Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low quality?

[Chungalin]

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 doesn't 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. It's 500mA rated. I decide to change it with a much powerful SS8050. It gets hot too, but it'll 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 that's 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 I've received another Thump15 for repair, same symptom. I'll report if it's the same case. If so, it will be clearly a design quality issue.

[budm]

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?

[Chungalin]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

I've already returned the speaker. ±15V were obtained from the ±B 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 it's unlikely that there're more than 40V anywhere.

[Chungalin]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

I've found schematic for this speaker/amp. ±15V generation circuit is in first page.

[R_J]

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.

[Chungalin]

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 couldn't determine anything bad there. If something else is really bad, I'll know in a matter of days or weeks. NP if it's the regulator. NP if it's an OpAmp. But if it's that huge Analog Devices ADSP-BF531, I think I'll be toasted.

[Vintage Amp Revamp]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Hey. New to the forum here. Just would like to thank Chungalin with this post. I had the same issue with a Thump th-15a last week. The +15 volts would drop out after a few seconds. I upsized s8550 and the problem went away. While i had the power supply board out, I also replaced c118 and c120 with 25 volt rated caps as the 16 volt rated ones were slightly under sized.

[denormalizer]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Apologies for joining late on this subject, but it appears I have a Mackie Thump 15-A with the same issues as described in this post: +15v is out (even with preamp disconnected) and OL Light stays on with a hiss coming from speakers but no sound from input.

Background: Speaker was working connected to the right output of a Mackie mixer in my music practice room from Dec 2010 until Mar 2018. The speakers were used several times a week, never saw the road, and very rarely had the level above 1/4. Interestingly, they failed with the same symptoms in Aug 2017 but when I opened the unit for testing, it started working on its own, so I just re-assembled and reconnected. I did not check voltages at the time of the first failure.

Current State: I have the amp removed from the cabinet, speakers and from LED disconnected; power board disconnected and removed. I have replacement SS8050 and SS8550 transistors and two 1N5245BTR zener diodes from Digikey, but the 25v 220u caps and 100u caps are too big to fit on the board, so I ordered 16v that should be delivered Monday. I even have the 2.2k surface mount resistors, just in case. So basically parts for the entire 28v to 15v circuit. I inspected all boards and caps, nothing obviously leaking or burnt anywhere (no stink either).

I am a bit concerned about what might have caused the 8050 transistor to fail. Op amp, diode, or cap on the preamp board possibly or maybe it just died on it's own.

Any thoughts on steps I should take? Should I just replace Q104, C118, C120 and try it or is there further testing I should do on the preamp board before powering up?

Thanks.

[denormalizer]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

I was able to get the +15v working replacing Q104. Replaced Q105, the caps and zener diodes for a exercise in soldering desolder I suppose since Q104 was the only bad part.

Still not working, power LED cycles from on to dim about every second. If I disconnect the output of the preamp from the input of the power amp, this stops.

I am thinking the next step is to inject a signal to the power amp with speakers connected and preamp disconnected to verify that it is working. If it is, move on to the preamp board.

[denormalizer]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Power Amp not working; I went ahead and tested preamp including DSP, all good there. Swapped preamp section with my working amp and it worked there too.

So looks something on the main power amp board.

[MaxnanosuitV3]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

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 doesn't 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. It's 500mA rated. I decide to change it with a much powerful SS8050. It gets hot too, but it'll 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 that's 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 I've received another Thump15 for repair, same symptom. I'll report if it's the same case. If so, it will be clearly a design quality issue.

Have you had any issues with it? I've got the exact same problem, I've checked if I had the +15V and they werent there, so I just wanna make sure before desoldering stuff.

[Chungalin]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Have you had any issues with it?

No returns so far, but remember that repairs are not recipes. Every case may be different, regardless of showing same symptoms. In case of quality issues or design flaws, a certain repeatability is expected.

[Cocco]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Hello guys, same problem here.
I solved replacing that little piece (replace S8050 with SS8050) and it worked !!
Thank you so much for the help!

[Jnahanna]

Re: Mackie Thump TH-15A repair, no sound and OL LED always on, power supply, low qual

Hey folks I had the same issue with two of my Thump amps. Thanks for all the previous troubleshooting work. I appreciate the ideas. I supplied both of them with my bench supply and found they draw 60mA. This equates to about 900 mW of dissipation which is really too much for either transistor in a warm environment which these would be in. Instead I simply installed a 7815 linear regulator and mounted it to the chassis. I cut out all the old normal 15v supply parts and made my own.
Works great!
Thanks, Jim