Repairing my old cassette deck
- joepampel
- 3 days ago
- 7 min read
Updated: 19 hours ago
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 wheel) and playback was pretty terrible (belt). 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. 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
Q-tips (lots of them)
Iso alcohol for cleaning metal parts (heads, capstans)
Ethanol or Caig rubber treatment (rollers & wheels)
Sewing machine oil/hammond organ oil or 3 in 1 oil (oil that won't gum up)
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 27 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. The two wheels are not the same size so it matters where they get put back. They can just fall out once the cover has been removed, so just be aware. (ask me how I know...)
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.
Make sure you have a picture of which capstan goes where, they are not the same size. Nothing holds them in so they will fall out if you are not careful.
When you do remove them, pay special attention to the tiny washers that help act as oil seals for the capstan bearings. They will probably fall off. Refer to the parts diagram in the service manual if you are not sure where something goes.
(orientation here is looking down at the rear of the mechanism).
If you are not sure where each capstan goes, line up the indents and then rotate them in the direction of normal tape travel (clockwise from the rear of the unit) The wheel on the right should fall behind the wheel on the left very slightly. This is how the unit maintains tape tension. If you have a set of calipers, the capstan that goes on the left will be a tiny bit smaller in diameter.
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.
Clean any rubber parts you are not replacing (such as rollers) with ethanol. Vodka works in a pinch.
Clean any metal parts in the tape path with iso alcohol (heads, capstans, guides)
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.
I also cleaned up the capstan wheels where the belt makes contact. 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.
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, line up those 2 holes and then rotate the left hand one (take up reel) a few times clockwise. The right hand capstan should fall just a tiny bit behind (as shown). I am not sure if all of the smaller capstans had painted marks on them (?) although it would make sense.
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 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.
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)
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. CN-8-1 and CN-10 noted above, and below:
This is page 26 in the manual below. The HX is a slightly later model but identical for these repairs. 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. 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.
Behind the backing plate in the cassette transport there is the backlight LED (under the yellow) 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.
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: 4218324313
Part# 112 is the capstan belt. Denon part: 4238028119
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 probably work, as would the belt from a DR-M22. One is simpler, one is top of the range and all 3 share a closed-loop dual capstan transport. There may be more, I didn't try to ID them all.
The belts might also be shared with turntables. If you search on "denon belt 4238028119" you will find a number of sources to try.
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.
Resources:
PDF Service Manual:
Online Service Manual: https://manualmachine.com/denon/drm33/198140-service-manual/
Idler Tire:
OD: 16mm
ID: 11mm
Thickness: 2.5mm
Sourced from: https://fixyouraudio.com
Replacement Belt: https://webspareparts.com/products/capstan-belt-for-cassette-deck-denon-drm-33hx-1
Video I watched. Not the same deck, but similar, and some good info to help get me started. https://www.youtube.com/watch?v=ALBl98tcZs0
Book on recording history that I really enjoyed which also covers the history of modern tape decks: