Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff


Chapter 4) Audio Cassette and Open Reel Tape Equipment

  4.1) Parts of an audio tape transport

The following description applies to most cassette and open reel tape
transports including those used in portable and microcassette recorders,
Walkmen, and telephone answering machines.

Looking at the top of the deck such that the tape heads are at the bottom:

* Supply reel table - left hand side platform on which the supply tape reel
  sits.  Edge which contacts idler tire (if used) should be cleaned.

* Takeup reel table - right hand side platform on which the takeup tape reel
  sits.  Edge which contacts idler tire (if used) should be cleaned.

* Idler - assembly which swings between supply and takeup reels and transfers
  power to the appropriate reel to wind the tape up during play and record
  and often to drive FF and REW.  In some designs, this uses gears or some
  other type of mechanism.  In very expensive decks, individual motors are
  used for each reel and there is no intermediate drive.

* Idler tire - the black rubber ring on the outside of one part of the idler
  which actually contacts the reel edges.  This is not as common in audio
  tape decks as VCRs.  If one is used, it should be cleaned and inspected
  for deterioration, dirt, and wear.

* Capstan - right side after tape exits from area of record/playback/erase
  heads.  The capstan is a shaft (about 1/16" diameter in cassette decks,
  recorders, and Walkmen, 3/16" or larger diameter in open reel machines)
  which during play and record modes precisely controls tape movement when
  the pinch roller is pressed against it.  For autoreverse transports,
  there will be two capstans - one on each side of the head assembly
  so that the tape is always pulled across the heads as this is most
  precise.  (In a VCR, there is only one capstan and it is also used for
  reverse play or search modes.)  Clean to assure proper tape movement
  during play and record modes.

* Pinch Roller - black rubber roller which spins freely and is pressed
  against the capstan during play, record, and search modes.  For autoreverse
  decks, there will be two pinch rollers, one for each capstan.  A hard,
  shiny, cracked, or dried out pinch roller can lead to tape edge munching
  and erratic or wavering sound.  Clean thoroughly (until no more black
  stuff comes off).  Inspect for cracked or deteriorated rubber.

* Tape heads.  Most low to mid priced tape decks have two - an erase head
  and a combined record/playback head.  High-end decks will have separate
  record and playback heads supporting sound-on-sound mixing to the same track
  and allowing recording quality to be monitored off of the tape.  These may
  be physically independent assemblies or combined into a single unit.
  Autoreverse decks often have a head assembly that rotates 180 degrees
  depending on tape direction.  This is less expensive than having two erase
  heads and two record/playback heads or a single record/playback head that
  shifted position to align with the appropriate tracks and electronic
  switching of the record and playback signals.

  Play-only transports such as found in car cassette decks and Walkmen
  do not need an erase head.  Autoreverse play-only decks often do just shift
  the position of the playback head a fraction of a mm depending on playback
  direction to line up with the tracks and interchanges the connections
  for L and R channels.

  Clean the polished surfaces thoroughly (DO NOT use anything abrasive!).

* Various other guide posts - vertical stationary metal posts which tape
  contacts.  Should be cleaned but rarely need adjustment.

* Belts - various size black rubber bands - a typical transport will have
  between 0 and 4 belts, usually below decks.  These will require
  replacement after a few years.  Clean and inspect.

  4.2) General guide to tape deck cleaning and rubber parts replacement

The following procedures apply to boom boxes, cassette decks, microcassette
and other portable tape recorders, open reel tape decks, and telephone
answering machines.  While the tape transports used in these devices are
less complex than those used in VCRs and other helical scan recording
equipment, some routine maintenance can go a long way towards preventing
future problems.  All the guideposts, wheels, and rubber parts should be
inspected and cleaned periodically - how often depends on usage.  Of course,
no one really does this unless something goes wrong.

