AUDIO TELEX DCM250 Operating and installation instructions

Australian Monitor
1 Clyde Street, Silverwater NSW 2128 Australia
[phone]
www.australianmonitor.com.au
SERVICE INFORMATION
AUDIO TELEX
DCM250
POWER AMPLIFIER
CONTENTS:
OPERATION MANUAL
CIRCUIT DESCRIPTION
SET-UP / TEST PROCEDURE
SCHEMATIC DIAGRAMS
PCB OVERLAYS
COMPONENT LISTS

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DCM250,250 Watt Power Amplifier
Product Description
The DCM250 is a 250 watt power amplifier in a two rack unit (2RLI) chassis suitable for table or direct 19" rack
mounting. The DCM250 has outputs for 100 volts & 4 ohms. It has a balanced input of 10K ohms. The DCM250 will operate
fron 240 VAC @ 50 IIz or 110 VAC @ 60112 (not user selectable, internal, factory adjushent only, specify at the time of
ordering) or 24 YDC' and will meet it's full performance specification on either voltage supply. The DCM250 also features a DC
battery trickle charge facility, autosensing frn cooling, plus overload, short circuit and over t€mperature protection. The
maximum recommended load for the DCM250 is 40 ohms.
lnitial Set Up
There is an unlabelled screwdriver adjustable output level control located centrally on the front panel of the DCM250.
Turning this control in the clockwise direction will increase the power output, turning this control in a counter-clockwise
direction will reduce the power output. The factory default setting for this control is such that a I volt input will give a 100 volt
output.
Front Panel Controls
Output Level: The output level control is unlabelled recessed (scewdriver adjustable) and is located in the centre of the
front panel, just to the right of the DCM Series logo. Turning the c.ontrol clockwise will increase the ou@ut of the DCM250
towards it's maximum output level while turning the control counter-clockwise will decrease the output level. Adjust this control
for the desired output level dependittg on the level of the input sigrral (from a mixer or other signal source). The factory default
setting fc this control is such that a I volt input will give a 100 volt output.
Powef Switch: The rocker switch located in the front ce,ntre of the panel turns AC power on to the DCM250. Rocking the
power switch to the right to turns the AC powetr 'on'. When the AC power is 'on', a green LED will glow in the amplifier status
display window. Please note that this switch does not switch DC voltage. If a DC voltage supply is connected to the DCM250, the
amplifier will o,perate as stx)n as the connection is made, regardless of the position of the AC power switch. If both an AC and
DC voltage supply are connected and you rock the AC power switch to the 'off position, the DCM250 will automatically
continue to operate normally from the DC supply (and the 'mains failure' LED in the amplifier status display window will also
glow under these conditions; see the amplifier status display window section under Front Panel Controls later in this manual).
Cooling Fans (Air lntake): The cooling fans (two installed in the case of the DCM250) are temperature sensitive and
only switch on when the temperature of the DCM250 had reached a pr+determined range. The fans will stay on and only switch
off again once the te,mperature of the DCM250 has frllen below a pre-determined level. So, the hct that the fans axe not
operating at any time (and most noticeably to the operator at turn-on) does not mean that the amplifier is faulty in any way, just
that it is operating within a temp€rature range that does not need frn cooling for adequate heat dissipation. If the DCM250 is
operating continually at conservative levels and proper load conditions, it is possible that the cooling fans will not switch on at
any time during normal operation. Whe,n operating, the fans cause air flow from the front to the rear of the DCM250.
Continued noct page3

