Help with Akai Amp Output

Hi,

I am working on an old Akai amp and would like some advice before I use a replacement STK0050II as an expensive fuse!

Symptoms are:

No output
No protective relay click

checked relays, both are ok

I apologize in advance for the tables below not lining up properly in the post. They looked fine when I submitted!

Measured voltages on STK-3042

Pin** Voltage Measured ** Voltage on Schematic
1 ** 7.42 ** 8.0
2 ** 7.43 ** 8.0
3 ** 45.8 ** 47.2
4 ** 45.5 ** 46.9
5 ** -5.01 ** -4.97
6 ** 5.42 ** none
7 ** -47.3 ** -48
8 ** 0.4 mV ** 0
9 ** 46.4 ** 47.7
10 ** 30.76 ** none
11 ** 18.7 ** -4.97
12 ** 44.8 ** 46.9
13 ** 46.1 ** 47.2
14 ** 6.53 ** 8.0
15 ** 8.63 ** 8.0

Voltages on STK0050II (A and B, Left and Right channel)

***** Left ****** Right
Pin ** Measured ** Measured ** Schematic
1 ** 21.76 ** -1.6 ** none
2 ** -43 ** -43.5 ** -44.7
3 ** -43 ** 167 mV ** 0
4 ** 21.8 ** -1.6 ** -1.7
5 ** 0.5 ** -0.41 ** -0.5
6 ** 36 ** 0.79 ** 0.6
7 ** 25.6 ** 2.04 ** 1.8
8 ** 37.1 ** 210 mV ** 0
9 ** 43.3 ** 43.2 ** 44.8
10 ** 25.65 ** 22.4 ** none

TA7317 Voltages

Pin ** Measured ** Schematic
1 ** 0.76 ** -0.6
2 ** -0.78 ** 0
3 ** -0.72 ** 0
4 ** 0.6 ** GND
5 ** -0.71 ** -0.7
6 ** 48.5 ** 0.9
7 ** 3 ** 0
8 ** 5.7 ** 1.3
9 ** 3.01 ** 3.1

I suspect based on above voltages that the that the LEFT channel STK0050II is bad, but I am not sure if the STK-3042/TA7317P killed it! I do not want to replace the 0050 and have it get destroyed by the 3042 or TA7312P.

Any advice/additional test ideas from more experienced members of the forum would be greatly appreciated.

Thanks in advance for your help with this.


Mitch

What model Akai do you have?

Ooops.... I can't believe I forgot to include that info! It a AM-U04.

Sorry!


Mitch

I have seen quite a few STK-3042 chips being defective and NO STK-0050II.
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.

best,

/tri-comp

Thanks for the reply.

I had read about failed STK-3042 units as well, but I am not sure the voltages measured support that mine has failed. It seems that one STK-0050ii has the Vcc on the outputs which suggests its bad, but I am just not sure.

As to tapping the 3042, speaker protection is on at start up and the output relays do not ever click in to connect the module outputs to the speaker terminals. If the 3042 is bad it seems dead bad!

If it needs replacing then I will definitely go with the new version, as you suggest.

Thanks for your inoput.

Mitch

It should be fairly easy to check if the STK-0050II is shorted or not.

Pin 9 is the B+ supply voltage and Pin 2 is B- supply.
On Pins 8 & 3 are the output transistors emitter-connections.
Any short from 9 to (8 or 3) or from 2 to (8 or 3) indicates a blown chip.
8 to 3 are suppose to be shorted as long as the chip is inserted into the board because of the external emitter-resistors (<1 Ohm). When removed from the pcb this short should go away.

have a look at the STK-pdf's here:

http://www.audiokarma.org/forums/showthread.php?t=103737
http://www.audiokarma.org/forums/showthread.php?t=41504


rgds,

/tri-comp

I really appreciate your help with this and thanks for the attached photos. Very helpful.

I'll try to follow your advice this week end and let you know what I find!

Thanks again,

Mitch

Hi All,

Well followed tri-comp's advice and checked the two STK-0050II modules.

Unsoldered pins 2,3,8 & 9 and found a short across pins 2 and 3 on one module. Also found that the resistor across the outputs (pins 3 and 8) was open. This amp uses a double 0.47 ohm emitter resistors to ground from pins 3 and 8: one side of the resistor measured open out of circuit.

Do you think that the resistor opened and killed the module or is it more likely that the module took out the resistor?

BTW, the heat sink compound on the back of the bad module was very, very dried out...could be a heat related death?

It's really a question of curiosity as both the module and the resistor needs to be replaced.