Qtips and alcohol (91% medicinal is ok, pure isopropyl is better. Avoid
rubbing alcohol especially if it contains any additives) can be used
everywhere EXCEPT on the rotating heads of VCRs and camcorders (and other
helical scan devices like 8mm and 4mm (DAT) storage drives) - see the
document: "Notes on the Troubleshooting and Repair of Video Cassette
Recorders" for detailed procedures on cleaning of video heads - you can
destroy the most expensive part of your VCR by improper cleaning techniques.
Dry quickly to avoid leaving residue behind.  Sometimes good old fashioned
water (just a damp cloth) will work better on sugar based gunk and other
kids' grime.

Cleaning may get your machine going well enough to get by until any replacement
rubber parts arrive.

Things to clean:

(Some of these components may not be present in your particular equipment).

1. Capstan and pinch roller.  These collect a lot of crud mostly oxide which
   flakes off of (old) tapes.  Use as many Q-tips (wet but not dripping
   with alcohol) as necessary to remove all foreign matter from the capstan
   (the shiny shaft that pulls the tape through the unit for play and record).
   Just don't get impatient and use something sharp - the crud will come off
   with the Qtips and maybe some help from a fingernail.  On autoreverse
   decks, there will usually be two capstans and pinch rollers.

   Clean the pinch roller (presses against the capstan in play and record)
   until no more black stuff comes off.  Use as many Qtips as necessary.

   If the pinch roller is still hard and/or shiny or has a cracked surface,
   it will probably need replacement.  Many are available from the sources
   listed in the section: "Recommended parts suppliers".

2. Various guideposts that the tape contacts.  Clean like the capstan.

3. Idler tire (idler swings between reels and transfers motor power to
   reels - clean until no more black stuff comes off.  A dirty or worn idler
   tire may prevent the takeup reel from turning resulting in spilled tape.

   Also, the idler assembly includes a slip clutch.  If this weakens, the
   idler may not have enough force to press on the reel table edges.

4. Reel table edges - surface on the reel tables where the idler contacts.

5. Audio head(s) and erase head.  Q-tips and alcohol are ok for these.
   Do not use anything sharp or abrasive!

6. Anything else that the tape contacts on its exciting journey through your

7. Rubber belts.  Access to some of these may require the services of a
   Swiss watchmaker (if any still exist).  Some boomboxes seem to be designed
   specifically to be difficult to service.  After noting where each belt
   goes, remove them individually (if possible) and clean with alcohol and
   Qtips or a lint free cloth.  Dry quickly to avoid degrading the rubber
   from contact with the alcohol.  If a belt is trapped by some assembly
   and not easy to remove, use the Qtip on the belt and/or pulley in place.
   However, if it is stretched, flabby, or damaged, you will need to figure
   out how to free it.

   Note that on some equipment like dual cassette boomboxes and telephone
   answering machines, the belt(s) may follow a highly circuitous path -
   make a detailed diagram!

   Any belts that appear loose, flabby or do not return instantly to
   their relaxed size when stretched by 25% or so will need to be replaced
   and may be the cause of your problems.  Belts cost about $.30-$2.00.
   Meanwhile, the belts will function better once they are cleaned, maybe
   just enough to get by until your replacements arrive.

  4.3) Lubrication of electronic equipment

The short recommendation is: Don't add any oil or grease unless you are
positively sure it is needed.  Most parts are lubricated at the factory and do
not need any further lubrication over their lifetime.  Too much lubrication is
worse then too little.  It is easy to add a drop of oil but difficult and time
consuming to restore a tape deck that has taken a swim.

NEVER, ever, use WD40!  WD40 is not a good lubricant despite the claims on the
label.  Legend has it that the WD stands for Water Displacer - which is one of
the functions of WD40 when used to coat tools for rust prevention.  WD40 is
much too thin to do any good as a general lubricant and will quickly collect
dirt and dry up.  It is also quite flammable and a pretty good solvent - there
is no telling what will be affected by this.

A light machine oil like electric motor or sewing machine oil should be
used for gear or wheel shafts.  A plastic safe grease like silicone grease
or Molylube is suitable for gears, cams, or mechanical (piano key) type
mode selectors.  Never use oil or grease on electrical contacts.

