Repairing my old cassette deck

joepampel
Dec 26, 2025
10 min read

Updated: Jan 31

Prompted by our youngest becoming interested in making a 'mix tape' but not knowing what was involved. Literally...

Me: "Do you have a cassette deck to use?"

Them: "What's that?"

I found my old tape deck in 'meh' condition at roughly 40 years of age. I hadn't used it in a very long time. It's a Denon DR-M33, from roughly 1984. It's pretty well built for a 'home' unit, it features 3 discrete heads and a dual capstan direct drive transport. It depends on a belt for the capstans and an idler wheel for the drive motor, both of which are rubber and both of which were pretty dried up at this point. Fast forward and reverse were very slow (idler tire) and playback was pretty terrible (belt & tire). With nothing to lose, I went for it.

Part of my coursework at IAR involved aligning and servicing 24 track studio decks so the basics are familiar to me.

ps: Modern magnetic tape dates to WW-II and it's a neat story if you like that kind of thing. (link below)

Tools & Materials

Small philips head screwdriver set

Tweezer set

Find the service manual for your mfr/model or as close as you can get.

Q-tips (lots of them)

Iso alcohol for cleaning metal parts (heads, capstans)

Ethanol or Caig rubber treatment (rollers & wheels)

Sewing machine oil/clock oil/hammond organ oil or 3 in 1 oil (oil that won't gum up)

Bonus: a head demagnetizer

Finding replacement parts was a bit of a saga which involved lost orders, incorrect parts, EU suppliers who wouldn't ship to the US (thank you, tariffs!) and so on. But in the end I wound up with a belt & wheel I could use. (specs below)

Getting to the belt and wheel are the hard part here. You have to get the tape transport partially removed to get to either of them. That's about 24 screws worth, give or take. All those screws are different.

Make sure you have some kind of organizer tray and take lots of pics to help you get the thing back in 1 piece. I picked this one up at Harbor Freight.

The tape transport fully removed from the deck and the rear cover has been removed. The capstans are the two big steel wheels, their momentum helps prevent tape speed variation. You'll notice they've been made about as large as they can be. If they get mixed up, the one with the less smooth (more of a matt finish) capstan should be the takeup side.

To get the transport loose so you can work on it, there are:

4 large phillips heads sheet metal screws holding the cover on.
2 machine screws below the transport (accessible from the bottom of the unit)
3 phillips screws holding the bottom of the faceplate on, and another 3 on top with countersunk heads
5 screws holding the metal brace across the top of the front of the chassis, 2 self tapping (on the ends) 2 machine screws (into tape transport frame) and 1 longer machine screw with a washer roughly halfway across the brace.

Remove the cover, then the front faceplate and finally the top brace. Move the on/off switch lever over to the left & then you should be able to manuever the tape transport assembly enough to get to get to the belt and idler wheel. Watch the timer switch wiring (you can unscrew it from the faceplate to simplify things).

You can replace the belt by removing the rear cover of the tape transport assembly (4 screws, 3 metal and 1 self tapping for the plastic fitting it goes into) and just putting the new belt around the capstans. I used a dental pick with a 90 degree angle on it to lift the belt over the motor's pulley when re-assembling it. You get that done when the back cover is nearly in place. Spin the wheels a few times to make sure it is properly aligned with the pulley. Be careful not to lose the washers on the capstan shafts. They are there to keep oil in the bearing area and keep it from seeping on to the rollers etc.

Stuff you will want to do while you are in there:

Oil the bearings of important spinning parts. Sewing machine oil or 3 in 1 oil is fine. You need an oil that will not gum up over time.
Clean any rubber parts you are not replacing (such as rollers) with CaiKleen RBR or ethanol. Vodka works fine in a pinch. (really)
Clean any metal parts in the tape path with iso alcohol (heads, capstans, guides)

To remove the drive motor, you first need to remove the bias mode switches (upper RH of the tape transport assembly as you face the rear) The board is located by 2 fragile plastic standoffs that each have a 'catch' on them. Push them out slowly and gently to free the board. Once the board is out of the way you can unscrew the 2 phillips heads machine screws that hold the drive motor in place. The idler tire is in front of the drive motor.

You want to put a drop or two of oil into the capstan bearings. It does not take much. In this picture the capstans have both been removed as has the drive motor (the motor in the white jacket). To replace the idler tire you need to detach the spring (at the right) and lift out the little "arm" assembly the tire is on. The tire itself just pushes off or on with your fingers.

Be careful around the servo board (upper LH of the above pic). It is naturally "loose" but its overall alignment is critical if FF & RW are to work properly.

I also cleaned up the capstan wheels where the belt makes contact with some metal cleaner. They were a bit gummy from the old belt. The belts dry out over time and stretch. Don't we all.

The metal parts in the tape path get cleaned up with Iso alcohol and q-tips, never anything abrasive. It is very important not to scratch the heads. The pinch rollers get cleaned with either rubber cleaner or ethanol and q-tips. Iso will dry out the rubber which is not what you want. Be gentle, go slow. (The capstans are reversed in this picture. You can see how much less shiny the one on the left is - it should be the takeup capstan, eg: on the right. )

Give the rubber time to dry off before trying to play a tape. The tape gets moved along via pressure from being squeezed between the capstan and the pinch rollers. If the rollers are not dry and grippy the tape won't play and the deck will auto-stop - it will think the tape is over.

keep going until the q-tips come out clean. In the case of the rubber parts, you may get a little material coming off even when clean but the color will change from brown to black when you're done. "Brown" is tape residue.

