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Old 13th May 2015, 09:20 PM   #1
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Default Amplifier Akai am-u33, powers on, but no sound

Hello, I have an Akai am-u33 amp that has a problem.
When I bought it, I noticed a crackling sound coming from the speakers. After the logical tests (speakers ok, input source irrelevant, thorough cleaning of all pots and switches) the problem located in the preamp section of the amp (one channel worse than the other). An inspection for solder joints problem didn't solve anything.

My first decision was to recap the amp (electrolytic only), starting with the preamp section. During testing and probing for faulty parts and before the recapping process, the unit stopped working. I proceeded with the recap anyway (a merely 3euro cost) and now the situation is as follows:

The unit has new electrolytic caps in the preamp section.
The power transformer works great and the power supply pcb board delivers as expected. The unit powers up and the input source indicator lights work just fine.
BUT... no sound at all! The headphone output is also dead.
I tried to use the preamp and power sections independently... NO luck...
And by dead, I mean dead silent. Absolutely no sound at all, not even a hiss.

Any thought's what to look for? Where to look first?
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Old 13th May 2015, 10:22 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by dlamprou View Post
I tried to use the preamp and power sections independently... NO luck...And by dead, I mean dead silent. Absolutely no sound at all, not even a hiss.
Akai AM-U33 Manual - Stereo Integrated Amplifier - HiFi Engine

Neither the preamp by itself, nor the power amp by itself work? Have you checked all the internal fuses?
If the power supply voltages are ok, it's hard to say what would cause this.
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Old 13th May 2015, 10:45 PM   #3
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Hi,
Thank you for your answer.
I already have the service manual, I used it to confirm the caps characteristics.
I've checked all fuses, no problem there.

Hmm, my only idea is to take apart and start checking all diodes and transistors. Any thoughts?
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Old 13th May 2015, 10:56 PM   #4
anatech is offline anatech  Canada
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Hi dlamprou,
Have a look at TR12, 2SC313, +15 V regulator. Make certain it is working. TR1 and TR2 are your +/- roughly 20 V regulators - check-em.

You also have a bunch of fuse resistors - measure them because they normally look fine when burnt. You do have a speaker protection relay with power delay turn on via IC3, a TA7317P. If the relay doesn't go click after turn on, you might have a DC offset (measure the output of the STK2145 at the inductors (L1)) that would keep the relay from closing. There has been enough disturbed in there to cause a problem almost anywhere in your travels.

-Chris
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Old 13th May 2015, 11:59 PM   #5
tvi is offline tvi  Australia
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Quote:
STK-3042 will even give VERY LOUD crackling noises in the speakers when heating up before the speaker protection kicks in (if any). Gently tabbing the case of the STK-3042 with a screwdriver-handle with also provoke these bursts. The STK-3042 has been replaced with STK-3042II and it doesn't have this weakness afaik."]The STK-3042 will even give VERY LOUD crackling noises in the speakers when heating up before the speaker protection kicks in (if any). Gently tabbing the case of the STK-3042 with a screwdriver-handle with also provoke these bursts. The STK-3042 has been replaced with STK-3042II and it doesn't have this weakness afaik.
STK3042/3062/3082 series seem to be problematic
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Old 14th May 2015, 02:31 AM   #6
anatech is offline anatech  Canada
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Hi tvi,
Quote:
STK3042/3062/3082 series seem to be problematic
Yes, they sure can be! But I normally don't suggest even thinking about this until more is known. Everyone wants the quick fix. Pinching these will normally cause DC shifts in the output. Don't forget about the solder joints to the PCB though. Same for the regulator transistors.

The voltage amp can be tested by taking the current stage out of circuit and some rewiring with a couple diodes added.
-Chris
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Old 14th May 2015, 09:33 AM   #7
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Thank you all, for your answers.
I'm also afraid of STK-3042's condition. Is it posible to substitute STK-3042 with STK-3042II ?
As for the crackling, I used a second (power) amp to check the preamp section and it was still there (the crackling sounds). I assumed that the preamp section was guilty. My mistake?

I'll get back with more info.
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Old 14th May 2015, 04:49 PM   #8
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Quote:
Originally Posted by anatech View Post
Hi dlamprou,
Have a look at TR12, 2SC313, +15 V regulator. Make certain it is working. TR1 and TR2 are your +/- roughly 20 V regulators - check-em.

You also have a bunch of fuse resistors - measure them because they normally look fine when burnt. You do have a speaker protection relay with power delay turn on via IC3, a TA7317P. If the relay doesn't go click after turn on, you might have a DC offset (measure the output of the STK2145 at the inductors (L1)) that would keep the relay from closing. There has been enough disturbed in there to cause a problem almost anywhere in your travels.

-Chris
Back with more info.
First of all the relay doesn't click.
As for TR12, 2SC313, +15 V regulator sorry but, didn't find any TR with these specs on board.
I'll check TR1, TR2, TR3 and TR4 after desoldering them.

Some issues with fuse resistors and a couple of diodes though. All measurings were done "on board". Later I'll desolder and measure the prime suspects.

I also need a critical advice. If my test shows bad diodes or fuse resistors what should I do? Replace them first and then proceed to IC's checking?
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Old 14th May 2015, 10:25 PM   #9
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STK3042 fail noisy regularly.
The only solution is to replace and make/fit a heatsink to keep temp down.

Dan.
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Old 15th May 2015, 09:23 PM   #10
anatech is offline anatech  Canada
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Hi Dan,
Nope. Those voltage amp ICs have internal heaters. Do not install a heat sink!

Hi dlamprou,
You are doing fine. Your problem sounds like it is before the amp section. Still, give the power supply a good going over.

Quote:
First of all the relay doesn't click.
Okay, that means a power supply problem, but please check for DC offset to be certain. It still could be an amplifier problem
Quote:
As for TR12, 2SC313, +15 V regulator sorry but, didn't find any TR with these specs on board.
My mistake. The manual I see covers more than one model.
Quote:
If my test shows bad diodes or fuse resistors what should I do?
Desolder the part(s) and check again out of circuit. Bad diodes and transistors normally fail short, resistors can go low, then high and finally open. You can have open junctions in transistors, E-B would be the normal open failure. Be aware that voltages long after the power has been off can affect your readings. Always test for voltage across your components as well. First, in fact!
Quote:
Replace them first and then proceed to IC's checking?
Yes, solve issues as you find them without checking with pull power applied. Sometimes you can test an STK IC out of the circuit. That way you aren't always waiting before moving on.

-Chris
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