issue with AKAI AA-1030 Receiver

[rotco]

Hi everyone,

I will say that i have quite a new hobby of fixing audio gears.
i might solve some basic problems and would like to have your help with this one.

This receiver i've got, i already dioxide all its potentiometers, so i would say the issue is not related to dirty contacts.
the issue is, that in low volume, the sound is fine.
but when i raise the volume for average music volume, its start making "boom" like small explosions (especially when it's should playing bass beats)
findings:
1. it's happening in both L/R channels
2 it's happening in all audio inputs (tape, aux, phono and also tuner)
3. it's happening also in the headphones jack.
4. it's not abut the speakers/cables i use. all the peripheral equipment is working Properly.

i guess it's related to the preamp circuit?
what should i do?

Thanks a lot!

 

[elnaldo]

Hello.

You need to find and download a service manual or schematic, so we can refer to exact components and see the circuit.

Please check DC at the speakers terminals before connecting speakers again. Speaker selector enabling the terminal where you are measuring. Expect less than 50mV. If not, post back.

When you say "both channels", does that mean exactly the same noise at the same time in both channels, like in "mono", or both channels have a problem but not exactly the same?

Problems at both channel at the same time usually point to the power supply, which usually feeds both channels.

What tools do you have to trace a fail?

If your amp has "pre-out" "main in" connectors, you can discard the preamp or power amp sections.

IF YOU PLAN TO KEEP TRYING LISTENING TO YOUR SPEAKERS, I SUGGEST TO INSTALL A 2A FUSE OR A POLYSWITCH IN THE SPEAKERS CABLE. So if the amp has a short, doesn't burn your speakers. A polyswitch is an auto-resetable fuse, i have one installed in my test speakers and saved them a couple of times.

 

[rotco]

@elnaldo
Thanks for your reply.

Actually i cannottell if it was from both l/r at the same time. I could check it later..

I have DMM and able to do soldering..
Sure i downloadd the service manual.
The receiver dosent have "pre out main in" connections (see back panel).

Thanksfor the advice of polyswitch, i will try to get one.
Meantime.. I thought of checking it on cheap headphones before i connect my spreakers again.

B.t.w
I already performed idle current and zero dc potential adjustment as described in the S.M.

It didnt help.

 

[elnaldo]

Small explosions are usually caused by transistors failing. A freeze spray is useful to identify which ones.

 

[rotco]

Small explosions are usually caused by transistors failing. A freeze spray is useful to identify which ones.

So
I will start by validate the zero DC at speakers terninal.

Then, i would like to check the power suppply.
Can you tell me what are the desired voltages of it's outputs?

At last, i will use the spray.
Over the 4 output transistors (the TO-3 tansisgors)?

Since i hear the noise in both channels...
Is it correct?

 

[blhagstrom]

Low volume OK, higher volume boom to me indicates a part that cannot handle higher voltage demands.

If it’s both channels, I would look to the power supply since that would effect both channels.

The weakest link in vintage gear is electrolytic capacitors. The second most common is transistors, the third most common but much less so is diodes.

You have a semi-working system and finding the weak part will be challenging. Without experience with electronics repair, it will be a lot of guess work. An o-scope and basic skill to use it would make it a bit easier but it would still be tough without experience with vintage repair.

 

[rotco]

@blhagstrom
I really like your detailed and the explanation you gave for each detail.

Your words making lot of sense to me.

But, for me, it would be much cheaper and easier to replace even 12 caps.. Rather then take it to proffesional lab technician.

So, you said about the power supply, and electrolytic caps.
You mean to caps in the power supply circuit?
Can we point of few spesific caps?
Given that ita happen on both channels.. Maybe it's reduces my possibilities to few specific caps.

Thanksssss!

 

[elnaldo]

So, the noise is identical at both channels, that points to the power supply first .

I agree that tracing an intermittent fail can be difficult without some tools like a dummy load and an oscilloscope.

Power supply board is small, you could start replacing the electrolytic caps ( I see 7) in that board, then 3 transistors and one 13V Zener.

It won't hurt if you are CAREFUL, work very clean, and don't install something backwards (specially transistors, sometimes the leads are not in the same order than the originals). If you do it step by step, everything should be fine.

Your problem could be as simple as a dirty power switch or a dirty fuse holder, so make sure the simplest things are OK.

 

[rotco]

Thanks guys!

I think i've got the right orders to do.
I guess it will took time until i make ot done.. But promise to give an update.

Actually, i see 6 caps in the p.s board.
I visually looked for demeged caps.. But none looks suspect.
Attached pics.

Last tihng please!
Couldn't it be related to the two big main filter caps?

Thanks
@elnaldo @blhagstrom

 

[elnaldo]

Top - right capacitor in your last pict doesn't look OK. The sleeve looks shrunk (that means the capacitor was heated or over heating.

I'd replace all those electolytics anyway.

Never seen a capacitor causing the noise you describe. But fresh caps always help the circuit to work as expected.

 

[sregor]

Not sure it applies to this model, but some Akai (and a few other) amps use one of the screws as a single ground point for the amp board. With age and temperature fluctuations, they can get loose and can cause a loud cracking sound, as well as intermittent protect issues. So - try tightening screws on the amp and power supply boards. If it does take care of problem, you might try a little deoxit on it, then retighten My 2 cents....

