DYNACO Stereo 120 Use and care manual

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UPDATING THE
DYNACO STEREO 120
SOLID STATE
POWER AMPLIFIER
© 2011, Daniel M. Joffe
All rights reserved
Revision 10.0

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Table of Figures .................................................................................................................. 2
Section 1: About This Manual ............................................................................................ 4
Section 1: About This Manual ............................................................................................ 4
Who Should Attempt this Project? ................................................................................. 4
Tools You’ll Need........................................................................................................... 4
Project Overview ............................................................................................................ 4
Important Safety Notes ................................................................................................... 5
Section 2: Saving the Stuff that Should Be Saved .............................................................. 6
Opening Up the Amplifier .............................................................................................. 6
Cutting the wires on the old amplifier modules .............................................................. 6
Removing the LEFT channel module ......................................................................... 6
Removing the RIGHT channel module ...................................................................... 7
Removing C7 RIGHT ............................................................................................... 11
Removing C7 LEFT.................................................................................................. 11
Removing the Output Zobel Networks ..................................................................... 11
Salvaging the Heat Sinks from the Amplifier Modules ................................................ 13
Section 3: Checking the Stereo 120 Power Supply .......................................................... 13
Prepare and Connect the Test Load .............................................................................. 13
Test the Power Supply .................................................................................................. 14
Just In Case – Trouble Shooting ................................................................................... 14
The Voltage is Much More than 75.6 volts .................................................................. 14
The Voltage is Much Less than 68.4 volts.................................................................... 14
There’s No Voltage at All............................................................................................. 15
Section 4: Building the Updated Circuit Boards............................................................... 16
Install the ¼ Watt Resistors .......................................................................................... 16
Install the Small Capacitors and the Diodes ................................................................. 18
Last Capacitors and the Transistors .............................................................................. 19
Winding the Output Inductor ........................................................................................ 20
Solder the LM3886 into the Board ............................................................................... 22
Final Inspection of the Circuit Board............................................................................ 23
Mounting the Updated Amplifier Board to the Heat Sink ............................................ 24
Section 5: Wiring In the Updated Amplifier Modules...................................................... 26
Building and Installing the Power Wiring Harness ...................................................... 26
Building and Installing the Ground Wiring Harness .................................................... 27
Wiring the Inputs .......................................................................................................... 27
Wiring the Outputs........................................................................................................ 28
Making the Speaker Binding Posts User Friendly ........................................................ 28
Section 6: Testing the Completed Amplifier .................................................................... 29
Test #1........................................................................................................................... 29
Test #2........................................................................................................................... 29
Test #3........................................................................................................................... 30
Test #4........................................................................................................................... 30
Resistor Color Code ...................................................................................................... 32
Table of Figures
Figure 1-Location of the four screws that hold the cover to the base ................................. 6

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Figure 2-Location diagram for cutting wires ...................................................................... 9
Figure 3-After the Amplifier Modules have been removed.............................................. 10
Figure 4- Removal of components is completed .............................................................. 12
Figure 5- Separating heat sinks and circuit boards ........................................................... 13
Figure 6-Connecting a test load to the power supply ....................................................... 14
Figure 7-The first four resistors are installed with leads bent on the back side, but not yet
soldered. ............................................................................................................................ 16
Figure 8-leads soldered and clipped ................................................................................. 17
Figure 9-showing LED cathode orientation...................................................................... 19
Figure 10-Starting to wind the output inductor................................................................. 21
Figure 11-finishing step for the output inductor ............................................................... 21
Figure 12-LM3886 installation step one, just the corner pins are soldered ...................... 22
Figure 13-The LM3886 sits up off the board to avoid shorts between the front and back
rows of pins. The body should be at right angles to the circuit board .............................. 22
Figure 14-attaching first bracket to the board ................................................................... 24
Figure 15-both brackets attached to the board .................................................................. 24
Figure 16-circuit board mounted to heat sink ................................................................... 25
Figure 17-a completed amplifier module.......................................................................... 25
Figure 18-wiring harness .................................................................................................. 26
Figure 19-Ground wiring harness ..................................................................................... 27
Figure 20-Sometimes the input connector needs to be bent to clear the PCB .................. 28
Figure 21-Schematic of amplifer module ......................................................................... 31
Figure 22-demonstrating the resistor color code .............................................................. 32

