DYNACO ST-70 Assembly Manual - Page 8
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UNMARKED eyelet near the left edge of the driver board about 7/8 inch from the BACK of the
board. Do not press the wire in too deeply as it may short against the chassis below. (S)
6. Connect a 2 ½ inch wire from pin #6 of V6 (S) to the UNMARKED eyelet near the right edge of the
driver board about 7/8 inch from the FRONT of the board. Do not press the wire in too deeply as
it may short against the chassis below. (S) Connect a 3 ½ inch wire from pin #6 of V7 (S) to the
UNMARKED eyelet near the right edge of the driver board about 7/8 inch from the BACK of the
board. Do not press the wire in too deeply as it may short against the chassis below. (S)
7. Connect the RED-BLACK wire from the power transformer to the eyelet on the board marked "-
50 VAC" near the back edge of the board. Do not press the wire in too deeply as it may short
against the chassis below. (S)
8. Connect the free end of the 5 inch wire coming from the grounding lugs to the eyelet marked
"GD" on the rear edge of the driver board. (S)
9. Connect the wire coming from the quad cap TRIANGLE terminal to the eyelet marked "B+" near
the center of the driver board. (S)
10. Route the AC cord through the rubber grommet on the rear face of the amp. Split the two wires
for about 6 inches and tie a knot in the two wires. Strip ¼ inch of insulation from each wire and
connect one wire to terminal "B" of the fuse post (S) and the other wire to terminal #2 of the
on/off switch. (S)
1. Make sure that the 3 amp SLO-BLO fuse is in the fuse holder.
2. Place the three 12AT7 tubes in the sockets on the top of the driver board. DO NOT PLUG IN THE
RECTIFIER TUBE OR THE EL34 OUTPUT TUBES at this time. Plug in the amp to an AC outlet
and turn on the amp. Wait about 20-30 seconds to see if the three 12AT7 tubes light up. If they
do and they stay on for a minute then proceed to step 3. If the 12AT7 tubes don't light up or if
the fuse blows check the wiring below and measure the AC voltage where both sets of twisted
wires connect to the driver board. Check to see that you get about 5.9 - 6.5 volts AC ACROSS
THE TWO WIRES. If not, check the wiring back to the power transformer for an error.
3. Get your multitester and set the range for 0 to 2 volts DC.
the GZ34 rectifier tube and just the LEFT EL34 output tubes in their socket. Turn on the amp.
Allow the amp to warm up for a minute and check to see that the EL34 and GZ34 tubes light up
properly. Measure the bias on the LEFT output tubes by placing the BLACK NEGATIVE probe
anywhere on the chassis and the RED POSITIVE probe in the hole marked "Biaset 1.56 v". With
a small screwdriver adjust the FRONT LEFT bias adjuster on the driver board closest to the front
left tube. Turn the adjuster CLOCKWISE TO REDUCE BIAS and COUNTERCLOCKWISE TO
INCREASE BIAS. Set the bias for .400 volts if you are using EL34. Now place the positive probe
in pin 4 (which is on the opposite side of the power take off socket) and use the BACK LEFT
bias adjuster to measure bias on the LEFT REAR output tube. Set it the same as the front tube.
Turn off the amp.
4. Plug in the two RIGHT channel EL34 tubes, turn on the amp and repeat the bias procedure as
outlined above on the RIGHT channel tubes. Now go back to the left channel tubes and notice
that the bias is now a little LOW. ADJUSTING BIAS ON ONE TUBE HAS A SLIGHT AFFECT ON
THE OTHERS. Go back and forth between all output tubes until all tubes have the proper bias.
5. If at any time a fuse blows IMMEDIATELY check carefully the power transformer wiring for an
error. Check all wiring against the pictorial. If a fuse blows AFTER A FEW SECONDS and/or the
Initial startup
CONNECT THE SPEAKERS.
Place