Tips for Wireless Enthusiasts: No1: Servicing Roberts radios which use germanium transistors

Roberts R600, R404 and R303 (for example)

1 Most Roberts models look very nice from the outside but as the models got newer, and as discovered when they are taken apart, get progressively more and more horrible. A repair on some specimens will often take days (as one slips in remunerative jobs) and often there may be a strong urge to bin it. The following specific information has been gained fairly recently fixing an R600 an R404 and an R303 although I have been fixing Roberts for many years on and off.

2 A radio may arrive with the complaint that it eats batteries. The last one I bought had a little piece of paper glued to the underside with a history of its last ten batteries! It may be bought with the casual remark that "I've put a new battery in it but of course it needs an aerial so I can't let you hear it working" or "it works OK because it makes a loud click as you turn it on".

3 Roberts radios from the 60s use a range of RF transistors which fail in a rather strange way. The transistors are the AF117 style (various varieties are used) which employ germanium, rather than silicon, are fatter than average and employ a fourth wire (in addition to those connected to the emitter, base and collector) which is connected to its metal case. The material between the case and the transistor chip should be an insulator but over the years may have degraded and started to conduct, shorting one of the electrodes to the case. As the fourth lead is invariably connected to circuit ground, if the short is to the collector, leakage across the battery connections will arise. The first indication of this will be an ever-reducing battery life and the owner will often put the set on one side shortly after fitting the last battery. If the owner is very rich and doesn't mind buying batteries, the problem usually gets to the point where the rogue transistor(s) will cease to perform its circuit function and the set will stop working. There is usually a click as it's turned on and little else. At this point the radio is put away and often later turns up at a car boot sale.

4 Although Roberts receivers work well, when they are working, their mechanical design does not lend itself to easy servicing! Their design is reminiscent of what we used to call "rat's nest" construction and it's important to make copious drawings and notes before removing any of the assemblies otherwise you may have difficulty working out where all the wires go. Some wires are uninsulated lengths of 22SWG wire and special care must be exercised as these are easy to short against adjacent bits of the circuitry or metalwork.

5 Sorting out the transistor problem is not always straightforward as later models utilise a screened metal box, in which the front end and IF strip are fitted, and this box is not always very accessible. For example (and for the following description) in the R600... the box fits under the four way push-button assembly. The latter is wired (intimately) to the adjacent circuitry and the whole push-button unit must first be removed before the screened box can itself be removed. You may also wish to remove the plastic dial plate but try not to disturb the dial cord which is underneath. In the R300 (typically), which uses a rotary switch, the box can be removed much more easily.

R600 receiver: bottom view of chassis showing push button assembly which masks solder tags of the screened box. All the wires need to be detached before this can be pulled off its locating tags on the front panel. The latter need to be straightened in order to remove the push button frame.

R600 chassis: top view showing IF screened box. The label covers adjusting holes.

6 When you've unsoldered the metal box (which has a warning label to frighten off the fainthearted!) it can be detached, sometimes only with a some difficulty, because of the mechanical arrangements, and as various pins are extended with bits of thin bare wire. Next you have to unsolder the metal base cover and then remove the insulating paper and the circuit board. Typically you will see three AF transistors. Make a sketch and mark their orientation, so you can see how to refit them, then remove each one in turn. There were little or no circuit board markings included by the Roberts' Drawing Office so you will need a little luck to unsolder just the four leads required per transistor. When the first device is out you can check its condition. The case connection is easy to find as it will be electrically connected to the case (zero ohms). The other leads may, or may not, be connected to the case by a lowish resistance. Germanium transistors will show something like 150millivolts between base-emitter and base-collector when appropriately polarised test meter leads are applied across the legs. You may also see a low resistance between the case and one or all of the legs due to the fault which has arisen in the internal construction of the transistor.

7 The most straightforward repair is to cut the case leg off and to refit the transistor. If you have qualms, and possess a transistor tester, by all means check it before you refit it. If the transistor does not exhibit the fault I would advise to cut off its case leg anyway (unless you are a masochist and wish to repeat the whole performance two or three times in the future). When all the transistors on the circuit board have been processed, reassemble the box and refit it (not forgetting the metal lid). Carefully rewire the assemblies and test. Battery drain should be around 15-20mA with no signal.

