Home   Projects   For Sale   Contact     News  
     

 


 

Cassette overview

 



The cassette was born in the mid 1960's, invented by Phillips and mainly intended for use in dictation machines - the format was inherently inferior to full sized reel to reel and hence wasn't originally intended to be used for reproduction of high quality audio in Hi-Fi systems. The problems came from actually fitting the tape inside a compact cassette format case, only so much magnetic data could be stored on the narrow tape and only so much tape could be wound onto the spools inside the case; to get a reasonable recording time the tape would need to be run much slower than conventional full sized reel format, under 2 inches per second.

Noise would prove a major problem for the cassette format, it wasn't until the efforts of Dolby laboratories that the cassette finally began to gain acceptance into the realm of Hi-Fi.

 

Wow and flutter?

In audio cassette players this is a term given to the fluctuation in frequency introduced into a sound because of changes in speed of the tape passing over the record/playback head. Record a steady tone onto tape and then play that tone back and what you hear will be a close resemblance of that tone but distorted in frequency as the mechanical parts of the deck vary speed slightly, altering the speed which the tape passes over the playback head hence the frequency of the tone.

It's impossible to remove this phenomenon completely, some technology reduces it considerably but on any analogue audio format (reel to reel, vinyl, etc) it will always be there to an extent.

Flutter is most noticeable at mid/treble frequencies, mostly on passages of music with solo instruments in that pitch range. Play a piano solo through digital format and then record the same passage onto tape and play it back and you might find it painfully noticeable.

 

Advantage of dual capstans

Most basic (lower end) cassette decks share the same overall design, being 1 motor, 2 head machines. The record and playback heads are combined with the erase head being separate (the erase head is basically an electromagnet which, when powered by AC at high frequency, effectively demagnetizes a portion of the tape which passes over it, erasing any recorded media). The motor on a single motor transport drives both the capstan and the tape spools. We're talking the majority of low/mid range Japanese decks here, auto reverse decks are usually similar but use a different type of head and mechanism.

An improvement on this basic design is to add a second motor to drive only the capstans and/or add a second capstan and pinch roller on the supply side (left side) of the transport. The right hand roller still has the most importance as it performs the function of pulling the tape through at a constant speed, the main reason for adding the second capstan/pinch roller to the left side is to keep constant tension on the tape so it stays flat against the heads and runs smoothly, this design is sometimes coined as 'Closed loop' or similar.
Pinch roller pressure is critical on these machines, on the majority of designs the right hand roller has around 300-500 grams of pressure, the left hand roller usually a quarter or third of this.

 

A few points about restoration

Depending on the deck, it can take a lot of work to restore to working order. In the case of the average Pioneer, getting the actual transport working involves disassembly to change all the belts and tyres, then a case of cleaning and re-greasing the mechanism, replacing a couple of microswitches which invariably go faulty, cleaning and regreasing the main play/record switches and starting the reassembly. Motors are usually rebuilt.

Rebuilding a Technics RS-M03. This one is quite simple, single capstan, two head

Once everything is reassembled the deck is powered up to check everything is working as expected and if so, it's then on to setting everything up again (tape speed, azimuth, playback levels, record bias etc etc). The final thing I always do is a wow and flutter test, even if the deck sounds like it's running well. I'm not after a perfectly accurate flutter test, I don't have access to the laboratory conditions which the original manufacturer did - just an indicator that everything's running smoothly.

As with most vintage equipment there's always the dilemma over ageing electrolytic capacitors (elcaps), there's a section devoted to these in the amplifier rebuild section. There is a case made for replacing everything but this can get expensive because of the sheer amount of time it can add to a rebuild, as a cassette deck generally runs cool and most of the elcaps are relatively safe in low voltage circuits, there's not usually a board full of dried elcaps causing noise or frequency problems.

Actually, most noise problems I encounter are down to leaky transistors in the low level signal circuits and nothing to do with elcaps whatsoever. Most vintage equipment was assembled with tie wrap pins where the single core wiring looms were connected to the board via pins which the wire was coiled around as the unit was assembled during production, with age these oxidize in the space between wire and pin and can cause noise problems and odd faults, I prefer to leave the pins in place where possible but to solder them.

Vintage decks do have a problem with rubber components including the pinch rollers hardening with age, if your deck has bad wow and flutter then new belts alone are unlikely to cure it. Yes, belts can (and do) cause running problems but the common advice around that wow and flutter is mostly caused by loose belts is misleading. The tape is actually pulled through the mechanism by the pinch roller(s), the take-up spool merely turns in playback mode to keep the slack taken up as the tape passes through from the feed spool to the take-up, in the same way that they are driven when rewinding but at higher torque and speed.