Amplifier Status Display WindOw: The status displaywindowhighlights the operating conditions of the DCM250.
Please refer to Figure #l on the back page ofthis instruction sheet with nlmerical reference as follows:
Hig! Temperature: This red LED glows if the fan has failed and the amplifier has been shut dov,rn by its, te,mperatwe
control circuitry. If this LED is glowing and the fans have not failed, it means that the amplifier is operating in an ambient
environment that it naturally too hot for fan cooling to make any difference to the temperaturi of the amplifierl
Power: This LED glows green if AC power is switched on to the DcM250. Note that this LED does not indicate the
pres€,lroe of a DC supply voltage
MainS FailUre: The LED glows red if there is a failure in the AC mains power supply. However, this LED will only
glow if there is a DC supply voltage present. If no DC supply voltage is present then this I.fb wiff not glow.
Output Level VU ileter: A 10 segment LED VU meter is provided to give an indication of the output signal level of
the DCM250 from -25 to +3 dB. For normal operation the LED's should oscillate in and out of the red zon!. If the LED's in
the red zone are lit continually, then the output level control (or the level of the input sigrral to the DCM250) should be
adjusted to reduce the output level. Too much output level can cause distortion and positt" Iu-"g" to the connected speaker
systern.
Rear Panel Connections
Please refer to Figure #2 on the back page of these instructions with numerical refere,nces as follows:
3 pin IEC' AC mains power inlet The operating voltage is 240 vAC @ 50 Hzor l l0 vAc @ 60 trz. The AC
power voltage level is not externally user selectable but is factory pre-set (via transformer selection). tre intet is equipped
with an inbuilt AC fuse holder fitted with a 6 Amp slow blow fuse plus one spare frrse. Power consumption is 400 VA. please
ensure that the mains power cord is disconnected before attempting to check or replace this fuse.
Output terminal strip . Reading from left to right these connections are as follows:
Common for low impedance
4 Ohms
Corrmon for constant voltage systems
100 volts
24 VDC power connectao[. The left side red post is the + (positive) terminal wtrile the right side black post is the
- (negative) terminal. The DC curr€nt drain is 15 Amps, maximum at full power. This socket also provides trickle clarge to a
DC battery supply (if connected) wtren the DCM250 is operated from AC mains power. The level of t i"kl" charge is 30b mA,
maximum.
Twin DC low voltage, fuse receptacles. Access the DC tuses is by turning the cap half a turn cormter-
clockwise with a screwdriver. The value of the fuses is l0 Amps slow blow. Please ensure that the AC power switch is in
the 'ofr position and that the mains power cord is disconnected before attempting to check or replace this frrse
!!PUt (& paralleloutput)ILR signal connection. The input to the DCM250 is transformer balanced @
lOK ohms' The pin configrnation of both sockets is as follows: pin #l-earth;-pn #Z-agtive (high, +); pin #3-active (low, -j.
The output socket is to allow the original input signal to be fed on to another amplifier. Rs ttrese two sockets are wired in
passive parallel, the failure of any one amplifier will not affect the signal flowing though that amplifier to another amplifier.

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DCM Series
Circuit Description
The DCM series are power amplifiers designed for commercial installations. They can be used for
either low impedance (4 ohm/8 ohm) or constant voltage line speakers (100v/70v). These amplifiers
can be mounted in a standard 19” equipment rack or they can be used on a shelf or table. The DCM
series feature line level input (with parallel output) and are normally used with mixers, mixer amplifiers
or other power amplifiers. The DCM series will operate from mains voltage or 24VDC. The DCM
series also feature a DC battery trickle charge facility, auto-sensing fan cooling, plus overload, short
circuit and over temperature protection.
Power Switch
This switch controls the switching of AC power to the amplifier. A blue ‘On’ LED will indicate
whether the amplifier is switched on or off. This switch will not switch DC power on or off in DC
operation. In DC operation mode, the amplifier is always on and the blue power LED will always be
illuminated. If both AC and DC voltage supply are connected and the AC power switch is in the off
position, the amplifier will continue to operate normally from the DC supply and the mains fail LED
will indicate.
Level Control
The output level control is located in the centre of the front panel. It is a fully recessed screwdriver
adjustable pot. Turning this pot cw will increase the gain of the amplifier. At maximum setting the
input sensitivity is 300mV. The amplifier ships from the factory with the sensitivity set to 1V.
Amplifier Status Display
This VU meter indicates the output level of the amplifier. The sensing for the circuit is taken on the
amplifier side of the output transformer. The 0dB level is referenced to 100V. This is an RMS meter,
not a peak meter.
Protect
The protect LED will illuminate when the amplifier cuts out because of either over current or high
temperature. The amplifier will switch back on after approx 4 sec for an over current trip. The amplifier
will switch back on after the amplifier has cooled to 60degC for a thermal trip.
Limiter
The limiter is a hard limiter with an attack time of about 1msec. It is defeatable by removing the jumper
on the solder side of the front pcb. This however is not recommended as voltage overload and speaker
transformer current saturation may cause the amplifier to cut out under normal program material.
Current Limit and Setup
Current limit is controlled by a microprocessor (PIC12C509A). The detection is done by sensing
voltage across the emitter resistors. Trimpot P1 on the front pcb is accessible through the hole in the
top right of the chassis return (only visible with the lid off). Turning the trimpot ccw will decrease the
point at which the amp cuts out ie the amp will cut out earlier. (P1 resistance is increased.)
To set the current limit:
1. Reset the trimpot P1 turning fully clockwise.
2. Connect the amplifier to half it’s minimum load (10ohm for DCM500, 20ohm for DCM250,
40ohm for DCM120).
3. Run an rms 1kHz sine wave into the amplifier and set the input level so that you read
425mVDC (DCM250/500) or 825mVDC (DCM120) across the emitter resistor, measuring the
side which has the higher current (measured as a voltage across the emitter resistors).
4. Turn the trimpot P1 ccw till the amplifier cuts out. The amplifier is set to the factory default.
Thermal and Fan control and Setup

