Infinity Infinitesimal IV, Service Manual

TROUBLESHOOTING (cont’d) 

 
B.     If all voltages are normal: 

Check for +25 to 30 volts across relay terminals. (See drawing) If voltage is present, replace 
relay. If little or no voltage is present, check for+15 volts on pin I of IC3 (LF 347N on PC-1). 
Check for +2.9 to 3 volts on pin 2. 
Check for +12.7 volts on pin 3. 
Use main filter caps, (common point), or black power supply wire for ground. 

If

any

of

the

above

voltages

are

abnormal:

Check DC voltage across R4 (33K

Ω

). (See drawing for location) Less than 50mV should be 

present. If higher, relay will not energize.   DC is coming through the amplifier section.   Check 
and replace semiconductors as necessary. 

If

there

is

negligible

voltage

across

R4,

 (normal condition) replace LF 347N IC on PC-1. 

III RELAY

CLOSING

NORMALLY,

BUT

NO

OUTPUT

OR

DISTORTION

EXISTS

 
A.  Re-read or follow the instructions mentioned on the first page concerning other issues which 

could result in no output: (bad woofer or mis-wired high level input). 

B.  If those items check out, follow the signal path with schematic and DMM (oscilloscope is usually 

not required), checking for these approximate voltages at the points indicated below.  Use main 
filter caps, (common point), or black power supply wire for ground: 

TEST CONDITIONS: 

Signal: l00mV @100 Hz connected to the low level input with a Y-cable (both right & left 
inputs).  Both control pots (level and frequency) Full Clockwise. 
Turn UUT on. 

No

speaker

load

should

be

connected

at

this

time.

ICI (LF347N) pin 7:        388 mV 
ICI (LF347N) pin 1:        392 mV 

Check C16 (.15

µ

f Capacitor). One or both leads must be unsoldered for proper reading. Replace if 

defective. 

IC2 (LF353N) pin 7:        380 mV 
IC3 (LF347N) pin 14:       138 mV 

R53 (See drawing):         117mV 
R48 (See drawing):        2.7 volts 

Loss of signal at this point means Relay contacts are defective. Replace if necessary. 

Nominal Output at speaker cable:      2.7 volts 

Note: When a loss of signal occurs between amplifier stages of different IC's, it's easy to be fooled 
when one amplifier stage is shorted out, "loading down" the previous section of another IC. 
 

NOMINAL POT POSITIONS ON MAIN AMP BOARD — (VR3 - Full CW), (VR4 - Half-way 
position) 
 
Another

procedure

for

confining

the

problem

to

one

board

or

the

other

is

as

follows:

1)  Remove both IC's on smaller filter board (if they are in sockets), if not unsolder 5 - 

conductor ribbon attaching PC-1 to PC-2. 

2)  Follow "TEST CONDITIONS" above regarding signal injection except lead should be 

connected to C19 (See drawing for location). 

3)  Turn UUT on.  Output at speaker cable should be .7 - .8 VAC.  If output is normal, then 

problem or loss of signal is on smaller filter board. 

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