AKAI TRACK CHANGE

[Joe Nechanicky]

AKAI -ROBERTS
Problems with not changing tracks using automatic track changing feature:
AKAI CR80 (D) CR 81 (D) CR83D AKAI Quad (and most AKAI RR Combos) ROBERTS
808 (D) the AKAI series of recorders listed above and the Roberts versions
may encounter automatic track change problems. These problems could be
electrical component failure however most likely there is another cause as
outlined below.
The electronic channel changing circuitry of these decks is not of the
straight forward design found in most 8 track players and recorders. In
common design for triggering the channel change solenoid, a direct path is
used from the sensing contacts to the solenoid. One side of the sensing
contacts is usually grounded and the other side goes to the ungrounded side
of the solenoid. When the sensing foil passes, a connection is made and the
solenoid activates.
In AKAI/Roberts 8 track decks this is not the case. These decks use an
advanced method of triggering the channel change solenoid. In short, the
electronics incorporates a charged capacitor, when discharged, switches on a
transistor which completes the current path for solenoid operation. This is
a good system and works well under most conditions. The purpose of this
circuitry is two fold. It allows positive triggering of the solenoid by
using the charged capacitor to provide a strong voltage change to the
solenoid switching transistor. If the sensing foil stops on the sensing
contacts, no damage will be done to the solenoid as the capacitor will
discharge and will not recharge by circuit design under those conditions.
Over the years, one problem I have noticed with these decks is the inability
of the automatic track change to function properly. At times the automatic
change would at best just put out a weak "click" but would not change
channels. The manual track change button on the front panel would function
normally if used during this occurrence. I tried many methods to get the
automatic track change to work. I tried cleaning the contacts, modifying the
contact area, checking all the electronics for malfunctions, but nothing
really seemed to correct the problem for the long term. I took a hard look
at the problem and came up with some concrete facts and information that I
want to pass on to the 8 track news group.
Analyzing the problem:
With the player in play mode, and no tape in the recorder, I could always
trigger the automatic mode when making a contact manually by placing a piece
of metal across the sensing contact surface inside the tape recorder. When
running a tape in the recorder, the automatic changer would not trigger. I
tried new sensing foil, longer sensing foil, more pressure on the pads,
checked for sensing contact protrusion and so forth. None of these items
contributed to the cause of the lack of head switching.
Analyzing further:
The circuit which fires the solenoid is a simple circuit yet is complex in
theory. Basically what happens when things are operating normally is this:
1. The sensing foil contacts sensing contacts
2. There is fast discharge of the capacitor which (through various circuits)
causes the transistor in the solenoid circuit to conduct, and in return this
causes the solenoid to operate.
3. Once the capacitor is discharged, it will not recharge until the foil has
passed from the contact strip. If the foil should stop on the strip, there
is a long time constant R/C circuit recharge circuit which prevents the
capacitor from recharging in that mode. This prevents the solenoid from
burning out should the foil stop on the contacts. These decks stops cold
when it goes into automatic stop mode, so protection is needed for the
solenoid.
Why the automatic change will not work but the manual change does:
The manual change button does exactly the same electrical function as the
foil over the contacts, the circuit is the same. So why then will not the
automatic system work? It could be one of the usual things, and these should
be checked first, pressure pad, contacts need to be cleaned, new sensing
foil installed. If that does not do the trick, here is what I have found to
be the problem. This problem really had me stumped until I did some real
down to earth research on what is going on. The only way the automatic
feature could not work is if the capacitor would not hold a charge, so I
changed the capacitor, put in a lot larger value of microfarads, did some
circuit changes. The results of this varied the operation somewhat but it
still was not the answer. I took even a harder and longer look at the
problem. I put a volt meter probe on the ungrounded terminal of the sensing
contact, the other on the ground of the chassis. I wanted to see if the
voltage on the capacitor dropped when the tape was running for what ever
reason. I was amazed to find that as the tape turned, the voltage at this
connection fluctuated erratically. What this indicated was that the magnetic
tape surface was conducting electricity from the positive side of the
capacitor to ground, and causing the capacitor to gradually discharge
leaving it in a discharged state in a short time. I removed the tape, I
tried to measure resistance with my volt meter probes across the recording
tape, however I didn't see much for conductivity. Yet when I put the tape
back in the recorder, I could see the voltage being bled off the capacitor
by the conduction process. The magnetic particles on the tape I suppose have
some conductivity, but not normally enough to cause any problems.
The Graphite backing:
The recording tape I was using was an older, well used TDK AD series tape.
Close examination on all continuous loop tapes will revel that the graphite
will get embedded into the oxide surface over time, and I suspected the
graphite and oxide together are doing the electrical conducting.
To prove this theory, I cracked open a brand new TDK AD tape, put it into
the recorder. Guess what, absolutely no conduction what so ever! The problem
was in the older well used tapes. Brand didn't make any difference. The
backing and oxide on old well used tapes will not affect standard 8 track
channel change electronics as the conduction is so small it's negligible on
these types of circuits. It does affect the transistorized channel change
electronics of the AKAI decks.
Conclusion:
If you have an AKAI or ROBERTS's deck which will not change tracks for you,
do all the basic things first, then try the newest tape you can find to see
if it will work for you then, it will. You do not have to use brand new
tapes in these decks, however you should be aware that the use of some well
used tapes may cause the auto track change function to not operate
correctly. I did go on to measure the heavy graphite build up on the pinch
rollers of a few carts; the substance is conductive in the Meg ohm region,
but conductive. The channel change system can be modified so even older
tapes can be used with out problems; however this is at the cost of having
the solenoid locked on during auto stop, not a good thing to do. This may be
the answer to another question also which has surfaced concerning a
scratching static noise heard some times when playing tapes. This noise has
been associated with the sensing contacts, but no one including me has ever
been able to really figure out why, perhaps the same conductivity may be
causing the noise problem.
Keep Trackin'
Joe Nechanicky

[Danspeakin]

>Close examination on all continuous loop tapes will revel that the graphite
>will get embedded into the oxide surface over time, and I suspected the
>graphite and oxide together are doing the electrical conducting.

Wow Joe, great sleuth work. This info might help solve some of the noise
problems as well.
Dan2

[Daniel Modlin]

I just yanked the sensor out, got rid of the noise problem *almost* completely.
I have 3..or maybe it's 4 of these Akai units and they have all had this noise
problem and all have been helped by yanking sensor.