OUTPUT TRANSISTORS
Output transistors Q9 through Q12 (Figure 13, Page 13) are located on the black, finned heat sinks. The replace- ment procedure is the same as that used for driver tran- sistors Q7 and Q8.
NOTE: Output transistors Q9 through Q12 must be matched for current gain. When replacing output transis- tors, be sure to replace with devices which have the same gain and part number as the original transistors. Shure transistors have a letter suffix in the part number (i.e., 86A360A) which serves as a gain code. Transistors with the same part number and suffix letter have equal current gain.
DIODES
Diodes D l 5 and D l 6 (Figure 13, Page 13) are located on the black, finned sinks with the output transistors. Special care is required to insulate these diodes from the heat sink while providing good heat transfer from sink to diode. Heat- shrinkable tubing or "spaghetti" should be placed over the diode and connecting leads; the diode should be securely clamped to the heat sink with the clamp provided.
DIODE BRIDGE RECTIFIER
Silicon diode bridge rectifier D l 1 is mounted on the underside of the chassis as shown in Figure 14, Page 14. When replacing this component, apply Wakefield Type 120 thermal joint compound between the heat sink base of the rectifier and the chassis to provide good heat transfer to the chassis. Note that the base of the rectifier is provided with a locating pin. When installing the rectifier be sure to position it such that the locating pin fits into the hole provided for it in the chassis. The terminals are marked to indicate the ac connections to the power transformer and the (+) and
CHECKING TRANSISTORS AND DIODES
Defective transistors and diodes may be located by use of a standard ohmmeter such as a Simpson Model 260. (Polarity of the ohmmeter must be verified before these checks are made.)
With a known diode orientation, measure the diode re- sistance in the forward and reverse directions. The lowest meter reading establishes the probe at the cathode end (schematic symbol arrow points to cathode) as the "minus" probe while the other probe is "plus." Some ohmmeters are not polarized in this manner with relation to "volts plus probe" and "volts minus probe." With the ohmmeter "plus" probe on the anode end of a diode and the "minus" probe on the cathode end, the ohmmeter should read ap- proximately 2 kilohms or less. With the meter probes reversed, a reading of about 10 kilohms or more should occur. If either of these conditions is not met, the diode should be replaced.
When checking the bridge rectifier disconnect all leads to the assembly, noting the terminals to which they were connected. Check each diode leg of the bridge in the same manner as described previously for individual diodes. Refer to the preceding section, Diode Bridge Rectifier, for instal- lation instructions if replacement is required.
To check transistors, the ohmmeter should be set to the 100- or
with screws (Q7 through Q12) may be tested in place; how- ever, the base and emitter leads to these transistors must be removed.
If all conditions in the following table are met, the tran- sistors may be considered free of any major defect; if any of the following conditions are not met, the transistor should be replaced. See Notes to Circuit Diagram (Figure 20, Page 23) for transistor terminal codes.
Ohmmeter Connections | Ohmmeter Reading | ||
|
| NPN | PNP |
"Plus" Lead | "Minus" Lead | Transistor | Transistor |
Collector | Emitter | High | High |
Emitter | Collector | High | High |
Collector | Base | High | Low |
Emitter | Base | - | Low |
Base | Collector | Low | High |
Base | Emitter | Low | - |
SR105B AMPLIFIER CONVERSION FOR 70-VOLT OUTPUT
If it becomes necessary to equip a Model SRIOSB Power Amplifier for
W A R N I N G
This conversion should only be performed by quali- fied service personnel or the Shure Factory Service Department.
Once the conversion is performed, the Amplifier is identical to the SR105A Amplifier and all instructions and diagrams pertaining to the SR105A Amplifier apply.
The following parts, materials and tools are required to perform the
Remove the line cord from the ac outlet and remove all input and output cables. Remove the metal grille cover as described previously in this section. Remove the two lock- nuts securing the two lamp socket assemblies (see Figure 16, Page 17). Remove the lamp socket assemblies from the mounting studs and move them away from the studs to permit mounting of the
Turn the Amplifier