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Section 1: About This Manual
This manual gives you all the information you need to bring your Solid State Dynaco
Stereo 120 Power Amplifier up to date using UpdateMyDynaco replacement modules.
The only part of your Stereo 120 that really needs to work is the power supply. With this
kit, you’ll build good sounding, highly reliable amplifiers to replace the original amplifier
modules.
Who Should Attempt this Project?
You can build this kit and update your Dynaco Stereo 120 if you can:
1. solder (using normal rosin core solder and a soldering iron).
2. use simple hand tools like screwdrivers, wire cutters, and pliers.
3. read and follow directions.
It helps if you:
1. know a bit about electronics, or
2. have a friend who knows a bit about electronics
3. can get to YouTube to watch a few helpful videos about the assembly process (not
available as of this edition of the manual).
Tools You’ll Need
You’ll need the following tools to update your Stereo 120:
1. flat blade screwdriver for #6 screws
2. Phillips screwdriver (#2)
3. pliers or nut drivers suitable for #6 hardware (5/16” nut driver or hex wrench)
4. needle nose pliers (helpful, but not strictly necessary)
5. pencil type soldering iron of 25 to 50 Watts (no huge honking soldering guns or
blowtorches)
6. multi-meter (helpful, but not strictly necessary)
Project Overview
The project consists of the following steps:
1. Removing the old amplifier modules, saving some parts for re-use.
2. Checking out the old Dynaco Stereo 120 power supply.
3. Assembling the updated amplifier module PC Boards.
4. Mounting the updated amplifier modules to the heat sinks.
5. Wiring in the updated amplifier modules.

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Important Safety Notes
By purchasing this kit, you have agreed to hold UpdateMyDynaco harmless for any
injuries you may receive in its assembly and/or use. To prevent injuries:
Wear safety glasses when soldering to prevent eye injuries.
Always unplug the power before working on the amplifier.
Large capacitors hold lots of energy for a long time. Before you put your hands
into the amplifier:
oPull the AC plug!
oWait 1 full minute for the capacitors to discharge!
Remove jewelry and rings from your hands and wrists, or anything that might
dangle into the amplifier.
If working in the amplifier, keep one hand in your pocket, especially if you’re
near the power supply or power supply wires. This can prevent serious shocks.
Build with a buddy nearby. If you’ve ignored all the previous advice, they can
dial 911 or get you to the hospital.

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Section 2: Saving the Stuff that Should Be Saved
Opening Up the Amplifier
1. Make sure the amplifier is unplugged!
2. Remove the four screws along the outside edge of the bottom that hold the cover
in place (see Figure 1).
Figure 1-Location of the four screws that hold the cover to the base
3. Holding both the top and bottom of the amplifier, flip it over.
4. Lift the perforated metal top off of the amplifier.
Cutting the wires on the old amplifier modules
Make your life easier by following this advice:
1. Print out a big copy of the pictorial diagram (Figure 2).
2. Set the print out near the amp.
3. Align the picture and the amp in the same orientation.
4. Verify twice before cutting once!
Removing the LEFT channel module
To remove the LEFT channel module, cut the following wires in the following places:
Step
Which Wire
Where to Cut
Done
1
The wire from PC-15 eyelet 5 to LEFT
PC-14 eyelet 10
Close to PC-15 eyelet 5
2
The wire from C12 + terminal to C of
LEFT Q5.
Close to C of LEFT Q5
3
The wire from LEFT PC-14 eyelet 9 to
RIGHT PC-14 eyelet 9.
Close to LEFT PC-14
eyelet 9.
4
The wire from C7 LEFT + terminal to
LEFT PC-14 eyelet 8.
Close to LEFT PC-14
eyelet 8

