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Old 9th January 2012, 04:47 AM   #11
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What's driving the base of Q40?

What happens if you remove Q40?
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Old 9th January 2012, 11:35 PM   #12
Dr Zeus is offline Dr Zeus  United States
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Theres a 220 ohm resistor from ground, connected to a 10k resistor, connected to the base of Q40. Between the 220ohm and 10k resistor is a 2n5064 at U4. The cathode (Leg 1) of this thyristor is connected b/t 220ohm and 10k resistor.

I'm not sure if I can test a Thyristor using diode check.

With Thyristor 2n5064 removed
Red on Leg 1 (Cathode)
Leg 1-2: OL
Leg 1-3: OL

Red on Leg 2
Leg 2-1: 0.641 (beeps)
Leg 2-3: OL

Red on Leg 3
Leg 3-1: OL
Leg 3-2: OL

With both of these TO92 transistors removed, and the jumper b/t Pin 8&16 removed on the SG3525, the amp powers and stays powered without flickering, no voltage drop.

The MPSA06 measures good off the PCB. I put it back in (With the Thyrister still out of circuit and the jumper still removed) the amp powers on forever.

I'll trace what this thyrister is connected to after I get back from an errand
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Old 10th January 2012, 02:19 AM   #13
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The SCR is likely being used as a latch. With DC, if you charge the gate of the SCR, it conducts and continues to conduct until voltage is removed from the other 2 terminals.

Yes. follow the circuit back to see if you can determine what's driving it.

With it out of the amp and everything clamped down, does it produce clean audio into a load up to clipping?
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Old 10th January 2012, 02:35 AM   #14
Dr Zeus is offline Dr Zeus  United States
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I think I figured it out. I had noticed the two 50v caps close to the thyristor were a tad loose before, but now they are broken of the board. They may not have been connected up before. I will replace them and glue the new ones down but need to buy these new. I don't think amp is gonna work otherwise.
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Old 10th January 2012, 03:41 AM   #15
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'Thyristor' is a type of semiconductor that covers multiple components. You have an SCR (a type of thyristor).
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Old 10th January 2012, 04:18 AM   #16
Dr Zeus is offline Dr Zeus  United States
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Much to learn

I carefully put a pair of temporary caps in place of the 50v/1000uF units. The temp ones are 50v/2200uF. They are way to big to fit under the cover, but they allowed the amp to power up with all components installed. Time to order-up some of the proper size.
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