The thermal cutout and fan is controlled by a microprocessor (PIC12C509A). The temperature is
sensed using a 10k@25degC NTC. The fan is normally off and turns on to full speed at 60degC. This
temperature is fixed and not adjustable. The thermal cutout temperature is set using the trimpot
accessible through the hole in the top left side of the chassis return (only visible with the lid off).
Turning the trimpot cw will decrease the point at which the amp cuts out ie the amp will cut out earlier.
Power Amp
The power amplifier is a push pull single supply amplifier driven by a class A transformer coupled
front end. The drive is provided by HEXFETs (RF9520/9530) into NPN BJTs (TIP35C). When
replacing the FETs it is recommended that you replace both FETs. The matching of these FETs
determines the balancing of the emitter currents in the output devices. For optimum performance the
emitter currents in each side should match to within 30% of each other.
Bias Setup
The amplifier is set with a bais setting of 1mV measured across the emitter resitors.
Bias is set using the trimpots located on the power pcbs on each side of the amplifier.
Turning the trimpots cw increases the bias.
If the HEXFETs have been replaced the resistor in series with the pot may need to be changed. Use a
lower value resistor if the bias cannot be turned off or a higher value if the bias cannot be turn on.
AC Power Inlet
The operating voltage is 230/240 VAC @ 50 Hz. The 3 pin IEC power inlet is located on the bottom
left of the rear panel and accepts a standard mains power lead fitted with an IEC connector. Before
plugging in a power lead, please check the rear panel of the amplifier to ensure that the voltage switch
is set correctly for your part of the world.
The inlet is equipped with an in-built AC fuse containing the rated fuse and a spare.
24 Volt DC Power Inlet
The DCM series feature optional 24VDC power to run off a battery back-up if required. This is
connected via the rear binding posts. The front panel Power Switch will not switch DC power ‘on’ or
‘off’ in DC operation. In this mode the amplifier is always ‘on’.
The trickle charge resistor across the diode is a 47ohm/5watt wire wound resistor. The maximum
trickle current is 300mA supplied from internal 35V rails.
230V/240V Slide Switch
The operating voltage of the amplifier is user selectable between 230V and 240V via a slide switch
located on the center of the rear panel. This switch should be set to match the AC voltage of your
country. The mains transformer is wound with a 230V winding plus a 10V winding internally
connected.
Speaker Output Terminal Strip
The screw terminals located on the top left of the rear panel allow access to the direct speaker outputs
of the amplifier. Reading from left to right the terminals are:
COM Common or “-” for low impedance speaker loads (4 or 8 ohms)
4 Positive “+” for 4 ohm speaker loads (use with common)
8 Positive “+” for 8 ohm speaker loads (use with common) DCM120 only
COM Common or “-” for 70v or100v speaker loads
70 Positive “+” for 70v line speaker loads (use with common) DCM120/500 only
100 Positive “+” for 100v line speaker loads (use with common)
Please ensure that the correct “Common” is used. Low impedance and 70/100v loads can be used
simultaneously but please pay careful attention to the overall speaker load.
Note: The minimum impedance (or maximum load) at 100 volt line should be no less than

