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.

Disclaimer

The following articles are not to be treated as do-it-yourself tutorials on how to fix, restore, rebuild, or improve the unit in cause. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to launch into such an adventure.

The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.

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Denon GT-700: Technical Data

This turntable has the following technical characteristics.

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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.

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.

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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

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.

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.

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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.

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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.

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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!

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