SONY TC-K850ES: Introduction

This is one of the good cassette decks. It is one of my master cassette recorders. Second to the TC-K555ESA. I have intensively used it for a lot of years so far. But only for recording purposes. Rarely I have been playing cassettes through it. It sounds very nice, typical of Sony of the late '80s - early '90s. But considering that it records so damn well, I decided to switch to AKAI GX machines for playback only purposes.

This cassette deck was in pristine condition when I first bought it back in 2010. It even had the original packaging material. As it still happens now, back then people were getting rid of cassette decks. I mean decks are nice but they require constant maintenance and care in order to deliver good sound. Otherwise they can -- and will -- sound like compressed sh*t.

One of the problems of this machine is that from times to times it chews tape. Mostly chrome tapes. I have carefully observed the tape path and found out that the supply side pinch roller was very worn. This is a known issue of the TCM-200D class mechanical blocks. Thankfully it is pretty easy to fix with the right tools.

Electronics-wise, this machine is still operating in top condition. So there is nothing to fix here. But some improvements can be brought nonetheless.

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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SONY TC-K850ES: Technical Data

This cassette deck has the following technical characteristics.

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SONY TC-K850ES: Parts List

I have assembled a partial parts list for this cassette deck.

Descriptions and Parts Listings

The SYSCON printed circuit board occupies the left area of the steel chassis. It is home to the power supply and the system control circuit.

The main audio printed circuit board occupies almost the entire right section of the steel chassis. It is the large board that implements the record and playback amplifier, the BIAS oscillator, and various adjacent circuit sections.

The Capstan printed circuit board is bolted to a steel plate that goes to the mechanism block on the back of the two flywheels. It implements the capstan motor driver circuit. There are two mandatory capacitor replacements on this board.

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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SONY TC-K850ES: Better Capacitors

While this cassette deck is not defect and everything is all right, I decided to change some capacitors in key positions on the main audio printed circuit board. I state again that this operation is not necessary at this point. The old capacitors were OK and the sound is probably not improved along with this operation. But I am targeting functional stability in time.

So I have changed every electrolytic capacitor rated 1 uF / 50 V on the main board with Panasonic ECQ series stacked film parts of the same rating. In total there are nine parts to replace. Eight of them are part of the Dolby CX-20188 integrated circuits. The ninth is marked C608 and is part of the positive rail of the power supply regulator circuit.

See the eight electrolytic capacitors interleaved with the other Panasonic film parts next to the Dolby circuits.

These are the ECQ series parts. I have mounted glass beads on their legs as spacers. They help relieve mechanical stress. Needless to say that I hand selected them to be within 1% capacity tolerance.

Here is an overview of the audio main board with the parts exchanged. Those with eagle-eyes will spot something else as well. Read the next article for more information on this modification.

And some close-ups.

Old parts.

Reliability is increased now. Less electrolytic capacitors, less faults on the long term. Again I don't actually believe there is any sound improvement. And let's get this straight again: this is not what I originally aimed.

That's about it for now.

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SONY TC-K850ES: Preparing for CD4066 Removal

This deck produces quality sound at its output. But it could produce even greater sound quality with a minor modification. As most decks of this vintage utilize digital integrated circuits as analog signal switches, the sound quality slightly suffers from the internal variable resistance with pass-through frequency. This translates into audible signal distortion. There is nothing we can do here. Of course one could hunt 4066 variants with the lowest distortions. But I chose the old school method: use a relay.

The CD4066 integrated circuit is the last audio stage in the TC-K850ES machine. So it is just before the line output RCA jacks. It switches between source and tape signals at the press of the MONITOR switch.

My machine has a Mitsubishi M4066BP part installed.

Mounting a relay requires a little bit of ingenuity. But no matter what, I decided to keep the deck as original as possible. This means I need to remove the CD4066 integrated circuit and provide some kind of interface to the relay board that I will build. The simplest interface is an integrated circuit socket. Pay attention that Sony engineers used a slightly different integrated circuit raster when they designed the layout of this board. So I had to slightly bend the pins of the socket. 0.5 mm is sufficient.

This is the socket.

Soldering side looks good as well.

But I will leave the actual modification as a subject for a future article. For now I have installed the 4066 switching circuit in the socket.

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SONY TC-K850ES: Producing a M4066BP Replacement

With the help of a bit of simple reverse engineering of the original TC-K850ES schematic, I have produced a solution based on a miniature relay and a command transistor. There is nothing fancy about this circuit. But I wanted to build it on a miniature printed circuit board that will be equipped with augat pins with a raster identical to a 2 x 7 pin integrated circuit. Like the original M4066P circuit.

