CAE Dynaco ST-70 Restoration Module Guide

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 1 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
I. Introduction
Thanks for your purchase of our ST-70 Driver Restoration PC Board. It has been
designed to provide an exact replacement for the original Dynaco input driver card (Dyna
PC-3) using dramatically improved construction materials (Mil Spec FR-4 fiberglass). It
has been designed to drop into the original PC-3 position without any electrical or
mechanical changes to your ST-70. Once installed, the reliability of your ST-70 will be
greatly improved and its stability significantly enhanced.
This document describes the specific instructions and steps associated with our Dynaco
Upgrade board (PC-3R). Additional documents referenced (and available from our web
page) are:
a. General Overview Assembly Notes (CAE Tech Note # 1)
b. Soldering Tutorial (CAE Tech Note # 2)
c. Recommended Tools & Test Instruments (CAE Tech Note # 3)
Please take the time to familiarize yourself with all of these documents in particular Tech
Note # 1.
II. General Overview
Take time to review all of the documentation provided including the aforementioned tech
notes related to soldering technique and proper tools and test equipment. In the following
sections we will guide you through several project phases. Each section has been
carefully prepared with our own notes and comments from our customers to be sure that
any anticipated question has been considered. The sequence is identical to that which we
follow when we complete the fabrication for our customers. Please follow the same
sequence to maximize efficiency and eliminate errors. It is also very helpful to read each
section before beginning to gain a visual idea of the construction.
Your project will be built in stages. If you have purchased the PC Board in its
unassembled form (PWB-PC3U) you will first assemble the components onto that board.
Next, you will remove the original Dynaco PC-3 board and carefully label each of the
associated wires. You will then install the CAE replacement upgrade PC Board and
reconnect the original wires to their corresponding terminals followed by a few electrical
tests and power up sequence.
Finally you will check and set the bias of the output stage.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 2 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
Please note that this module houses components having lethal voltages present on
their leads and cases. Please take the appropriate care in both installing and
operating this module.
Although you are unlikely to encounter any problems, should they occur they would
almost certainly be based on an improper assembly of the PC Board or the wiring to the
Stereo 70. Therefore to avoid the time consuming and frustrating task of troubleshooting
afterwards please remember these simple suggestions:
a. Proper soldering is crucial. Please refer to our soldering tutorial and practice on
surplus boards until you master the technique. Be especially careful to avoid solder
bridges or “cold” solder joints. Our experience shows that nearly 95% of all
problems are associated with soldering related errors.
b. Diodes, transistors, IC’s, (all semiconductors) and many capacitors (always
electrolytic types) are polar - this means that there is a right and wrong way to
insert polarized components in the board. If placed backwards, the component and
probably others nearby will be damaged upon initial power application. Double check
each step associated with these parts and once again later after you have had a chance
to take a break.
c. Some components are color coded with their value (mostly resistors, but there may be
others). If you are not proficient at reading these codes, use an ohmmeter to double
check the value of each resistor before insertion. It is very easy to confuse a 100 ohm
metal film resistor with a 1000 ohm resistor or worse with a 100K ohm resistor.
III. Internal Wiring
Unless otherwise specified, you should use 20 to 22 gauge stranded wire with Teflon
insulation. Thicker wire, solid wire, or PVC insulation will cause problems - if not now
(too much strain on the boards or parts) or later (reliability issues). This can’t be
overemphasized.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 3 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
III. Assembly
A. Building the PC Board (PC-3R)
Components are to be inserted on the non foil side and soldered on the foil side. The
sequence of assembly has been chosen so that the components with the lowest profile
(such as jumpers, low power resistors, IC’s etc.) are installed first with higher profile
components added until the highest profile components are installed last.
Refer to the component placement drawing (figure 1). Be sure to observe correct
orientation of polar components (capacitors). Be sure also to check with an ohmmeter
resistor values if you are not absolutely sure of how to read metal film resistor color
codes.
1. Begin by installing all of the ½ Watt
resistors. There are a total of 16 ½ Watt
resistors specified. Refer to Figure 1 (Top
View PC Board Component Placement
Diagram). The board has been designed so
that a gentle radius near the body of the
resistor will position the leads in alignment
with the associated holes in the PC Board.
