Audio Preamplifier produces glitches (cracks)

[Bigfoot]

I have 1979 Realistic STA-2000 Receiver. The amplifier section works fine (when I separate PREAMP from AMP section there is no problem). The TONE AMP however produces random (very frequent) glitches (cracks) with maximum power output. It sounds like cracking or with other words like when you have short circuit. The glitches are visible on the output power meter (Power VU meter jumps to the top). Visually there is nothing that suggest which component might be wrong.
Any help is more than welcome

 

[shrtrnd]

Sounds like something is actually arcing (sparking) inside the unit. I'd open the
chassis up, and look very carefully for any tell-tale burn marks on the components.
(Capacitors are prime candidates)
A secondary consideration is a dirty or damaged potentiometer. Turn the pots and
see if any of them produce the symptom (I say this because I've seen this happen
without touching the pots, and just seeing the effect of sonic vibration from the speakers
causing the main chassis to vibrate).
That's the simple first things to check.
If you don't see a problem, it's probably going to take detailed component troubleshooting
to find the problem.

 

[Harald Kapp]

Does the cracking noise come from the amplifier itself or is it a sound emitted by the speakers? In the latter case I'd second shrtrnd's guess concerning the potentiometer.

 

[Bigfoot]

Does the cracking noise come from the amplifier itself or is it a sound emitted by the speakers? In the latter case I'd second shrtrnd's guess concerning the potentiometer.

I separated preamp_out and amp_in on the back panel and the glitches disappeared. The problem is in the tone preamplifier board. It is creating impulse signal (glitches) which can be heard as cracking (loud cracking without touching the potentiometer) and it is also registered by the output power meters. The frequency of the glitches is approximately 40 times per minute.

I bought it used and when I first turned it on it worked fine for 15 minutes and then started to produce this signal ....

 

[Bigfoot]

Sounds like something is actually arcing (sparking) inside the unit. I'd open the
chassis up, and look very carefully for any tell-tale burn marks on the components.
(Capacitors are prime candidates)
A secondary consideration is a dirty or damaged potentiometer. Turn the pots and
see if any of them produce the symptom (I say this because I've seen this happen
without touching the pots, and just seeing the effect of sonic vibration from the speakers
causing the main chassis to vibrate).
That's the simple first things to check.
If you don't see a problem, it's probably going to take detailed component troubleshooting
to find the problem.

It is already opened and checked. No visible defects ... Pots are cleaned and checked ...the tone preamplifier doesn't even have potentiometers ...

 

[Nobody]

You've mentioned toneamp. Do you have turntable connected? If so, is it happening when you play LPs ?

 

[Bigfoot]

You've mentioned toneamp. Do you have turntable connected? If so, is it happening when you play LPs ?

No turntable and it is happening as soon you turn it on no matter which source is selected

 

[Harald Kapp]

Check the power supply of the pre-amp. Do you have an oscilloscope? Is the voltage clear or noisy? For a test, add some electrolytic capacitor in parallel to the power supply's capacitors.

 

[duke37]

I think Harald is on to something, I have had electrolytic capacitors which short over internally. This gives a crack and then they then take some time to recharge. Is there a very short time with lack of sound after the crack?

You may be able to see a voltage change with a fast analog or digital meter.

 

[Bigfoot]

I think Harald is on to something, I have had electrolytic capacitors which short over internally. This gives a crack and then they then take some time to recharge. Is there a very short time with lack of sound after the crack?

You may be able to see a voltage change with a fast analog or digital meter.

You may have a good point. I am suspecting either on capacitor or transistor. The problem is that the whole board has over "zilion" capacitors
I will check them one by one.
And to answer the first question - it makes crack after crack. The signal goes on through the channel but I get this cracks (spikes or glitches on the power meter)

 

[Harald Kapp]

Look for the "big" ones called electrolytic capacitors. You should find them spread across the board, but probably a few very big ones at the point where the power supply enters the board.

 

[Bigfoot]

Look for the "big" ones called electrolytic capacitors. You should find them spread across the board, but probably a few very big ones at the point where the power supply enters the board.

The comic situation is that I am pretty much familiar with electronics but I have never seen something like this. So consider my question as a dilemma posted by a very competent person.

If it is the power it would affect the amplifier section too but it doesn't so I don't think it is in the power supply. This receiver has the beauty of separate pre-amp & amp sections which allows you to hook something (equalizer) in between. I thoroughly checked the schematics and the signal can bypass all the other boards with proper selection of the switches which leaves only the tone amp circuit as a suspect (and eventually the main potentiometers which would be horrible if true). The amplifier part when separated works perfect.

Unfortunately I do not have nor I can find oscilloscope, and as I mentioned all the components look fine (and believe me I have disassembled the unit and checked everything visually). Unless somebody had similar situation I have to go part by part which means a lot of money and time.

It might be a potentiometer as somebody suggested, burned transistor (which I suspect) or dead capacitor (none of the electrolytes looks suspicious )
I will try to change the transistors (if I find them) then I will see ...

 

[KrisBlueNZ]

Does the sound come through both channels at the same time?

 

[duke37]

I have had a thought on how a slow meter can see a short, sharp negative transient. i have not tried it.

 

[Bigfoot]

Does the sound come through both channels at the same time?

95% is on the left channel only occasionally the right. The balance doesn't affect the glitches, is works with the audio signal but doesn't cut of the glitches ...