Unless the unit was not properly lubricated at the factory (which is quite
possible), don't add any unless your inspection reveals the specific need.
Sometimes you will find a dry capstan, motor, lever, or gear shaft.  If
possible, disassemble and clean out the old lubricant before adding fresh
oil or grease.

Note that in most cases, oil is for plain bearings (not ball or roller)
and pivots while grease is used on sliding parts and gear teeth.

In general, do not lubricate anything unless you know there is a need.
Never 'shotgun' a problem by lubricating everything in sight!  You might
as well literally use a shotgun on the equipment!

  4.4) Tape head demagnetizing

With audio tape decks, demagnetizing is often recommended to improve
sound quality and frequency response.  There is some debate as to
how much benefit there is to this practice but if done properly,
there is little risk.  Demagnetizing removes the residual magnetic
fields that can build up on ferrous pole pieces of the tape heads and
various guideposts and other parts in the tape path which may affect
frequency response.

Use a small demagnetizer designed for a tape deck or cassette deck.  See
the section: "Homemade audio tape head demagnetizer" if you don't have one
or don't want to buy one.  However, do not use anything that might be too
powerful or a bulk tape eraser which would certainly be too powerful.

Make sure the tip is covered with a soft material to prevent damage to
the finely polished surfaces in the tape transport.

Turn power on to the demagnetizer when a couple of feet away from the unit.
Then, slowly bring it in close and slowly go over all surfaces of anything
that the tape contacts or comes close to in the tape path.  The key
word here is **slowly**.  Move fast, and you will make the magnetic
fields stronger.  When finished, slowly draw the demagnetizer away to a
distance of a couple of feet before turning it off.

  4.5) Homemade audio tape head demagnetizer

A perfectly serviceable tape head demagnetizer can be easily constructed
using a large nail, 100 turns of insulated wire (just guessing here) and
an AC wall adapter (from an obsolete modem, for example).  Grind down the
end of the nail so that it is not sharp and coat it with a soft material
or cover the end with electrical tape to protect the finely polished heads
from scratches.

Adjust the number of turns and input voltage for desired strength.  How
strong should it be?  A direct comparison with a commercial unit would be
best but when in close proximity to a steel surface, you should be able to
feel the 120 Hz attraction but it shouldn't jump out of your hand!  Sort of
like "Use a pinch of salt you will know how much" :-)

  4.6) Building a bulk tape eraser

A variety of approaches work for this - all based on strong magnetic fields.
These will erase floppy diskettes, audio and video tapes, and all your credit
cards and Turnpike passes!

* Magnets removed from large loudspeakers (including the pole pieces where
  the voice coil went) and microwave oven magnetrons.

* Some motors, transformers, the butt-end of some soldering guns, etc.

(From: Steven L. Bender (buqu35d@prodigy.com)).

You need a Power Transformer about 3" in each direction, can be like a 
low voltage 12 volt / 3 Amp unit or rated higher.  Remove end bells if 
any, remove all the metal laminations (break the first one, yank it, 
and the rest will come easier).  Re-insert all the metal laminations 
facing in the same direction, with the "E" all pointed the same, 
re-glue, varnish, or whatever.  Connect AC Plug to the Primary, then 
insulate the whole works with Plastic tape and outre layer of Duct 
tape. After insulating it with several layers of tape - Instant Bulk 

Warning - Do not apply power for more than 60 seconds at a time!  (It will get
hot and burn your hand after two minutes.)

I had one of those for some years, but accidentally left it plugged in, 
(pulled the wrong wire out of the 6 to 1 outlet box) and after a few 
minutes, it smelled and was too hot to touch, and made a nasty noise as 
the copper started to melt...  (Sounds Effects of Liquid Krell Metal
in the distance...., Forbidden Planet - Paramount, 1956).

Luckily I didn't walk out, another few minutes and it would have caught fire..

I am not liable for any personal, profession, or consequential damages from
use of this information !!!

