AKAI integrated amp - blows a fuse and has a low gain TUNER input

[michalism]

Hello

I'm working on an AKAI AM-U22 integrated amplifier. The original issue I had was that the tuner input was very low. ie, when I connected the tunner on AUX or TAPE inputs, it was playing much bettter than when I was using the TUNER input.
 The amplifier is based on the STK463 ic, which seems to be a single input i/c. So I would expect that if the issue was on the ic, then all inputs should be weak.
In any case, I started taking some measurements on all pins, and I saw some pins were much higher than expected (almost at 60V instead on 33,7 or 35,5). So, I will need to check the power supply.

However somewhere in the process, my probe slipped and I sorted something  Now everytime I try to power up the unit, it blows one of the fuses.
I checked the path of the fuse, and I could not find anything that could cause an excesive current draw, appart from the ic itself. With the IC removed from the board, the fuse is not blown.

So, my questions to the fellows here:
 - is it safe to assume that there's a short somewhere in the IC and by replacing it I can move forward with my troubleshooting?
 - what could cause a low amplification on the TUNER input. Maybe a bad selector switch?
 - not sure the title of the post is accurate or confusing. please forgine my ignorance...still low on the learning curve.
I can provide the service manual if requested. I am not sure if I can just post it here...can I?

Thank you very much in advance!
Michalis

[SpecialK]

I understand getting STK modules is getting tough.  Hopefully you can get a good one.  Check the power supply voltages before installing the new one.

I understand that there are three fuse holders, but only one should have a fuse dictated by your utility voltage.

[floobydust]

Check the STK463, it might have shorted transistors inside. Check for a short between pins 9-7, 9-10 or pins 11-12, 5-6: the output transistors inside, i.e. TR 11, TR10 for both channels- using diode-test or ohms.
The Tuner, Aux, Tape 1&2 inputs all go straight to the volume control after the swithces, so I would clean the switch contacts.

[michalism]

I understand getting STK modules is getting tough.  Hopefully you can get a good one.  Check the power supply voltages before installing the new one.

I understand that there are three fuse holders, but only one should have a fuse dictated by your utility voltage.

I am able to find a good STK from at least two reputable physical stores here in Greece for about 10 Euros. That's not expensive at all; ebay is much cheaper, but then I need to wait for more than a month (in some cases), and may get a bad module.
The unit has maybe 6 fuses, and it is using 4 of them. One is for the mains, and there are 3 more on what I assume are the power lines for the IC. One of those is the one that is blown.

Check the STK463, it might have shorted transistors inside. Check for a short between pins 9-7, 9-10 or pins 11-12, 5-6: the output transistors inside, i.e. TR 11, TR10 for both channels- using diode-test or ohms.
The Tuner, Aux, Tape 1&2 inputs all go straight to the volume control after the swithces, so I would clean the switch contacts.

I did check many pin combinations and I was not able to find a short. However, following your advice and checking again, I found out that pins 9-10 are shorted for sure!!! Aparently I missed that combo.
Now I now for sure the IC is toasted and can order a new one.
I will check the voltages on each pin before inserting the new module in.
Any other advice to make sure that the new module does not get toasted as well? (Apart from being more careful ?  )


As a side note and trying to understand better what happened:
 Is there a chance that this short was there from the beginning, and my actions had no affect? I do not know the previous working condition of the amplifier since I bought it on a flea market as unknown working condition. The area I was working with my probes was close to pins 4-5-6 and not 9-10. Checking the datasheet of the STK, I can't figure out how I may have caused this short on pins 9-10. Also, after the spike from the slipped probe, a capacitor near the fuse blew, and that made me notice the fuse was blown. I need to check if the capacitor was on the same track withthe fuse...

Sorry for the long post, I am just curious to find out what happened 

[PKTKS]

If you want to spare the money for new ICs...

Remove the IC and put in the "TEST" circuit suggested
on the SANYO datasheet.

It is simpler (far simpler) than the real amp and
a 25W load is not that hard to achieve.

Once checked properly with a BIAS and sine wave
you can move on.

Typical step in expensive integrated power amps

Paul

[michalism]

If you want to spare the money for new ICs...

Remove the IC and put in the "TEST" circuit suggested
on the SANYO datasheet.