DCM120 – 80 ohms
DCM250 – 40 ohms
DCM500 – 20 ohms
XLR Audio Input and Parallel Output
The DCM series includes both male and female 3 pin XLR connectors per channel. While the female is
normally used as the input to the amplifier, both XLR’s are connected in parallel so either will work.
The XLR’s inputs are transformer balanced and wired as:
Pin 1: Shield.
Pin 2: Hot, +, Positive
Pin 3: Cold, -, Negative
Fuse Sizes
(DCM120)
Mains: 230 VAC 4 Amperes Slow Blow HRC 20x5mm
DC: 10 Amperes Slow Blow HRC 20x5mm
(DCM250)
Mains: 230 VAC 6.3 Amperes Slow Blow 20x5mm
DC: 2 x 10 Amperes Slow Blow HRC 3AG
(DCM500)
Mains: 230 VAC 10 Amperes Slow Blow HRC 20x5mm
DC: 2 x 35 Amperes Slow Blow 3AG

TESTING PROCEDURE OF DCM 120/DCM 250/DCM 500
I. PRE-TESTING (of complete sets).
•Check
1.1 All screw for tightness (Bridge rectifier and transistor bolts)
1.2 Earth Connection for good contact (solder and crimpling)
1.3 This setup has signal input to the Amplifier through male (XLR)
1.4 Check with Multimeter that there is a DC resistance of about 250 Ωbetween
Pin2 & Pin3 of each of XLR’s. Also between (Pin 1 & Pin 3) and (Pin1 & Pin2).
There should be very high resistance. (IE: no reading).
2. Electrical Check
2.1 Fuse Check:
DCM 120 DCM 250 DCM 500
Mains fuse: 4AT 6.3 AT 10 AT
DC fuse: 10AT (x1) 10AT (x2) 35AT (x2)
2.2 Connect the Amplifier to the setup (Variac voltage = 0V) set all presets on front
board (only) fully clockwise, voltage selector switch to 230V
2.3 Slowly increase the input voltage to 230V, keep watching the input current
should not exceed 0.1A for DCM-120, 0.1A for DCM-250, 0.1A for DCM 500.
2.4 Check and reset if necessary all emitter resistor voltages with the help of preset.
(Each emitter resistor voltage should be between 0.5 mV to 0.8 mV )
2.5 Check DC voltage Main rail = 33V
7815 input = 30V
7815 output = 15.5V
2.6 Give input signal of 500 mV to get outputs as follows (@ 4Ωoutput load)/ 22V
for DCM 120, 32VAC for DCM250, 44.7VAC for DCM 500. Check 100V O/p
at 100vV line, Remove the input signal

II. FINAL TESTING
(This setup should have signal input to Amplifier through female (XLR)
(The limiter link should be out of circuit initially.)
1. Connect the Amplifier to the setup, set voltage selector switch to 240V
2. Switch ON the set to 240VAC.
3. Slowly increase the input signal (of 1kHz) keep watching the 24dB LED, it should
glow at approx 9V output. Increase the input signal to get 70VAC output.
4. Check Dc voltage of all emitter resistors, Minimum value should be within 30% of
the maximum value.
5. Slowly increase the input signal, keep watching the 0db LED, it should glow at 100V
±5V output voltage.
6. Set 100V 1kHz as 0db reference. Change frequency to 10kHz check dB level drop. It
should be 2.5dB ±0.5dB.
7. Change the frequency to 1kHz, reduce signal level to get 10VAC output. Half the
output load.
8. Overload setting: - Check the DC voltage at the emitter resistors having the
maximum voltage value. Increase input signal to get 820mV for DCM 120, 425mV
(for DCM250 & DCM 500). Turn preset (P2) anticlockwise such that it just mutes
the output signal and signal returns back slowly after 2 seconds.
9. Reduce the signal & re check whether the signal mutes at the corresponding above
stated voltages.
10. Again make the output load to original full value. Turn volume preset fully
anticlockwise, set input signal strength to 1V, set volume preset clockwise to get
100V output.
11. Set input signal strength to get output 110V VAC. Insert limiter link, the signal
should reduce to 100V ±5V.
12. Remove the input signal and check noise. It should measure less than 25mV.
III. THERMAL & SOAK TEST
1. Connect the Amplifier to the setup : Output load = 4ΩOutput Voltage: (DCM 120):
14V. (DCM 250):20V. (DCM 500): 40V
2. Set the Amplifier thermal cut off temperature at around 105°C with the help of preset
P4.
3. Leave the unit “ON” (with lid fixed, if possible) for 24 hours.
IV. Sound Test/Listening test.
1. Switch On the set. Check for any switch on thump.
2. Connect CD player to the input, listen for irregularities if any.
3. Switch off the set check for switch off noises.