When the pinch rollers harden they lose the ability to grip the tape properly and pull it through the capstan and past the heads, if it keeps losing grip then the speed will vary and you will get the dreaded wow and flutter or worse the tape will slide off of the roller and get chewed (imagine trying to drive a car and then steer and brake with old, hardened tyres with worn treads). Many people don't realize that it's the capstan and pinch roller part of the mechanism which controls the tape speed.

Most transport faults will cause the machine to eat tapes, occasionally at first but more often as the part in question wears more. Most tape eating faults are caused by the tape take up spool not keeping up with the tape as it rolls through the pinch roller, as the tape comes through it needs to be wound onto the take up reel in the cassette and kept fairly taught, otherwise it ends up wrapping around the pinch roller and making a mess, probably ruining that portion of the tape too.

Usually it's the rubber idler tyres which fail (or harden to the point where they can no longer transmit enough torque to the reels to drive them). Usually the idler tyres are buried nice and deep inside the transport and take a lot of disassembly work to actually gain access to, CT-F Pioneers are offenders here. The tape can also slip off of the heads and/or the pinch rollers and become eaten, this can be caused by worn pinch rollers but more usually happens in dual capstan decks and is caused by lack of friction on the tape feed spool or from the heads being out of line with the pinch rollers (usually the erase head is the culprit as the playback head has a forked guide attached to prevent this).

 

Getting everything apart, and back together again afterwards

Toshiba PC-6030. Probably one of the more difficult decks. Main power supply and control boards have been removed, hence the loose connectors

Sounds simple, until you pull the case and start looking at all the wires, screws and start trying to figure out how to remove the transport from the case even if you ever manage to get it unbolted in the first place. You're not alone, cassette decks are complex, cramming all of that complexity into a small sized case makes the whole situation worse.

With most decks there's little you can do with the transport still in place, you can sometimes just about change the main belt if you don't mind scraping your knuckles; for anything more though, the chances are that the whole transport will need to be removed. Besides, the transport itself often contains dozens of small springs, washers and clips, you'll really need it on a clean, well lit surface to make the job easier, and to find and tiny parts that may inadvertently get dropped. If you are determined to have a go yourself, good luck.

 

Pinch rollers

Just to be difficult, most manufacturers reserved some really oddball components for their top end machines, pinch rollers for high end cassette decks are almost always difficult to find and sometimes expensive - as a strange twist of fate, it's the higher end decks where these usually cause problems and need replacing.

The pinch rollers in one of my own Pioneer decks had degraded badly. When I bought the deck about 5 years ago not working I replaced everything else and soaked the rollers in restorer for a few days to soften them. For a while it was fine but they gradually hardened up again. Most later high end Pioneer (CT-F) decks use twin 9.5mm rollers, new replacements are unavailable from Pioneer nowadays but are still in the catalogue priced at a heart stopping £35 each, all for a tiny lump of brass/rubber. I did find some high quality replacements for higher end vintage decks, have used them several times on decks for customers but quality costs and they are expensive.

I tried a pair of 'generic' 10mm rollers from the US. These aren't expensive brass like the originals (these are nylon) but are still good quality, I've used these parts in other decks and they are still perfect after a year of regular use. Problem is that these are a size meant for modern dual cassette decks, not only are they slightly narrower than the originals, the bores aren't drilled large enough to fit on the Pioneer axles either. I bored them out to 2mm on a lathe (without a lathe of some kind you'll never get them bored straight and they will be unusable), on initial fit they were surprisingly near perfect. I stamped out some shims to pack them out to the same width as the originals and the 0.5mm difference in diameter wasn't noticeable at all, once in the deck they worked perfectly.

 

New pinch rollers fitted on Pioneer CT-F850 with shims. Originals are brass bodied but the rubber hardens...
You can see the left hand roller is shiny, flutter on this deck was unbearable
 
Old pinch rollers from Akai GX-95. Rubber material often degrades into hard, brittle plastic as is the case here, these are so hard and brittle that you could crack the rubber portion with a hammer New rollers in place in the GX-95 transport during rebuild


 


Back to cassette decks page

 


All photographs and text on this website unless otherwise stated are © 2008-2014

Briarsfield Hi-Fi - www.briarsfieldhifi.co.uk