Transistor Radio Alignment and Adjustment

MW/LW alignment

With transistor sets it is very unlikely that the alignment will change by itself. If you think the set needs realignment, ask yourself why. If someone has been fiddling with the adjustments, this is a good reason. In practice this is less common on transistor sets because the adjustments are too small for the average bodger to get a terminal screwdriver into. If components have been replaced with the same types, this should not cause enough variation to require realignment. If components have been replaces with similar parts this may cause enough variation to require the alignment of the stage in question to require slight adjustment, but it would be better to obtain the correct replacement part if possible.

The service data for the set will generally contain alignment instructions, and these should be followed if the required equipment is available. If a signal generator and output level indicator are not available, it is often possible to adjust the alignment on broadcast stations, as long as it isn't too far out to begin with. Some people may object to this idea, but it really isn't that difficult or dreadful if it's done carefully.

Tune the set to a station on MW that is fairly weak but still clearly audible. Music is generally better when adjusting for maximum volume by ear, so find a distant station of the "Classic Gold" variety or something similar. Carefully adjust each of the IF transformer cores, starting at the mixer-oscillator and working forward, for maximum volume. Only adjust each core once then move forward. Don't go back round and do them all a second time.

Now tune to a known station near the low frequency or high wavelength end of MW (such as Five Live on 693kHz or 433 metres), and check it is at the right position on the band (bearing in mind the likely scale accuracy of the set). If it is not correct, make sure the tuning mechanism is working correctly and that the pointer or whatever is moving properly from one end of the band to the other. Also make sure it's going the right way - highest wavelength or lowest frequency when the tuning capacitor is fully closed. If the station is a little way out, adjust the oscillator core to bring it into line. You'll probably need to adjust the oscillator a bit then the tuning a bit, as you move the station back to where it should be. If the station is some way away from where it's supposed to be, you won't get it in by alignment - there must be something else wrong.

Now tune to a known station near the high frequency or low wavelength end of MW (such as Virgin on 1215kHz or 247 metres), and check it's in the right place. If not, adjust the trimmer capacitor on the oscillator section of the tuning capacitor to move it back. Now recheck the first station again. You may have to readjust each of them two or three times.

Having got the stations in the right place we need to peak the RF alignment. Tune to a weak station at around 1500kHz or 200 metres, and adjust the trimmer capacitor on the RF section of the tuning capacitor for maximum volume. Tune to a weak station at the other end of the band and adjust the position of the MW coil on the ferrite rod aerial for maximum volume.

Switch to LW and tune in Radio 4 at 198kHz or 1508 metres. Adjust any trimmers and cores that are only used on LW for maximum volume. Also move the LW coil on the ferrite rod for maximum volume.

With all these adjustments, if anything is sealed assume it's correct. The only reason for an adjustment to be out is if someone has been at it. If there's no evidence of this, leave it alone.

Often the only adjustment needed is the IF alignment. It is rare to find that the RF and oscillator are out of alignment, even when the bodger has been at it.

Many Far-Eastern sets from the 60s and early 70s benefit from slight readjustment of the final IF to make the set sound less shrill. These sets normally have coloured cores - the final IF is the black one. Remove the wax and try adjusting it a little bit each way. About a quarter-turn is all that's normally needed, and the set will sound significantly better for it. Melt the wax with a soldering iron to reseal the core. Don't adjust anything else - that's as good as it gets!

VHF/FM alignment

The VHF band can also be aligned by ear, but this is not as easy as MW/LW since increasing the signal level doesn't necessarily increase the volume. A more meaningful level indicator can be obtained by connecting a multimeter on the 10V DC range across the electrolytic capacitor in the ratio discriminator circuit.

With the set tuned to a medium strength signal, carefully adjust each IF core in turn (again starting nearest the front-end) for maximum indication on the meter. As with AM, go through once only - don't go back around and do them a second time. Don't disturb the final IF yet.

If the sound is distorted, and you have already checked the diodes and capacitor in the ratio discriminator, very carefully adjust the core of the final IF transformer slightly in each direction and see if you can make any improvement. With a quiet programme (such as Radio 3) this should correspond with maximum reading on the meter, but the peak may not be that clearly defined. If you find that you get better audio quality with the adjustment significantly away from the maximum meter reading, something else is wrong with the ratio discriminator. Try adjusting the diode balancing control with the transformer adjusted for maximum meter reading, again aiming for minimum distortion.

You really need to take things very carefully with VHF/FM alignment, since it is easy to make a mess of a good set by too much fiddling. Of course if the set is already working badly, you haven't got anything to lose! As with AM alignment, if an adjustment is sealed, assume it's OK and leave it alone.

Output stage bias adjustment

With many sets the bias of the output stage is fixed and no adjustment is provided. If adjustment is provided, it is normally only necessary to adjust it if the output transistors have been replaced, or if someone has been fiddling. The service data will normally detail the procedure and state the current. If not, assume 5mA.

The procedure normally calls for a link to be removed from the PCB and the meter on a DC mA range to be connected in its place. The relevant control is then adjusted to give the correct current reading.

We can save ourselves a bit of messing around by connecting the meter in series with the battery instead of disconnecting the link. Adjust the variable resistor for minimum current. At this position the output stage will be drawing zero or virtually zero current, and the meter will be showing the current drawn by the rest of the set. Add the required bias current onto this (or add 5mA if the actual figure isn't known), and adjust the control to give this value on the meter. Leave it for a few minutes to stabilise then check and readjust again if needed. The adjustment is quite sensitive, and the current will rise rapidly as it is operated, so be careful.

If the set sounds distorted at low volumes when the bias current is set to 5mA, then maybe it needs a bit more. Try increasing it to about 7.5mA. If it's still a bit distorted, try 10mA. Don't go any higher - get the service data or find out the correct value.

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The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
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Last updated 14th April 2006.