Scott 299 DC AC Balance

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All electrical equip-ant requires maintenance. The 299 waa designed to operate
troublarrree (or nany yeers. certain checks done at the end of every year, houever,
will help to keep the amplifier ln good condition.

A. Check the tubes' particularly those in the power output stage and the recti-
fier every year. If the tubes are outaide the lanuracturer's ratings or show
gas, they ahould be replaced. Oaaay tubea may damage other conponenta or the
circuit.

5. When the anpliiier in being checked yearly, clean the tubes 0! duat no that
they nay radiate their heat nore effectively.

C. it at any tine the hun or noiae increases noticeably, check the power tubea.
Thia ayepto- is often an indication o! gaaey tubea. l! both tubes check properly
and the anplixier in without hue when the anpliiier is disconnected, then check
the amplifier. If the hue continues. have a asrvice nan check the amplifier.

NUI'S: DO NU? US! was O'lflfl THAN THOSE SPICXFIFD Fm TfllS PI-"1. TH! INTRODUCIIW
OF ANY UNSPBCIFIBD PART VOIDS YOUR WARRANTY.

D. l! the anplifier hlaea tuaes frequently. have a aervice nan check the appli-
tier. If no trouble ia apparent, check the line voltage. Should the line
voltage rise above 125 volts, drop the line voltage by Ileana or an auto-
tranaior-er or place a voltage regulation tranaloraer between the a-piitier and
the line.

NOTE: DO NOT USE FUSE SIZES OTHER THAN THE FUSR SIZE SPECIFIED. TO DO OTHERWISE MAY
RESULT IN FERNANENT AND COSTLY DAMAGE TO THE AIPLKFIBE IF THI FUSE 15 T00 IABGE AND
WILL REQUIRE FREQUENT RBPLACEIINT 0F FUSES IF THE FUSE SIZE IS TOO SHALL.

E. PO'CI Tube Bias Control:

This control ia located on the rear apron o! the a-plifier. To adjust the control,
uncover the button plate of the anplifier; place a DC voltmeter across iiltor resistors
R209 and R210, two 330 ohm 10 watt wire wound reaiators connected in parallel; and adjust
the control until the voltage acroaa the reaiators is 24.5.

P. 00 Ballnce Controls:

Theae control: should be adjusted when the power tube: age or are replaced.
Their location: are between the power tubea on the top 02 the 199 chaaaia. To
set these controla use the following procedure:

(1) Connect a Dc volt-eter whoee nininuo range ia 1 volt lull acale
between both platen of the power atagea.

(2) Adjust the DC Balance Potentioeetera tor Iinilun reading on the DC
voltaeter.

6. AC Balance controls:

These controls ahould be adJuated when the power and phase inverter tubea age
or are replaced. They are located to the rear 01 each pheae inverter tube
on the top of the 299 chaaaia. To adjust then. ule the following procedurea:

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.9.

Two methods {or setting the control will be described. The first method, which
makes use of tools that are available to the ordinary service man. is less accurate
than the second method. which makes use of equipment that is {ound only in complete
electronics laboratories. The difference in accuracy between the two methods is very
smnll, but it lay be significant for none laboratory nonsureleuts.

51351- mnog

A. Equiggent Neceaaary

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Low distortion audio oscillator, sine-wave. Note: Many oscillator
kits are apt to have high dietortinn.

Resistive load of proper value (for instance, 16 oh-s for the 16
one tap) and wattage. Note: Many contercial wirevound resistors
have considerable residual inductance; care must be taken to avoid
these .

Oscilloscope. Rate: Kany oscilloscope kits have nonlinear sweep
circuits and are apt to give distorted displays.

8. Procedure

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Connect audio oscillator to proper input Jack. 0n the complete
amplifiers a high level input is normally used. There are two
reasons for this: First,-the (requency dependent of the recording
equalizer circuits are avoided; second, better distortion measure-
ments can be made with the lower noise level available. Connect
audio oscillator to the 0.5 volt input in the power amplifiers.

Adjust all tone con! rols to their FIAT position; all rumble and
scratch filters, including the dynaural, OFF; and the loudness
volume control to VOLUIE.

Turn the loudness control on complete amplifiers or the level
control on power amplifiers to their maximum position.

Turn dynamic power monitor (where included) to OFF, and set damp-
ing control (where provided) to nor-ll operating position.

Connect resistive load to proper output terninala, and connect the
oscilloscope input in parallel. Note: lake sure the oscilloscope
ground is properly oriented. (See Figure 5.)

Turn all equipment on.

Set audio oscillator to Irequency desired if the amplifier is to
he used at a fixed frequency. Otherwise, set the audio oscillator
to 1000 CPS.

lncrelse the output 0! the oacillntor until the sine-wave just begins
to clip. (See Eigure 6.) with screwdriver, adjust balance control