 

[rotco]

Hi,

So i made some measurements on live current.
i did it to the p.s board, and to the power amp board.
all measured points fits to the values on the diagram, except one (see pics attached)
i was lucky the manufacturer wrote the desired Potential values on some areas on those boards.
on the p.s board, i checked the values on the diodes bridge (rectifier) all was good.
also i checked the three transistors there, also good.

Maybe you guys can help me from here?
Maybe the fact i didnt get the desired 37.8VAC on point #3 points to any faulty Cap?
Thanks

 

[elnaldo]

looks like a mistake in the PCB diagram. The yellow wire, according to schematic, is connected to ground (ground symbol on the schematic), that means 0 Volt at yellow wire, and 38V at Violet wire (connected to F4).

Anyway, I think your noises and explosions will be difficult to find checking voltages with a meter ,since you reported the unit works OK at low volume.

You could try tapping on the LIVE UNIT (BE CAREFUL) (BE VERY CAREFUL) with a non conductive tool, like a plastic stick, a wood pencil, a plastic screwdriver handle, even a plastic toothbrush, checking if some transistor, connection, or board, reacts to the stress.

As suggested, check screws and connections to the chassis and Power Supply ground.

BE CAREFUL, FILTER CAPACITORS CAN HOLD THE CHARGE FOR LONG TIME IN SOME CIRCUITS, ALWAYS MEASURE WITH A METER THE VOLTAGE ACROSS THE LARGER CAPS, AND DISCHARGE THEM WITH A RESISTOR IF NEEDED.

 

[rotco]

I will wait for the 6 caps to arrive and give an update soon.

Meanwhile, i wanted to take advantage of this post, and ask regarding the transistors i have in other rceiver.
I noticed in many stereo receivers, they are part of the final amp circuit.
They are always comes in couples.
What are exactly they do or responaible for?
Are they was actually couse the signal to be amplified?
I know they are one different from the other in meaning of PNP/NPN.

And most willing to know, whats can be inffered from their name?
If i want to buy and replace them, and cant find the exact couple, what can they be replaced with?
Is the name has a meaning of physical property?
For example, look on this couple i took off Rotel RX-830.
2SB633/2SD613.
Also they marked with E5H/E5K

Thanks!

 

[elnaldo]

I'd leave the originals in, if they are not burned. You can do more damage if desoldering and resoldering components. If you forget an insulation washer or mica when reassembling, you'll short and burn one or more transistors. The same if you make a solder bridge or disconnect some wire when handling the PCBs.

Regarding the function, we need to see them in a schematic. Those TO-220 devices are used as output transistors in small amplifiers, or output drivers in bigger amplifiers. They can be also voltage regulators in power supplies. Without a schematic is not possible to guess the application.

 

[rotco]

"output transistors in small amplifiers, or output drivers in bigger amplifiers."
what is the different? (in short explanation).

i know how to solder carefully.
those transistors i took off from Rotel RX-830. they are shorted already, i have to replace them anyway.
regarding my question... the "model name" which is written on the transistor. is it have any meaning? any physical value?
I already find the exact ones to buy in ebay, but, i wanted to know what is the thing i need to check when i buy replacement.

last thing, i already have in my home a heat conductive paste for CPU i had.
is it recommend to use it also for transistors?
the one iv'e got is:

High quality 80g Kafuter K-5204K High Thermal Conductive Paste Silicone Adhesive Grease Paste Glue for CPU Silion Rubber Gel.

I was reading online that after a period of time, this paste might becoming electrically conductive, and i need to make sure the one that i use will not be like that.

b.t.w
i buy new replacement capacitors in same exact values.
it's turn to be like in physical size (volume) the new ones are about 1/3 of the size.
is it normally?

 

[elnaldo]

the output transistors drive the speakers, they handle the current going to the speakers. The "driver" transistors, drive the output transistors which drive the speakers. Just google some amplifier circuit and you will see it easily.

Your transistors are this ones 2SB633/2SD613:
http://pdf.datasheetcatalog.com/datasheet/sanyo/ds_pdf_e/2SB633.pdf
85V-6A-40Watt
hFE 40 to 320
fT 15MHz

You can replace them with similar transistors, Voltage, Ampere and Watt can be higher, but I'd stay close to the fT and hFE ratings. Check on line some 6A to 10A TO-220 devices, and post back your finds.

I use CPU grease, but yours says "glue", I don't know how strong is that glue. The thermal grease must be NON CONDUCTIVE (there are some with silver inside, which conducts electricity, but it's usually gray, not white)

Newer capacitors are much smaller, that's normal. If voltage and uF is the same , they are fine.

 

[rotco]

Thanks a lot for your detailed answear and your patient for my many questions.
Thats make things much more clear to me.

So, i see no imply from the "name" of the transistor to its electrical properties.
Its only if you look inside the datasheet.
Unlike capacitors which you can clearly see the electrical values.

 

[elnaldo]

Actually if you search papers on the web, there is some coding implied in those names . They are not random numbers. But nothing you can see clearly like the capacitor rating.

 

[rotco]

If i may, A companion question.

regarding the Rotel RX-830 which directly burned the fuses.
i found the issue is bad transistors.
lets say, after i will replace them, i would like to check if it solve the issue.

i'm affraid maybe there is other couses, and still it will couse the fuses to burn again.
is there any way in this situations to predict/measure anything to determine before installing new fuses which might burn again.

i know it's chip, and may just buy few more... but i sure technicians has a smarter solution.
maybe one of the solutions is using the Polyswitch you told me regarding the speakers protecting.
if so, install it in series instead of the fuse?
and which values should i look for?
thanks.