issue with AKAI AA-1030 Receiver

[rotco]

Pin 12 calculates to be around +42 V.
Unsoldering collectors of muting transistors should allow full output if this is where the problem is.
Checking base voltages should tell something
No idea what other transistor is doing. Good Luck
View attachment 1288993

Steve! you are the best!
thanks a lot!
i will be able to do it only after my return in 2.5 weeks. watch ahead.

i will unsolder this two collectors pins, i believe this will solve the issue!
because when i bypass the filter board, the power amp board worked just fine!

but. should i just give up this mute transistors?
isn't it better to solve the issue instead of bypassing it?
in that case, is it mean that most likely that this transistors is bad and replacing them will solve the issue?
(sure, after taking them off the board and test them).

or, the issue might related to other components feed the base & emitter of these transistors?

thanks again!

 

[sregor]

Unsoldering is to verify problem. To fix you have to check base voltages, and if positive, trouble shoot the rest of the circuit. The only problem is that you may have to figure out the circuit differences from the original, and figure out if the extra transistor does anything.

 

[rotco]

Unsoldering is to verify problem. To fix you have to check base voltages, and if positive, trouble shoot the rest of the circuit. The only problem is that you may have to figure out the circuit differences from the original, and figure out if the extra transistor does anything.

i took muting transistors out of the board. now i get high output at the speakers.
but, i have a strong buzzzzz noise.
i tested the 2 transistors i took out, and they seems to be good.

so, now i wonder.
will it help to change them with new ones? or maybe some other components are the problem?

thanks

i still have another transistor which we dont now it's use yet, and 3 electrolytic capacitors in this board.

btw, i also replaced all 6 electrolytic capacitors on the p.s board.

 

[Nuel]

@rotco, I just posted my findings for the same exact problem in the other thread. Hit me up if you need something. I was away from AK for a while, hence did not see your message. Hope this fixes the AKAI.

 

[rotco]

Summary of everything until now. (almost a year past i started working on it)
it started with no signal at outputs
i found an open cap in filter board.
now i find new issue - the receiver make buzz noise (i stop it by speaker selector switch)
in the video it connected thru DBT, and can hear this noise.
i was assuming its related to the p.s board. during all my work on this receiver i replace all electrolytic capacitors in it.
so i started measure voltages according to the service manual (without DBT), an found at the output of the diode rectifier bridge one point has -39vdc (as expected) and the other has only +26vdc (instead of +39vdc. by the way, i also set the DMM to VAC and found at this point together with the 26VDC also 20VAC).
so, as i understood probably one of the diodes malfunction?
if so, do i need to replace all the 4? according to s.m they are "SILICON DIODE HIFI SPECIAL 400V 3A)
how can i find replacement?

link to the video:

 

[sregor]

20 VAC screams open filter cap or bad connection to filter cap (also 26 VDC suggests filter caps.) Diode is not likely problem.

 

[rotco]

20 VAC screams open filter cap or bad connection to filter cap (also 26 VDC suggests filter caps.) Diode is not likely problem.

Hi @sregor
Thanks a lot for your reply!
Im not really recall the exact Vac value i measured but it was about 20-30vac.
I was really surprised to see both ac and dc on the same point.
My attention was all focused the 26vdc i found which is significally low then i expected to find (+39v)

If something wrong with the filter cap, how is it possible i got desired -39v (negative) on the other "side" of the recitifier?
Maybe only one of them is bad and thats why i got correct for negative and miss-correct for positive?

Thanks!!

 

[sregor]

Hi @sregor

Maybe only one of them is bad and thats why i got correct for negative and miss-correct for positive?

Thanks!!

I think you are correct. Two separate filter caps for plus and minus supply. The one for the plus is open somewhere.

 

[sregor]

If you have anything larger than 500 uF/ 50 volts around, you can parallel it (I use jumpers) and the hum should go away. Just a quick test.

 

[rotco]

Finalllllllllllyyyyy!!
This receiver work come to an end!

I found the cable from filter cap to the p.s board disconnected. Probably break while i lift and turn the p.s board while soldering new caps to it.

Thanks a lot for everyone here who helped me! And also big thanks @sregor (btw, can you please tell how did you know that the filter cap is the couse?)

Bottom line, this reciever weakness point is that filter board, elct. caps, diodes and transistors. (If your issue is no amplifing the signal but receiver seems to be working properly)

 

[sregor]

can you please tell how did you know that the filter cap is the couse?

Primarily experience. Once you've seen the symptoms (loud 120 hz buzz, low DC voltage, high ac which shouldn't be there), then think about how the circuit works, it all makes sense. It is even more obvious if you look at it on an oscilloscope, but that's a luxury not all people have. Glad it's up and running.

 

[petehall347]

well done . all the best for the new year

 

[Nuel]

Good job. Glad it worked in the end.