Customer Repairs

Grundig TK5

This piece of equipment is an "Open Reel" tape recorder and was said to just twitch rather than play.

 Grundig made a series of machines like this one over a relatively short period of time in the post-war pre-transistor era. You could buy one of these in 1956 for 46guineas. You could always tell better quality stuff because it was quoted in "guineas" and it always came as a bit of a shock when the salesman presented you with the bill; in this case £48:6:0. Most Grundig models of this date are very similar and have a heavy chassis with relays and valves liberally sprinkled around. This model uses two fat circular cross-section belts for play/record and winding functions and a thinner belt for driving the counter. The TK5 uses a single large motor known as a squirrel-cage induction motor. This type runs at a synchronous speed no matter what the load (within reason). The name derives from the fact that the external rotor looks like a cage (the text books say like a squirrel's cage but I guess not many people have ever seen one so the full description is a little superfluous!). Basically the construction is similar to a transformer with a normal wound primary fed from the mains supply and a secondary winding comprising lots of single turns of heavy metal short-circuited top and bottom. The cage is free to rotate and does so in conformance with one of Faraday's laws relating to electro-magnetic field. The primary winding has tappings to deal with start-up as a disadvantage of this type of motor, particularly very large varieties is excessive start-up current. In fact very big motors must have a switching arrangement called "Star-Delta" which allows a different running configuration to that used for starting. In factories where these types of motors abound it used to be a major problem if mains power dipped off then came back on again. If motors restarted in Delta mode their windings would burn out. No doubt with clever circuitry nowadays the problem is not as serious as it was 60 odd years ago.

 Anyway....this machine uses a tiny motor compared with those used in power stations. The left hand drive belt was broken with age, the rubber having cracks every quarter of an inch. The right hand belt was in better condition but had stretched and no longer turned the reel. When this machine was built the designers paid little heed to changing drive belts and if you ever contemplate doing the job before it is vital to either make a detailed sketch of how it comes apart, or like me, take a photograph of the deck first. Under the main operating knob, on its square shaft, are three cams and fitted under the winding knob are others. These cams must be fitted correctly otherwise the deck will operate in very strange ways. The cams are used for positioning the pressure roller, positioning the erase head pressure pad, and setting the un-locking mechanism for the winding knob. Under this knob are the cams for sorting out the deck mechanism for fast forward and reverse and setting the left and right hand reel brakes.

I couldn't find proper replacement drive belts so had to make use of smaller cross section square belts. These are not ideal because they don't engage with the knurling on the pulleys which is provided for extra grip. Consequently the tension of the smaller belts is probably greater than that of the originals and puts a slightly greater load on the motor. As the motor normally runs pretty hot it is likely to run even hotter so the machine should not be used for very long periods without being occasionally set into its "rest" condition. Probably this setting is is provided on the main operating switch for that very purpose as with modern cassette decks no such provision is made and cheaper machines allow their motor(s) to run continuously. Modern motors are usually DC and are fed by a stabilised power rail either external or sometimes internal to the motor and on some there is a little hole in the top through which a screwdriver can be inserted and its speed altered.

The TK5 now works normally. It's single speed and appears to have provision for just two tracks because when I played an old tape which I recoreded on a Philips machine with four tracks the TK5 plays two at the same time. The piano keys at the bottom are used to select the source of data to be recorded and are nothing to do with the mechanical operation of the deck. On the left next to the magic eye is a small gear wheel. This is for setting the audio level and mates with the underside of the knob giving about 180 degrees of movement. The centre spindle is for overall on/off and tone.

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