Based on what I found any thoughts on whether the STK-3042 might be the cause or is it likely to be ok?

Thanks again for all the help sorting this out.


Mitch

Mitch,

The chip killed the resistor, not the other way around.
So, at least you have one defective STK-0050II with a short between pins 2 & 3 and a defective dual-resistor.
You may even want to pry open the defective chip. Remove the black plastic front (Wise, Flat Screwdriver and some luck) and you can most likely see the blown transistor easily. It will have some kind of crater like from a mini-explosion. Obviously it will do you no good, as it can't be replaced but still I think it's interesting.
Beware, the STK-3042 may still be the root-of-all-evil if it has some temporary fault, usually temperature dependant. Keep the opened amp. under surveillance for a while after replacing the 0050II.

Good luck,

/tri-comp

Hello tri-comp and others!

Great idea to pop off the top of the module. Came off ok and really nice inside! Amazing...guess its what's called a thick film hybrid?

One transistor (TR5 on the diagram you referred me to) to pin 8 was actually shattered, with 3-4 cracks in the substrate of that transistor mounted on a heat sink...pretty amazing! I have a dissecting microscope and had a close look at it. So, that transistor was obviously damaged, but no burn mark...I did find a small black crater in the film itself...probably a resistor burned up. Looks like it bridges pins 5 and 6, so resistor R6

So yes half a dual resistor damaged...any idea where to find another with values of 0.47 ohms each or should I just replace it with 2 0.47 ohm 5 watt resistors? Do they need to be in the same package to track thermally?

BTW, I had a closer look at the STK-3042 and it is actually an STK-3082II. This was a pre-owned amp and someone must have been in there before me. Is the 3028II a direct sub for the 3042? If not, should some components in the circuit e changed to accomodate it? The schematic clearly states a STK-3042.

Thanks for any advice you can offer,

All the best,

Mitch

Parts Express used to carry the dual emitter resistors.

You can go with a dual emitter-resistor as was used originally, and that will look more neat, or you can just take 2 separate resistors. No temperature tracking here.
The STK-3042 was already changed which imo makes good sense. The models w/o II to the typeno were not stable. Yours was replaced with an overkill-for-this-purpose chip but that's all right. It was just designed to drive a bigger output-chip with higher supply-voltage and it will run fine in you amp.

Perhaps you should give some consideration to re-cap the circuit around the STK-3042/3082II. There are some small electrolytic caps I would replace now the lid is off. Look for cap's connected to pins 1,2,7,9,14 & 15.

Yes, isn't looking inside a thick-film hybride interesting!?

rgds,

/tri-comp

Hi All,

Just wanted to update you on the status of my amp. Sorry for the delay but family health issues, my job and holiday shopping got in the way of my fun!!

Anyway, ordered the STK0050II from B&D (great service and fast shipping), and the dual emitter resistor from MCM (ordered a few, $7 for resistors and $20 for shipping!! Ouch!). Cleaned up the heat sink with alcohol, put new sink paste on the back of the new module and attached it to the heat sink. Replaced open emitter resistor and success! Both channels operating fine! A very nice sounding amp.

But, output only if I bypass the pre-amp! Pre-amp output was low to none. Looked at pre-amp voltages and found B+ to be very low. I traced the supply line back to TR12 (2SA968), replaced it with a new transistor and valtage came up to spec and the pre-amp is working fine now. Note that the old TR12 passed a test with several transistor testers (B&K), so a pass is not guarantee!

A couple of power supply caps (6800 uF @50V) have somewhat bulging tops, so I have ordered 4 new ones and will replace as soon as they arrive.

I just wanted to thank everyone for their help with this project. You pointed me in the right direction, I fixed problems and I learned a lot in the process! I appreciate everyone's kindness and willingness to share their knowledge with we newbies!

Thanks and I wish everyone happy holidays and a wonderful New Year!

Mitch

Hello! Sorry if my english is not good! Thank you from the bottom of my heart! I had AKAI AM-U310 since 1982 till few days ago. This amplifier means a lot for me. Before it died out it was connected to PC, I was listening music at a relatively high volume and my PC is setted to enter in standby after no commands is performed and HDD and monitor stopped. In that moment I've heard a big boom sound and no sound in speakers after I've exit in standby. I stopped the amplifier and turned on and the click start sound no longer appeared and no sound in speakers. Also sometimes I had sound only on right channel together with a crackling sound from speakers on left channel. All being said you think that STK / STK's is broken? I want to make this amplifier to work as good as it can be. I'm sort of audiophile person. Kind regards!