Denon DRM-740 Cassette Deck Repair

I was recently in the market for a low-cost dual capstan cassette deck to digitise a couple of degraded tapes. The concept is simple even if the execution is anything but; in a common tape mechanism, the tape is pulled across the heads by a single capstan, attached to the flywheel, and a rubber pinchroller beneath. A sprung pressure pad in the tape itself maintains pressure against the playback head.

In a so-called ‘closed loop’ system, a second capstan and pinchroller are located to the left of the playback head assembly. The tape is then drawn along the tape path from both sides and even head pressure is maintained without the pressure pad playing an integral role. In theory the system is preferable over a single capstan transport as even tension is maintained even when the tape is stretched or its pressure pad has failed. But to properly execute a closed-loop mechanism added a degree of cost and complexity to the machine.

Denon’s DRM-740 is one such effort. Produced from 1995 to 1999, the DRM-740 was a 3-head, dual capstan cassette deck near the top of the Denon range and roughly mid market price wise, at a time when the peak of cassette deck manufacture was nearing its end. It’s a fairly basic unit with Dolby B and C noise reduction, HX Pro, an MPX filter and a few calibration controls for recording. Being a 3-head deck you can monitor recordings in realtime as well as via the massive fluorescent display that dominates the front panel.

A mint DRM-740 untouched (I.E unserviced) will currently set you back around £100. I acquired mine for £75 in need of fresh belts, but electrically in working order. The electronics in these machines are quite simple, mainly condensed into integrated circuits with a basic power supply and a smattering of supporting components.

The mechanism is another story. It’s quite a nice one, albeit not a patch on Pioneer’s ‘reference master’ or the earlier Denon models with a cassette drawer and cassette stabiliser. The take-up reels are entirely gear-driven so there are no rubber idlers to perish besides the pinchrollers, which were fine in my unit. The head had a bit of wear but nothing of concern.

There are three motors in this machine. One drives the reels directly via toothed plastic gears. Another actuates the mechanism via a belt, which is the most common cause of faults with these. If your Denon appears to play but won’t produce sound and stops after a few seconds, that belt is probably slipping. If that belt slips, the mechanism can’t fully engage the heads and pinchroller, and if the electronics don’t detect audio or tape movement the mechanism will shut off to prevent tape damage.

The third motor drives the two capstan flywheels. The capstan motor is a Matsushita MMI-6S2LK 2400RPM counterclockwise motor. These are nice motors and reputedly better than the Sankyo and Mabuchi motors found in other tape decks including Nakamichi of the time. The speed of the machine is set via a potentiometer accessed through the hole in the rear of the capstan motor.

Changing the belts in this is a real chore. Not because the mechanism itself is difficult to work on, but because this deck was built at a time when Denon’s components were ‘plastic fantastic’ construction hidden behind metal decorative fascias. I always thought of Denon as a poor man’s Marantz; they coexist under the same company. This deck did nothing to dissuade me of that notion.

The front comes apart in two sections. Three screws on the top, two of them securing grounding wires and tabs, hold the decorative front panel in place. Once removed it can be unclipped – first from the bottom and then the top, with great difficulty. A plastic pry tool helps immensely.

With that removed, four screws, two on each side secure the inner front panel to the plastic shell of the machine. WIth those out, the cable ties cut and the connectors removed, the front panel can be unclipped. There are plastic clips to either side, and slots at the bottom into which you slide a flat-blade screwdriver to release the plastic clips. It’s a pain to remove without breaking anything. The best advice I can give is to take your time and look into every nook and cranny to find the hidden clip you’ve missed. Some gentle coaxing, and profanity when that doesn’t work, will help.

Once you have the front removed the rest is plane sailing. I removed the mechanism as I wanted to lubricate the reel gears and the reel motor, but this is not necessary to change the belts. All you need to do to change the belts is remove the rear plate holding the actuator and capstan motors. There are 4 screws, one of them considerably longer than the others, which allow that plate to lift free.

You’ll note that the two flywheels are different sizes. The entry side capstan runs slightly slower than the exit side of the tape to maintain correct and even tension. This allows both flywheels to be driven by a single belt. In other tape decks, the secondary flywheel is driven by a separate belt from a smaller section of the primary. Either method works.

The DRM-740 takes two belts. The flat capstan belt is a 70 mm x 4 mm flat belt, and the square belt driving the actuator is a 20 x 1.5 mm belt. It’s important to look for the correct metric sizes as the closest imperial size (8.5 inches) applies too much tension on the two flywheels. The 70 mm belt gives ample torque but allows a small degree of slip between the two flywheels if needed to re-tension the tape.

The belts for this project were supplied by DeckTech. I used their belts in the Sony TC-WE675 and Yamaha KX-580SE I repaired recently and found them to be excellent. The belts for the Denon were no exception.

