AM Radios

By J. Richard Johnson

Tricky wiring in the filament circuits of these receivers
sets them off from conventional radios.

Everyone talks about television — yet, even in this era, radio receivers continue to outsell TV sets. Most of the radios sold nowadays are portables of various types and, among these, the “three-way” portables account for a very large number. That being the case, they are worth the attention of anyone earning a living in service work, especially if he is looking for something to bolster his income from TV service.

A “three-way” radio is one that may be operated from battery power, from an a.c. power line, or from 117-volt d.c. sources. These sets are tricky in some ways, and distinctively different from “portable only” or “power-line only” radios. Yet each specimen of this kind is enough like the others so that, once the tricks are mastered, the technician who goes in for handling them can turn them over fast and profitably.

When problems arise in servicing g these sets, it’s usually because someone was too impulsive about slipping a new tube in to replace one which burned out. In a fraction of a second, out goes the new tube! Yet a simple check of the circuit before tube replacement would have revealed exactly why this had to happen. Let’s analyze ~ why so simple a circuit can sometimes be so irritating.

The main thing that makes three-way portable sets different from other types is the way filament power is provided. For battery operation, of course; there’s no problem. One just uses a battery of the right voltage. However, the filaments are usually connected in series instead of in parallel, as are the filaments of battery-only sets. The filament ratings usually add up to 7.5 volts, which is then provided by a battery of this rating when the switch is in battery position.

In a.c. operation, the series string of battery-type filaments is powered by current from the “B+” supply, as illustrated in Fig. 1. The battery-type tubes don’t have separate cathodes, so excessive hum would result if they were powered by alternating current Therefore, current for the filament string is obtained from the same rectifier that rectifies power line current for the “B” supply.

Since the current rating of the filaments of the tubes employed is ordinarily 50 ma., a vacuum-tube rectifier or a selenium rectifier can supply direct current for both the filament and the plate and screen circuits of the receiver. A dropping resistor limits filament current to the desired 50 ma Any parallel connection would require so much more current that the ratings of ordinary low-power rectifiers would be exceeded. This is why the heaters are connected in series for a.c. operation. Once they are so connected, they are kept in series for battery operation simply to eliminate the need fox switching in the filament circuit when changing from one mode of operation to the other.

Most three-way portables now use selenium rectifiers. When vacuum-tube rectifiers are used, they also have filaments that require power; however, these are a.c.-type filaments, and their current doesn’t need to be rectified. The filament ratings are usually 50, 70, or 117 volts and they are supplied from the power line through dropping resistors (except for the 117-volt tube, whose filament can be connected directly across the power line). Examples of such rectifiers are the 50Y6 and the 117Z6.

Now let’s consider some of the more common servicing problems connected with three-way portable sets. Most involve the filament circuit.

Filament Burn-outs

The filaments in battery-type tubes have hardly any glow. Don’t expect to see light in them except in a dark room. If you suspect filament burn-out (and you nearly always should) be careful about using an ohmmeter to check filament continuity. The operating current in many ohmmeters exceeds the safe current that can be passed through 50-ma. filaments. Using one of these may dispatch all the other tubes to a premature death! The best thing is to check the tubes on a checker or else check d.c. voltages in the filament circuit with the d.c. voltage scale of a v.t.v.m. or v.o.m. If one of the filaments is open, it will show full filament-battery (or filament-supply) voltage across it when the receiver is turned on, and the voltages across the other filaments will show zero. If the receiver is connected for power-line operation, the voltage across the open filament will be the full d.c. voltage available as “B+” because there is no current and therefore no voltage drop across 113 in Fig. 1. Set your v.o.m. scale accordingly. The voltmeter test is OK if the meter is the generally used 20,000 ohms-per-volt device, or even if it has 1000 ohms-per-volt sensitivity, but don’t use a low-resistance voltmeter. On a low-voltage scale, the meter resistance may be enough to overload the other filaments and, besides, the readings will be way off.

Fig 1
Fig. 1.
The two ways of deriving filament
power in 3-way receivers.

Filament Current Resistors

In most receivers of this type, it will be found that there are resistors connected between some of the filaments and ground or across some of the filaments. Such connections are illustrated in Fig. 2. The reason for this practice is that, since there is no separate cathode in a battery-type tube, the plate current must enter each tube through its filament lead. In so doing, for some tubes, it must pass through the filaments of other tubes.

Fig 1
Fig. 2. Resistor network is needed in the
filament circuit of a 3-way radio.

For example, in Fig. 3, the broken-line arrows indicate the flow of plate current; the solid-line arrows indicate the flow of filament current. Note that the filament of V0 has to carry the plate currents of V1 and V2. as well as the filament current. In actual receivers, a string of four tubes is usually used and pentodes add screen-grid current to the load. Overload of the filaments due to the extra plate and screen currents would occur if equalizing resistors were not used. Fig. 2 shows the equalizing resistors R4, R5, R6, and R7. These resistors shunt parts of the filament circuit and carry the extra plate and screen currents.

Fig 1
Fig. 3. The path of plate current through
the filaments of a 3-way radio.

If one of these equalizing resistors should burn out or otherwise open up, it might take a filament along with it because of the overload caused by plate current. If this does happen, and a new tube is substituted for the burned-out one without prior attention to the resistor, the new tube may also burn out. It is thus very important, before any tubes are installed in one of these receivers, to check the whole filament circuit to make sure all components match the schematic.

Filament Filter Capacitors

An even more likely cause of tube burn-out in replacement is an open condition in protective resistor R., and consequent release of the charge from filter capacitor 02 into some of the filaments. This capacitor is used to filter the hum out of the directly heated cathodes, so that such hum cannot modulate the plate currents of the tubes during a.c. operation. It ordinarily has a value from 50 to 100 mfd., and may be connected at any position along the filament string. Notice in Fig. 2 that, if R5 were to open up, the capacitor would charge to a high proportion of the full voltage from the power supply, if one of the filaments is also open. Even if R5 is not completely open, but just high in value, excessive voltage can still appear across this capacitor. The 3V4 filament is particularly vulnerable, since it would carry full charging current into the capacitor. If we are not aware of this possibility, and substitute a new tube for the one burned out, the charge from the capacitor is released through the filament string and the new tube, or one of the others, burns out. Note that this can happen with the receiver turned off! The capacitor can hold its charge for some time.

Thus, it is best to make sure this capacitor is discharged and then to check the protective resistor R5 before any substitutions of new tubes are made. Discharge the capacitor by shorting right across its terminals with a screwdriver to keep the charge from passing through any other components. Even though it seems like a lot of trouble at the time, it pays to check the whole filament circuit each time a burned-out tube is replaced. After a new tube has burned out due to an open resistor, we really appreciate why this is an important precaution!

Some technicians make it a rule to trace out the filament circuit before making any repairs or replacing any components. It’s very helpful to know where each shunt resistor and bypass capacitor is connected in the circuit before servicing operations start.

Other Bypass Capacitors

As can be seen in Fig. 2, the large filter capacitor C3 is not the only one in the filament string. You may encounter one or more mica or paper bypasses on the order of 0.05 to 0.25 mfd. Since these are usually of a much higher voltage rating than the filament circuit, or even than the “B+” voltage, burn-out is rare. However, if one should short, it can cause full series filament voltage to be applied to less than the full filament string and take one or more tubes with it. This sort of thing can happen on either battery or power-line operation.

Not much has been said about sections other than the filament circuit of the three-way receiver. This is because it is the filament portion which is distinctive. Other parts of the receiver are subject to troubleshooting procedures applicable in other types of sets.

Source: TV & Radio News December 1957