Pop on shutdown. Problem?

[Brillo]

Since adding MQ iron, I get a fairly loud pop when I power down my Paramounts. Switching inputs on the FPIII prior to shutdown occasionally helps, as does turning the volume down, but most of the time I get a pop. Should I be concerned? Anything I should test for?  I don't believe I have grounding issues in the system, but perhaps something is not right. Paranoia?  Shut down procedure: Paramounts, then FPIII, then source.

As always, appreciate the input from the forum.

[Doc B.]

If the new factor in the system is that you changed iron, that would probably be the first place to look. My suggestion would be to check that you have continuity from the laminations to the safety ground. It might be that the star washers have not penetrated the varnish on the stack and maybe the pop occurs because they have a small potential on them. It's a little tricky to check as you need to make sure your probe is not insulated from the lams by the varnish when you touch them. You might need to scrape away a tiny spot on the lam stack to make good contact.

[Brillo]

Got it, Doc.  When I installed the MQ upgrade, I ground away the varnish on the frame to ensure good contact with the ground bus, and checks indicate the frame is grounded well.  Still, I'll poke around and see what I can determine.

[Doc B.]

When you do the check, be sure to measure from the lams themselves to the safety ground, not just the channel frame to safety ground. Mike uses a varnish that really penetrates well, and it could potentially insulate the lams from the channel frame. You might also double check the continuity of the connection from the black binding post to ground.

If that's all good, the next place to check would probably be any connections that were removed and replaced during the installation.

[Brillo]

Update: Continuity checks indicate the lams are grounded too, so good there. All replaced connections are wired correctly as well, I believe.  I did confirm continuity from black binding post (-) to ground, so that's okay, but I also have continuity from (+) to ground.  Actually, all connections on the OPT secondary indicate continuity with ground, and to each other.  Can that be right?

[Paul Birkeland]

all connections on the OPT secondary indicate continuity with ground, and to each other.  Can that be right?

The secondary is a continuous piece of wire.  It wouldn't be alarming to see ~1 Ohm or less between each pair.

-PB

[Brillo]

OK Paul.  That's what I have, 0.4 ohm.  Any other possible sources of, or solutions to, this pop on shut down I can research?  Is this even a concern, other than an annoyance?

[Paul Birkeland]

It is odd to me that the Foreplay can impact whether you get a pop or not. 

If the Foreplay is unplugged from the Paramounts, do you get a pop?

[Brillo]

Nope. Unplugged the right side. No pop in that amp at shut down, but I do get a smaller pop in the left channel. I then unplugged the left channel RCA. No pop at all from shut down. So, something in the Foreplay obviously. I've been updating parts in the FPIII for a long time, so may have messed something up along the way.

[Grainger49]

Are you shutting down the FP III at the same time as the Paramounts?  If so, they have always emitted a pop, or thump at shutdown.  This is why the Shutdown/Startup sequence is recommended.

[Brillo]

No.

[Grainger49]

That is really odd.  You seem to have eliminated the Paramounts.  If you do not shut down the FP III there should be nothing coming from it but silence. 

The output transformer in the Paramount is wired to the Parafeed cap and returns to the top of the cathode resistors and capacitor.  That last point is also the wiper of the hum pot.

Is this how the new transformers are wired?  It is worth a check.

I have Paramours with MQ iron.  I have a SPST switch that shorts the primary of the transformers at startup and shutdown to prevent any magnetization of the transformer when the DC leaks through the Parafeed cap.  It probably isn't a Bottlehead approved mod but it surely makes the amps quiet.  I short the same points as described above, from the output of the Parafeed cap to the top of the cathode resistor(s). 

[Brillo]

Yep

[Grainger49]

With monosyllabic responses I will stop trying to help you.