  4.7) Splicing of audio tapes

If a tape is broken or seriously crinkled, cutting out the bad section and
joining the remaining ends will be necessary.  There are special splicing
kits for this.  I don't know if a place like Radio Shack carries these but
an audio dealer or electronics distributor should have one.    In a pinch, you
could very carefully use a razor blade or Xacto knife to cut the tape an a
45 degree angle and ordinary transparent to mend it.  Then, it is best to copy
the tape to a new one.  At least with an audio deck, you don't really have to
worry about ruining the heads with an improperly made splice though you do
want to avoid depositing adhesive from the mending tape onto parts of the

  4.8) Tape or cassette deck, recorder, or Walkman transport problems

The following are common problems with audio tape transports:

1. No movement in PLAY or REC - most likely capstan is not turning or not
   engaged.  If the motor is not working (listen for a hum from inside the
   transport), refer to the chapter: "Motors and Relays".  Otherwise, see
   the list below.

2. Tape eating - the capstan is turning but the takeup reel is stationary
   or not turning rapidly enough to take up the tape as it feeds from the
   capstan/pinch roller.

2. FF and/or REW are inoperative or sluggish - assuming the motor is working,
   the driven reel is not being powered at all or does not have sufficient
   torque to overcome the tape friction.  The driven reel alone must
   pull the tape through the transport.

Note that the required torque for the driven reel is much less for PLAY
and REC compared to FF and REW as the capstan in contact with the pinch
roller pulls the tape from the supply reel.

The most likely causes are similar for all of these symptoms.  The driven
reel and/or capstan is not turning due to:

* A broken or stretched belt, an old and deteriorated, dirty, or worn idler
  tire.  Refer to the section: "General guide to tape deck cleaning and rubber parts replacement".

* Worn or broken.  For example, a spring may have popped off an idler clutch
  or a press-fit gear or pulley may have split.

* Gummed up lubrication which is preventing the idler gear or tire that
  operates the takeup reel from engaging.  See the section: "Lubrication of electronic equipment".

* A solenoid that is not engaging properly due to a weak spring, insufficient
  drive, or lubrication problems.

If the cause is not immediately evident once the bottom of the transport
is visible, try to observe exactly what is happening when you play a
garbage tape or run the deck with no tape present.  Look for broken parts
or bits of parts that may have failed off.

If the transport shuts down shortly after entering any mode, check for
a missing or stretched tape counter drive belt or a defective reel
rotation sensor.  The tape eating protection circuits are shutting
down the unit improperly due to a lack of reel sensor pulses.  A related
symptom will be that the tape counter (mechanical or electronic) does not
change during the period when the tape is moving.

If the logic is not properly controlling the various solenoids or other
actuators in a 'soft touch deck', then a service manual will be needed
to proceed much further.

  4.9) Tape transport azimith adjustment

When prerecorded tapes or tapes recorded on another deck sound muddy, the
azimith alignment of the suspect deck may have shifted or be misadjusted.
Azimith refers to the angle that the record/playback head gap makes with
respect to recorded audio tracks.  This angle should be exactly 90 degrees.
If it is not, than high frequencies will tend to be reduced in amplitude
during playback of a tape not recorded on this machine.  Similarly, a tape
recorded on a transport with an improper azimith setting will sound muddy
on a properly adjusted deck.

A simple test to determine if azimith alignment is your problem is to
record some music on your machine and immediately play it back.  If this
recording sounds fine but it sounds muddy on another deck, then improper
azimith alignment is the likely cause.

If the recording is still muddy, your deck may have electronic problems
like excessive bias (check to make sure you have selected the proper type
of tape or bias setting), a worn record/playback head, or the heads or
other parts may be magnetized (see the section: "Tape head demagnetizing".,
However, dirty heads as well other mechanical problems can also result in weak
muddy sound.  See the section: "General guide to tape deck cleaning and rubber parts replacement".

The best way to adjust azimith is while playing a recording that was made
on a known good deck - commercial tapes are usually (but not always) a
good choice.

Warning: once you adjust the azimith, any tapes previously recorded on
this transport may sound muddy.  If you only record and play your own
tapes on this deck, you may want to just leave it alone.

