Fender 30 Channel-Switching "Pop"

[idylldon]

I recently acquired a fairly rare Fender amp from the early 80s.  It was one of a trio designed by Ed Jahns and Freddie Travaras, the other two being the 75 and the 140.  The 30 is the pick of the litter and not many were made.  The lead dress is kind of abysmal like the rest of Fender's offerings from the Silverface era on, and I've done quite a bit to clean it up.  I've got the amp running well now (new filter caps, bias, etc.) and it all works well except for a rather loud "pop" when I switch between the normal and drive/reverb channel.  The channel-switching setup is rather primitive so I'm not sure much can be done to minimize the "pop," but I thought I'd post here in case someone might have some ideas I can try.

I've attached the schematic.

Cheers,
--
Don

 

[JohnRoberts]

Maybe look for a leaky dc blocking cap...

JR

 

[PRR]

If it really is rigged like that.....

Design flaw. (Unusual but stuff happens.)

Perhaps not a flaw but an unintended worst-case.

Make opto "B" low-resistance and opto "C" hi-r. What is the DC voltage on the path shown?

If "C" is OPEN-circuit, we can not say what DC is on this path. But best-guess is a blend of the leakage from 210V though an old .047u cap and the leakage from 82V through an old 1000p cap. Just like that, we expect this path to drift up to 100V-150V or so.

Now hit the switch, make "B" hi-r and "C" low-r. What DC V? Unless caps are very leaky, the 220K+999K will hold things down very near zero VDC.

This 100V-0V transition is a POP. A pretty severe one.

Against that is the fact that opto "C" never really goes to INfinite resistance. But IMHO it may go well above 100Meg after some time in dead-dark. A healthy 0.047u may not leak much into 100Meg, but I leak more now than I did in the 1980s, and maybe the cap does too.

*Maybe* Fender thought its caps never leaked, and indeed maybe their optos leaked more than their caps. When new.

I know the original Fender-picked caps may be sacred, but I'd really look at that 0.047u. Maybe un-do one end and tack in a fresh new top-quality cap. Maybe a fresh happy cap leaks much less than the opto, and the pop is mild.

As a design thing, there really should be a bleed resistor on that node. A simple dependable hunk (film) of carbon, not trusting opto-wafers to leak DC. 10Meg where I show might be the least intrusive hack.

 

[idylldon]

Thanks, guys!  I appreciate it.  I'll swap out some caps (have them on order) and will report back.  Fender was all about saving money at this point so the caps in this amp aren't anything to write home about.  They look like blue Orange Drops! 

PRR, a special thanks to you for this and all that I've learned from your posts over the years.  You have an amazing depth of knowledge about all things electronics!  I appreciate your willingness to help a great deal.

Cheers,
--
Don

 

[idylldon]

To update this in case anyone else has this problem, PRR's suggestions worked brilliantly!  The switching is dead silent now and works flawlessly. 

Thanks again, PRR! 

BTW, if anyone has any of these early 80s Fender amps with the blue caps that look like Orange Drops, replace them all with your choice (I used Mallory 150) and I guarantee you'll hear quite a difference for the better. 

I highly recommend one of these amps if you can find one.  They are truly a real sleeper and are still affordable for a non-PCB constructed amp.

Cheers,
--
Don

 

[PRR]

JR's diagnosis is almost always the correct one.

Caps always leak. Old caps often leak more. In tube amps the caps may be blocking 200V. A 0.010V leak when fresh can rise to a 1V or 10V or more leak decades down the road. Suspecting and test-replacing coupling caps IS the best first guess when you have POP in an old amp.

Caps that can "float" when switched-out "should" have a leak path. You see it on this amp: a "brite" cap has an associated 10Meg resistor, so it won't POP when left in one setting a while and then switched to the other setting.

So it is odd that this Normal/FX switch does not have any bleeder.

Maybe they thought the opto leakage was ample.

Maybe they didn't think. It may be the result of a marathon session with solder-iron and parts-drawers. The specific parts they used played well, didn't pop (or it was not noticed in short testing), so was pushed to Production without analysis, extended testing, or Design Rules. I think many newer amps do have extensive Design Rules, because sometimes I see parts which might "be a good idea" generally but are not needed in this specific case. Yes, in today's economics it may be cheaper to put-in parts than to think about it too hard (for guitar-market runs; you could not afford this in cell-phone production).