Popping in the Old Harman Kardon 930

This issue is not with the notorious startup and shut down pop on this receiver, but rather a popping sound on start up. It lasts for about 2 minutes and then goes away I presume a warm up. It happens in both speakers even with the volume at 0. I am thinking power supply. I did the old freeze spray trick on the TR903/2SC932 bipolar transistor. I am fairly sure that this is the culprit. After a spray it stopped the popping. It maybe on it's way out. Any suggestions for a replacement" I have found this a BD241A/C and need confirmation as to it's compatibility from the more knowledgeable. http://www.mouser.com/ds/2/389/CD00000938-14108.pdf

FYI most of the lytics have been replaced on the pre driver, power supply and preamp boards.

Schematic attached.
 

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yes ...

TR903 provides AUX voltage to AUX circuits of the amplifier operation and failure trace for this circuit will come to an end if in the output of circuit provides the expected voltage there .

To my understanding a failure in the area of TR903 will cause some trouble to the amplifier IE some of the function will not work but nothing like you said .( example : the main amp should work without any of these problems mentioned ) .

To be able to trace the fault you need to separate main amplifier with pre

if the main amplifier doesn't produce the problem then failure lays in the pre .While the main amplifier has DC blocking capacitor in the input often there is many things that can happen there that can be perceived as a thump or so.

But let me for a minute be more deep and practical ...From the pictures i can tell that the capacitors of your unit have already been replaced which is a very good thing, then comes a question ?

Is the values you used proper ? you can beef most of the capacitors for higher capacitance if those are related to decoupling and power supply circuits but this rule does not apply in R-C circuits and feedback /bootstrap circuits . Any mistake there or wrong choice will result to some form of unbalance .

Usually turn on and of thumps appear when LTP pairs ( first stage of amplifier /preamplifier ) do not have current sources to preserve proper current flow through the circuit at any given voltage .

That will mean in both start up and shut down it will take a few seconds until LTP "equalize" its current so something that you done there when it comes to connections and size of capacitance can be an issue

Though ""a few seconds "" next to 2 minutes is a different thing

Existing bleed resistors in the main capacitors should be there R901-2-3 and there also is a place to look for connections and "voltage abnormalities ""IE voltage B1+B2 should be brought up directly after power on to the stated voltage . If it takes time to bring up the voltage something is wrong .

Kind regards
Sakis
 
yes ...

TR903 provides AUX voltage to AUX circuits of the amplifier operation and failure trace for this circuit will come to an end if in the output of circuit provides the expected voltage there .

To my understanding a failure in the area of TR903 will cause some trouble to the amplifier IE some of the function will not work but nothing like you said .( example : the main amp should work without any of these problems mentioned ) .

To be able to trace the fault you need to separate main amplifier with pre

if the main amplifier doesn't produce the problem then failure lays in the pre .While the main amplifier has DC blocking capacitor in the input often there is many things that can happen there that can be perceived as a thump or so.

But let me for a minute be more deep and practical ...From the pictures i can tell that the capacitors of your unit have already been replaced which is a very good thing, then comes a question ?

Is the values you used proper ? you can beef most of the capacitors for higher capacitance if those are related to decoupling and power supply circuits but this rule does not apply in R-C circuits and feedback /bootstrap circuits . Any mistake there or wrong choice will result to some form of unbalance .

Usually turn on and of thumps appear when LTP pairs ( first stage of amplifier /preamplifier ) do not have current sources to preserve proper current flow through the circuit at any given voltage .

That will mean in both start up and shut down it will take a few seconds until LTP "equalize" its current so something that you done there when it comes to connections and size of capacitance can be an issue

Though ""a few seconds "" next to 2 minutes is a different thing

Existing bleed resistors in the main capacitors should be there R901-2-3 and there also is a place to look for connections and "voltage abnormalities ""IE voltage B1+B2 should be brought up directly after power on to the stated voltage . If it takes time to bring up the voltage something is wrong .

Kind regards
Sakis

Thanks Sarkis.

I will go point form here to see if I understand you correctly...

1. Since freezing t903 the problem has disappeared. I do understand that freezing can alter a components behaviour.

2. There is a thump on startup and shut down and that is normal for a 930. I mute the speakers when turning off and on.

3. I will need to check the values of the 2 caps close to T903 and report back to you on that. I have never gone under value on a cap and try to get close to values in anything other that of the Power Supply. So if I understand correctly the values should be exact, yes?

4 . I can take a shot of the resistors on R901-2-3. They are new. Just hope they are the right wattage. I think I put in 1 watt Takman's.

5. I should check the voltages on the rails.

6. Since I am not getting the issue anymore, now what?

7. If it does occur again, I should install a separate power amplifier to isolate the problem to either the PS or Pre circuits.

Thanks again,
Cha
 
a more tech approach will be to verify voltage and behavior of circuit around TR903 freezing it will alter but will not tell much

B3 is responsible for the tone control section an volume buffer since cap there is rated at 16V i don't expect there anything more than that

B5 is +14 volt dc for the stereo indicator

B3 might be an issue focus there but don't forget hardware issues like a missing cable or ground and so on

Kind regards
Sakis
 
a more tech approach will be to verify voltage and behavior of circuit around TR903 freezing it will alter but will not tell much

B3 is responsible for the tone control section an volume buffer since cap there is rated at 16V i don't expect there anything more than that

B5 is +14 volt dc for the stereo indicator

B3 might be an issue focus there but don't forget hardware issues like a missing cable or ground and so on

Kind regards
Sakis
Thanks again,

I'll see what I come up with. I am a little perplexed though, since I did the freeze that it appears fine :confused: