Operating instructions

5.3.7 Audio/Bias Mixing Circuitry

PB-16KOA (PB9A577)

A master bias signal is generated on the Bias Control board and distributed to

each channel's electronics, where it is amplified and mixed with the pre-

conditioned audio input. A 10-turn 1- kΩ potentiometer (VR301) serves as

the channel's Bias Level adjustment. The bias current is applied to IC301,

the amplification stage, through R301 and C302. L301 and C301 are wave

shapers. IC302 is provided with controls for the record mode and A amp

voltage in Control circuit by IC516 and Q206. All the controls for the record

mode are carried out by clock pulse to provide gap-free punch-in at both high

and low speeds. At 30 ips, it takes about 30 ms for the bias to turn ON. The

clock pulse provides an additional delay of approximately 45 ms to

compensate for the distance between the erase and record heads. TP1 is a

check terminal for the record bias current. This carefully synchronized turn-

on not only provides gap-free punch in, it permits the machine to enter

record mode without "pops" or other noises. Once in record mode, the bias

and erase current ramp down in the inverse sequence.

The bias signal enters a high speed Op Amp (IC302) which mixes the audio

and bias signals. Q302 and Q303 form class AB output amplifier that mixes

the audio and bias (the head driver amp). The mixed bias and audio signal is

applied to the record head through C308, parasitic trap L303/R317 and relay

RL510. Each track on the Record Head has two identical windings. RL510

switches the windings into series for Sync Repro mode, and into parallel for

Record mode to provide low impedance load for increased bias current

capability.

5.3.8 Erase Head Driver

PB-16KOA (PB9A577)

The erase current is also generated on the Bias PCB Assembly and

distributed to each Amp PCB Assembly. RV401 mounted on the audio PCB

Assembly, is a gain adjustment for the erase current. The erase current is

then fed to IC401. The clock pulse provides all the rise and fall times of the

erase current. L402 and C404 are wave shapers.

The erase current is amplified at IC402 and CLASS AB AMP (Q402 and Q403)

then supplied to the erase head through RL502. TP2 is a check terminal for

the measurement of the erase current.

MTR-90III Operation and Maintenance Manual

October 1990

Section 5 Maintenance

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