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]

On 2021/06/24 8:13 p.m., Phil Allison wrote:

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

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

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John\'s Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
\"Old pinballers never die, they just flip out.\"

 

[John Robertson]

On 2021/06/24 8:13 p.m., Phil Allison wrote:

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

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

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John\'s Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
\"Old pinballers never die, they just flip out.\"

 

[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

 

[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]

On Thursday, June 24, 2021 at 8:13:50 PM UTC-7, palli...@gmail.com wrote:

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.

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.

 

[whit3rd]

On Thursday, June 24, 2021 at 8:13:50 PM UTC-7, palli...@gmail.com wrote:

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.

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]

On Thursday, June 24, 2021 at 11:13:50 PM UTC-4, palli...@gmail.com wrote:

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

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

 

[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]

On 26/6/21 8:44 am, ohg...@gmail.com wrote:

On Thursday, June 24, 2021 at 11:13:50 PM UTC-4, palli...@gmail.com wrote:
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



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.

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

 

[Clifford Heath]

On 26/6/21 8:44 am, ohg...@gmail.com wrote:

On Thursday, June 24, 2021 at 11:13:50 PM UTC-4, palli...@gmail.com wrote:
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



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.

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]

On Fri, 25 Jun 2021 16:10:09 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

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

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

RL

 

[legg]

On Fri, 25 Jun 2021 16:10:09 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

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

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

RL

 

[legg]

On Fri, 25 Jun 2021 16:10:09 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

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

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

 

[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]

On Sat, 26 Jun 2021 19:56:58 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

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

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