The azimith adjustment is usually a screw that pivots the record/playback
head.  It may be spring loaded and possibly fixed in place with a some
Loctite or varnish.  Often it will be accessible through a hole without
removing any covers but not always.  Look for it while in play or record
mode in back of any holes (which you had no idea had a purpose until now).
If there are no access holes, you will have to remove the loading door,
cover, or front panel.  Be sure you have the correct screw before turning
wildly - others may affect critical height or simply be mounting screws.

Play a tape with lots of good highs - classical instrumental music or jazz
are excellent.  Now, simply set the azimith adjustment for best sounding and
strongest high frequencies which should result in most natural sound.
Go slow - a 1/16 of a turn is significant.  Turn the screw back and forth
and leave it in the best sounding position.  Carefully put a dab of Loctite
or nail polish on the screw to prevent it from moving.

  4.10) Walkman/Discman power or sound intermittent

Note: for actual tape speed, operation, or sound quality issues, start with
the section: "General guide to tape deck cleaning and rubber parts replacement".

The socket that the AC adapter or headphones plug into is often quite
abused during normal operation.  This can lead to broken solder connections
where it joins the circuit board inside the unit. Test for this possibility
by wiggling the plug without moving or flexing the cable itself.  If the
sound cuts in and out or the tape player starts and stops or the radio
goes on and off, or the CD player resets or stops, then there is likely a
bad connection here.  Note: eliminate the alternate possibility that the
AC adapter or headphone cable is bad by wiggling and tugging on the cable
while holding the plug steady.  Further verify that it is not simply a
matter of dirt or grime interfering with a good connection.

The connections can be easily resoldered but you will need to open up the
case using.  Hopefully this will only require jeweler's screwdrivers and
great care.  (However, some Walkmen are constructed such that access to
the interior is virtually impossible without a hand-grenade.)  To repair
the connections, use a low wattage iron and fine rosin core solder.  Make
sure you do not introduce any solder bridges.  Try not to lose any of the

  4.11) Cassette or tape playback - one channel dead

This could be a bad playback head, bad connections, or a bad component
in the playback electronics.

First, confirm that the problem is not in your headphones, patch cables,
or the remainder of your audio system - try an alternate audio source
where possible.

To determine if the playback circuitry is working, gain access to the
terminals on the playback head  - a metal cased little cube near the
center of the tape side of the cassette.  There should be four wires coming
from it.  While the machine is supposed to be playing, touch the end of
a jeweler's screwdriver gently to each of the four terminals in turn.  When you
touch the good channel, you should hear a buzz from the appropriate speaker.
If you touch one terminal and get a buzz from the 'dead' channel, then
it is possible that the head is bad for that channel.  If you can touch
two different terminals and get a buzz in the bad channel for both, the
it is likely that the ground connection to the input preamp has fallen off.
If you do not get anything from the bad channel, then there is likely an
electronic problem in that channel.  Bad connections aside, the most common
problem area would be the audio amplifier - bad IC or capacitor.

  4.12) Distorted or erratic recording

First determine if it is a record or playback problem - play a tape
recorded on another machine or a commercial prerecorded tape.
Try a tape from this machine on another known working tape player.

If record is the problem and it has very distorted sound, this may be
a sign of a bad bias oscillator or switching circuit or record switch.
The bias is an ultrasonic signal that is impressed on the tape along
with the input signal.  Without it, the sound will be highly distorted.
In effect, it is a linearizing signal.

Check that the record select switch is clean - it may have many contacts
and may have collected a lot of crud.  If behavior changes with each
activation of the record switch, get some contact or tuner cleaner spray
and use the extension tube to spray inside the switch (with the power
off), put the switch through its paces several times and allow to dry
before powering it up.

If it is a portable subject to abuse, check for bad connections as well,
especially if, say, one channel comes and goes.

Beyond this, you can try to measure the signal going to the record heads
while in record mode.  You should be able to see a high frequency signal
in addition to the input signal.  If the either of these is absent, then
you need to trace back to its source and at this point will probably need
a schematic.

