Kenwood Basic M1 User manual

Kenwood BASIC M1 Audio Power Amplifier
Current Source Modification
www.drmaudioht.com

Kenwood BASIC M1 Current Source Modification
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! ! ! IMPORTANT ! ! !
This document intended for use by individuals experienced in electronics
repair.
! ! ! CAUTION ! ! !
The power supply in the Kenwood BASIC M1 amplifier
generates hazardous voltages that can cause electrical
shock.
The user of this document assumes all responsibility to follow safe
practices to prevent shock or injury while making the modifications
described in the document.
This document is the property of Daniel R. Mattis and may not be
copied, reproduced or distributed in part or entirety

Kenwood BASIC M1 Current Source Modification
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Overview
The Kenwood BASIC M1 uses a 56K resistor for a current source in the differential input stage.
The configuration is known as a long-tailed pair. In the ideal world, the resistance of the current
source resistor would be infinite. This is impractical, so an active current source with very high
impedance is used in most high end amplifiers.
Looking at the Kenwood Basic M2 schematic, we can see that Q3 and the related components
are used to implement a current source for the input differential amplifier:
This modification replaces the 56K current source resistor in the Basic M1 input differential
stage with a LM334Z 3 terminal adjustable current source device. The LM334Z is specified to
operate between 1 uA and 10 mA, which is well in the range of the 350 uA operating current
with the 56K resistor.

Kenwood BASIC M1 Current Source Modification
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The V+ and V- leads of the LM334 are formed to have the same spacing as the original
56K resistor. The center lead is formed to allow a 150 ohm resistor to connect from the
R lead to the V- lead. This resistor sets the current of the LM334 current source.

Kenwood BASIC M1 Current Source Modification
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Parts List
The following table contains a list of parts included in the kit.
ID Value Part #
IC17 LM334z LM334z
IC18 LM334z LM334z
R17 150 ohm ¼ watt 5% resistor 3011151
R18 150 ohm ¼ watt 5% resistor 3011151
VR3 500 ohm linear trim pot horizontal – Hex adjust 3090501
VR4 500 ohm linear trim pot horizontal – Hex adjust 3090501
WR1 2mm allen wrench 9001101

Kenwood BASIC M1 Current Source Modification
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Instructions
Step 1 - Form the leads of the two LM334Z devices that are in a TO92 plastic case and solder a 150 1/4
watt carbon film resistor to the formed leads as shown in this photo:
Note that the resistor is connected between the center lead, which is pin 2 of the device and pin 3. You
can form the leads as you desire, just maintain the lead spacing of .400 inches so the devices will fit in
the holes in the boards that were originally occupied by R17 and R18.
Step 2 - Remove R17 and R18. This photo shows the top side of the amplifier pc board assembly with
the original current source resistor R17 removed:

Kenwood BASIC M1 Current Source Modification
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This photo shows the top side of the amplifier pc board assembly with the original current source resistor
R18 removed:
This photo shows the bottom side of the amplifier pc board assembly with the original current source
resistor R17 removed:

Kenwood BASIC M1 Current Source Modification
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This photo shows the bottom side of the amplifier pc board assembly with the original current source
resistor R18 removed:
The first two set of photos are intended to assist in location of the resistors R17 and R18.
Step 3 - Insert the pre-assembled LM334Z devices with the 150 ohm resistors in the original location of
R17 and R18.
This photo shows the top side of the amplifier pc board assembly with the original current source resistor
R18 replaced by the LM334Z:

Kenwood BASIC M1 Current Source Modification
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This photo shows the top side of the amplifier pc board assembly with the original current source resistor
R17 replaced by the LM334Z:
IMPORTANT!
Notice that the orientation of the devices is different for the left and right channel.
This is due to the fact that the pc board layout is mirrored for the left and right channels. This becomes
evident upon close examination, as most of the semiconductors are orientated 180 degrees between the
two channels.
Idle Current Potentiomenters
Replace the open frame idle current pots VR3 and VR4 with the new sealed pots in the kit.

Kenwood BASIC M1 Current Source Modification
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Testing
Congratulations! At this point, you have installed current source modification in your Kenwood
Basic M1 amplifier! Before using the amplifier, measure voltages as defined in the service
manual schematic to verify that the amplifier is operating properly.
IMPORTANT
While the Kenwood Basic M1 has speaker protection, it is imperative that speakers are not
connected to the amplifier until it has been verified that the amplifier is operating correctly.
Measure the voltage at the speaker terminals and verify that it is approximately 0 volts. There
may be a negative or positive voltage present that could be as much as 100 millivolts or more,
but it should not exceed 200 millivolts.
Once it is determined that he amplifier is operating correctly, connect the speakers and power
the amplifier up. Allow time for the amplifier to heat up before proceeding.
After the amplifier has reached operation temperature, follow the instructions in the service
manual to adjust the idle current, using the 2 mm allen wrench supplied in the kit.
Support & Assistance
Please send requests for assistance to
[email protected]
I will answer emails within 24 hours or sooner if possible.
I hope that you enjoy your Kenwood Basic amplifier with the current source modification
installed as much as I have!
Thanks!
Dan Mattis