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grooves of the record? Has the problem appeared suddenly or has it been building up and growing worse over a long period? Has the set had any rough treatment lately either through spring cleaning or handling by outsidera? These ąuestions themseWes suggest the reasons for asking them.

Having heard the customer’s story, turn on the set, gather your own im-pressions, and hope they correspond with the customers. Whatever your conclusions. your next step will prob-ablv be to reach for your tool kit. The contents of this kit is important. In addition to the usual hand tools, acces-sories. and other materials used for othcr types of electronic service, it should carry the items listed in Table 2. The exact naturę and purpose of some of these will come up for discus-sion.

If the phonograph appeara to be in-volved, for example, you should have with you at least one known recording of good quality for comparison pur-poses, in addition to a conventtona) test record. as you can’t rely on the customer to own a disc with which you are familiar.

A good routine for turntable. tone-arm, and cartridge checking should in-clude inspection of the turntable drive mechanism for dried-up rubber com-ponents; also a cleaning of the inner drive surface of the turntable platter and the rubber driving wheels with alcohol. Check the Stylus for align-ment (both horizontal and vertical) and the condition of the damping blocks (if present). A smali mirror placed face-up on the turntable. with the tonę arm resting on it. should suf-fice for the alignment check. The older G-E cartridges may suffer from hard-ening of the blocks or actually have the blocks tom away from the Stylus har. The barest touch of Duco cement will usually serve to flx the blocks securely in place.

Check for a hard dust accumulation around the Stylus tip. Cartridges that use an internal viscous damping fluid are particularly prone to accumulate '‘gook*’ which is not easily removed by flicking the Stylus with the finger. A dab of alcohol on a pipę cleaner will dissolve the accumulation.

A spot of oil on the turntable bear-ings and petroleum jelly (Vaseline) thinly applied to the friction surfaces on geara and levers will freąuently do wondera in bhnging a slow turntable up to speed. There are cases, however. when a replacement motor or morę stringent overhaul methods are neces-sary: it then frequently becomes a toss-up as to whether the mechanism is worth repairing.

Turntables using rubber-belt drives tsuch as the frequently encountered Garmrd RC-80) may slow down in the change cycle or not operate at all on one or morę of the three speeds. Dried-out drive belts are the culprits and replacement is in order. These belts should be a standard item in the well-stocked repair kit. Another frequent causc of turntable slippage and slow-

October. 195?

5U4	6SN7
5Y3	6V6
6AN8	6X4
6AQ5	12AT7
6BQ5/EL84*	12AU7/ECC82*
6C4	12AX7/ECC83*
6CA7/EL34	5879
6L6/5881/EL37*	GZ34
6SC7	KT66
6SL7	KT88
Z729/EF86

Table 1. Common ampllfisr-proamp tubę*. See text for listing* with asterisks.

down is glazing of drive wheels, that is. their friction surfaces become too smooth. An emery board such as is used for trimming milady’s finger nails can rough up a drive wheel nicely without removing too much rubber. Use a gentle hand in this operation.

The strobe disc and tone-arm stylus-pressure gauge are another pair of service kit "musts” for turntable inspection. A smali pocket microscope or powerful magnifying glass, while not entircly adequate for checking a Stylus for wear, serves nicely in providing a close-up look in checking for Stylus alignment.

A light source satisfactory for ob-serving the pattern on a stroboscopic disc is also essential. A fluorescent lamp or neon bulb with sufficient light output will do. The NE-57 neon bulb listed in Table 2. which has a built-in limiting resistor. will do nicely. Its screw-in candelabra base fits the socket of many pencil-type soldering-iron handles that accommodate screw-in element-and-tip combinations. If this type of iron. which is a handy one for the outside cali in any case, is not used. a sępa ratę socket with a.c. cord can easily be arranged.

A replacement cartridge or Stylus of the same model as the customers is another valuahle service kit accessory. Direct A-B listening comparison be-tween the old cartridge or worn Stylus and the new one you have temporarily installed will convince the most recal-citrant penny pincher that rcplace-ment is in order. Make your A-B comparison on a good. new recording, preferably featuring a solo female voice or solo violin. Either one will sound "raspy around the edges" or seem to crack or break up if every-

Table 2. Add these special item* to

the reguiar tool kit lor hi-fi work.

Alcohol (in dropper Container)

Cement (Duco or similar)

Pipę cleaner*

Light oil

Petroleum ielly

DrWe belts (record changer)

Emery board Smal) mirror Pocket microscope Stroboscopic disc Stylus-pressure gauge Cartridges Styli

Test record*

NE-57 neon lamp Phono plugs and jack*

Thin phono cabłe. insulated & shielded.

25 it

thing isn’t right with the Stylus assem-bly. There aren't that many popular types of cartridge around that you can’t afford to stock a spare selection. A couple of good magnetics and a couple of good ceramics will certainly facilitate testing even when they are not identical replacements.

From the turntable we come to the phono preamp section of the ampli-fier. The big problems here are micro-phonics, hum and noise; and the culprit is usually the first, low-level stage, frequently a 12AX7/ECC83. While the quality of these tubes has improved steadily over the years. hard service and old age show up as a gradual inerease in the noise level and a tendency toward microphonics.

It’s a good idea to pre-test your service kit tubes in your own bench amplifier for their hum and noise prop-erties. It’s embarrassing to replace one bad tubę with an equally noisy one while the customer looks on. Some of the other preamp tubes. such as the Z729/EF86 or 5879. require the same pre-testing technique. The 5879. in par-ticular, which is found in a number of tape-recorder preamps. ofttimes may require extensive pre-testing before a quiet one is found.

All the hum and noise problems in a preamp. of courae. are not due to faulty tubes or components. Frequent-ly buzzes and hum will appear coin-cident with the opening up of a new TV channel in the vicinity or some renovation or change in house wiring. Bad grounds on the a.c. linę are a fre-quent but little suspected cause of “in-curable" hum conditions.

A good test for the ground condition of an a.c. linę is to measure between the ground side of the linę and an ex-ternal cold water pipę or other avail-able element at earth ground with a v.o.m. A five- to ten-volt reading can cause a lot of noise in a sensitive phono Circuit, especially in stereo.

A problem which seems allied with the bad ground condition is TV sync buzz pickup. An examination of the phono-input position of the new stereo preamps will reveal a number of “pur-poseless" resistors spotted about the stage. The theoretical explanation for these resistors functioning the way they do is rather complicated but. prac-tically, they serve to eliminate TV sync buzz and sometimes TV and FM sound from the phono channel. You will find values from 10.000 to 100,000 ohms at the input triode installed directly on the tubę socket between the grid-re-tum resistor and the tubę pin. See Fig. 1. Older preamps. whose input Circuit configuration involves the magnetic cartridge load resistor across the input jack coupled to the high-value grid-return resistor by a capacitor (Fig. 2). are particularly prone to sync pickup and benefit by the installation of a “stopper” resistor as described.

Most stereo preamps have all the Circuit precautions possible already installed. There is little that you can do with these in stubborn hum pickup (Continued on page 128)

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