Mapletree Audio Design S70A Special ed Driver Module for Dynaco ST-70 Installation instructions ev. Jan. 9/ The Special ed S70A driver module is a drop in replacement for the original driver board of the Dynaco ST-70 power amplifier. Unlike the original, all wiring on the S70A is point-to-point with connections to the amplifier made to turret terminals. The schematic diagram of the S70A module identifies the turret connections to the ST-70 circuitry. The instructions identify which of the original wires and components are to be removed and discarded and the new wiring that is required using the silver-plated Teflon wire supplied. When making a new connection from one of the S70A turrets to a point in the amplifier, first route the wire as shown in the diagram in order to obtain the correct length. Then strip ¼ inch of insulation from each end and melt a small amount of solder on the exposed wire by heating it with your soldering iron and melting the solder on to the wire (pre-tinning). The best connection to a turret is made by inserting the pre-tinned end of the wire into the hole in the top of the turret, leaving a bit of wire exposed, then heating the turret with your soldering iron while adding solder so it runs down the wire inside the turret. Note: It may be necessary to remove the s on the front apron of the ST-70 to allow room for the S70A board. If this has not already been done in previous modifications to your amplifier, the following instructions should be followed.. Unsolder both ends of all wires connected to the lugs of the two s on the front apron of the ST-70.. emove the sockets.. The stainless steel filler plates supplied can be attached to the inside of the chassis using the original socket machine screws or the # sheet metal screws supplied.. If it is desired to provide external access to the bias test voltage for the ELs, you can mount suitable test jacks in the filler plates, making sure you maintain clearance for the S70A board. These jacks should be wired to lug of V (L) and V (). The bias test voltage (. V) is then measured from each jack to the chassis (ground). efer to Figure showing the original wiring to the printed circuit board, with the s removed.
. Unsolder all wires and resistors from the input CA jacks and stereo-mono switch on the front panel. This assumes that the stereo-mono switch will not be used.. Unsolder both ends of all wires connected to the eyelets on the original board that are marked with an. The two wires connected to the Ohm taps of the output transformer should only be unsoldered from the eyelets on the circuit board.. emove the original board from the chassis.. Install the S70A board using the original screws/nuts or the -0 screws and hex nuts supplied.. Unsolder the K resistor between lugs and of the original -section filter capacitor and discard. efer to the wiring diagram for the S70A board connections (Figure ).. Solder the twisted pairs of wires pre-soldered to lugs 7 and of the SL7 tubes V (left) and V (right) on the S70A board to lugs and 7 of V (L) and and 7 of V ().. Solder the wires coming from the Ohm speaker terminals on the rear panel to the FBL and FB turrets on the S70A board.. Measure, cut, and strip silver-plated Teflon wire lengths and make the following connections: i. CA left input jack ground lug to CA right input jack ground lug ii. CA left input jack ground lug to the S 70A ground (GND) bus iii. CA left input signal lug to turret iv. CA right input signal lug to IN turret v. Filter capacitor lug to turret B vi. Filter capacitor lug to turret B vii. Chassis ground lug (next to filter capacitor) to the S70A ground bus viii. Center lug () of the right bias potentiometer to the Bias turret ix. Center lug () of the left bias potentiometer to the turret x. Lug of V (EL) to turret G xi. Lug of V7 (EL) to turret G7 xii. Lug of V (EL) to turret G xiii. Lug of V (EL) to turret G This completes the installation of the S70A module. e-check all connections using the wiring diagram as a guide. Install the four tubes with the SJ7s toward the front of the chassis. It is recommended that you re-adjust the bias for the output tubes in the usual manner. Notes:. The S70A can be supplied with a heater voltage of VDC, 0. A for each channel if desired.. If non-stock Dynaco output transformers are installed, it may be necessary to adjust the value of C (70 pf) in each channel for optimum transient response (as evidenced by the best square wave reproduction and/or the flattest high frequency response).
and wiring removed and wiring removed Unsolder and remove all wires and components from input jacks and stereo-mono switch original driver board Unsolder both ends of all wires and components marked and discard.. unsolder at eyelet end only unsolder at eyelet end only S Board Installation FIgure
stereo-mono switch filler plate input jacks L filler plate TWISTED WIES L L V FBL GND BUS B B B B FB gnd bus 7 77 7 7 V V G G G G7 G FBL G G V Bias Bias G7 G G Bias G G7 FB IN IN B B TWISTED WIES S Board Installation FIgure
Mapletree Audio Design S70A Special ed driver module for Dynaco ST-70 Copyright Lloyd Peppard 00- rev. Jan. / IGHT. Input IN FB To Ohm tap on right OPT SJ7/9/SJ7GT.K 00 Gnd Bus C 70 pf 9 V C 0.07/00.K V 00K 7 C /00 7 SJ7 7 SL7 V SL7GT/9 C 00p To. heater voltage (Lugs and 7 of V) V.K 9 0 C 0./00 C 0./00 G7 Bias G Pin of V7 (EL with white/blue plate lead). bias pot -. V Pin of V (EL with blue plate lead) 0K W % L. Input V SJ7/9/SJ7GT. V.K 00 Gnd Bus C 70 pf V C 0.07/00 00K FBL 9 V.K To Ohm tap on left OPT B LEFT C 00p 7 C /00 7 SJ7 7 SL7 V SL7GT/9 To L. heater voltage (lugs and 7 of V) V.K B C 0./00 9 0 C 0./00 G G Pin of V (EL with white/blue plate lead) L. bias pot -. V Pin of V (EL with blue plate lead) V V 97 V remove + V + K +.K + + Gnd lug on chassis Quad filter capacitor Plate voltage = V Screen voltage = V