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5
The wire from C7 LEFT - terminal to
LEFT PC-14 eyelet 11.
Close to LEFT PC-14
eyelet 11
6
The wire from the LEFT input socket to
LEFT PC-14 pin 12 (typically a green
wire).
Close to LEFT PC-14
eyelet 12
7
The wire from the LEFT input socket
ground to LEFT PC-14 pin 13 (this is
typically a black wire).
Close to LEFT PC-14
eyelet 13
8
The wire from LEFT Q6 E terminal to
LEFT Speaker ground terminal.
Close to the LEFT Q6 E
terminal. This may be
difficult. If so, don’t worry
and read on!
You may not be able to reach your cutters to LEFT Q6 E terminal. Don’t worry. We’ll
make it easier now, while we check your work to this point. With the exception of this
one wire, the LEFT power amplifier module is free to lift out as soon as you remove the
two #6 screws that hold the LEFT heat sink to the chassis. Unscrew the screws, lift out
the module, and cut the wire that you couldn’t reach in the previous step.
Save the LEFT module and mounting hardware. We will re-use the heat-sink and some
other bits as described in “Salvaging the heat sinks from the amplifier modules”.
Removing the RIGHT channel module
To remove the RIGHT channel module, cut the following wires in the following places:
Step
Which Wire
Where to Cut
Done
1
The wire from C12 + terminal
to C of RIGHT Q5.
Close to C of RIGHT Q5
2
The wire from C11 + terminal
to RIGHT PC-14 eyelet 10.
One inch from C11 + terminal
3
The wire from C7 RIGHT +
terminal to RIGHT PC-14
eyelet 8.
Close to RIGHT PC-14 eyelet 8
4
The wire from C7 RIGHT -
terminal to RIGHT PC-14
eyelet 11.
Close to RIGHT PC-14 eyelet 11
5
The wire from the RIGHT input
socket to RIGHT PC-14 pin 12
(typically a black wire).
Close to RIGHT PC-14 eyelet 12
6
The wire from the RIGHT input
socket ground to RIGHT PC-14
pin 13 (this is typically a green
wire).
Close to RIGHT PC-14 eyelet 13
7
The wire from RIGHT Q6 E
terminal to RIGHT Speaker
ground terminal.
Close to the RIGHT Q6 E
terminal. This may be difficult. If
so, don’t worry and read on!

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Similar to before, you may not be able to reach your cutters to RIGHT Q6 E terminal.
Don’t worry. We’ll make it easier now, while we check your work to this point. With the
exception of this one wire, the RIGHT power amplifier module is free to lift out as soon
as you remove the two #6 screws that hold the RIGHT heat sink to the chass is. Unscrew
the screws, lift out the module, and cut the wire that you couldn’t reach in the previous
step.
Save the RIGHT module and mounting hardware. We will re-use the heat-sink and some
of the other bits as described in “Salvaging the heat sinks from the amplifier modules”.

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Figure 2-Location diagram for cutting wires

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Figure 3-After the Amplifier Modules have been removed

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Removing C7 RIGHT
1. Remove the 3 sets of #6 hardware that hold C7 RIGHT to the bottom of the
chassis.
2. Unroll one foot of the wire that’s wrapped around C7 RIGHT and connects to the
RIGHT RED Binding post.
3. Cut this wire near C7 RIGHT so that about 1 foot of wire remains connected to
the RIGHT RED Binding post.
Removing C7 LEFT
4. Remove the 3 sets of #6 hardware that hold C7 LEFT to the bottom of the chassis.
5. Unroll one foot of the wire that’s wrapped around C7 LEFT and connects to the
LEFT RED Binding post.
6. Cut this wire near C7 LEFT so that about 1 foot of wire remains connected to the
LEFT RED Binding post.
Removing the Output Zobel Networks
1. Clip the lead of the 0.1 uF capacitor that connects to the RIGHT RED binding
post, near the RIGHT RED Binding post.
2. Clip the lead of the 4.7 Ohm resistor that connects to the RIGHT BLACK binding
post, near the RIGHT BLACK binding post.
3. Clip the lead of the 0.1 uF capacitor that connects to the LEFT RED binding post,
near the LEFT RED Binding post.
4. Clip the lead of the 4.7 Ohm resistor that connects to the LEFT BLACK binding
post, near the LEFT BLACK binding post.
5. Remove the terminal strip and the two attached resistors and capacitors.
Figure 4 shows what the amplifier should look like when you’ve completed this step.

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Figure 4- Removal of components is completed

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Salvaging the Heat Sinks from the Amplifier Modules
Figure 5- Separating heat sinks and circuit boards
Remove the four numbered nuts, bolts, and standoffs. Cut the wires that connect the
power transistors to the circuit board. Remove the four screws that hold the power
transistors to the heat sink. Remove the power transistors. Feel free to save ever ything for
a later project. Remove the Mica insulators from the heat sink. After 40 years, they may
be pretty stuck on...a single-edged razor slid between the insulator and the heat sink may
be the easiest way to separate the two.
Save the old heat-sinks! They’re part of the new assembly! Clean off the old thermal
compound using Isopropyl Alcohol and paper towels. The best kind of isopropyl alcohol
for the job is 90%. The typical 70% is ok, but will take more scrubbing. One motorhead
who assembled the kit reports that carburetor cleaner is very effective. When you use
Isopropyl alcohol or any other solvent, remember the following:
1. It’s flammable! Keep it away from your soldering iron and don’t smoke!
2. Use it in a well ventilated area.
3. Try not to get too much on your skin!
Section 3: Checking the Stereo 120 Power Supply
The only part of your old Stereo 120 that really needs to work is the power supply. After
you’ve completed the previous section, only the power supply remains in the chassis.
This section describes how to verify proper operation of the power supply.
Make sure that the power cord is out, and has been out for at least 1 minute before
starting this section.
Prepare and Connect the Test Load
We’ll recycle parts of the amplifier boards to make a test load resistor for the power
supply.
1. Keeping the leads as long as possible, remove two 300 Ohm power resistors from
one of the PC-14 circuit boards.
2. Connect the two resistors in series, twisting and soldering.
3. Connect the series resistors across C12. Figure 6 shows how the test circuit will
look. The ground side connection is a bit round-about, but it’s there none the less.

