McMartin LT-80A User manual

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Transistor Audio / PA Amplifier
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SPECIFICATIO NS
Power Output ...... , . , . .. . 8 watts rms @ 1000 cps
12 watts music
power
20 watts peak power
Frequency Response .... . . . 30 to 1.5 ,000 cps ±2 <lb
20 to 20,000 cps ±3 db
Distortion . .. . ....... . . .... Less than l '< 50 to 15,000 cps @ l watt
Less than l % @ 1000 cps @ 8 watts
Il um & 1oise
Microphone . . .
-60 db below 8 watts
Program . . .
........ .... -7 0 <lb below 8 watts
Phono (with P
II -6A) ... .. -60 db below 8 watts
Inputs
Microphone . .
......... . . Hi-Z; 150 K ohms, .008 v for 8 watts
Program . . . . . . ........ Bridging; 10 K ohms, 0.8 v for 8 watts
Phono (with PH-6A)
..... Magnetic, ceramic, crystal cartridge
600
ohm (with MT-6) ..... Balanced: -10 clbm for 8 watts
Tm~er (w
ilh PII-7) ....... HiZ ; 270 K ohms, + 5 db gain increase
Outputs . . ......... . .... 4, 8, 16, 150 ohms; 70.7 v/ 600 ohms
Controls
... . ... . ......... Microphone Cain
Program Cain
Tone (cut to -20 db @ 10 ,000 cps)
Power Of\'-OFF switch
Semi-conductors
.... Q-1 2N5Q8;T2391
Q-2, Q-3 2 1508/ SA-100
Q-4 2 ll 371'R69 1
Q-5 2 1
508/ Cll36
Q-6 2Nll02
Q -1, Q-8 2Nl.501
SR-1, SR-2 1Nl693
Power Supply . . . . .
.. . .. 120 VAC, 50 60 cps, fused, 18 watts max
Ambient Te
mperatur e . . .. Full pC'rformance to 130°F
Dim ensions & Finish .... .. 9'' w x 7" d x 4½" h, Black & Natural Cray
Shi pping ,veight ......... 6 lbs.
No t e: • Th,• program Bridginp; Tnput mny be com1<•t • ted directly to n low im-
pedance
or telephone line without lo, ~ of )!nin or re~vonsc diaractcr-
i~tics.
•
The tone con tr ol t!oC'.s not affect the mi cropho ne c ir cuit.
WARRANTY
Md,Iartin Broadcast and Audio Pr od ucts arc warranted to be
free from
def ec ts in workmanship - FOREVER.
At our discretion, we will exchange or r epair any defective
un
it or component, at an y time, without charge. Material and com-
ponents are
guaranteed for a minimum period of 90 days from th e
date of original purchase. Transportation charges must be pr e paid
on eq uipment returne<l for warranty service.
This
warranty does not extend to any of ou r products which
have
been subjccte<l to misuse, neglect, accidents, incorrect wiring
not our own, improper insta ll ation, or to use in violation of t he
instructions fu rnished by us; nor to units that have been altered
outsi
de our factory.

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CONNEC TIONS
Care should be taken to see that input and output connections
arc made properly and sec ur el y. Adequate shi ck l ed ethic should be
used especially with a microphon e or phonograph. Pr eferably use
spade lug s for connectio n to all sc r ew terminals. To provide maxi-
mum ve r sa tilit y consiste nt with minimum cost, the c:ircuib·v of the
LT- 80A is arranged so that only one (1) of fom (4) available t>rogram
inputs can be used simulta n eous ly with the microphone input ,\t
~rny one time.
l ) 10 Kohm Brirl gin!{ Jnput : Cunnn·liun is mndc to the 1:ihck·cl 'l 'rcw tl · nninnl
anoss any low or mnlium im1wtl:111ct• ~ourcc. \Vilh u \'Cry high impedance
tht·re may be somL' lou<linl,( cffoct. In thii. t·asL', me thL· 1'11-7°.
2) (
JOO ohm B ala n ce d lnput: For GOO ohms , insert the 1ilu!(-i11 1'tod(•l ~IT-6°
transformer into the 9-pin ,oeket and connt·ct the li111• to tilt' labeled screw
ll'rminals. Either sill<' m ny be grounded if unbt1lnnt·L•d line is desired.
3)
Phon ograp h: Fnr n m:ignetie, ceramic, or crystal phono input, in,erl tlw
plug-in l\loclcl PH-6AO Phono Preamp into the 9-pin ~ockt•t an<l connect ,I
~hielded <:able from the carlTid~e to the phono jack.
4) Hi Z ( Tuner ): A 5 db l,(nin incrPasc and 270 K ohm input impedance i,
available by inscrlinl,( the Pll-7° Preamp into the 9-pin ~ocket. The input
connection is then made by shielded cable to the phono jack.
0
Optionally available
l\Iicrophone
Hi gh Impedan ce : T he microphone ioput circu it is factory wired
for operation w ith any high imp edance microphone. Th e micro-
phone connecto r is the shorting type.
Low Impedance: If operation wit h a low impedance micro-
phone is desired, modi6cation at the input c:ircuitry is simple. R efer
tu the schemat ic indicating the "Changes for Low Z ~l icrophonc.''
OPERATION
A specia l feedback circuit is incorporated in the amp lifier to
protect the transistors in case the speaker loud is acci<lentally re-
moved or sho rt e d.
The power tran sistors are externally fused for protectio n
against overload.
:o warm-up time is necessary. Sufficient space should be a l-
lowed around the amplifier for air circulation. After volume and
tone controls are set to the desired levels. the knobs may he re-
moved and replaced with the plastic Contro l Guards (suppli ed with
your amplilier) to prevent tampl'l'ing.
SERVICE
l\ f aximum power ou tput with a sine wave signa l may be checked
at any freq11cncy 11p to 10.000 cps. A sustained tone above approxi-
mate ly 5000 c: ps should not be applied for more than a few minutes
or excessive transistor h eating will result. This cond ition will blow
th e prot <'c ti ve fuse (r ear chassis). ll igh frequencies at l ow p ower
output will hav e no adverse effect. The LT -80A will supp ly full
power co ntinuousl y with normal program material.
Dama ge to tran sistors may result if any wiring is shor t ed wh ile
servicing or circuit h·acing. Avoid removing transistors from thefr
sockets while th e power is turn ed on. lf r ep la cemen t of ei th er power
transistor is necessar y, be sure to coat b ot h sides of the mica insu-
l ator with Silicone Jel. A powe r line fuse is lo ca t ed und er the chassis
and is accessible by rem oving th e bottom cove r.

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SCHEMATIC
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