The lesson learnt (or learned) by Barney in this Mac's Radio Service Shop saga is one that he has learned (or learnt) before, if you are an regular reader of the feature. As always, the story is a combination docudrama and tutorial concerned with troubleshooting, handing customers, giving air to some newfangled device, or instruction on circuit theory. Proprietor Mac McGregor is usually the teacher and Barney the pupil, but on rare occasion the roles are reversed. As you will see in this episode, radio and television set designers ginned up all kinds of ways to accomplish the same end objective - whether to avoid patent infringements or preferred exercising of creativity - and the nuances between them could cause no end of frustration to even highly trained and experienced troubleshooters. An additional inconvenience set upon technicians working on vacuum tube equipment was needing to wait for the tubes to warm up to determine whether the intended "fix" actually did the job. That was in addition to having to avoid getting nailed by lethally high voltages both when the set was powered up and for a short while afterward while the high voltage supplies drained their stored charges.
Mac's Radio Service Shop: The Not-So-Simple Sets
By John T. Frye
"Midget Day" was what Barney termed it. He was referring to the fact that every set slated for repair that day was of the small a.c.-d.c. variety, and the youth was looking forward to a pleasant afternoon of "turning 'em out like hotcakes," as he put it.
However Mac, his boss, noticed that the boy seemed to be taking an unconscionably long time to cook the first hotcake. In fact, after probing and snipping and soldering on the small chassis for nearly an hour, he was looking more puzzled and exasperated every minute.
"Surely an electronic engineer of your caliber can't be having trouble with a simple midget set," Mac taunted him.
"Simple my eye" Barney exploded. "This thing is about as simple as Sanskrit. It don't make sense. When you first turn it on, you can barely hear the local station; but the longer it runs, the better it gets. After it has been on twenty minutes or so, the sensitivity and volume gradually improve to where you can pick up several out-of-town stations, although the receiver is still not as hot as it should be."
"And what kind of a message do you get from these symptoms?" Mac asked with a quizzical grin.
"First I decided it must be a tube with low emission that took a long time to get hot enough to put out; so I changed all the tubes - but that was the only thing that changed. Next I concluded that perhaps an i.f. transformer had a defect so it was out of line when cold and sort of drifted in as the set warmed up, but that was a bum steer, too. I got to thinking perhaps there was a plate or screen-dropping resistor that had a real high resistance when cold but came down to somewhere near normal when warm, but a careful check with the ohmmeter proved that hunch was no good. The 'B-plus' voltage is around seventy-five or eighty volts, which is a little low but not low enough to account for the deadness of the set when you first turn it on. That brings you up-to-date on my thinking. No reasonable suggestion from those present will be ignored."
Mac pulled down the service manual that contained the diagram of the set and took a long look at it. Then he asked mildly, "Have you looked at this ?"
"Who needs to look at diagrams of a.c.-d.c, receivers?" Barney scoffed. "I can draw everyone of them with my eyes shut and recite 'The Shooting of Dan McGrew' backwards while I'm doing it."
Mac did not argue, but he took a small electrolytic condenser from a drawer and bridged it across one of the condensers in the little set. Immediately the volume increased at least four-fold, and the receiver acquired that between-stations hiss that goes along with good sensitivity.
"Let me have a gander at that diagram that tells you so much," Barney muttered as he reached for the service manual. After glancing at it for only a minute he looked up with a sheepish grin. "Now I get it," he said. "That dog uses a voltage-doubling rectifier, and where I was measuring 75 volts I should have been getting 190. One of the current-storing condensers is about gone. When cold, it has practically no capacity; but as it warms up it does achieve a microfarad or so, and then the voltage goes up and gives the set more pep. And before you say it," he hurried on as Mac opened his mouth, "I know I should have turned to the circuit diagram when I first began to feel stumped, as you so often have told me to do."
"I might as well have saved my breath to cool solder joints," Mac complained. "Your trouble is that you are always letting what you think you know get in the way of a real chance to learn. Don't ever sell these little sets short when it comes to giving you headaches. Many of them use almost identical circuits, but that just lulls you into a condition where the occasional different circuit can waste a lot of time.
"That set I just finished was a good example. It would play fine for a few minutes, and then the volume would slowly fade away. I shucked it out of the cabinet so I could get a good look at it, and then I noticed that just before the volume started to fade some of the tubes lighted very brightly while a couple of them became very dim."
