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Old 05-18-2017, 12:27 PM   #1
Chungalin
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Default Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low quality?

This powered speaker has some hiss that reveals that power amp is working, but OL (Overload) is always on.

Power supply has -15V but not +15V. No shorts in preamp part. Even disconnecting +15V line, it doesnít show up.

Line regulation is only a BZX55C15 15V zener and a S8050 TO-92 NPN. Zener seems OK, but transistor is not conducting (not shorted though).

I replace it and it works, but... the transistor gets hot as hell. Itís 500mA rated. I decide to change it with a much powerful SS8050. It gets hot too, but itíll stand longer. -15V regulator (S8550) gets hot too, but not so much. +5V for DSP (a large IC!) is produced from +15V via linear regulator. Perhaps thatís why +15V has extra load while -15V is not so stressed.

Anyway, everything else looks pretty new, so it seems that someone designed this PSU cheaply. Today Iíve received another Thump15 for repair, same symptom. Iíll report if itís the same case. If so, it will be clearly a design quality issue.
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Old 05-18-2017, 10:30 PM   #2
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Default Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

What is the input Voltage feeding that linear regulator circuit? 45V?
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Old 05-19-2017, 05:58 AM   #3
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Default Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Iíve already returned the speaker. Ī15V were obtained from the ĪB for IC AMP, a ST TDA7294. I think that big caps were 25V rated. Or they were 35V? Normal voltage for that IC AMP is around 20~30V, so itís unlikely that thereíre more than 40V anywhere.
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Old 05-19-2017, 02:21 PM   #4
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Default Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Iíve found schematic for this speaker/amp. Ī15V generation circuit is in first page.
Attached Files
File Type: pdf Thump TH-15A Schematics.pdf (1.28 MB, 6 views)
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Old 05-19-2017, 04:25 PM   #5
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Default Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Its a good chance U1, U2, U4 were bad, or even U6 regulator? or something on the +5 volt line, I guess we'll never know.
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Old 05-19-2017, 06:14 PM   #6
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Default Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

I think I checked all these ICs with alcohol, just to see if anyone heated faster, but I couldnít determine anything bad there. If something else is really bad, Iíll know in a matter of days or weeks. NP if itís the regulator. NP if itís an OpAmp. But if itís that huge Analog Devices ADSP-BF531, I think Iíll be toasted.
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