Note that the ½ Watt resistors should be
inserted so that the body of the resistor
rests gently against the PC Board. After
inserting the resistor leads thru the board, a
slight bending outward of the leads will
hold the part in place until you can solder it
in place.
2. Install the Six (12) 3 Watt metal Oxide resistors. You should also gently bend the leads
of these resistors similar to the ½ Watt resistors (but of course with a greater radius). But
unlike the ½ Watt resistors, the # Watt Resistors should be spaced away from the plane of
the PC Board by 1/16” to allow for heat dissipation.
4. Install the 2 nine (9) pin miniature tube sockets. Be careful to insure that the leads fully
extend thru the PC Board and that you have sufficient tab length to obtain a solid solder
connection.
5. Install the 10 capacitors. There are 6 large values and 4 small values.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 4 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
6. Visually inspect all of your solder
connections. Refer to the adjacent diagram.
Quality solder connections should be
smooth and shiny. Dull surface or large
blobs on the PC board should be cleaned
and resoldered. In this case remove the old
solder with solder wick or similar and
resolder the connection.
8. Install the 2 7199 Tubes.
B. Removing the Original Dynaco Stereo 70 PC-3 Board
OK – that was pretty easy.
1. To begin removing the original Dynaco PC-3 board, remove the top cover (should
be held in place with either 4 or six screws (depending upon the vintage) along
the side. These screws will release both the top perforated metal cage and the
bottom “U” cover.
2. Remove the 4 output tubes (and label them for reinstallation later). I recommend
that you clean the dust and grime from the topside both inside and outside of the
amplifier. A vacuum cleaner with a soft paintbrush will work very well here.
3. Next begin desoldering each wire from the original PC-3 card. The original
Dynaco wires were solid PVC insulated. The PVC is not very tolerant of heat and
therefore you should be very careful not to melt the insulation. Normally a
“wetted” soldering iron tip will have better thermal transfer causing the solder to
melt quickly before the heat travels up the lead of the wire compromising the
PVC insulation. Also, when you remove the wire from the PC Board, it is not
uncommon for the wire to “snap” at the other end. I suggest having some
replacement wire available. You can use solid PVC wire as in the original
however if possible, however I strongly recommend using 22 ga. stranded Teflon
Insulated wire in this application. Please do not use anything larger – it is not
necessary and it will compromise the reliability of your amplifier as you attempt
to make reliable connections on terminals unable to withstand the strain. Note that
you may encounter original wiring that is “less than professional” in its execution,
If so, it may be necessary to lengthen or replace one or more of the original wires.
4. Desolder each wire and using masking tape (a temporary identifier) label each
wire to make installation of the replacement module much easier.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 5 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
5. After you have desoldered all of the wires from the original Dynaco driver board
(PC-3) remove it by removing the four (4) 4-40 nuts from the bottom of the board.
The board should fall loose (if not identify if any other wires still remain).
6. Remove the two (2) 470K ohm connected to the audio input connector tabs. These
resistors are no longer necessary.
C . Installation of the New CAE Stereo 70 Driver Board (CAE-PC-3R)
1. Locate the CAE replacement Driver board (PC-3R) and orient it so that the TWO
330K ohm 3 Watt resistors are near the rear of the amplifier (nearest the amp
center). The board should be mounted so that the top surface of the PC Board
rests inside the Dynaco ST-70 chassis cutout (as per the original). Be careful not
to capture any wires and secure the board in places with the original 4, 4-40
screws/nuts.
2. Begin reconnecting the wires originally removed for the original Dynaco PC-3
board to the corresponding terminal on the CAE replacement Board (PC-3R). I
suggest placing the stripped and tinned wire into the hole in the board and then
quickly soldering the wire to the terminal. Be sure to inspect each connection as
discussed before. As you complete each wire, remove its masking tape identifier.
Although I have designed the replacement board’s terminals to be very near the
original board, some amplifiers may have been constructed so that lengthening
may be required.
