Vacuum Tube (Valve) Radio and Audio Repair Guide: Repairing FM and AM/FM receivers

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To a large extent the same techniques as used for AM-only receivers will apply on both AM or FM to the main chassis of the receiver, bearing in mind that on FM the IF is increased to 10.7 MHz and the detector works in a different way. The AF amplifier and output stages are similar to those of AM sets only in respect of extending the treble response. More expensive sets were fitted with special high frequency ‘tweeter’ loudspeakers alongside the ordinary unit. Experience has shown that the chief causes of trouble other than those to which any receiver is heir have been breakdowns on the AM/FM switching due to the presence of HT on the wafers and a greater susceptibility to self-oscillation in the IF amplifier stage.

For the first problem you may be lucky enough to find some unused switch contacts that can be pressed into service, otherwise some kind of replacement will have to be made. Rather than change the entire switch unit, which may be difficult or impossible with press button types, you may well be able to fit a separate small switch on the rear of the chassis.

IF instability is usually due to a faulty decoupling capacitor on the HT feed to the anode or screen grid of the IF amplifier — or to someone having twiddled the cores in the transformer. We shall look at realignment in a moment.

By far the greatest problem with VHF tuner units has proved to be deterioration of an ECC85 or UCC85. The best way of proving this is by substitution, but if you haven’t a spare to hand you could try the old service engineers’ trick of gently tapping the tube (valve) with the handle of a small screwdriver. It is surprising how often this treatment will restore the performance, if only temporarily. Don’t overdo the tapping!

If there appears to be trouble within the tuner unit you may or may not be able to check voltages, etc., because the usual small size and near totally enclosed construction makes it extremely difficult to get at the tube (valve) bases and components. Each tuner will have to be considered on its particular merits.

Be warned that the sensitivity on FM of many sets was not much better than poor, and a good outside aerial is called for unless you live in a very good reception area.

Realigning FM and AM/FM receivers

Contrary to what might be expected, an FM signal generator is not necessary. Although most manufacturers issued their own particular instructions for alignment the following procedure usually will serve well enough. With combined AM/FM sets carry out the AM alignment first, if necessary, in exactly the same way as for ordinary AM-only sets.

On FM the 10.7 MHz IF AM signal needs to be injected into the grid of the VHF frequency changer which more than likely will be inaccessible. In this case, try partially removing the almost invariable screening can from the tube (valve) so that it no longer is in contact with the chassis, then clip the live generator output lead to it. You may have to use a fairly high output from the generator to make this work but it usually will do the trick.

De-tune the secondary of the last IFT (i.e. the winding that feeds the ratio detector) by unscrewing its core outwards until it is level with the outside of the can. This will render it sensitive to AM signals. Connect a multimeter on its 10V AC range across the loudspeaker and tune all the IF cores save the last for maximum output. Reduce the output from the signal generator as necessary to maintain about 1.25 V. Now rune the last core for minimum output, which should coincide with the point of maximum sensitivity to FM signals.

Remove the signal generator input, replace the can on the ECC85/UCC85 and try the set on an aerial to see if stations can be received satisfactorily. Sometimes, against the rules for AM alignment, it may be necessary to fine tune the last IF core by ear to achieve completely undistorted sound.

On the RF side you really need an FM signal generator to carry out detailed alignment. Fortunately in most cases this will not be necessary and a little careful adjustment on actual stations should enable you to position them correctly on the dial and to realize maximum sensitivity. When a VHF tuner has been mistuned badly in the past it will be necessary to obtain the manufacturer’s servicing data and to follow it faithfully.

Bear in mind that UK tubes (valves) FM receivers covered only from about 87MHz to 100 MHz, although it may be possible by a little adjustment of the local oscillator trimmer or core to raise the upper limit sufficiently to receive Classic FM, if so desired.

In all cases, seal cores and trimmers with wax or paint after alignment is completed Note that this was done in the radio trade not only to prevent accidental movement but also to reveal if the customer or a ‘knowledgeable’ friend interfered with them.

Special note: be warned that a number of cases have been recorded of Murphy A252 AM/FM receivers having left the factory with a 1000 pfd decoupling capacitor on the HT feed to the VHF tuner omitted, suggesting that there may have been a bad batch which escaped the factory quality control. Check for the absence of this capacitor in the event of oscillation being experienced on the tuner unit or the AM frequency changer when acting as a VHF IF amplifier.

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Updated: Saturday, 2020-05-23 8:15 PST