500B Snap, crackle, pop

[Rockyhill]

A few weeks ago while listening to FM I noticed a faint crackling/static sound from right channel. The sound resembles an LP with a scratch. The frequency was intermittent but I'd say several seconds between occurrences. I mmediately turned it off. When things cooled down I pulled the power tubes and tuner tubes and placed them back. It's in a case so the rest are not accessible. Subsequent use displayed none of this sound.

FF to last Sunday. I put it on the bench. All tubes removed, pins and sockets cleaned with Deoxit. Adjusted bias. Placed back into service. All was fine until yesterday. Same crackle sound. Specifics:

Right channel only.
Heard when Balance control is all the way to Left.
Heard when Volume is at minimum.
Heard regardless of Input selection.

Thoughts for trouble shooting?

Thanks

Oh, there are no Rice Crispies in it!

 

[njcanuck]

Swap the tubes from channel to channel to see if the noise follows the tubes.

 

[RS Steve]

I also got some crackle from my latest acquisition (500-B) so far I found some resistors that were way out of spec. I'm in the process of installing new capacitors, and upgrading the power supply section now.

I would check to see if you notice any tube arcing, and have the voltages checked out. You didn't mention if this receiver has been restored or is all original.

 

[Rockyhill]

I also got some crackle from my latest acquisition (500-B) so far I found some resistors that were way out of spec. I'm in the process of installing new capacitors, and upgrading the power supply section now.

I would check to see if you notice any tube arcing, and have the voltages checked out. You didn't mention if this receiver has been restored or is all original.

This receiver has been restored with a Jim McShane rebuild kit. IBAM is installed and bias set on Sunday. The odd thing is the crackle is very intermittent. Last noticed yesterday morning; nothing last night or this AM. No tube arcing observed.

 

[RS Steve]

This receiver has been restored with a Jim McShane rebuild kit. IBAM is installed and bias set on Sunday. The odd thing is the crackle is very intermittent. Last noticed yesterday morning; nothing last night or this AM. No tube arcing observed.

Does the dial lights dim when the unit crackles, if so it could be the power switch going bad.

 

[Rockyhill]

Does the dial lights dim when the unit crackles, if so it could be the power switch going bad.

I don't think so but will monitor. I don't use the switch to turn on & off anymore after reading about its "delicacy".

 

[RS Steve]

I don't think so but will monitor. I don't use the switch to turn on & off anymore after reading about its "delicacy".

The current still has to pass through it, so it doesn't have to be turned to have problems. I connected jumper so both legs get used. One leg goes to the outlets on back, and the other the transformer, better contact using both.

 

[vendo81]

I've found bad resistors that make the crackling sound you mention that is only in one channel. If you have a scope or signal tracer you can trace it to the bad stage/resistor.
I know you have the bias set but try swapping around the output tubes and see if it follows a tube. I had the same problem with a 500c with a faint crackle in the left channel that would come and go. After checking everything in the receiver it ended up being an output tube.

 

[Rockyhill]

I've found bad resistors that make the crackling sound you mention that is only in one channel. If you have a scope or signal tracer you can trace it to the bad stage/resistor.
I know you have the bias set but try swapping around the output tubes and see if it follows a tube. I had the same problem with a 500c with a faint crackle in the left channel that would come and go. After checking everything in the receiver it ended up being an output tube.

Thanks for the advice. Unfortunately I have neither piece of equipment. The output tubes are EH and are 18 months old +/-. Even before IBAM they weren't run hot. Which upstream tubes might be suspect? All the 12AX7 tubes are original. The original tuner 6AU6 tubes were replaced with NOS Philips as was the 6AQ8. I have back up 12AX7 available.

The current still has to pass through it, so it doesn't have to be turned to have problems. I connected jumper so both legs get used. One leg goes to the outlets on back, and the other the transformer, better contact using both.

Something to consider, thanks.I stopped using the switch as it was a little stiff when turning on/off. Mainly to prolong its life.

I'm using the receiver now and it sounds as nice as ever.

 

[petehall347]

i would gently rock the tubes if it does it again .. might be a corroded connection or dry solder on a socket .

 

[Rockyhill]

i would gently rock the tubes if it does it again .. might be a corroded connection or dry solder on a socket .

Thanks Pete. Maybe I'll reflow some solder joints on sockets if the symptoms persist; even if they don't I'll do it at next bias check.

