Tascam M 50 Service Manual

Page 1

Effective: December: 1982

*

MAINTENANCE

NOTES

All procedures refer to all inputs and outputs wit-
hin the circuit or system discussed. For conven-
ience, the procedures are described referring to a
single input-output chain, but must be followed
for all inputs and outputs of the circuit or system
being discussed, unless expressing stated other-
wise

All resistors are 1/4 watt, 5%, unless marked oth-
erwise. Resistor values are in ohms (k=1.000
ohms, M=1,000,000 ohms).

All capacitor values are in microfarads (pzpicofar-
ads).

A Parts marked with this sign are safety critical
components. They must always be replaced with
identical components-refer to the TEAC Parts List
and ensure exact replacement.

0 dB is referenced to 1 V in this manual unless
otherwise specified.

PC boards shown viewed from component side.

73

Page 2

1. LEVEL SETTING AND OPERATION CHECK

1-1. MIC IN -> PRE OUT (ACCESS SEND)

1. In the MIC IN connector, pin #3 is "hot", pin
#2 iscold", and pin #1 is shield".

2. Remove the jumper between the ACCESS SEND
and the RCV jacks for the channel being tested.

3. Plug an AC voltmeter into the PRE OUT or the
ACCESS SEND jack. The voltmeter should have
an input impedance of 50k ohms or more, input
capacitance less than ZOpF, a sensitivity selec-
tor switch and a maximum sensitivity of -90dB.

4. Set the front panel control for the channel being
tested as follows:

INPUT select switch : Set to MIC

MIC ATT switch : Set to 0

PHASE switch : Set to NORMAL
PHANTOM power switch: Set to OFF

5. Apply a 1kHz, -60dB (1 mV) signal to the MIC IN
connector on the back panel.

6. Adjust the MIC TRIM for a -10dB (316mV) read-
ing on the voltmeter. After this adjustment, the
MIC TRIM knob should be between 6.5 and 7.5
(between 1 : 30 and 2 : 30 o'clock).

raw l
Min
RM:

1

m.
LOAD

TRIM Knob
at normal setting

ALL MIC INPUTS ARE USED

WITH TRANSFORWRLESS TYPE BALANCE CIRCUIT. 600 OHMS.

CHAN._| ,

PHANTOM I C
POWER

CHAN. 2

M_C ATT

74

a to EUWTAPE
SELEC

7. If there is any malfunction, refer to the preampli-
fier (01 N 04 and U1) circuit schematic and
check the IC and transistor voltages and signal
levels.

8. Check all remaining channels in the same man»
her.

9 Set the MIC ATT switch to 30 and apply a 1kH2,
-30 dB (31 .6mV) signal to the MIC IN connector
on the back panel.

10. If the output is not -10dB, readjust the MIC
TRIM for a -10dB (316mV) reading on the volt-
meter. After this adjustment, the MIC TRIM
knob should be between 6.5 and 7. 5.

11. Measurement of S/N ratio, T. H. D.:

S/N ratio . More than 70dB (using
"A" weighted network)
or : 68dB (measured with a
bandpass filter, ZOHz
~ 20kHz)

or : Morethan 62dB, on an

AC voltmeter with a
100kHz or greater
bandwidth

: Less than 0.025% (at
1kH2, ZOdB above
nominal input level
measured with 30kHz
L.P.F.)

12. Set the PHASE switch to REVERSE.

Confirm that the phase of the output signal is in-
verted using an oscilloscope connected to the
PRE OUT jack.

T.H.D.

m Ems:

T SWITCH