Sony’s DTC-75ES, DTC-700, and DTC-55ES are essentially identical machines. The ES models have copper plated chassis instead of zinc plate, and very slightly better specs. They were Sony’s high-end, high price models. The DTC-75ES was sold in North America and the DTC-55ES in Europe.

The mode switch on my DTC-700 broke several years ago. I cleaned it with alcohol and glued it back together. I hoped to steal that gear from the DTC-75ES, but it was missing. So far, the glued gear is holding together.

What to do about the DTC-75ES? I found a gear in a broken and discarded VCR which was close to the right size, 23 instead of 21 teeth. I had to shave it down in thickness and drill a center hole to fit over the switch shaft. I pressed a #6 flat washer onto the switch shaft and glued the gear to the washer and shaft.

Since in these decks, the switch only signals three adjacent mechanism states, the wrong number of teeth isn’t too critical, but positioning is. The switch has to be installed with exactly the right tooth engaged with the driving gear, and then adjusted into position so that in STOP the tape is fully wrapped around the drum but the pressure roller is 1mm back from the capstan. When it’s correct, STOP, PLAY, and REVERSE PLAY are correctly transmitted to the control logic.

The latest fix I’ve tried is bridging the break by taking a short piece of wire and melting it across the break to reinforce the gear. This seems the strongest method but only if you find the broken gear inside the chassis.

A piece of wire melted into the plastic can also serve sometimes to replace a broken tooth or a worn area.

Sony uses tiny bits of felt glued to plastic levers as brake pads. These by now are probably gummed with dried lubricant, worn smooth, or missing entirely. If you’re lucky, you may be able to rough up the surface and get acceptable action. If not, you need to find some high density felt, the sort used in hats for instance. Something that won’t fall apart. Cut out little bits to replace the old pads and glue them in place with gel type super glue or urethane glue. The feed reel holdback brake is a loop of felt-lined plastic attached to a tension arm. It needs to brake evenly as the reel table rotates. You may be able to clean it with 91% isopropyl alcohol. Getting the right holdback tension is critical to tape tracking. Too loose and the tape wanders up and down the guides and capstan. Too tight and everything wears out quickly. The service manuals say to set tension with torque measuring cassettes which aren’t available, so you’re sort of left to tinker and hope you’re close. If you’ve worked around tape recorders, you’ll have a sense when it’s right.  There is a small felt pad on the take-up side which serves as the holdback brake in reverse play. It is often dirty, out of position, or missing. If the tape wants to wander up and down the capstan, the holdback tension is wrong. Usually a brake pad is missing.

If as in the case of the three “parts” machines I bought, someone has been messing about, there may be missing springs and other items to make your day more interesting.

Plugs don’t work the same when jammed sideways into their sockets.

Copper plated steel chassis

Head preamplifier

Power supply

Rotary head drum

Mode switch

Main circuit board

Inside the older top-line Sony DATs.

What does the raw digital signal off the heads look like? Here’s the RF while the Sony is playing a somewhat wrinkled and abused bench test tape. You’ll see a lot of drop-outs in the signal and watch them move from left to right as the creases move across the drum. The remarkable thing is there is enough redundancy and error correction that the music is played without distortion.

A properly aligned DAT produces a flat, square RF envelope. Since the tape wraps 1/4 of the way around the drum and there are two heads, you get two bursts of data per drum revolution. These are decoded, error corrected, and sent to a RAM buffer where they are clocked out by a stable crystal oscillator into the DAC and sent to the digital outputs. As badly as this tape has been mangled by the broken decks it’s been in, the data is corrected, and what comes out the digital outputs is exactly the same as was originally recorded from my computer DAW. All told, it’s a remarkable system, but it’s a lot more complex and delicate than open reel analog tape. On the other hand, noise and distortion are like a high-quality preamplifier, orders of magnitude better than analog tape is capable of. There are folks who swear that analog tape sounds better, and just as with tubes vs transistors, they sound different. If you like the way analog tape and tubes sound, that’s fine. If you like the way an Edison wax cylinder sounds, that’s also fine. Just don’t claim the fidelity is better. High fidelity means reproducing the original sound faithfully. Absolute fidelity isn’t necessary for listening enjoyment, however. If you like extra bass, go for it. MP3s throw away some of the signal, but if you enjoy the music, who cares?

Useful links:

12voltvids DTC-75ES repair