I appreciate all the suggestions.

 

[dcgillespie]

Rocky -- Your excellent description of the symptoms has pin-pointed the problem to being somewhere within the power amplifier section (only) of the right channel. This limits the issue to V10, V11 and V13. Since all the tube sockets have already been cleaned, one thing to check (if it hasn't been cleaned or replaced already) is the Phase Inverter Adjustment control for the right channel. When they need cleaning, it will do exactly what you are describing: create an intermittent crackle sound only in the channel the dirty control is associated with. If that has not been addressed, then there's a good chance that this could be your problem.

Good luck with it!

Dave

 

[Rockyhill]

Rocky -- Your excellent description of the symptoms has pin-pointed the problem to being somewhere within the power amplifier section (only) of the right channel. This limits the issue to V10, V11 and V13. Since all the tube sockets have already been cleaned, one thing to check (if it hasn't been cleaned or replaced already) is the Phase Inverter Adjustment control for the right channel. When they need cleaning, it will do exactly what you are describing: create an intermittent crackle sound only in the channel the dirty control is associated with. If that has not been addressed, then there's a good chance that this could be your problem.

Good luck with it!

Dave

Thanks for the kind words Dave. When I rebuilt the receiver the two potentiometers that weren't cleaned were those for the phase inverters (my bad). I will address that. I may also remove the associated "noose" at that time.

If I decide to replace those pots do you have a recommendation?

 

[dcgillespie]

If they have not been cleaned, then the pot for the right channel becomes the prime target as the culprit for your noise.

If you replace the pots, any good quality linear taper .5 watt pot will work. It does need to be of good quality, as it operates at some 350 volts above ground. Therefore, I would stay away from the really cheap stuff.

If you do remove the noose, be sure to move the B+ supply for the AF Amplifier section down to the next lower B+ supply point.

Dave

 

[Rockyhill]

If they have not been cleaned, then the pot for the right channel becomes the prime target as the culprit for your noise.

If you replace the pots, any good quality linear taper .5 watt pot will work. It does need to be of good quality, as it operates at some 350 volts above ground. Therefore, I would stay away from the really cheap stuff.

If you do remove the noose, be sure to move the B+ supply for the AF Amplifier section down to the next lower B+ supply point.

Dave

On a 500B the "nooses" are R99 & 100, correct? And the B+ is represented by connection "E"? With the resistors removed where does "E" connect?

Thanks for clarifying.

 

[dcgillespie]

The nooses are in fact R99 and R100. As for altering the B+ supply to the driver stage, point E would still power R88 and R91 (R99 and R100 are gone), but R86 and R87 would now be connected to the power supply point powering the line/tone stages (point F).

Dave

 

[Rockyhill]

The nooses are in fact R99 and R100. As for altering the B+ supply to the driver stage, point E would still power R88 and R91 (R99 and R100 are gone), but R86 and R87 would now be connected to the power supply point powering the line/tone stages (point F).

Dave

One more clarification, please. Point F connects at the RED or BLUE X's?

 

[dcgillespie]

The red X. R86 and R87 get DISCONNECTED from their tie to the point "E" B+ feed, and then get connected to the point "F" B+ feed. The connection of these resistors to V11 and V12 must remain in tact.

When you're finished, the only thing that point E will supply on your partial schematic will be R88 and R91.

I hope that helps!

Dave

 

[HiFiHarv]

Rocky, the phase inverter pots in my 30As have been the cause of intermittent noise in the past. I'll bet Dave is right (of course)

 

[Rockyhill]

Rocky, the phase inverter pots in my 30As have been the cause of intermittent noise in the past. I'll bet Dave is right (of course)

As I reflect back on the original overhaul I did clean these pots, just not as thoroughly as all the others. I didn't flood them with Deoxit and work the full range multiple times like I typically do. I don't recall my logic for applying that limited technique. It may have been that I didn't want to move them far from the settings they were at.

The red X. R86 and R87 get DISCONNECTED from their tie to the point "E" B+ feed, and then get connected to the point "F" B+ feed. The connection of these resistors to V11 and V12 must remain in tact.

When you're finished, the only thing that point E will supply on your partial schematic will be R88 and R91.

I hope that helps!

Dave

Thanks Dave. That definitely helps this "paint by numbers" guy! I'll mark up those changes and post one more time.