Fender amp chassis non conducting?

[Phil Allison]

Hi,

just saw a circa 10 year old Fender " Blues Junior" amp.
The chassis is of the usual plated steel, bent sheet construction - looks like thousands of others.
But the surface is insulating - DMM does not continuity beep and shows megohms between adjacent points on the chassis.

To get a ground connection you must use one of the threaded bolt holes for the back panel. Never seen this before.

Surely this makes grounds on speaker jacks and even the safety ground connection unreliable.
I have read that the plating may be " chromated zinc passivated " which is high resistance.

Any ideas?

..... Phil

[John Robertson]

It might be an anodized aluminum skin. Anodized aluminum is very tough...

John :-#)#
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[Phil Allison]

John Robertson wrote:
===================

>>
> It might be an anodized aluminum skin.

** Completely wrong colour.

> Anodized aluminum is very tough...

** I am very familiar with anodised Al, this is nothing like it.



..... Phil

[Peter W.]

> Any ideas?

Blue tint? That would be your passivated zinc-based material.
Yellow tint? That would be *chromated* material.
No tint? See Applied Varnish.

Given that the chromated material requires an extra step in the plating process, if this is a passivated plating (which I doubt) I suspect the former rather than the latter. Further, the latter is mostly done for corrosion protection, not insulation. Would Fender indulge in a costly process for corrosion protection?

Note that the tints are quite subtle, but definite.

Given that both are rather more costly than a simple applied varnish, of which there are dozens, I would look to one of those rather than something exotic.

https://www.mcmaster.com/electrical-insulating-spray/
https://www.cable-technologies.com/productdisplay/3m-%E2%84%A2-scotch-%C2%AE-1601-and-1602-electrical-insulating-spray-1 Two of very many.

Now, imagined it being done at an industrial level - the goal would be achieved at a very low cost.

Note also that anodizing comes in many colors from clear to black and all between.

https://www.ashevillemetalfinishing.com/2019/04/22/electroplating-and-anodizing/

But of all the options, that is the least likely as it is the most expensive. Somehow I do not see a guitar amp manufacturer doing this for something that is very nearly invisible to the end-user.

Peter Wieck
Melrose Park, PA

[whit3rd]

To get through paint or plating, a star-type lockwasher is good; screw/lug/lockwasher/chassis
is a reliable grounding connection, even if you replace 'screw' with 'rivet'. Loctite screw thread
application doesn't seem to hurt the grounding with a machine screw, either.

[ohg...@gmail.com]

I called my brother who is a research chemist. He said it could very well be the chromated zinc passivated finish you suspect. He said the steel is first zinc plated, then a trivalent chromate finish is applied over it. If it's colorless or slightly blue, that's probably what it is. He did say that this trivalent chromate finish would also have a bit of sheen to it as well, and it most definitely would be resistive in nature. He said it's for protecting the steel from corrosion as we would suspect it would be. He also said the plating could be dyed other colors if wanted.

[Phil Allison]

ohg...@gmail.com wrote:
=======================

>
> > just saw a circa 10 year old Fender " Blues Junior" amp.
> > The chassis is of the usual plated steel, bent sheet construction - looks like thousands of others.
> > But the surface is insulating - DMM does not continuity beep and shows megohms between adjacent points on the chassis.
> >
> > To get a ground connection you must use one of the threaded bolt holes for the back panel. Never seen this before.
> >
> > Surely this makes grounds on speaker jacks and even the safety ground connection unreliable.
> > I have read that the plating may be " chromated zinc passivated " which is high resistance.
>

> I called my brother who is a research chemist. He said it could very well be the chromated zinc passivated finish you suspect. He said the steel is first zinc plated, then a trivalent chromate finish is applied over it. If it's colorless or slightly blue, that's probably what it is. He did say that this trivalent chromate finish would also have a bit of sheen to it as well, and it most definitely would be resistive in nature. He said it's for protecting the steel from corrosion as we would suspect it would be. He also said the plating could be dyed other colors if wanted.

------------------------------------------------------------------------------

** Thanks for that. Here are some pics showing the chassis details of one:

https://www.mojotone.com/blog/fif-blues-junior-power-transformer-swap

Look *exactly* like ordinary zinc plated.

FYI:
The top, where the controls knobs are, has a thin, nickel plated steel label folded over at the back and held by glue.
The threads of the various pots and power switch effectively ground it by passing through holes that are free of plating.

IME whenever such a chassis has a non conducting coating ( eg anodised Al or paint ) makers normally mask off any small areas that need to conduct. Not the case here.

I reckon the metal shop simply got batch of plated steel that look normal but was not correct.
Nobody with a DMM being around to check it.