R303 receiver chassis: The screened box tags are readily accessible.

8 I have also had problems with the audio output transistors. These are mounted under a copper heatsink screwed to the metalwork. If a short develops between the case and the internal bits the transistor will need replacing. Some types are no longer available but alternative types are. AC187 and AC188 types are still sold by CPC. AC127 and AC128 are difficult to get now, even though they are optimistically still listed by a few suppliers. The former types can be used, as they are roughly equivalent to the earlier types, but you might have to bend the heatsink as some arrive in slimmer packages (in fact a pair of the last ones I got had two different case styles). It may be possible however to swap one of the driver transistors which have the same code for one that clips under the heatsink as it doesn't matter if the case is live in the driver position. The photo above shows the output transistor heatsink mounted to the plate through which the ferrite rod passes. There are two potentiometers, one sets the standing current through the output pair and the other sets the mid-point voltage to balance the push-pull circuit.

9 Sometimes a receiver will have distorted audio. If the battery is OK check the loudspeaker by substitution as I've had to replace a few.

10 It is relatively easy to fit a mains power unit to early models which don't have one. For safety reasons use a black 13 amp plug type. A small type with a rating of say 500mA will be adequate. You can use a phono socket or a co-axial power plug mounted in the plastic grille to carry the power. If a socket without an integral switch is used, switching between internal battery and external power can be made using a pair of germanium diodes wired with their anodes connected to the positive battery input and the power unit positive respectively. The cathodes are connected together to the positive circuit power input. Germanium diodes waste less battery voltage than silicon types. Depending on the choice of power unit and the level of ripple at its output you may need to fit a 3-pin voltage stabiliser, some smoothing capacitors and last but not least you may have to fit one or two small capacitors (say 0.47uF) to eliminate hum from VHF reception. Where the decoupling capacitors are wired may need to be done by trial and error as the path of the currents resulting in the hum may be elusive. Even with a perfectly clean DC power output you can get modulation hum on FM signals. A noisy type of power unit may need to be set or selected for 12 volts to give headroom for cleaning up its output. A clean type of supply not requiring extra smoothing can be a nominal 9 volts. Unfortunately some Roberts models need plus AND minus 9 volts. This can be achieved with a 20 volt unit and a little ingenuity. Others such as the R404 need 18 volts; this is reduced to about 4volts for the RF stages and the full 18volts for the audio section.

11 Now some problems with an R404 which will be fairly typical I guess. The receiver was bought by a customer at a car boot sale and didn't work... the usual click from the speaker and nothing else. Unfortunately someone had been fiddling and I found in the output stage one 350uF capacitor had been roughly replaced and a 100uF with a long dangly lead attached. I fitted new components and cut the screen to the case of the RF transistor then removed the IF box and cut the screen leads to the two AF117s inside. One had actually failed as well as having a shorting case so I fitted the RF transistor in its place so that there was no chance of oscillation which may have arisen if I'd fitted an equivalent replacement in an IF stage. After reassembly I found to my dismay that someone had twiddled the transformer slugs.

12 Apart from one being broken the problem was that there are no threads for the slugs to screw up and down in. Instead there are pieces af elastic which had perished and once the slugs had been wound down (or rather pushed down) they wouldn't come up again. Out came the IF box and I had to remove each of the three transformers. I put a piece of plastic tape about an eighth of an inch wide into each coil to provide a thread (the thread is pressed into the plastic as the slug is screwed up and down). I turned each of the slugs upside down to give me a new slot because the twiddler had caused some damage to the top slots and reassembled the IF box etc.

13 Realigning is a bit of a problem as five out of tune transformer windings provide a lot of rejectivity to the required 470kHz signal but looking in the top of the R404 IF box there's a resistor lead available to which the signal generator can be attached enabling you to tune the last IF transformer and then the centre one. After this the generator is set back to the external aerial connection and the first transformer can be set to 470kHz as well.

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