"Doesn't sound so tough," Barney broke in. "Probably one of the heaters was shorting through a cathode to ground and cutting heater current off the tubes that went dim. Since the voltage that used to be divided among all the heaters was now across only a part of them, these heaters naturally got brighter."
"That makes me wonder if I'm starting to slip," Mac replied, "for my first thoughts followed exactly along the same channel as yours. Ignoring the obvious fact that if a short of that nature was occurring the tubes without heater current would have gone clear out instead of just growing dim, I replaced all tubes that either grew brighter or dimmer. When this made no difference, I looked at the diagram and found out the two dimming tubes had 300 ma. heaters while the remaining 150 ma. tubes were arranged in two parallel strings in series with the 300 ma. jobs.
"Normally this arrangement gave each tube its proper heater current, but now one of the 150 ma. heaters was intermittently opening. When it did, it left only one 150 ma. string in series with the 300 ma. tubes. This meant that the current through the smaller heaters was greatly in excess of what it should be for them, but at the same time this current was inadequate to light the 300 ma. heaters to normal temperature. It so happened that the tubes in the string that was cutting on and off had very dark bulbs that hid this action from sight. This is the first time in a long, long time I have run across this deal in a radio, although it is pretty common in TV sets."
While Mac was talking Barney had replaced all the filter condensers in the set he had been working on and had located an output transformer with an open primary in an overgrown table model. He quickly removed the defective unit and replaced it with a new transformer, but when he turned on the receiver it gave out with very poor quality and produced an audio howl at certain settings of the volume control. No checking of filter or audio bypass condensers turned up anything wrong, nor could any of the normal causes of audio amplifier instability be discovered.
"I should have stood in bed," Barney said with a sigh. "And this was the day I thought was going to be an easy one! I can't pick a set that has one nice simple little thing wrong with it like an open transformer. Oh no! I've got to draw one with complications."
"Maybe it didn't have complications until you replaced the transformer," Mac suggested.
"What do you mean by that crack?" Barney demanded truculently. "Are you insinuating I can't even replace an output transformer without making a mistake?"
"Could be," Mac said with a shrug as he picked up the speaker with the new transformer mounted on its frame. "What's this wire going from one side of the voice coil to the speaker plug?"
"I didn't trace it out, but I guessed it went to a terminal on the chassis for connecting an output meter across the voice coil," Barney replied. "Lots of sets use that, you know."
"But does this one?" Mac insisted as he pointed at the service manual shelf.
Reluctantly Barney hauled down a service manual and studied the diagram. "It doesn't go to a terminal," he said slowly. "Instead it seems to go back to the input of the first audio tube."
While the boy had been looking at the diagram, Mac had picked up the solder gun and had reversed the leads coming from the output transformer secondary, leaving all the rest of the connections intact.
Then he turned on the set, and the tone quality was excellent. No sign of amplifier instability could be found at any setting of the volume control.
"Since when has a guy got to observe polarity in output transformer secondary leads?" Barney demanded. "If that's necessary, why don't they color-code them?"
"Ninety-nine times out of a hundred it's not necessary," Mac explained; but this is the hundredth time. This set uses negative feedback, and the feedback tap is taken from one side of the voice coil. There is, of course, a 180 degree phase shift from one end of the voice coil to the other. When you replaced the transformer, you happened to solder the tap to the wrong side of the voice coil and so started feeding back voltage that was "positive" rather than "negative."
This, of course, accounted for the poor quality and the tendency to oscillate. When the leads from the transformer" were reversed, all was well. If you had just looked at the diagram in the beginning to make sure of the purpose of that lead from the voice coil, you could have figured all this out for yourself."
"I'm sure a prize dope," Barney admitted. "As many times as you have told me, it looks like I would learn to use the manuals. I've always been like that, though. When I'm looking for something in a catalogue, I only use the index as a last resort, even though I know I can find what I want much quicker by using it. I kind of feel like I've been licked if I have to use the index.
"It's the same way with the manuals. I guess I'm trying to prove how smart I am by showing that I can get along without them. What I end up proving, as I've just shown, is how dumb I am."
"Don't be too hard on yourself," Mac consoled. "A mistake that's recognized is half corrected, and I'll make a manual-thumber out of you yet - I hope!"
Posted October 16, 2018
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive 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 Radio & Television News
magazine (which itself started as simply Radio News), and then changed
its name to Mac's Service Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney
is his eager, if not somewhat naive, technician assistant. 'Lessons' are taught
in story format with dialogs between Mac and Barney.