  4.13) Previous recording not erased

In this case both the original and new audio appear on the tape.  The most
likely cause (assuming your deck doesn't have some fancy sound-with-sound
or sound-on-sound modes that may be engaged) is a faulty erase head or its
driving signal.

The erase head precedes the record head and probably uses the same high
frequency signal as that for record bias to totally wipe the previous
recording.  (However, on really really cheap tape recorders, erase may just be
performed by a permanent magnet.)  If the new recordings are really distorted,
the bias oscillator itself may not be working.  The erase head is either part
of the REC/PLAY head assembly or a totally separate head.  Check for broken
wires to this head as well.  If you have an oscilloscope, monitor the signal
during record.  The erase head could also be defective or really dirty.

  4.14) Cassette player erratic autoreverse

Some of the autoreverse decks use a rotating magnet under or part of the
each reel and a reed switch or hall effect device to detect lack of motion
and do the autoreverse thing.

I had one from a Toyota where the plastic drive gear which included the
magnet and was part of the reel split and was getting stuck at the broken
tooth causing a reverse and eventually eating the tape.  It was $9 for
that little plastic gear.

Others are entirely mechanical and if there is a lack of lubrication,
dirt, tired belts or idlers, or broken parts they may start acting erratically.

Although there could be an electronic fault, carefully examine the mechanism
for obvious or subtle problems before breaking out the 'scope.

The following methods are use for autoreverse:

1. Optical sensor detecting the clear leader on the cassette.  Better tape
   decks use this for sensing at the end so that the reverse occurs just
   quickly at the end of the tape rather than waiting for the leader to go
   by and a second or two for the tape to stop.

2. Totally mechanical where a lever arm presses against the tape and when the
   tension increases with the reel stopped, it trips a mechanism to reverse.

3. Optical sensors on reel rotation.

4. Magnetic sensors on reel rotation - either hall effect devices or
   simple reed switches.

If the transport will run without a tape in place, see if the takeup reel
is rotating properly and whether the reverse still occurs.  If reel rotation
is normal but it still reverses, the either you have the optical tape
end sensor or there is some fault in the sensors for the reel rotation.
If the takeup reel does not rotate, then as suggested above, check for
bad belts or idler tire.

Belts and idler tires are readily available from places like MCM Electronics.

  4.15) Autoreverse audio not correct for either or both directions

This may mean that one or both directions is weak or erratic or that both
sets of tracks are playing simultaneously (one in reverse).

There are three common ways of implementing autoreverse with respect to
the tape heads:

1. Locate both the record/play heads and erase head on an assembly that can
   rotate (flip) 180 degrees depending on the direction.  Mechanical stops
   determine the precise position.

2. Locate both the record/play heads and erase head on an assembly that
   can shift transversely across the tape by one track distance depending
   on direction.  The connections to the L and R channels must be interchanged
   electronically in this case for one of the directions.

3. Provide a complete set of heads for both directions.  Selection is then
   done electronically or via a set of switch contacts controlled by the
   direction reversing mechanism.  (This would require duplicating 6 heads
   for a full record/play deck so it is more likely with a simple player which
   would then only require a total of 4 heads.)

Problems may be mechanical or electronic.  However, it is probably not what you
would consider head alignment.

In either design, the mechanism could be gummed up and not being properly
positioned in one or both directions.  There could be broken cables or
bad connections since (particularly with (1) and (2)) there could be
significant cable movement.

Check, clean, and lubricate the mechanics first before considering electronic
faults.  However, since all of these must select channels based on direction,
electronic or switching problems are quite possible.

  4.16) Walkman plays both sides of tape at once

One set of tracks will be playing backwards which may make for interesting
conversation!  There are two possibilities:

* Where a single pair of heads is used, the head assembly is misaligned
  and straddling both sets of tracks.  This would be the case with a
  non-autoreverse player or with an autoreverse player that shifts head
  position when it reverses direction.

  This is a mechanical problem with head alignment (height) or the shifting
  mechanism (autoreverse).

