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This installment of Mac's
Radio Service Shop, entitled "A.C.-D.C. Bread and Butter," could stand alone as a chapter in a troubleshooting manual
for vacuum tube radios. John Frye's (or a trusty consultant's) knowledge of circuit
operation is abundantly clear here. Recall that Mr. Frye later authored the
Carl & Jerry teenage techno-sleuth series in the next decade. A couple
things stood out to me. The first is the heretofore unfamiliar phrase "play hob with,"
which means to cause trouble for. The second is mention of a metal tuning dial indicator
needle making contact with a part of the metal faceplate and thereby affecting the
tuning of the radio. What that means is the needle had some level of voltage on
it that could short to the chassis. It was not unusual to have very high voltages
on the dial and button shafts of user-accessible controls, with only a plastic or
phenolic knob or button separating the user from a potentially lethal shock. Safety
grounds were not part of the supply line cord, further increasing the danger.
In fact, I am currently restoring a 1970s vintage record turntable that has a
2-wire line cord and the electronics are mounted to the metal chassis that serves
as the base of the mechanical assembly. The input transformer is isolated from the
chassis with rubber grommets around the mounting screws. They are in good condition,
but out of an abundance of caution I am installing a 3-wire power cord and an inline
fuse.
Mac's Radio Service Shop: A.C.-D.C. Bread and Butter
By John T. Frye
 Barney, returning from picking up the mail
at the post office, was so intent upon looking at the small object he was holding
up toward the sky that he walked straight into the screen door of the service shop.
The squeaking protest of the stretched wire made Mac look up from the chassis on
which he was working.
"Could it be that you need glasses - or maybe a whole new head?" he inquired
solicitously.
"Neither," Barney promptly retorted. "I was merely trying to look at these transparencies
that just came back. They are all shots of the fall leaves that Margie and I took
on our trip to the southern part of the state a few days ago."
Mac picked up the box of slides and held some of them toward the light. "Say!"
he exclaimed, "you've got some beauties here. I really get wound up about the show
Old Mother Nature puts on in October in this part of the country, and these color
shots of yours really do justice to the autumn foliage."
"Why all these kind words?" Barney asked suspiciously as he looked around. "Oh
I get it now," he said as his eye lighted on a great stack of small a.c.-d.c. sets
piled on the receiving bench. "You're trying to butter me into wading through that
bunch of cigar-boxes."
"My boy," Mac said solemnly, "sit yourself down and let me tell you some of the
facts of life. Those 'cigar boxes' as you disparagingly call them are quite likely
to be a major part of your bread and butter for some time to come. Have you noticed
that while the number of consoles we get is less and less since TV started in this
fringe area, the number of these a.c-d.c. sets seems to be on the increase?"
"Now that you mention it, I have," Barney assented. "How come?"
"The television receiver has almost entirely thrust the console radio out of
the place of honor in the living room," Mac explained; "but the little radio is
still holding its own in the kitchen, basement, bedroom, or any other location in
the house where a person may want to listen to music, news, soap opera, or what-have-you
while going ahead with work, reading, or other similar activity. That is why it
looks as though the small set will be the last to go under before the onslaught
of television - if it ever goes under. I've been glancing back through our records,
and I find that we are actually getting more of these little fellows to service
now than we did before TV started. Since a very high percentage of the radio work
we do is going to be on these sets, I thought it might be a good idea if we sort
of overhauled our a.c.-d.c. service procedure and made sure our technique in handling
these sets is the best possible."
"I know all there is to know about working on those sets," Barney stated flatly.
"I'm sure that you do," Mac said meekly. "If there is one kind of set that the
technician should know like the back of his hand, it is the five or six tube a.c.-d.c.
receiver; but I am getting kind of old and in my dotage and do not remember things
as well as you young fellers do; so if you don't mind, we'll just run over a few
points about working on them for my benefit. After being in this game for a quarter
of a century, the one thing I am sure of is that you never know so much about service
work but that you can learn something new - present company excepted, of course."
