Akai Receiver problem

richardpp

Active Member
Hi,

I'm trying to repair an Akai Receiver, Model AA-R22L.

I've replaced the STK-1844 module and everything is fine except for a low level noise which is common to both channels and not responsive to volume control, but can be made worse by manipulating the tone control. It follows the audio signal, i.e. no signal no noise and is similar to the sound of radio station when not tuned in properly. It is present on all inputs.

I have the schematics but with so many IC's on board, (it's logic controlled so no pots either), it's beyond my ability to troubleshoot. I have tried freeze spray on a few of the transistors on various boards with no effect. Could it be caused by oscillation from the replacement module?

Any suggestions of where I should be looking would be kindly appreciated.

Cheers
 
Yes, quite possible. I tried to purchase from the most reliable source I could find but who really knows!

I'm repairing this for a friend who hasn't used it for a long time and don't know if the problem was there beforehand or not.

Cheers
 
Noise common to both channels suggests power supply. Assume you don't have an oscilloscope. Does the unit have pre-out/main in RCA's.
I will download the sm.
 
Yep, way too many IC's for me.

low level noise which is common to both channels and not responsive to volume control
Likely indicates problem after volume control

can be made worse by manipulating the tone control
Preamp issue

It follows the audio signal, i.e. no signal no noise
A gain issue? no input, no issue?

Suggest take a close look at voltages on opamp in preamp, IC1, pins 4 and 8.

You will need a cro to troubleshoot. A wild guess would be faulty IC1/replace with modern equivalent, use socket for opamp rolling.
That would be a waste of time, better to troubleshoot and find root cause. PC SW versions of a CRO and sound generator are
available for free download at,
https://www.zeitnitz.eu/scope_en

no affiliations.

Warning, not to be used where dc voltage is present, ok for audio path.
 
Noise common to both channels suggests power supply. Assume you don't have an oscilloscope. Does the unit have pre-out/main in RCA's.
Correct, no oscilloscope. I've been looking for some time at a low cost DSO as the need arises more and more now. No pre-out/main RCA's.

Suggest take a close look at voltages on opamp in preamp, IC1, pins 4 and 8.
I'll try and identify and locate the preamp IC. Thanks for the scope link, I will need look into it!

Just to expand on the problem, normal listening volume masks the sound, but at low volume it can be heard. In the second or two of pre programme, i.e. no signal, it can't be heard. However, if I turn off the source and boost or cut the tone control I get noise, sometimes intermittent and pulsing, almost like radio frequency beeping, not buzz or hum.
 
Cell phone close by?
Thanks, good suggestion but no cell phone nearby.

It is clearly interference of some kind, which I can only describe as being like radio signal interference. With so many logic controls stacked in the amp, I'll probably need a lot of luck to find the problem. The next step, following mbz's suggestion, is to get some form of CRO and signal generator and try to isolate the problem.

Cheers
 
In the second or two of pre programme, i.e. no signal, it can't be heard.
I've got no idea what you mean by this. Are you saying there is a number of pre-programmed buttons and on the second one there is no noise.
Have to ask, this may be important. Also to rule out problems with the TUNER, use the simplest config, ie, AUX/CDP.
 
I've got no idea what you mean by this. Are you saying there is a number of pre-programmed buttons and on the second one there is no noise.
Have to ask, this may be important. Also to rule out problems with the TUNER, use the simplest config, ie, AUX/CDP.
Sorry, I meant the couple of seconds of silence before the musical track begins. I am using AUX/CDP as main source, but also testing on PHONO and TUNER to be sure.

Also try using/connecting source to TAPE 1 and/or 2 Playback. Noise present?
Noise still present on these inputs.

On further testing, with nothing connected to the inputs, the receiver is noisy. Further complications are that touching any of the controls alters the noise and there is a 'pop' at power on and at shut down which is not normal. I still wonder if these symptoms are 'oscillation' or high gain? Or possibly even a ground issue.

Thanks for your patience with this!
 
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Problem could be "anything" however I'd be leaning to either power
supply related and the analogue/audio path rather than the digital
control part (however both channels are affected). Certainly plenty of
candidates as tone and volume control handled by chips and opamp based
preamp, power supply to these chips should be checked.

The psu appears to have the following feeds,
+/-15.4Vdc preamp opamp
+/-31.2V STK

The schematic is not real good, hopefully there will be markings on the
board so you can test for these voltages.

Not interested in tuner/display supplies


The pop at power ON/OFF is due to an inbalance in the positive/negative
voltage rails. One rail charges up/discharges quicker than the other due
to higher current usage/lossess. Problem could be an out of spec resistor, a
leaky cap or the new/fake(?) STK or ... Difficult to track down, workaround
would be to install a protection circuit/relay (maybe AUD15 from china)
 
Thanks mbz,

Supply to the preamp board = +15.1/-15.4 Vdc
Supply to STK (rectifier output) = +37.0/-37.9 Vdc

No access to measure voltage directly at IC1 pins but supply is to spec.

The schematic shows the unit has some kind of a 'power on' mute.
 
The pop at power ON/OFF is due to an inbalance in the positive/negative
voltage rails. One rail charges up/discharges quicker than the other due
to higher current usage/lossess. Problem could be an out of spec resistor, a
leaky cap or the new/fake(?) STK or ... Difficult to track down, workaround
would be to install a protection circuit/relay (maybe AUD15 from china)
Thanks for explaining this.

I measured the 'fail safe' resistors on the amp board to be at their correct values. I believe these can become unstable.
I've also measured various voltage points as marked on amp and control boards and these appear to be ok.
 
Supply to the preamp board = +15.1/-15.4 Vdc
Supply to STK (rectifier output) = +37.0/-37.9 Vdc
PSU voltages look good. Don't recommend probing between IC pins to measure voltage.

The schematic shows the unit has some kind of a 'power on' mute.
I see what you mean. Appears to be a clamp tied to the input of the STK. It wont stop a dc surge produced by the STK.
Suggest diode checking etc components in this circuit, especially the TR, cap and diode
 
TR, cap and diode are ok, I probably should re-check the transistor but I'm getting resistor measurements I'm not so sure about first, R30 and R33 (edit: correction R30 and R31) appear to be misbehaving.
It's in the area where I repaired circuit board traces which were damaged possibly at the time the STK module blew. I will need to check my work again. I need access from underneath the transformer chassis, so a little work ahead. I should have realised/reported these repairs sooner.
 
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Have you checked for voltages on the inputs to the STK pack ? Why was it replaced originally ? Have you checked voltage on your speaker outputs ? I had a similar issue on a Technics receiver and it was the DC filtering caps that were to blame for letting low level DC voltage through into the power amp. I recapped the amp board and all was good again.
 
I replaced the STK pack because I found it was internally shorted. I think this caused the rail fuses to blow and damaged the pcb tracks as well. With the new STK, apart from the noise, everything seems to be working as it should.

Voltage on the speaker outputs is about 9mV on each channel. Haven't been able to measure voltage on the inputs yet.

It sounds like a good idea to recap the amp board regardless. As mentioned the amp hadn't been run for years and would not power up. All the more reason to recap, no doubt.
 
I replaced the caps in the signal path on mine and that cured the problem. I did have much higher DC getting through though so YMMV.
 
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