crackle and pop but no music [solved]

[ike]

Hello,
I am not sure where to trouble shoot.  My extended FP3 is popping and crackling on both speakers. I try swiping tubes and still the same.  Any thoughts to where i may diagnose.
Thanks in advance.
ike

[fullheadofnothing]

Did you swap tubes from side to side, or try another pair?

[Paul Joppa]

As always, more information is needed to begin guessing. Here are some starters:

Did the preamp ever work? If so has anything changed in the preamp or system?

What do you mean by popping and crackling? How loud? Does any music come through?

How do you know it's the Foreplay? i.e. have you disconnected all inputs and used shorting plugs? Is the power amp and speakers silent if the FP is disconnected from them?

Is the preamp operating correctly, i.e. does it pass the resistance and voltage checks in the manual?

[ike]

Hi Paul,

Yes, the preamp has been operational without any problems for 3+ years.  My amps are Paramounts. 

There was no music coming through last might and I panicked and wrote the note... since then, music is coming through.

Though, the symptom persists, swooshing sound and occasional crackle from both channels that comes and goes and occasional loud pops from both speakers (like when you accidentally turn the amps on first and then turn on the preamp).

Recent changes to the FP3: added 6SN7 with adapter, five months ago,  A couple of weeks ago, I disconnected my system and cleaned all connectors (spring cleaning).  I'm not sure if I have loosen something... Although, I believe the swooshing and popping noise was apparent prior to spring cleaning.

I suspected the issue is coming from the preamp because the additional noise is in stereo. 

I have not not gone under the chassis and check resistance and voltage.

Thanks in advance for your advice.

ike

[Doc B.]

Try changing out the center tube in the FPIII.

[Paul Joppa]

Also inspect the boards for signs of heat around the large transistors (discolored board surface). I don't remember when we switched to the larger transistor (MJE5731A) in the shunt regulator - if you have the smaller MJE350, and if your power line voltage is a little high, that could be the problem.

[Doc B.]

Oh yeah, I had forgotten that. Good call, Peej.

[ike]

I just try changing out the center tube, problem persist... I'll try the other methods when I have more time.  thank you all for your suggestions
Will report back.

[fullheadofnothing]

Have you tried pulling the adapters and returning to 12AU7 operation?

[ike]

Yes, I tried returning back to 12AU7, no change
So, I made some shorting plugs, and tested the inputs.  First on the amps, no noise, moved on to the preamp inputs, and on one channel has a low hum, barely audible .  I replugged the same spot and the hum was gone.  So far, the system is on for over an hour and no swooshing noise, no pop and crackle just music.  Weird

[ike]

Hi there,
I turned on the system last night after a 10 day business trip... no music nor sound just static.  I just check under the hood and one of the LED light on the center board is not lit (D1 on the A side of the board).  How do I fix that?  Do I need a replacement part(s)?

See attached pic
thanks,

[Paul Joppa]

Diagnosis first, prescription later.

The fist thing I always do is wiggle the tubes in their sockets, in case it's just a socket connection problem (trace of corrosion on a pin).

We'll need voltages in order to diagnose what's going on. Since most of the board LEDs are glowing, we know the power supply is working - so most important are the tube pin voltages. These are hard to get to, so for a first step try this: A) are the LEDs on the left and right sockets glowing? B) measure the voltages at OA and OB on each of the three boards. This will give us an idea of what might be wrong.

The puzzle is that the dim LED is a problem on only one channel, but you say both channels are not functioning. So there are probably at least two different things wrong. Check to make sure you have the selector on the right setting, cables are connected, etc. I know this seems too simple, but it makes things easier if we eliminate as many things as possible before getting deep into the circuit behavior.

[ike]

Hi Paul,

Here's what I did: I removed all 3 tubes and with hit with the soldering iron on the center tubes socket pins.  The dimmed LED on the center board came back on!  Plug back into the system, I have MUSIC... 20 minutes later, the left channel start spitting and crackling, right channel is still playing music.  I checked the LEDs, and like before one if the LED on the center board is not working.  All the LEDs including both the sockets are on. 

i plugged back into the system just to see if any change - No music on the right channel and spitting and crackle on the left

Here are some voltage measurements:
Left channel board - OA 67.6V, OB 66.7V
Center board - OA 84.5V, OB 84.6V

thanks
Right channel board - OA 68.4V, OB 67.4V

[Paul Joppa]

Good info!

It appears the shunt regulator is not working - on the center board, OA and OB are the regulated voltage outputs, which should be 150 volts.

I suspect those MJE350s on the reg board. Contact Eileen (spareparts...) for the replacement MJE5731A. I think there's a sticky somewhere about mounting them - the metal faces the opposite direction.

If this does not solve the problem, we'll search further - but it's still a good idea to replace those transistors.

[Paul Birkeland]

Eh, e-mail replacement parts for a new board, and say that PB said he'd take care of it.

I'll throw a replacement PC board together for you - it is rough to retrofit the MJE-5731A's into the circuit, and we would be happy to help.

-PB