaning fluid. I was very skeptical at first but I bought it anyway.

Once the thing arrived I decided to clean the ATN150Ea stylus. I examined the nude shank with a magnifier glass before and after the cleaning. Before it was black and had all these ugly deposits around the tiny suspension shaft (or cantilever). After following the instructions in the manual to clean up the needle I inspected it again under magnification. All black deposits vanished away. Then I did this operation on the ATN20 stylus as well.

It might be bogus but the crispiness and the reproduction of high frequencies improved a bit.

I'm pleased with this cleaning solution. And I also think it will last a lifetime. And I thought I knew a lot on turntables.

if you will

Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

Copyright © 1998- Alexandru Groza