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Barney, Mac McGregor's trusted technician sidekick, would be in a heap of
trouble in today's workplace. His complimentary and sincere remarks to office
secretary Matilda are considered as harassment and even
misogynistic by
current standards. The unintended consequences - or maybe they are
intended - has been to cause tense and guarded environments where traditional
interpersonal behaviors and attitudes are avoided rather than risk offending
someone and paying a steep price for it. Mentioned in this "Tape Recorder
Tips" from a 1958 issue of Radio & TV News magazine is the chemical
compound carbon
tetrachloride, aka carbon tet. At one time it was commonly used as a
cleaning agent because of its ability to efficiently clean oily and fatty
residues. Tape recorder heads, rubber drive wheels, and metal guide posts get
gunked up fairly quickly when the machine is used a lot, so service shops and
recorder/player owners used it. At some point it was determined that a health
hazard existed, and usage dropped considerable, and the next best thing -
isopropyl alcohol
- was substituted. I know as a not-so-active-anymore postage stamp collector that
carbon tet was/is used to expose watermarks on on gummed, mint condition stamps
without compromising the integrity.
Mac's Service Shop: Tape Recorder Tips
 By John T. Frye
This isn't going to do my figure a bit of good," Matilda announced as she dug
the little wooden spoon deep into one of the chocolate sundaes Barney had brought
back with him from a service call. He and Mac, the owner of the shop, were sitting
on the service bench. Matilda was perched on a high stool in front of them.
"There you are!" Barney mumbled with his mouth full of ice cream. "Matilda here,
with not a surplus ounce, worries about her figure; but when I was buying these
sundaes, there were two or three babes standing in line who looked big enough to
eat hay."
"Thank you, Barney - I think," Matilda said; "but that wouldn't be a sneaky way
of calling me skinny, would it?"
"You know doggone well it's not," Barney said with an aggrieved air. "I wolf-whistle
at you every chance I get, don't I? And I'm mighty careful who I whistle at. I've
got a reputation for taste to maintain in this man's town."
"Yes, and if you don't stop those leering whistles I won't have a reputation
at all," Matilda said with a dazzling smile that belied her reproof.
Mac set his little paper bucket down on the bench beside him and stretched luxuriously.
"That really hit the spot this sizzling day, Flame Head. It was mighty thotty of
you and it leaves me in a gabby mood. I've been wanting to talk to you about tape
recorder servicing. Think you can listen and eat at the same time?"
"I read you loud and clear; over," Barney answered.
"The whole thing is tape recorders have improved tremendously these past few
years. They are capable of excellent musical reproduction; and now that the hi-fi
crowd has taken them over, we have much more critical customers to satisfy. That
means we must be more critical, too, so that every recorder leaving the shop is
delivering the best performance of which it is capable."
"Sounds like sense," Barney observed.
"Always start by demagnetizing the heads. Magnetized heads will produce a five
to ten db increase in noise and distortion; but what is worse, such heads will gradually
erase high frequencies from tape passed over them. I don't want our expensive test
tapes ruined by using them on magnetized heads. By the same token, we should explain
to the customer that a magnetized head is as bad for his prize tapes as a worn-out
needle is for his fine records. Then we should sell him a head demagnetizer.
"Next, use the test tape to check head alignment. Don't forget that if a head
is badly out of alignment, such as might occur when a head is replaced or when someone
has tampered with the alignment adjustment, false peaks may be found on either side
of the true azimuth position. These false peaks, however, will be 15 to 20 db below
the correct position; so you will have no trouble locating the true azimuth setting
if the adjustment is moved far enough to be sure you are not stopping on one of
the false peaks."
"One thing that's always puzzled me is how you check the bias oscillator frequency,"
Barney said. "The frequency is too low for our grid-dip oscillator to reach; it's
below the lowest frequency put out by the r.f. signal generator; and it's higher
than the 20,000 top of our sine-wave generator. Just how do you check it?"
"Probably the best way is to feed the audio generator into the horizontal amplifier
and the signal from the bias oscillator into the vertical amplifier of the scope.
Then adjust the audio generator frequency until you get a stationary Lissajous pattern.
In getting this pattern, start at the upper limit of the audio oscillator and come
down. The oscillator frequency can then be read as the indicated multiple of the
audio generator frequency. For example, if counting loops of the Lissajous figure
indicates a 4 to 1 ratio and the sine-wave generator is putting out 16,000 cycles,
you know the oscillator is working on 64 kilocycles. While you're at it, turn on
the horizontal saw-tooth sweep and examine the waveshape of the bias oscillator
output. It should be close to a true sine wave for best operation.
