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?
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?
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.
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
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
Hi tvi,
The voltage amp can be tested by taking the current stage out of circuit and some rewiring with a couple diodes added.
-Chris
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
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.
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.
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?
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.
-Chris
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.
Okay, that means a power supply problem, but please check for DC offset to be certain. It still could be an amplifier problem
My mistake. The manual I see covers more than one model.
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!
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
Hello again,
thank you all for your time.
More results today! All measurements were done out of circuit!
All diodes are good!
TR3 and TR4 are good!
All, but one, fuse resistors are good. I have a single fuse resistor (FR1 68Ohm, in main amp schematics) with an open circuit DMM reading. FR2,3 and 4 are ok.
Still have to check TR1 and TR2.
I'll finish checking tomorrow and try to find a FR replacement. After that, all my efforts are towards STK3042 and STK2145. Should I try to check them out of circuit first? Any ideas what to look for?
Thank you, Chris for your help.
Dimitris
thank you all for your time.
More results today! All measurements were done out of circuit!
All diodes are good!
TR3 and TR4 are good!
All, but one, fuse resistors are good. I have a single fuse resistor (FR1 68Ohm, in main amp schematics) with an open circuit DMM reading. FR2,3 and 4 are ok.
Still have to check TR1 and TR2.
I'll finish checking tomorrow and try to find a FR replacement. After that, all my efforts are towards STK3042 and STK2145. Should I try to check them out of circuit first? Any ideas what to look for?
Thank you, Chris for your help.
Dimitris
Bunkum.
The device dissipation is 4W with max temp 100 C...keep them as cool as possible.
A simple flat plate attached is good enough to keep temps down.
Dan.
Hi Dimitris,
Sounds great! You're on the road to a working stereo now. Don't condemn those STK circuits yet, do you have a variac (variable AC transformer) so you can power it up gradually? Saves a lot of dead parts if you do.
Sometimes you can replace an STK pack with a different mark number, but other times you can not. They may even have different numbers of pins. I have replace lower voltage ones with higher in the same group.
Hi Dan,
All I can say is that they are designed to run at elevated temperatures to keep thermal drift down. I got that information from a design engineer who uses them. His information sounded very reasonable since there are no mounting holes to attach these voltage amps to a heat sink.
The spec book does not mention anything about external cooling and the only company I had ever seen attempt to use a heat sink was Yamaha. That was back in their bad old days when they used STK products. The actual failure point is where the pins are soldered to the substrate. The solder connections become very rough looking and release the leads. This probably also makes a lot of noise in some cases. I do agree that they run too hot.
-Chris
Sounds great! You're on the road to a working stereo now. Don't condemn those STK circuits yet, do you have a variac (variable AC transformer) so you can power it up gradually? Saves a lot of dead parts if you do.
Sometimes you can replace an STK pack with a different mark number, but other times you can not. They may even have different numbers of pins. I have replace lower voltage ones with higher in the same group.
Hi Dan,
All I can say is that they are designed to run at elevated temperatures to keep thermal drift down. I got that information from a design engineer who uses them. His information sounded very reasonable since there are no mounting holes to attach these voltage amps to a heat sink.
The spec book does not mention anything about external cooling and the only company I had ever seen attempt to use a heat sink was Yamaha. That was back in their bad old days when they used STK products. The actual failure point is where the pins are soldered to the substrate. The solder connections become very rough looking and release the leads. This probably also makes a lot of noise in some cases. I do agree that they run too hot.
-Chris
Ok, designed to run hot sounds reasonable......BUT.
It could be the internal solder joints going sour, but IME the noise sounds more like a transistor failed noisy.
Either way, I would recommend to reduce the operating temp by fitting some sort of additional surface area....like I suggest a flat plate of ally or copper doubling the device area would help.
IIRC there was an AIWA mod describing exactly this.
Dan.
Hi Dan,
Guess how I got into the conversation with said engineer in the first place. Believe me, we are both thinking along the same lines.
Yes, the noise does sound like a transistor. I have heard similar sounds from resistors and small ceramic capacitors. Have you ever peeled one open? I have done many, just to see if there was a way to find an intermittent crackle with more certainty.
One day I very carefully managed to peel the plastic off, then removed the sealer along the bottom where the pins connect to the substrate. That day I actually fixed the module by getting the solder reattached. Normally you can't solder that stuff. I think they use the glue to give the connection mechanical strength. Got it working, then replaced the chip anyway and tossed the good=bad part in the bin. No way was I going to be responsible for that "fix".
-Chris
Guess how I got into the conversation with said engineer in the first place. Believe me, we are both thinking along the same lines.
Yes, the noise does sound like a transistor. I have heard similar sounds from resistors and small ceramic capacitors. Have you ever peeled one open? I have done many, just to see if there was a way to find an intermittent crackle with more certainty.
One day I very carefully managed to peel the plastic off, then removed the sealer along the bottom where the pins connect to the substrate. That day I actually fixed the module by getting the solder reattached. Normally you can't solder that stuff. I think they use the glue to give the connection mechanical strength. Got it working, then replaced the chip anyway and tossed the good=bad part in the bin. No way was I going to be responsible for that "fix".
-Chris
I have opened and failure studied many many STK output amplifier hybrids, but never got around to opening/studying a 3042 or 3042II driver stage.....I now wish that I had known that the failure is the pinout solder joints......if that is indeed the typical failure mode.
Do they sound different running 'naked' ?
Dan.
Do they sound different running 'naked' ?
Dan.
Hello guys, thank for keep up posting!
More news today!
TR1 and TR2 both fine! Replaced the FR1 and.... ta da!!! We have sound again!
BUT crackling is still there and, I must add, worse than before (recapping).
I tend to thing that the preamp section electrolytic caps where not to blame!
Any suggestions about STK3042 and STK2145? Could I check them in or out of circuit? I should add that the crackling noises are in both headphones and speakers, the volume pot has no effect on them and the left channel is worse than right channel.
I think that I have enough caps to complete recapping in main amp.
Should I replace STK3042 anyway? Is there any alternative IC?
Sorry for all the questions, but I really want to make this amp work. I enjoy the procedure and I would like to give this budget amp a few more years of music!
More news today!
TR1 and TR2 both fine! Replaced the FR1 and.... ta da!!! We have sound again!
BUT crackling is still there and, I must add, worse than before (recapping).
I tend to thing that the preamp section electrolytic caps where not to blame!
Any suggestions about STK3042 and STK2145? Could I check them in or out of circuit? I should add that the crackling noises are in both headphones and speakers, the volume pot has no effect on them and the left channel is worse than right channel.
I think that I have enough caps to complete recapping in main amp.
Should I replace STK3042 anyway? Is there any alternative IC?
Sorry for all the questions, but I really want to make this amp work. I enjoy the procedure and I would like to give this budget amp a few more years of music!
Crackling is the usual failure mode of STK 3042.....read above posts about 3042 failures.
Yes.
No.
Dan.