Here is the schematic. Please note that this is *not* a universal *4066 replacement circuit. It is targeted only to TC-K850ES deck. It will probably work with other similar Sony decks that use the same signaling and voltages. But this needs to be verified before attempting to use it.

The trained eye will spot the fact that I chose to command the relay using the inverted command signal (/CMD) that appears on selector pins 5 and 6 of the integrated circuit. This is done in order to keep power consumption at minimum while in play mode. The relay coil will be energized and the armature will commute only when the MONITOR switch selects LINE IN as the monitoring signal source. Should the MONITOR switch select TAPE as a monitoring signal source, the relay coil will not be energized and the mobile armature will be idling. Thus, this circuit will draw close to 0 mA in play mode.

And the two layer miniature printed circuit board layout.

And simulation.

Bill of materials.

Now I am waiting for the printed circuit board that I ordered at a factory in U.S.A. that does extremely cheap prototypes. This PCB went for $ 3.95, free shipping. This is way cheaper than if I had done it myself.

Here is a link to the project on OSH Park: SONY TC-K850ES M4066BP Replacement

For fun, here is my simple reverse engineering page. There are a number of errors -- like relay pin numbering -- in my hand drawings. But who cares? I have corrected them all in the final circuit presented above.

Disclaimer: This circuit is provided AS-IS. In no event will I be liable for any damage whatsoever arising from the use of this circuit on your cassette deck. Please note that this is not a general *4066 circuit replacement and it cannot be used directly in place of any *4066 integrated circuit in any arbitrary position within a cassette deck, or other similar equipment. This relay switching circuit was designed exclusively for the SONY TC-K850ES deck alone. Its conception is the result of reverse engineering the audio and control signaling implemented in the original electrical schematic of this cassette deck.

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SONY TC-K850ES: Assembling the M4066BP Replacement

In the meantime the printed circuit boards have arrived. I like their quality and you can see the nice ENIG finish in the following picture.

Assembly is straightforward and goes smoothly.

See the nice array of augat pins. I had to trim the pin heads with a sharp electrical pliers so that I don't have any protruding elements on the relay side. Augat pins need to be pressed down in the holes. But everything is easily done by first inserting a pin then turning the board upside down on a hard surface and pressing on the board. Augat pin fits like a glove with a little bit of pressing force. Connection is already great at this point but adding a tiny bit of solder reassures permanent electrical contact. The results as follows.

The PCB assembly is carefully installed in place of the old M4066BP part.

Tests reveal very good sound and the rewarding relay click when I switch from TAPE to LINE IN -- and recto-verso -- resembles that of higher-end cassette decks.

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SONY TC-K850ES: Servicing the Mechanical Block

I described the procedure in Article #6 of the TC-K555ESA mechanical block restoration. Thus, I won't explicitly repeat the steps in this article. But I took pictures.

This is the mechanical block.

View from another angle. Notice the non-standard capstan belt.

Detailed view. We can see that the belt is not stepping inside the machined guidance on the take-up side capstan flywheel. Nonetheless the deck worked pretty well.

I decided to change the electrolytic capacitors on the two PCBs that are attached to the mechanical block.

Parts have been changed.

...PICTURE FOLLOWS SOON...

This deck however suffers from electrolyte leakage on the capstan motor controller PCB. The electrolyte from one of the 10 uF filtering capacitors partially ate away copper traces and I was forced to chemically and mechanically clean everything very well. The result is that part of one track disappeared completely.

I have used MLCC parts as replacements. Carefully thinking length and bending leads solved the missing track problem as well. It doesn't look that beautiful but it is functional. Notice the weird dot-like black spots next to the missing trace. That is some kind of corrosion which I failed to remove.

Here's a different angle view.

The mechanical block is stripped of the plastic ornamental plate. Both pinch rollers are out already.

The old supply side pinch roller is removed with my improvised rig. The new one is ready to be mounted in.

In the end, every subsystem of the mechanical block went back in reverse order. Then the mechanical block is ready to be mounted back in the deck.

The new capstan belt steps right in the machined guidance gutter on the take-up side flywheel.

At the end I secured the alignment nut with some nail polish.

And a picture with the flash activated so that you can see the new pinch rollers in their final position.

So far so good. The mechanical block is very stable. Again I have tested ferric, chrome, and metal tapes. All run smooth without skewing.

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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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