Distortion on one output channel of receiver

[TeMauri]

Hi All,
Was wondering if someone could lead me in the right direction to fix this issue. I am trying to resurrect a vintage receiver. The two ( speaker A / B) right sides work fine, however the left side for A/B is totally distorted. That is crackling , very loud when it comes through etc noise. The noise starts as soon as the receiver is turned on , this on all inputs. Selector change doesn't make a difference. I measured the mV on the correct channels, which fluctuates around 25 mV, as per spec. But the left channel shows max or very high mV and is very erratic ( -/ +) Are these my output transistors shot or something more? I suspected also the volume knob, and or stereo/mono button since it seemed to disappear for a moment, but that must have been a co incidence. Any ideas? Thank you in advance.

 

[rjenkinsgb]

Any potentiometer or switch is a possible culprit, just a bad connection due to dirt or tarnish etc.

What receiver is it (make and model) and what technology / construction? eg. Valve (tube) or transistor, built on tagboards or printed circuit boards etc?

 

[TeMauri]

It's a Akai AA1150, transitors on PCB. I did a bit of a spray on the pot meters and moving them a few times to max and min, at a certain point I thought it was the stereo/mono button , even the loudness one, since it worked for a moment as it should , but than the nasty noise came back. TO be clear , it comes on straight away when powering up. Music can be played through it , but is very loud and now after the "clean" is seems very distorted as in over powering. mVoltage was more than 200 if I can believe my multimeter.

 

[rjenkinsgb]

Spray all switches with suitable cleaner and operate them a few dozen times each. Bad switch contacts are extremely common on audio gear, where a switch has not been used for some time.

 

[rjenkinsgb]

Service manual available here:

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

 

[TeMauri]

Thanks for that. It's just a bit strange. It basically does it continuous, in one channel as an AM searching sound, but on the odd occasion it goes for a moment.I give the spray a first go , but better than I did. The receiver has likely not been used for a long time, deceased estate. Thanks for the help.

 

[rjenkinsgb]

Looking at the schematic in the manual, another possibility is a dirty bias pot in the power amplifier; the labels are not clear but I believe one is VR1, the other has no legible marking.

Try very gently tapping those with the end of a pencil and see if the noise changes or you get clicks/thumps? If so, move it very slightly back & forth with power off, but try to return it to the exact same position so the bias current is not changed.

 

[AnalogKid]

When all of the inputs are disconnected, does the noise volume change with the volume control, or is it a constant noise volume even when the volume pot is turned all the way down?

ak

 

[TeMauri]

Hello again. Tried the suggestions but unfortunately no joy. Bit more serious I assume? Transistors/capacitors on the amp board? Hello AnalogKid , as soon as you hear the relay kick in the noise is there at constant level. Once you start turning up the volume the music takes over but at a distorted level on that side. The noise, I noticed, does sometimes gets to the background like finding a radio channel almost but than comes back at constant level.

 

[mbarazeen]

It could be squelch control / auto muting issue, try adjusting IF frequency slightly (ferrite rod on IF transformer).

 

[AnalogKid]

Ignoring the signal loudness, if the noise is a constant across all volume control settings, then it is coming from a circuit component *after* the volume control - the power amplifier. My first suspect would be the voltage amplifier transistor in the power amp. TR6 - ?

ak

 

[TeMauri]

Cool thanks for the replies, I will besides getting some decent gear , start removing some bits and check it all out. It has been a while , I have been in this field, about 35-40 years) so a little rusty. Bear with me, once I found the issue I post back.

T

 

[unclejed613]

the left channel amp may be oscillating. since your output devices haven't gone up in smoke, i would guess your output bias transistor is shorted. i've seen this on Yamaha receivers and Onkyo receivers. the slight offset caused by one or the other output devices turning on gets "corrected" by feedback, and so the output can't settle at zero, but instead oscillates between the conduction states of both output devices. this produces an oscillation of about 1.5-2V on the output, but because the output devices are only being driven to minimum conduction, the oscillation doesn't damage them. troubleshooting distortion problems really require the use of an oscilloscope to properly diagnose, DVM readings can help, but really tell you nothing about what the nature of the distortion is. one good clue with a DVM would be to read the DC voltage between the bases of the output devices. it should be somewhere around 1.5-2V. if it's zero, your bias transistor is shorted.

 

[Ylli]

It is not unusual for those old transistors to develop a bad case of shot noise. Freeze spray might help locate a suspicious part. Jumper main board terminals 1 & 2 (and perhaps 23 & 24) shorting the inputs to the main amplifiers. Turn it on - is the noise still there? If so, the first place I would look would be those 2SA872's at TR1 and TR2. If the noise goes away, then look at the transistors on the Tone Board.