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Figure 6-Connecting a test load to the power supply
Test the Power Supply
4. Make sure that neither the resistors nor their connections rest on the chassis. This
is important.
5. Make sure that none of the other dangling wires is making contact with anything
else.
6. When everyone is clear of the amplifier, plug it in, and turn it on.
7. Measure the DC voltage on the terminals of C12.
8. The voltage should be between 68.4 and 75.6 Volts, with a nominal voltage of 72
Volts. If it falls in this range, your power supply checks out. Remove the load
resistors and move onto section 4.
Just In Case – Trouble Shooting
The Voltage is Much More than 75.6 volts
Most likely the pass transistor is shorted. However, repairing the power supply pass
module is beyond the scope of this manual. Don’t continue until you have fixed the
module. Check www.updatemydynaco.com , the Power Supply Circuit Description, for
more information.
The Voltage is Much Less than 68.4 volts
Most likely the pass transistor is open. However, repairing the power supply pass module
is beyond the scope of this manual. Don’t continue until you have fixed the module.
Check www.updatemydynaco.com , the Power Supply Circuit Description, for more
information.

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There’s No Voltage at All
Check:
1. Is the AC socket powered?
2. Is the amp plugged in?
3. Is the power turned on?
Remove the power, check and replace the fuse with a properly rated fuse. Sometimes,
fuses just get tired. Sometimes they blow to prevent a disaster. At this point, we’re not
sure what the case is here.
Maintaining a respectful distance, power the amp again, and observe if the fuse blows. If
it blows quickly, unplug the amp, and go to www.updatemydynaco.com , the Power
Supply Circuit Description, for more information. The FAQs at the end of the document
may be especially helpful.
At this point, if you’re scared, confused, or nervous, find someone who knows what
they’re doing to help you. Above all, be safe!

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Section 4: Building the Updated Circuit Boards
The next few pages detail the process of building the circuit boards. The general strategy
starts with the components that lay closest to the board, working your way toward the
taller components. In general, we will:
1. Install the resistors
2. Install the small capacitors
3. Install the diodes
4. Install the LEDs
5. Install the big resistor
6. Install the medium size capacitors
7. Install the transistors
8. Install the big capacitor
9. Wind and install the output inductor.
10. Tack the LM3886 in place.
Install the ¼ Watt Resistors
Install the resistors, placing the body on silk screen side of the board, and the leads
through the indicated holes. Bend the leads over on the back of the board to keep the
resistors from falling out until your solder them in place. Try to bend the leads in a
direction that won’t lead to solder bridges between traces that should remain
disconnected.
We recommend the following procedure:
1. Insert all the resistors of the same value, e.g. R14, R19, R20, and R21.
2. Bend the leads as described above.
3. Solder the leads on the back of the board.
4. clip the leads.
Figure 7-The first four resistors are installed with leads bent on the back side, but not yet soldered.

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Figure 8-leads soldered and clipped
When you’re done this section, there will only be two empty slots for resistors:
1. R16, a no-load, which will remain empty.
2. R15, a power resistor, which we will load later.
Keep track of your progress by placing a check-mark in the done column as every resistor
is installed. The resistor values can be checked with a meter, or by reading the color code.
Designation
Value
Color Code
Done?
R14
10
Brown, Black, Black, Gold, Brown
R19
10
Brown, Black, Black, Gold, Brown
R20
10
Brown, Black, Black, Gold, Brown
R21
10
Brown, Black, Black, Gold, Brown
R13
100
Brown, Black, Black, Black, Brown
R1
100K
Brown, Black, Black, Orange, Brown
R2
100K
Brown, Black, Black, Orange, Brown
R3
100K
Brown, Black, Black, Orange, Brown
R5
100K
Brown, Black, Black, Orange, Brown
R7
100K
Brown, Black, Black, Orange, Brown
R8
100K
Brown, Black, Black, Orange, Brown
R4
1K
Brown, Black, Black, Brown, Brown
R6
1K
Brown, Black, Black, Brown, Brown
R9
1K
Brown, Black, Black, Brown, Brown
R10
1K
Brown, Black, Black, Brown, Brown
R18
1K
Brown, Black, Black, Brown, Brown
R11
20K
Red, Black, Black, Red, Brown
R12
20K
Red, Black, Black, Red, Brown
R17
2K87
Red, Gray, Violet, Brown, Brown
R15
10 (1 Watt)
Brown, Black, Black, Gold, Brown. This
resistor has a larger body than the others.
R16
This Location Remains Empty
Note: R8 and R18 are near each other on the board. Be careful...don’t mix them up!

