Vacuum Tube (Valve) Radio and Audio Repair Guide: Repairing faults on automobile radios

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As mentioned earlier, the receiving sections of car radio receivers function in the same way as those of domestic sets so the same fault-finding procedures should be used in the output, detector/AVC/ AF amplifier, IF amplifier, frequency changer and RF amplifier (if fitted). Only the power supply stages are different and we shall address ourselves to them.

LT problems are infrequent as in general the heaters of tubes (valves) used in car radios are robust and reliable. In a set for 6 V operation all the heaters will be in parallel, and in the case of glass type an o/c tubes (valves) will be given away at once by the lack of heater glow. Metal and metal-clad tubes (valves) will, of course, need to have their heaters tested with an ohmmeter, but before removing them from the set, first narrow down the field by establishing, in the usual manner, in which part of the set a fault lies.

Twelve volt receivers using tubes (valves) with 12.6 V heaters in parallel may be treated just as for 6V sets, but as mentioned earlier there was a tendency for manufacturers to use 6.3 V heater tubes (valves) in series parallel. For instance, the frequency changer and IF amplifier tubes (valves) might have heaters rated at 6.3 V. 0.3 A each connected in simple series form. The heater of the popular 6X5/6X5GT rectifier is rated at 6.3V, 0.6 A. Typically, this would be placed in series with that of a 6V6/6V6GT output tetrode (6.3 V, 0.45 A) which itself is in parallel with the heater of a 6Q7/6Q7GT double- diode-triode (6.3 V. 0.3 A). As the total current of the last two is 0.75 A, a 42 W resistor would be wired in parallel with the 6X5 heater to pass the difference of 0.15 A. It will be seen that if any of the three tubes (valves) should go o/c the others are going to be affected. Should the 6X5 fail, for instance, the 6V6 and 6Q7 will receive no LT and also fail to light. On the other hand, should either the 6V6 or 6Q7 fail, the survivor will be over-run considerably due to excess current passing through its heater. Fortunately, as mentioned above, heater failures are rare and experience shows that when they do happen it is the 6X5 that is more likely to fall by the wayside. Nevertheless, it is necessary for the repairer to be aware of what could happen.

When the HT supply is absent the first thing to be checked should be the vibrator. In fact, this is in effect self-checking since it should reveal itself as in working order by emitting a characteristic buzz as soon as the set is switched on. If no buzz is heard you need look no further, but don’t assume straight away that it must be the vibrator itself that is at fault. Between it and the battery will be a number of components that could give trouble, such as a fuse, one or two RF chokes and the receiver on/off switch itself. The fuses in automobile receivers pass continuously much heavier currents than their counterparts in mains sets, and they can just wear out. The RF chokes, fitted to suppress interference from the car’s electrical system, are wound with thick wire and are unlikely to go o/c, but beware of bad soldered joints. The on/off switch is also called on to pass a heavier current than in a mains set and its contacts may simply burn away after a number of years. Multiway interconnection cables used with two unit receivers are a possible source of trouble due to fraying of individual wires.

If after all these items have been passed as satisfactory the vibrator still doesn’t buzz, use the voltmeter to trace the 6 V or 12 V, as the case may be, from the battery input to the pins of the vibrator. If no voltage is found look for the point at which it has disappeared, but if it is present things look bad for the vibrator. A test by substitution is now the only real way forward. It is possible for an experienced service engineer to dismantle and repair a faulty vibrator but it is a difficult job that may not prove to be very reliable, so regard this as a last resort. To open a vibrator the seal around the base has to be bent back with the aid of side cutters and no man is capable of restoring this to its original condition. If you find the vibrator base looking the worse for wear you may be pretty certain that someone previously has had a go at it, which more or less indicates that you might as well give up and look for a replacement.

The primary winding of the step-up transformer is robust and unlikely to give trouble. The secondary, however, like any other that delivers HT, is vulnerable. A feature of vibrator transformers not found in domestic receivers is that both primary and secondary windings are normally shunted by capacitors, which should also be inspected closely. A bad leak on one of those across the secondary could cause fatal damage to the winding. These are called ‘buffer’ capacitors and must be replaced by types having an AC rating of 800 V, due to the high peak inverse voltages involved. Ordinary coupling types will not do.