* For an autoreverse unit where the heads do not shift position (there are
  four heads gaps - one for each track but only 2 get selected for each
  direction), the head selection circuitry or switch is routing both sets of
  head signals to the amp.

  This is an electronic or switch contact problem.

  4.17) General tape speed problems - slow, fast, or dead

Are the speed problems sudden or gradual?  Over what period of time? Seconds,
minutes?  For portable devices, are you using a good set of their recommended
type of batteries?

Did this problem start suddenly or was this a tape recorder you found buried
under an inch thick layer of dust in an attic?

If the latter, then there could very well be multiple mechanical problems
due to deteriorated rubber parts - replace then or toss it.

Fast play could be an indication of a hard deteriorated pinch roller.
Or, you could have forgotten to turn off a 'fast dub' or 'quick copy' switch!

Clean and lubricate the mechanism.   Check for dry or tight bearings.

Is there any pattern to the problems - like with respect to the start
and end of cassettes?

If the tape speed has suddenly become excessive:

1. Mechanical.  If you had a recent tape eating episode, there may be
   a wad of tape wrapped around the capstan.  Remove it.  Alternatively,
   the pinch roller may not be fully engaging against the capstan
   and the takeup reel is simply pulling the tape through without any speed
   control.  Clean the mechanism, check for tired belts and springs.

2. Electrical.  The motor speed control is not working.  This may be either
   a mechanical governor inside the motor or a voltage regulator or other
   electronic control often also inside the motor.  In the latter case,
   you may be able to disassemble the motor and repair it.
   One possibility is that the series regulator has decided to turn into
   a short circuit.  This may be external or internal to the motor.

3. Cockpit error.  Some tape recorders and tape decks have various features
   (which you no doubt never use) that may have been inadvertently turned on
   or twiddled (perhaps by your 3 year old).  These include high speed dub as
   well as selectable and/or adjustable record or playback speed.

Slight tape speed error may simply mean that an internal adjustment is needed.
There may be an access hole on the motor or an external pot.  However, keep in
mind that any tapes you recorded on this machine (assuming it can record)
recently will play at an incorrect speed once you adjust the speed.

Is it slow and steady - no more wow and flutter than normal? Or slow and
erratic indicating that (1) the speed regulator is faulty, (2) some
bearings may need oil, (3) the pinch roller is glazed.

If the mechanics seem ok, then check for electronic problems with the
motor or regulator.  Sometimes there is a trimpot for speed adjustment
inside or external to the motor.  A faulty regulator or even a bad
connection may be the cause.

A variety of techniques are used to regulate the record/playback speed:

1. Mechanical governor inside motor - centrifugal contacts open at correct
   speed reducing current to motor.  If speed is too low, than springs could
   have weakened or contacts could be bad - open.  If speed is too high,
   contacts may be welded closed.  There may be a resistor and/or capacitor
   across the contacts.  An open resistor could conceivably cause unstable
   speed fluctuations.  A capacitor may be present to reduce electrical noise.

2. Voltage regulator inside motor case or external to motor.  The regulator
   or transistor may be faulty.  If power for the motor seems to come directly 
   from an unregulated supply, check across the motor terminals with an
   ohmmeter.  A low reading which is identical in both directions would 
   indicate a direct connection to the motor brushes with no internal
   regulator.  A high reading or one that is different in each direction
   indicates an internal electronic regulator - or you could just use your
   eyeballs to determine if there are any electronics inside the motor.  These
   can be disassembled and bad parts replaced.  There may be an access hole
   on the motor for an adjustment.  Alternatively, you could remove the guts
   and install an external regulator using an LM317 or similar part.

3. Active regulator with tachometer feedback from motor winding - there would
   be 4 wires instead of two coming out of the motor - 2 for power and 2 for
   tach.  Control circuitry could be bad or the tach output could be dead
   (speed too high).

4. If an optical strobe disk is located on the motor shaft, then it may be
   part of a speed control circuit. If it is on one of the reels - probably the
   takeup reel - then it simply operates the (electronic) tape counter or
   signals the controller that the takeup reel is turning - to catch tape

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]