Sound Board #1: 7-11-077-119 Rev A.
Handwritten tag stated that board was repaired on 12-15-1981 for $74.84. Had "Southwest" on tag: not sure if this was repair house or operator name. Three ICs had been replaced with sockets added: IC1(7414), IC6(7406), and IC12(74LS393).
IC25 had also been replaced at some point, this one without a socket.
Visual inspection showed resistor R122 was fried. Resistors R123 & R124 looked like they had taken some heat as well.
Looking at the schematics in the Star Castle manual, I could see that these resistors are in the final amp section of the board. From my experience with the Cinematronics monitors, I know that fried resistors can be caused by a transistor going bad. We need to check and replace any bad transistors before replacing the resistors. There are two in this part of the board, Q17 (2N6292) and Q18 (2N6107).
If your board looks like this, the bad transistors also probably fried your speaker. See below for how to test your speaker. Don't try plugging it into other cabinets or speakers to test it. Until the board is repaired, IT WILL KILL ANY SPEAKER HOOKED UP TO IT.
After testing the two transistors, I found the following:
Q17 (2N6292) - NPN
Test #1 showed OPEN - it should read between 0.45v and 0.9v on a NPN transistor- this showed the transistor was BAD.
Q18 (2N6107) - PNP
Passed all tests except test #5 - the test read 0v - shorted (same as just touching the two probes together) and should read OPEN- this showed the transistor was BAD.
If parts in your board have been replaced with NTE parts:
NTE196 crosses to 2N6292
NTE197 crosses to 2N6107
NTE857M crosses to TLO81
After installing a new 2N6292 and 2N6107, I plugged it in and heard - nothing.
Checked voltages - all good.
I checked to see if the speaker was good. The speaker tested as BAD.
Plugged in a good speaker and voila! - Star Castle sounds!
IC15 (TL081) was later socketed and replaced due to problems with the noise circuit.