It is simpler (far simpler) than the real amp and
a 25W load is not that hard to achieve.

Once checked properly with a BIAS and sine wave
you can move on.

Typical step in expensive integrated power amps

Paul

Can you please elaborate a little on this?
I have seen two datasheets for the STK, one completely in Chinese and the other is a single page, but I did not see any test circuit. Can you provide a link to the datasheet you refer to?
...also, since one of the internal transistors is shorted, how is this going to save me from buying a new module?

[floobydust]

[...] after the spike from the slipped probe, a capacitor near the fuse blew [...]

Which cap blew?

[PKTKS]

Can you please elaborate a little on this?
I have seen two datasheets for the STK, one completely in Chinese and the other is a single page, but I did not see any test circuit. Can you provide a link to the datasheet you refer to?
...also, since one of the internal transistors is shorted, how is this going to save me from buying a new module?

Use the test circuit by SANYO itself - simpler - cost effective

The english scan is not top quality but readable at least

[michalism]

[...] after the spike from the slipped probe, a capacitor near the fuse blew [...]

Which cap blew?

 I checked my notes and apparently it was C20 (100uF/50V) , which is connected to the fuse that is blown...I guess that is answering my question if the blown fuse was there from the past...(?) But still, I can't understand how I caused the short, while I was working where I consider a area that is not related to the shorted part of the IC...

Can you please elaborate a little on this?
I have seen two datasheets for the STK, one completely in Chinese and the other is a single page, but I did not see any test circuit. Can you provide a link to the datasheet you refer to?
...also, since one of the internal transistors is shorted, how is this going to save me from buying a new module?

Use the test circuit by SANYO itself - simpler - cost effective

The english scan is not top quality but readable at least

OK. we do have the same docs. I suppose by test circuit you mean the example setup that is on the top of the second page. I would like to do it just for educational purposes, but how is this going to help me to troubleshoot the amplifier (not the IC) itself? Especially since we know a part of the IC is shorted?

[PKTKS]

OK. we do have the same docs. I suppose by test circuit you mean the example setup that is on the top of the second page. I would like to do it just for educational purposes, but how is this going to help me to troubleshoot the amplifier (not the IC) itself? Especially since we know a part of the IC is shorted?

Quite simple.
That is not educational (only) it is indeed  a working draft
you can easily mount on a breadboard

Feed that with a 1K signal and check your load.
Done that several times on 80/90s with several power ICs

You will not measure THD or noise or such things.
Pretty much just check a faulty IC

Also works on BTL type ICs - if they are ok stand alone
but faulty on BTL configuration

Paul

[michalism]

Hello.

I have bought a new IC, but before putting it on the board, I did some measurements, and apparently someting is wrong.
PIN   | Volts
---------------
1   | 0,1
---------------
2   | 0
---------------
3   | 0
---------------
4   | -37,5
---------------
5   | -37,5
---------------
6   | 0
---------------
7   | 0
---------------
8   | 37,5
---------------
9   | 37,5
---------------
10   | 0
---------------
11   | 0
---------------
12   | -37,5
---------------
13   | -37,4
---------------
14   | 0
---------------
15   | 0
---------------
16   | 0,1
---------------

Pins 4 & 13 seem to be the ones that are way off (at -37,5 instead of -1.2. I am not sure if adding the IC will affect these voltages. If I am reading the schematic correctly, these are input pins, so the voltages are controled be the voltage dividers around the IC. But the resistors (R18/ R19) read ok on my meter.

The other issue is that I measured the voltages on TR1 and I was getting 0V on the emitter. I pulled the transistor off, and it seems to be faulty. At least that is when my component tester says. Base and collector voltages were close to the expected values (around 37.5V). Unfortunately, I do not have suitable replacement at hand, so I need to go to the store and this means another delay

I have two questions now:
Where should I look at for the erroneous voltage values on the IC pins?
Which transistor can be a good replacement for 2SC1627 in case I do not find the exact part?

Thank you!

[floobydust]

With the STK463 out of circuit, some nodes will float, so the voltages at pin 4, 13 will be way off but this is to be expected and not a problem. This will stress 47uF 35V capacitors C19 at those pins though.