"Sometimes the service manual gives you the bias or erase voltage that should
be present across a head winding. Ordinarily these must be read with a true a.c.
v.t.v.m. Make sure that the instrument you use has an upper frequency limit adequate
for measuring the oscillator frequency. Some v.t.v.m.'s, intended chiefly for reading
line frequencies and audio frequencies, will not give a true indication at the 50-100
kilocycle frequencies involved here. If you're not sure of your v.t.v.m., the scope
can always be used with a voltage calibrator to check the amplitude of the voltages
with sufficient accuracy.
"Keep in mind, too that some recorders change the voltage across the head with
the speed of the tape. For example, one Bell recorder specifies 35 volts bias at
7.5 ips, 14 volts at 3 3/4, ips; and 10 volts at 1 7/8 ips."
"What if erase and bias currents rather than voltages are specified?"
"Put a 10-ohm precision noninductive resistor in series with a lead going to
a head and measure the a.c. voltage drop across it. Multiply this voltage reading
by 100 to get the current in milliamperes. For example, if you get a reading of
0.3 volt, that means the current is 30 ma."
"Isn't there some quicker way of checking the bias oscillator without going through
all that stuff?"
"Well, a quick and dirty check of oscillator performance is to measure the d.c.
voltage developed across the grid leak of the oscillator with a v.t.v.m and compare
this reading with the data given in the service manual. If the voltage is too low,
try a new oscillator tube and check the other voltages applied to the tube elements
against the voltage data supplied in the service sheet. If the voltage is too high,
maybe the grid leak has changed value. More likely, the head winding may be open
or a switch may be making a poor contact so that the oscillator is running unloaded.
If you check the head winding for continuity with an ohmmeter, just remember this
d.c. current is certain to magnetize the head; so use the demagnetizer again before
running tape across the head."
"What should I use to clean the head and the rubber drive wheels? Some manuals
say to use alcohol; others say carbon tetrachloride."
"You know how I feel about carbon tet. That stuff is deadly dangerous when it
is used in such a way that you inhale the fumes. However, many manufacturers specify
that it be used to clean the tape residue from heads, capstan, and guides. It is
a good solvent for the material that collects on these. On the other hand, some
manufacturers say never to use carbon tet on any of the rubber drive wheels or pressure
rollers. They insist that alcohol be used in such places. Still others say that
carbon tet is perfectly all right for use in these places. I suspect that the difference
lies in the kind of rubber used. Always follow the manufacturer's instructions to
the letter. When in doubt, I believe it is safer to try alcohol."
"Check," Barney said as he spooned the last drop of his sundae from the paper
bucket.
"Many of the recorders have a hum balance control," Mac went on; "and there's
a lot more to adjusting this than just twisting a screw. In the first place, the
recorder should be in completely assembled operating condition when this adjustment
is made. This adjustment is intended to 'buck out' various sources of hum induced
from motor, transformer, choke, and other parts of the recorder.
"But even if the tape recorder is all together, you still have to be sure that
it is not picking up hum from some external source, such as an electric clock motor,
a fluorescent lamp ballast, etc. The way to check on this is to note whether or
not the level of the hum changes when the tape recorder is moved about. If it does,
you can be sure it is coming from an external source."
"Seems to me trying to judge the hum by ear would be pretty tricky."
"Don't try it. Hook our a.c. v.t.v.m. across the speaker voice coil and set it
to a low range. Usually the hum should be less than 0.1 volt with the volume and
tone controls full on. Keep your eye on the meter while making all checks and adjustments
regarding hum. It will give a more sensitive indication than your ear. Try reversing
the line cord, too, and adjust the hum control again. If the hum can be reduced
to a lower level with the cord in one way - due, no doubt, to some of the fields
cancelling themselves out - adjust the hum control with the plug in this way and
mention to the customer that the hum will be lower when the plug is properly inserted."
"One final question: is there any easy way to run down the source of wow?"
"Not really. There are some things that help. One is to have the tape transport
mechanism arranged where you can see it while a tape with a steady tone is passing
across the heads. Watch carefully and see if you can detect a connection between
the wow and the rotation of the capstan, pressure roller, or speed roller. If you
can spot any such relationship, the battle is half won. All you have to do then
is find out why that particular bit of the mechanism is running at an uneven speed."
"OK," Barney said. "Now that I've had food for the body and food for the mind,
I'm rarin' to go!"
Posted August 31, 2020
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
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