A quick flush spread over Barney's, freckled face. "Okay," he mumbled. "I had
that coming - and I got it!"
"I have noticed," Mac went ahead quickly, "that when a set comes in that quite
likely has a burned out tube, you try to locate the tube and replace it without
removing the set from the cabinet. In the future, I want the set removed in every
case. Doing so usually will actually save time. Trying to remove the tubes with
the set still in the cabinet often results in breaking loose the leads of the dangling
loop antenna. When you try to remove the tubes in the cramped and awkward quarters
inside the cabinet, often a tube will suddenly come loose and slip from your fingers
like a pinched watermelon seed and sail halfway across the shop. If it is not ruined
on the spot, the jarring will likely cause it to go out in the not very distant
future.
"Another reason for taking the set out of the cabinet is that it gives you a
chance to look it over for other trouble and to clean the dial glass - a small item
that looms surprisingly large in the eyes of the ladies. With the chassis out in
the open, a single glance is all that is needed to spot a frayed dial cord that
needs replacing or a slack tension spring that is getting ready to cause trouble.
The same quick look will reveal cracks in the speaker cone that require cementing.
Then you can flip the chassis over and take a look for charred resistors, especially
in the vicinity of the rectifier socket. While you are at it, notice if a lightning
surge may have blown apart the line bypass condenser. Look at the dial lamp. If
the filament is open, replace the lamp before turning on the set; and in that case
a good first guess on the tube with an open filament is the rectifier."
"I know that you mean that quite likely the portion of the rectifier filament
that the dial lamp bridges has gone out and let the full current pass through the
dial lamp and burn it out," Barney interrupted; "but I do not see why it is important
to replace the dial lamp right away."
"In the first place, I never like to subject that tapped portion of the rectifier
filament to an overload, even for a short while; and that is what it will get if
the dial lamp is not replaced. Secondly, a seasoned technician always watches that
dial lamp while the set is warming up. Since the rectified plate current as well
as the filament current flows through it, the brilliance of the lamp gives you a
rough but useful indication of any excessive current drawn from the rectifier. Many
a time I have seen a dial lamp blooming in time to snap off the set and save a rectifier.
"After the set has warmed up for a few minutes, align it. You can improve the
alignment in almost every case. The transformers used in the i.f. stages are usually
not the most expensive, and they are subjected to very high temperatures inside
the small cabinets. The alternate heating and cooling tends to change the resonant
frequencies of the windings. I kept a record of all the a.c.-d.c. sets serviced
for a week one time, and I found that in nine cases out of ten I could effect at
least a 20% increase in output measured across the voice coil simply by realigning
the i.f.'s. While you are adjusting the transformers, notice how sharply they resonate.
Of course, the winding across the diode load will tune broader than the others;
but the remaining three should peak up quite sharply and definitely. If they do
not, something is wrong. A weak i.f. tube, shorted turns in one of the windings,
a high resistance in the winding, or a defective trimmer condenser may be at fault.
"If the set oscillates when you try to bring it into alignment, that is another
headache; but never say to yourself 'I guess it was just made that way' and solve
the problem by throwing it out of alignment until it stops oscillating. Look for
open plate or screen bypasses. See if the output section of the filter condenser
- which doubles in brass as a plate bypass in many of the inexpensive sets - has
not deteriorated. Change the i.f. tube. Check the a.v.c. bypass condensers. Make
sure the plate and grid leads are dressed away from each other. See to it that the
metal shell of the i.f. tube is properly grounded through the socket connection.
If none of these causes is present, try changing the i.f. transformer. Often the
heat causes the top i.f. winding to slide down toward the bottom winding and cause
over-coupling."
"Hold up, now! Changing an i.f. is quite a little job just to see if it could
be the trouble."
"Not the way I do it. Simply cut loose the leads from the transformer to the
plate and to the grid or diode. Then tack the connections from the replacement transformer
to these points and also to "B-plus" and to the a.v.c. bus with the soldering iron.