As mentioned earlier, the 6X5 (and other rectifiers used in automobile receivers) has a highly insulated cathode, between which and chassis the rectified HT appears. An insulation breakdown here can have serious results, since it shorts the HT down to chassis and imposes a heavy load on the transformer which, if left too long, could burn it out. Always watch a glass tubes (valves) rectifier for signs of internal sparking when trying a set for the first time, even though its heater/cathode insulation may seem satisfactory when checked on a meter.

The 0Z4 heater-less rectifier needed a minimum of 300V (peak) to make it start when new, and this requirement increased somewhat with age. It can become erratic in time, and if a new one is not to hand it may be replaced by a 6X5 with heater supply derived from the battery, via a 10  5W resistor in the case of 12 V sets.

A certain number of sets in the late 1950s had small contact-cooled rectifiers, and in common with those found in domestic receivers, their output drops off sharply with age. If the HT voltage is found to be very low this is the component to suspect. Because of its small size this type of rectifier could be hidden away very effectively, and it may need a very sharp eye to spot it! If a similar unit is used as a replacement, remember that it is most important to bolt it down effectively, since the heat generated in a car radio has far less chance of escaping than in a table model. Do not replace with silicon rectifiers before reading the list of essential precautions regarding these devices given in Appendix 2.

A failure of the reservoir capacitor would drop the HT, but might not give as much audible warning as it would on a 50 Hz supply. As always, ensure that the working voltage and ripple current are sufficient for replacements.

Realigning automobile receivers

The usual procedure as used for domestic sets, and detailed elsewhere in this guide, should be followed but take this special precaution when adjusting the RF and local oscillator trimmers.

FIG. 1

Aerials used in automobiles are connected via screened cables which have significant internal capacities. The set designers are aware of this and adjust the inductance of the aerial coils accordingly. They also provide an aerial trimmer which is adjusted for best results in the particular vehicle in which the set is installed. To ensure correct alignment, use a special dummy aerial to terminate the signal generator output; a suitable design is shown in FIG. 1. This may also be used when ‘aerial testing’ a car radio, in conjunction with a short length of wire hung vertically above the bench. Don’t use a long wire aerial because this will give a false impression of the set’s sensitivity. It is both disappointing and frustrating when one goes to a great deal of trouble to reinstall an automobile radio which sounds fine on the bench, only to find that it has a very poor performance in the vehicle!

Can the polarity be changed?

Non-sync vibrators will work ‘either way round’ without any adjustment being required, but you may have to modify the battery input wiring. It was usual to have the RF anti-interference chokes by-passed by one, two or even three fairly large value electrolytic capacitors, typically 50 p 25 VW. Sometimes special reversible types were used which will work ‘either way round’ and don’t need to be touched, but these were probably in a minority. You are far more likely to find that the ordinary polarized types are fitted which will have to be unsoldered and reversed for a change of battery polarity. For obvious reasons, mark the case of the receiver very clearly to indicate positive or negative earth as the case may be.

Sets with synchronous vibrators always need to be adjusted for different polarity. In many cases it is necessary to withdraw the vibrator from its socket, turn it through 180° and reinsert it. Normally arrows and + and — signs are provided on the vibrator and surrounding metalwork to indicate its working position. Check for the electrolytic by-pass capacitors as for non-sync vibrators.

Predictably, Philips evolved systems of their own to make their receivers capable of working on either polarity and often on either 6V or 12V as well. The adjustments consisted variously of reversible plugs, terminals with sets of connecting links, and flying leads secured either by screws or by soldering. All are so complicated that no ordinary person can make sense of them, and it is absolutely essential to follow the maker’s instructions. Sometimes these will be found printed on a label within the case of the set, but if not you should be able to obtain servicing data from suppliers such as The Radiophile.

Occasionally you may encounter a set which bears a prominent marking ‘for positive (or negative) earth only’. Normally this means that although it is not totally impossible to change the polarity it is so difficult as to make it not worth while.

Note that many automobile receiver manufacturers adopted the American style of referring to the car chassis as ‘ground’, hence ‘negative ground’ or ‘positive ground’ receivers.

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Updated: Saturday, 2020-05-23 7:47 PST