TR1 2SC1627 appears to be a ripple filter only for the phono preamp power. ECB pinout 80V 0.3A 0.6W I would use whatever is in the junkbox it is not critical as long as it can handle the power.
I would check R8 2.2k fusible resistor is not open-circuit, and caps C35 100uF, C12 220uF are OK, in case  a short there overloaded TR1.

[michalism]

Thank you.
So I am assuming that it is safe to put the IC back on and take the measurements again.

I did check some resistors and caps and checked ok, I need to verify that they are the ones you are suggesting and reply.

Hopefully I will have some news later tonight.

[michalism]

Back with new findings...
I replaced the IC, and The voltages seems to be ok, although there are some deviations from the values in the schematic. (I will add the table with the measurements below.

What I'm dealing with now in one channel audio. Both the AUX and TAPE inputs of the amp sound ok, but only from one channel. I used my scope to check the inputs to the STK, and I do not get any signal on pin 16 (I think this is one of the inputs?) and of course no output on pin 11. I checked R12 and C16 but they are both OK. The balance and volume controls work fine, and therefore I am assuming that the problem must be on the signal path towards pins 14 & 16.

Another thing I can't explain due to ignorance is why I hear a buzz from the speakers when I touch pin1 with the multimeter probe.
I am pretty sure that both channels worked fine with the previous IC.
Voltage measurements (schematic value in parenthesis) taken with respect to point 16 of the schematic.
Pin1   -0.18 (0)
Pin2  - 0.18  (-0.18)
Pin3  0 (none)
Pin4  -0.7 (-1.3)
Pin5  -37,1 (-35.5)
Pin6  -0.137 (0)
Pin7  -0.122 (0)
Pin8  36,32 (33.7)
Pin9  37,34 (35.7)
Pin10  -0.18 (0)
Pin11  0 (0)
Pin12  -37,35  (-35,5)
Pin13  0,153 (-1.2)
Pin14  0 (none)
Pin15  -0.18 (0)
Pin16  -0,18 (0)

Not sure what else can be causing this behavior...

[floobydust]

You're getting 0.18V between pins 11, 10 across 0.47R resistor is 383mA which should make smoke or heat.
Pin 11, 10 (and pin 6, 7) should have almost the same voltage- unless the 0.47R 3W resistor R20 is open circuit. I hope it's a bad value, sometime these are fusible resistors. Otherwise I suspect the new STK might be different?
Pin 1 is the actual (STK) R-ch audio input, Pin 16 is the L-ch audio input, sensitive nodes that should hum and buzz when you put a probe there, this is normal.

[michalism]

You're getting 0.18V between pins 11, 10 across 0.47R resistor is 383mA which should make smoke or heat.
Pin 11, 10 (and pin 6, 7) should have almost the same voltage- unless the 0.47R 3W resistor R20 is open circuit. I hope it's a bad value, sometime these are fusible resistors. Otherwise I suspect the new STK might be different?
Pin 1 is the actual (STK) R-ch audio input, Pin 16 is the L-ch audio input, sensitive nodes that should hum and buzz when you put a probe there, this is normal.

I have not noticed any heat or smoke, but I will check again. I understand that this 0.18 Voltage may cause problems, but it still does not explain why I am getting no signal on pin 16, right?
the STK module is bought from a reputable store, I doubt it's problematic...but still, you never know.

EDIT: I checked the voltage across R20 and it is 0. Same for its pair on the other half of the amp... I am connecting a source again to check...

[floobydust]

Pin 16 should have audio unless there is another problem causing that. Inject audio on one channel and try the stereo/mono switch. That pretty much bridges pin 16, 1 together for AC signal (but before the volume/balance controls, not after). The LED meters would also indicate signal. A potentiometer or switch might be dirty.
You can also inject audio for the one channel say AUX in and just follow it, should be at tape output and the volume control.

[michalism]

Yeah..I will have to do the tracing...It's just that my space is so small and having two devices here is not as easy as it may sound...

[michalism]

well...it works...I'm holding my breath while re-assembling the unit, but for some reason it works now.
I did not do anything...
could it be that I wired the LEB meter in a wrong way and this somehow affects the circuit?
I don't know...
Hopefuly this thing keeps playing when I finish the re-assembly. Let's see...