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Mackie SRS1500: only a very low vol hum...

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  • Mackie SRS1500: only a very low vol hum...

    Hi all. First, total newbie, with just enough knowledge to probably make things worse. I appreciate any basic help/advice I can get here. I downloaded the schematic, for curiosity sake really. I’m not understanding the values though or how to test them. I’m very familiar with & work in residential electrical, and have a basic fluke meter for that. Amplifier circuitry is a whole another world...

    From other posts here - I started by making a light bulb limiter. Using a 75w bulb, when the amp is on the bulb is about half that of full brightness.

    The power indicator LEDs on the back and front do not light up. Other than the slight speaker hum, you wouldn’t know its on.

    Upon opening it up - 2 LEDs inside do light up (D24 & D36).

    I see 3 fuses - All continuity test ok.

    Disconnecting the speaker, and with power on - there’s no voltage at the speaker terminals (J2/J3). The speaker itself tests ok, using a 9v battery check. With the speaker re-hooked up, I ran signal into the amp, fairly cranked - got nothing but the same low hum.

    I completely took the circuit boards out and checked all around. Removing all the heat sink covers on the ?transistors - There is no obvious signs of burnt components, neither visual or smell.

    If anyone knows this amp and/or can point me in the right direction on how to test through it, I’m much appreciative...


  • #2
    I'm actually looking at my board right now. I picked a broken SRS1500 up for free. I've found the power MOSFETS to be shorted, the TIP36/35's shorted except Q3, the MJE's were both shorted. Diodes seem to all check ok. Main fuse is blown (to be expected). Wanted to see if I could repair it as a fun coronavirus refurbish project since I'm stuck in the house.

    I'm currently looking for recommended replacement parts for the obsolete parts, if someone wouldn't mind double checking my references, or helping.

    Any recommendations for the SA1478/2SC3788? I was looking at the KSA1381/KSC3503, but it looks like that pair the gains are mismatched for what i could find. KSC3503D gain is 60-120, KSA1381E gain is 100-200. Is that critical? The originals have a gain range from 40-320 based on sanyo's datasheet. Seems like a large range, so not sure how critical these are.

    Any recommendations for the MTW32N20E? I was going to maybe use SIHG40N60E-GE3-ND.‎ The max ratings are higher, the operational data looks very similar, and the curves look very similar.

    Anyone know what the voltage's for the collectors should be at idle once I figure out what parts I should get?

    Thank you in advance for any and all help!