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Install the Small Capacitors and the Diodes
Now we’ll install the following small capacitors:
Designation
Value
Marking
Done?
C3
220 pF
221
C4
220 pF
221
C9
47 pF
470
Here’s what these three caps look like:
Next we install diodes D1 and D4.
Be careful! Diodes have a polarity. Make sure the band on the diode aligns with
the banded end of the silk screen!
Designation
Value
Marking
Done?
D1
1N4004
4004
D4
1N4004
4004
Here’s what the diodes look like:
Next we install LEDs D2 and D3 (Light Emitting Diodes – note that in normal operation,
these diodes will be dark. However, in a fully darkened room, you’ll see a dull green
glow from the powered LEDs.)
Be careful! Light Emitting Diodes have a polarity, also! Read carefully to make
sure you’re putting the diodes in the right way! The cathode of the LED is
indicated by a bar (negative sign) molded into the package. On some packages,
there is no bar, but seen from the top, the circular outline with have a flat side.
That flat side is the cathode. Finally, if the leads are uncut, the shorter of the two
leads will be the cathode.
Designation
Value
Marking
Done?
D2
T1 style
none
D3
T1 style
none

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Figure 9-showing LED cathode orientation
Last Capacitors and the Transistors
Now, the medium tall polarized electrolytic capacitors:
C1 and C6 are polarized. Make sure the negative sign on th e capacitors faces
away from the positive sign on the silk screen!
Designation
Value
Marking
Done?
C1
47 µF, 63 V
47 µF, 63 V, and minus sign for polarity
C6
47 µF, 100 V
47 µF, 100 V, and minus sign for polarity
And the non-polarized capacitors:
Designation
Value
Marking
Done?
C2
1 µF, 63 V or
100V
1 uF, 63 V or 100V (box shaped mylar
capacitor)
C5
0.1 µF, 100V
104
C8
0.1 µF, 100V
104
C10
0.1 µF, 100V
104

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Install the transistors. Spread the outside leads a bit to make it easier to insert them into
the board.
Orient the transistor so its body shape matches the silk-screen outline. Leave the
top of the transistor about ½” off the board! The lead length prevents stress on the
body and keeps the transistor safe from too much heat during the s oldering
operation.
Designation
Value
Marking
Done?
Q1
2N5551, NPN
2N5551
Q2
2N5551, NPN
2N5551
Q3
2N5551, NPN
2N5551
Now install the speaker coupling capacitor:
Watch the polarity! Make sure the minus sign faces away from the plus sign on the
silk screen. That puts the minus side of the cap along the outer edge of the circuit
board.
Designation
Value
Marking
Done?
C7
3300 uF 63V
3300 uF, 63 WVDC
The speaker coupling capacitor will either snap or push into place. Verify once more that
you have polarity correct, and then solder both speaker terminals to the circuit board.
This will probably take more heat, time, and solder than anything you have soldered up to
now.
Winding the Output Inductor
This step takes just a bit of finesse, but if you’ve come this far, you have nothing to worry
about. If you’re a bit frazzled, take a break before proceeding.
There...feel better? OK, let’s go. Just follow these steps:
1. Cut a 21” length of solid wire. A great place to get it is from the output inductors
wound around the old Dynaco output capacitors (see Figure 10).
2. Strip back 0.5” of insulation from one end of the wire.
3. From the component side, place the stripped end of the wire into the L1A
terminal. Route it around the output capacitor as shown in Figure 11.
4. Solder the wire on the component side into the L1A hole.
5. Wind 5 turns of wire, close spaced, so it looks like Figure 11. When you get to the
end of the coil, carefully estimate the amount of wire you’ll need to make it from
the top of the coil down through the L1B hole in the board. You can make this
step a bit easier if you strip back a bit more insulation than you think you’ll need.
6. Pull the end of the wire through the L1B hole, and solder it on the back of the
board.