Then use a short clipped lead to ground the shield can to the chassis. This outboard
transformer can then be brought into alignment, and it can be quickly seen if the
trouble is corrected. If so, the transformer can be installed permanently; if not,
the leads of the transformer in the set can be reconnected in a matter of seconds."
"Say," Barney broke in, "when you're aligning the front end of a set and you
can't make the oscillator track both with the dial and the r.f. stage, which one
should you choose?"
"Well, if a choice has to be made, I'd say make it track with the r.f. stage,
for that would give you the best reception," Mac replied; "but usually you can shift
the i.f. or bend the plates of one or the other or both sections of the tuning condenser
so as to arrive at a happy condition in which the oscillator will track with both
the dial markings and the r.f. stage. Don't be afraid to bend those plates. That's
why they are slotted. Use the trimmers to line up the set at the high frequency
end of the band and then bend the plates to do the same thing at around 600 kilocycles."
"How about sets that are just low on pep?"
"Try substituting tubes, even though the ones in the set test good. This is especially
important with rectifier and mixer tubes, for a tube tester will often mislead you
on these. A bad i.f. transformer is often a cause, and the best test is to substitute
a new one as I mentioned awhile ago. Reversed antenna leads to a loop antenna will
sometimes play hob with the sensitivity, too. Of course, when you are aligning the
set, you should note if the gain from the mixer grid to the speaker is normal or
not. That helps a lot in knowing where to start looking for the trouble.
"Hum is another common trouble with these little sets. Usually it is caused by
weak or open filter condensers, but that is not always the case. Heater-to-cathode
leakage will often produce hum, especially in the output tube because the filament
at this point is considerably above ground. Changing tubes, of course, corrects
it. A hum that is only heard when a station is tuned in and one that is loudest
when the received signal is strongest is likely to be caused by an open line bypass
condenser. It will go away if this condenser across the a.c. line is bridged with
a good unit. On some sets the tuning condenser is isolated from the chassis by rubber
grommets around the mounting bolts. If one of these grommets is cut through or slips
out of position so that the tuning condenser can short to the chassis, you get an
exceptionally loud hum. Recently a receiver of this sort gave me a hard time because
it developed a hum only at certain points on the dial. The giveaway clue was the
fact that while the tuning condenser was being rotated a scratching sound was heard
just before the hum would start. I found the metal pointer on the end of the tuning
condenser shaft was shorting to the grounded metal foil on the face of the dial
at certain spots. Bending the pointer away from the foil cleared the trouble.
"While looking for strange noises, always jar the tubes lightly with that little
rubber tapper we use. These miniature tubes are bad ones for developing internal
shorts, and sometimes you can spot one developing by noticing a rattling sound or
change in volume when the tube is tapped. Especially notice if the rectifier makes
a noise when tapped. 35Z5's, 35Y4's, and the like often become noisy and should
be replaced; otherwise, the set will make a scratching noise every time it is jarred,
even by a loud sound from the speaker.
"Finally, after the set has been on for some little time, check the bias voltage
on the output tube. If it is low, cut the coupling condenser loose from the grill
and see if this makes any difference. If it does, the coupling condenser is doubtless
leaky, but you can double-check by seeing if the end cut loose does not display
a positive voltage. If the grid of the output tube remains positive with respect
to ground after the coupling condenser has been disconnected, you probably have
a tube afflicted with secondary emission. Try another tube and see if the condition
does not disappear. If the bias is developed across a resistor in the cathode circuit,
perhaps the resistor has changed value or the electrolytic bypass across it may
be partially shorted. The main point is that you should not let the set leave the
shop until the bias voltage of the output tube is within ten per-cent of its rated
value. A set cannot sound good unless this voltage is correct."
"You know," Barney reflected, "I am beginning to think I may have been a little
hasty in saying that I knew all there was to know about a.c.-d.c. sets. Two or three
of those points you made kind of brought to mind things I did know once but had
forgotten. Of course I did know about them."
"Of course," Mac agreed with an enigmatic smile.
Posted November 2, 2022
(updated from original post
on 12/5/2016)
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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