3. Once completed, inspect each connection again and carefully look for signs of
melted insulation near adjacent wires.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 6 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
D . Final Checkout & Initial Power Application
1. Replace the power fuse (3A Slo-Blo) with a 1A AGC (quick trip).
2. If you haven’t already, remove the rectifier tube. Plug in and turn on the
amplifier. Examine the filaments on the new replacement driver board. All three
should be illuminated. If the center tube is not illuminated, turn off the amplifier
(remove the plug from the AC socket) and reverse the wires labeled “4” & “5” on
the replacement board. Reapply power – all three driver tube filaments should be
illuminated. Remove power from the amplifier.
3. Remove the three driver tubes and insert the rectifier tube and the two left channel
output tubes. Do not install the right channel output tubes right now. Replace the
original 3A fuse. Rotate both original Bias Potentiometers fully counterclockwise.
Connect your DC Voltmeter to monitor the current thru the output tubes by
measuring the voltage across the 15.6 resistor located at pins 1 & 8 of the output
tubes - connect the negative lead (Black) to the chassis and connect the Positive
lead (Red) to the junction of the 15.6 ohm resistor and the Output Tube pins 1 &
8. Monitor the voltage as the tubes reach operating temperature. If the voltage
begins to exceed 2.5 volts quickly remove power. Otherwise adjust the Bias pot to
obtain 1.56 volts. Remove power.
4. Install the right channel output tubes, apply power, and repeat the procedure
outlined in step 3. Recheck the bias on the left channel. Remove power.
5. Install the two small 7199 signal driver tubes.
6. Replace the cover and enjoy.

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 7 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
Figure 1

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 8 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02
CAE Stereo 70 Restoration Module
7-02 R0
Stereo 70 Restoration Module, CAE# ASM-3R
PAGE 1 of 1
DESIG DESCRIPTION QNTY PART NUMBER VAL / VOLT
C1 & C2 CAPACITOR, MYLAR or POLYPRO 2 0.05uF/200VDC
C3, C4, C5, C6 CAPACITOR, MYLAR or POLYPRO 4 0.1uF/400VDC
C7 & C8 CAPACITOR, MICA 2 MSR#5982-5-300V390 390pF, 5%
C9 & C10 CAPACITOR, MICA 2 MSR#5982-5-300V82 82pF, 5%
R1, R2, R3, R4 RESISTOR, METAL OXIDE 4 DK#P47KW-3BK-ND 47 K , 3W
R5, R6 RESISTOR, CARBON FILM 2 MSR#29SJ500-18K 18K, 1/2 W
R7, R8, R9, R10 RESISTOR, CARBON FILM 4 MSR#29SJ500-270K 270K, 1/2W
R11 & R12 RESISTOR, CARBON FILM 2 MSR#29SJ500-10 10, 1/2 W
R13 & R14 RESISTOR, METAL FILM 2 MSR#29MF500-47.5 47.5, 1/2 W
R15 & R16 RESISTOR, METAL OXIDE 2 DK#P1KW-3BK-ND 1 K , 3W
R17 & R17 RESISTOR, METAL OXIDE 2 DK#P330KW-3BK-ND 330 K , 3W
R19 & R20 RESISTOR, METAL OXIDE 2 DK#P270KW-3BK-ND 270 K, 3W
R21 & R22 RESISTOR, METAL FILM 2 MSR#29MF500-620 620, 1/2 W
R22 & R23 RESISTOR, CARBON FILM 2 MSR#29SJ500-470K 470K, 1/2 W
R24 & R25 RESISTOR, METAL FILM 2 MSR#29MF500-1.5M 1.5 MEG, ½ W
R26 & R27 RESISTOR, METAL OXIDE 2 DK#P18KW-3BK-ND 18 K, 3 W
V1 & V2 PREMIUM TUBE 2 7199
X1 PRINTED CIRCUIT BOARD 1 CAE# PWB-3R
X4 PC BOARD TUBE SOCKETS, 9 PIN 2 CAE# PC-9
NOTES: CAE# - Curcio Audio Part Number
DK# - DigiKey Part Number MSR# - Mouser Part Number
AES# - Antique Electronic Supply Part Number

Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
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Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
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Dynaco ST-70 Restoration Module
Assembly, Installation, and Adjustment Manual
Page 11 CAE Dynaco Driver Restoration Module PC-3R – R0: 7-02