..... Phil

[Clifford Heath]

Thank for the response. To satisfy my curiosity, can you ask your
brother what process and chemicals are used to add the trivalent
chromate finish?

Yes, I realise that chromates are dangerous, hexavalent chromium is a
carcinogen.

Ch

[Phil Allison]

Clifford Heath wrote:
------------------------------

>
> Yes, I realise that chromates are dangerous, hexavalent chromium is a
> carcinogen.
>

** Before or after watching this?

https://www.youtube.com/watch?v=BGX4nMrnxg0


..... Phil

[legg]

Doesn't look like the chassis is used to carry any signal
grounding - audio plug has plastic body.

RL

[Phil Allison]

legg wrote:
===========

>
> >** Thanks for that. Here are some pics showing the chassis details of one:
> >
> >https://www.mojotone.com/blog/fif-blues-junior-power-transformer-swap
> >
> >Look *exactly* like ordinary zinc plated.
> >
> >FYI:
> >The top, where the controls knobs are, has a thin, nickel plated steel label folded over at the back and held by glue.
> >The threads of the various pots and power switch effectively ground it by passing through holes that are free of plating.
> >
> >IME whenever such a chassis has a non conducting coating ( eg anodised Al or paint ) makers normally mask off any small areas that need to conduct. Not the case here.
> >
> >I reckon the metal shop simply got batch of plated steel that look normal but was not correct.
> >Nobody with a DMM being around to check it.
> >

--------------------------------------------------------

> Doesn't look like the chassis is used to carry any signal
> grounding - audio plug has plastic body.
>

** Huh ???

The input jack sleeve is chassis grounded along with the common earth of the PCB.

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf

The majority of guitar amps have plastics input jacks, nearly all Marshall and VOX models for example.


....... Phil

[legg]

The mounting hardware for that jack is not counted on to make
an electrical connection to signal ground.

This was something that you said you were worried about, when
considering continuity of the box metalwork.

I think you'll find that PEM inserts have an electrical contact
to the metalwork, even though installed after the metal is
passivated. If the grounding stud is captive, it likely
also benefits from this contact, though star washers are standard
for a safety connection there, anyways.

RL

[Phil Allison]

legg wrote:
-------------------
>
> >> Doesn't look like the chassis is used to carry any signal
> >> grounding - audio plug has plastic body.
> >>
> > ** Huh ???
> >
> >The input jack sleeve is chassis grounded along with the common earth of the PCB.
> >
> >https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf
> >
>

> The mounting hardware for that jack is not counted on to make
> an electrical connection to signal ground.

** Irrelevant. So what if connects another way?

> This was something that you said you were worried about, when
> considering continuity of the box metalwork.

** Where ? I mentioned the speaker jack which is an all metal type.

> I think you'll find that PEM inserts have an electrical contact
> to the metalwork, even though installed after the metal is
> passivated.

** Yawnnnn - the holes are made post plating, which I mentioned.

Stop being such a PITA know all.


..... Phil

[legg]

Holes are usually formed at CNC, before metal bending.
Then passivation, then insertion.

To insert before passivation, you'd need either stainless
or compatible metal chemistry, unless you permit visible
discoloration. Messy.

Which is the more likely Phil com?

'you know, on second thought, there's nothing to worry about, sorry
for wasting your time with this.'

or

'I really don't want an answer to this question, so
piss off.'

RL

[Peter W.]

> 'I really don't want an answer to this question, so
> piss off.'

That would be the one.

[Phil Allison]

legg is a fucking but case asshole:

===========================

> To insert before passivation, you'd need either stainless
> or compatible metal chemistry, unless you permit visible
> discoloration. Messy.
>
> Which is the more likely Phil com?

** FFS you blithering idiot - I have the unit in front of me.

YOU DO NOT !!!!

I can see the hole for the metal speaker jack is not internally plated.
Go fuck your self.

[legg]

and is there a wired connection to that connector terminal?
There is usually a terminal provided to allow for it by the
connctor's manufacturer.

Simple chassis contact is not something you'd count on for signsl,
power or safety, unless specifically designed for the purpose.
That's why the ground stud is labelled as such, and electrical
contact is made with reliable conductors.

I don't think face plates are expected to have any electrical
function - they're designed for ease of use and cosmetic values.
They can't be specifically listed as a conductor or insulator
in their safety files, unless they meet certain requirements.
Usually these stop at legibility after cleaning and flammability.
The metal or other surface they are mounted on serve the chassis
function.

Zinc/chromate passivation on steel was considered to be an excellent
conductive finish for purposes of emc control, when it was widely
in use. Current substitutes may have issues.

RL