The new idler tire. I was able to order one by measuring my old one and picking the closest available size. You have to kind of hope that Denon did not custom order belts and tires, the pieces they used are probably used on other decks. If you know the sizes you'll have a better chance of finding something that works. Some more parts finding pointers are below.

Install the new belt around the capstans.

The switches that take care of tape bias and detecting if the record enable tab has been punched out are on a little board that sits on a couple of plastic brackets that both broke off in my case. The switch board has to be aligned both vertically & horizontally so that the switches engage properly. This was the worst part of the job. You can see the alignment was off in the picture above. The board has to be removed in order to get to the drive motor screws that let you access the idler tire.

My broken switch board mounts. I used JB Weld to re-attach them. Crazy Glue was not strong enough. I glued one side, let it seet and then used a small wood clamp to locate the board. I put glue on the other clip and lowered it into the hole. Once that set and the height was good, I used a small piece of foam to apply pressure to ensure that the 'metal' bias setting arm did not slip off the side of the switch. The board needs to be held firmly in place in order for the switches to action properly.

To fully remove the transport you will need to remove the wiring harnesses. Before removing the wiring harnesses, take lots of pics to capture both which connectors went where and also how the wires were routed. It can be very important for keeping the noise floor low. The power supply is in the upper LH corner and you can see they routed the various cables as far from it as they could. There is all of the motor wiring (3 motors), wiring to the tape heads (3 heads), wiring to the bias switches, and more.

The white motor in the middle is the drive motor you need to remove to access the idler wheel. First though you have to remove the back plate with the capstan motor, and then remove the tape bias switches (white switches to the left of the drive motor)

If you need to remove cables, go slow and take pics & notes. These "CN-xx" notations match up to the diagrams in the service manual. There are a lot of connectors with the same number of pins, and the service manual does not show color codes, so you will need pics. As a sample I have noted CN-8-1 and CN-10 above, and below:

This is page 26 in the manual below. The HX is a slightly later model but identical for these repairs. (The HX has a large daughter board that does automatic bias adjustment.)

All of the connectors are labelled although their actual locations and wire colors are not.

This is the servo motor which is located on the right side of the tape transport as you face the deck. It's used to move the heads up to the tape to play, or down and away from the tape for FF or RW operations.

On the back of the capstan motor there is a small adjustment screw for tape speed that is located under this rubber cover. To set the speed you use a calibrated test tape. You play a test tone from the calibration tape and then check the frequency being played back on a meter. The tones are usually around 3kHz. Alignment info is included in the service manuals (below) It is a good idea to check your speed after changing the belt and tire. You should not need to adjust the azimuth unless you have replaced the head. Azimuth is setting the tape head to be perfectly in line with the tape tracks. If the head is a little crooked you can get artifacts like comb filtering or even a loss of high end. If you want to check it anyway, there are a couple of methods. The most common in my day was using an oscilloscope to make lissajous patterns from the different playback heads (covered in the service manual). This is playing back a mono track and then comparing the two signals. You can also just listen to a music track in mono with some strong high end (cymbals are good) and tweak for the best (strongest) response. There is no setting that will over-emphasize the highs. Headphones are best here to hear what is happening accurately. There is a link to doing this adjustment below. The adjuster is the nut just to the right of the playback head. Use some kind of adhesive to keep the nut from moving afterwards. Nail polish also works in a pinch.

Behind the backing plate in the cassette transport there is the backlight LED (under the yellow plastic) and just to the left of it you can see the counter mechanism. It uses a slotted wheel, an LED & a photocell to count pulses as the take up reel rotates.

If you have some DeOxit D5, clean out the pots and switches. The front panel switches push plastic arms that acivate switches on the main PCB. There are holes in the switches for cleaning. The "TAPE/SOURCE" switch in my case was very cranky. One channel wouldn't work or they would both work but with low levels. A good cleaning and it's all fine now.

Finding Parts:

There is a good exploded parts diagram in the manual you can refer to that is paired with a parts list. It's also a big help seeing how it all goes together.

Part# 31 is the idler tire Denon part: 4218324312 (used in M22/M33/M44)

Part# 112 is the capstan belt. Denon part: 4238028119 (used in M33/M44)

Keep in mind that Mfrs like Denon might make a whole product line around these kinds of transports, so the capstan belt from a DR-M44 will work, as would the idler tire & belt from a DR-M22 (belt is a ?). The DR-M22 has the same part numbers for the capstan wheels. So one model is simpler, one is top of the range and all 3 share the closed-loop dual capstan transport. There may be more, I didn't try to ID them all. The belt part number for the DR-M22 is different although the rest of the thing looks identical so I don't know what that's about. (anyone?)

The belts might also be shared with turntables or CD players (they can use belts to eject the tray). If you search on "denon belt 4238028119" you will find a number of sources to try.

Alignment

Having now gone through a complete calibration process, I have put all of that information HERE. tl;dr - the speed was a bit high and the electronics had really drifted over the years and were badly in need of an alignment. Wow & Flutter are still higher than spec so I am thinking about having the pinch rollers re-built. [The capstan motor was dying which may have been my issue] Despite this, the deck sounds great.

Future Work

If I feel lucky I will replace the electrolytic capacitors in the power supply. They're 40 years old, about twice their expected service life. The trick is to find not only the correct values but also correct lead spacing for the circuit board. I have not noticed any symptoms (yet) so I'm going to just enjoy the thing for a while first. The power supply board does not have a panel to access the bottom (the audio board does) so replacing them will be a bigger job than it needs to be.