AKAI ROBERTS
Joe Nechanicky
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.
1. It allows positive triggering of the solenoid by using the charged
capacitor to provide a strong voltage change to the solenoid switching
transistor.
2. 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 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.
Keep Trackin’
Joe Nechanicky
Joe Nechanicky
home computer to see if it comes out better,
Joe Nechanicky
"Joe Nechanicky" <hone...@ll.net> wrote in message
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