Getting the capstan belt installed is easy as it has enough tension to hook over the two flywheels and stay there while you align the motor plate. The actuator belt is another story. The actuator pulley has to be dropped into the hole of the mechanism, the small square belt slid over it and hooked over the pulley of the motor. You then have to drop the motor plate fully into place and align the actuator pulley with its thrust bearing on the plate without knocking the belt out of place. It took me several tries and some creative language, but I got it in the end. Helpfully you can install two screws to keep the plate in place, and there’s enough play in the capstan belt that it will remain while you fiddle with the actuator assembly.

If you want to lubricate the reel motor you’ll have to remove the mechanism from the front panel. There are 4 screws holding it in place. The left-most screws secure the door latch mechanism which lifts out independently of the tape mechanism and has a tiny spring that is easy to lose. With the mechanism removed, you have access to the reel tables and gears.

The reel motor is held in place by 2 screws either side of the top-most gear. With those removed, flip the mechanism face down and remove the aforementioned plate covering the flywheels. You then have to remove the circuit board at the top of the unit, as it is soldered directly to the reel motor and is in the way.

It is possible to disassemble everything without unsoldering the motor. There are tiny clips holding the board in place, hidden deep beneath the wires. You have to lever them back gently with a flat screwdriver and lift up on the board, all the while supporting the reel motor. There is one clip located just in front of the two right-most of the three grouped detection switches, and another in front of the single detection switch on the right. Once the board is out both it and the motor will flip over to allow you access to the front of the motor for lubrication, though be careful not to break the wires running to the reel movement sensors.

Reassembly is fiddly. The key thing is to ensure you correctly situate the reel motor, and align the circuit board with its plastic guides before pressing down. You also have to ensure that the levers for the aforementioned detection switches at the top of the mechanism are in their upward position so that they clear the switches on the board.

With everything assembled, I flipped the mechanism over once more and lubricated the plastic gears with a plastic-friendly silicone grease. This made winding operations a lot smoother and quieter, and should keep the gears from prematurely shedding teeth.

Reassembling the deck is the reverse process. With the mechanism and door latch reinstalled into the front panel, slot the front panel into place and align it with the plastic body of the machine. Ensuring you’e aligned the shafts for the 3 potentiometers in the bottom right, press the panel into place until the clips latch. You can then install the 4 screws, and repeat the process with the metal faceplate until it too is fully seated. The 3 top screws go back in with their grounding tabs and cables. Take care to route the ground cables along the slots in the top of the front fascia or the top cover won’t fit correctly.

Having set the speed with a 3kHz calibration tape, checked over the electronics and put the deck back together, it was time to give it a listen. The DRM-740 is quite slick in operation. The mechanism operates smoothly and quietly, at odds with the flimsy door, imprecise controls and wobbly knobs. Once you’re over the feeling of cheapness it is up there with the best mechanically. It might not have the solidity of a Nakamichi or the genteel in operation of a Pioneer equipped with their ‘reference master’ mechanism, but it’s a far cry from the clunking and grinding of the typical latter cassette mechanism driven by solenoids.

But we’re not here to listen to the mechanism. With a tape in place the deck sounds clean and largely uncoloured. It doesn’t suppress tape hiss particularly well, but in other aspects it’s a refine, clean sound as is typical form for Denon. It verges on being dull though impresses in detail and speed stability. There’s an unwavering unflappability to how the DRM-740 handles sustained notes and complex stringed instruments.

It wasn’t all too dissimilar to the Technics direct-drive mechanism in its confident stability, which is quite a complement. The dual capstan direct-drive Technics mechanisms from the B900 and BX800/900 series’ are some of the few Technics components that I have no first-hand experience of, but would very much like to. It would be interesting to see how the speed stability of a dual capstan direct drive would compare to a dual capstan belt drive mechanism that is clearly well implemented, such as it is in the DRM-740. I do have plenty of first-hand experience with the single capstan Technics direct-drive decks however including most model from the B600 / B700 and BX600 / BX700 series’. The Denon is almost their equal where speed stability is concerned.

I’ve read complaints relating to the DRM-740 and other Denon decks that they cause a loss of high-frequency content on home-recorded tapes, especially normal bias tapes. This always happens to some degree with any machine but usually takes thousands of plays, whereas some users were reporting git after only a few plays after recording. I’ve not encountered this myself though I did demagnetise the heads and the tape path, and checked that the erase circuit was not functioning in playback which indeed it was not. I therefore can’t say definitively what causes these issues, only that they are not universal.

In summary. These decks are full of cheap plastic and not the most fun to service. But serviced with care they do sound very good, and cost less on the second-hand market than other 3-head, dual capstan models from Sony, Technics, Aiwa, Pioneer and Nakamichi et al. I’d probably still take a single capstan direct drive technics over one of these as a playback deck, even a 2-head model. But as a low-cost dual capstan machine for digitising some worn tapes, it adequately served its purpose and did so with a neutral sound.