JCM800, 4010 potentiometer

D

DesertRose

New member
Hi guys

I've got a JCM800 combo, model 4010(1983 if i'm not mistaken) which pot is screwed.
Pot is the Master volume of the amp.

On eBay I came across some pots, according to DrTube's website, its a VR2 1M, there are 2 pictures

P-M16-100KCX2_MED.JPG


Description 1:
Marshall amp potentiometer 16mm dual 100k reverse audio

Description 2:
Marshall amp potentiometer 16mm dual 10k linear PC mnt

any idea which one is the correct one?
1M should be 1mega, but looks tricky, just to play safe to ask first.
Thanks
 
Re: JCM800, 4010 potentiometer

DesertRose said:
Hi guys

I've got a JCM800 combo, model 4010(1983 if i'm not mistaken) which pot is screwed.
Pot is the Master volume of the amp.

On eBay I came across some pots, according to DrTube's website, its a VR2 1M, there are 2 pictures

P-M16-100KCX2_MED.JPG


Description 1:
Marshall amp potentiometer 16mm dual 100k reverse audio

Description 2:
Marshall amp potentiometer 16mm dual 10k linear PC mnt

any idea which one is the correct one?
1M should be 1mega, but looks tricky, just to play safe to ask first.
Thanks
Click to expand...

It's definately not a 10k pot..Notice(Variable Resistor) VR1(Volume) and VR4(Gain) are both shown as 1 meg values on the schematic.VR5 is also 1 meg..

http://www.drtube.com/schematics/marshall/2210prem.gif
 
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Re: JCM800, 4010 potentiometer

The 4010 is a single-channel master-volume 50W combo - i.e. a 2204 chassis in a combo cab. I know... it's my favorite amp of all time, and #1 on my "never shoulda sold it " list.

Still, both the preamp and master volume controls are 1 meg, probably audio taper.

That pot in the OP's picture is a weird one - some kind of double pot - and not what you're looking for. Based on this photo from Mark Huss' website:

Inputs.jpg


...you want a standard 24mm pot with solder lugs (not PC tabs), 1 Meg ohm, audio taper.

Available from Doug Hoffman: Alpha amp pots - 1ma, $2.00

or from Small Bear: Alpha 24mm 1 Meg Audio 313 in stock, $1.25
 
Re: JCM800, 4010 potentiometer

thanks guys

Its just replace and play kindda thing.
So trying to get the right thing and locals stores ain't selling vintage parts no more.

Hmm ...
I remembered it is a dual layer.
I'll take a picture of it(the pot), the whole thing and place it up soon.
Its already 1am here ...
Thanks for the inputs again.

Just asking, does faulty pots cause volume drop?
I gave it afew knocks and it came back on again. It isn't scratchy or noisy.
Here's a pic of my amp

th_DSCN3960.jpg
 
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Re: JCM800, 4010 potentiometer

DesertRose said:
th_DSCN3960.jpg
Click to expand...

That looks like it has too many knobs to be a 4010. 4010 has 2 jacks, Pre Gain, Master Volume, Treble, Middle, Bass, and Presence. Yours looks like a channel-switcher, in which case Stratdeluxer's schematic is accurate.
 
Re: JCM800, 4010 potentiometer

Rich_S said:
The 4010 is a single-channel master-volume 50W combo - i.e. a 2204 chassis in a combo cab. I know... it's my favorite amp of all time, and #1 on my "never shoulda sold it " list.

Still, both the preamp and master volume controls are 1 meg, probably audio taper.

That pot in the OP's picture is a weird one - some kind of double pot - and not what you're looking for. Based on this photo from Mark Huss' website:

Inputs.jpg


...you want a standard 24mm pot with solder lugs (not PC tabs), 1 Meg ohm, audio taper.

Available from Doug Hoffman: Alpha amp pots - 1ma, $2.00

or from Small Bear: Alpha 24mm 1 Meg Audio 313 in stock, $1.25
Click to expand...

Hi guys

The pots in that Marshall of mine is soldered on the board.
It has straight pins and not those like the above, circular.
It looks like this picture below.

Notice the pot 2nd from the left, it has a dual layer.
this picture is not mine, but it is very similar to it.
Will take a snap shot in a while

Ampchassis.jpg
 
Re: JCM800, 4010 potentiometer

I had a 4212 (2205 head in a 2x12 combo). That looks more like a channel switcher. Can't remember what number the 1x12 combo is; 4210?
 
Re: JCM800, 4010 potentiometer

Those pins can be bent:

http://elderly.com/images/accessories/GEPT/AT500.jpg

http://www.guitarnucleus.com/gnstore/ampparts/marshall/GNMAR16MM-DIM.jpg

Unsolder the old pot, and try to revive it.
If that doesn't work, get a new one.
Make sure to get the correct shaft length, so that your new put don't stand out.
Also get the right diameter (6 or 8 mm) so that knob can fit on it.

Standard pots are linear.
For volume, because of human hearing curve, logarithmic pots are used.
You need 1 Mega Ohm log pot.
The construction (one layer/dual layer) is not of much importance.

If you manage to get the same one, the better:
http://www.guitarnucleus.com/gnstore/elec/GN0386EL-500_meas.jpg

Just take the pot to the local Electronic Store, and they'll know what to give you :)


BTW, I didn;t know the pots were board soldered on JCM800?
Even the reissue has chassis mounted pots:
http://i200.photobucket.com/albums/aa191/badthingsrocks/100_0858.jpg
 
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Re: JCM800, 4010 potentiometer

cronnin said:
Those pins can be bent:

http://elderly.com/images/accessories/GEPT/AT500.jpg

http://www.guitarnucleus.com/gnstore/ampparts/marshall/GNMAR16MM-DIM.jpg

Unsolder the old pot, and try to revive it.
If that doesn't work, get a new one.
Make sure to get the correct shaft length, so that your new put don't stand out.
Also get the right diameter (6 or 8 mm) so that knob can fit on it.

Standard pots are linear.
For volume, because of human hearing curve, logarithmic pots are used.
You need 1 Mega Ohm log pot.
The construction (one layer/dual layer) is not of much importance.

If you manage to get the same one, the better:
http://www.guitarnucleus.com/gnstore/elec/GN0386EL-500_meas.jpg

Just take the pot to the local Electronic Store, and they'll know what to give you :)


BTW, I didn;t know the pots were board soldered on JCM800?
Even the reissue has chassis mounted pots:
http://i200.photobucket.com/albums/aa191/badthingsrocks/100_0858.jpg
Click to expand...


Whoa bro.
That is indeed lots of valuable info.
Now I perceive what a 16mm pot is defined as.
Thanks.
My local electronic stores here are kindda pathetic. The pots ain't Alpha pots but some crap cheapo. Talk abt cost savings....

Anyway what I will do now is to solder out channel 2 volume and swap it with the master volume pot. Both are 1M as indictated behind the pot.

I think the pot is beyond help, contact spray doesn't work either. I will upload a photo soon of the chassis interior. The pot is soldered on the dang board. I'm kinda worried the "tracks" coming off cuz I actually experienced that before and it ain't fun.

Thanks again.
 
Re: JCM800, 4010 potentiometer

DesertRose said:
Just asking, does faulty pots cause volume drop?
Click to expand...

If they are really dirty, yes...but it is almost always accompanied by a scratchy sound when the control is turned.

A cracked/cold solder joint is the usual culprit when the condition comes and goes. If you have a permanent volume drop, and have lost some gain too, there is a 33uF/450v electrolytic on the main board that has a tendency to dry up and short. It won't blow a fuse but it will drop the volume WAY down.
 
Re: JCM800, 4010 potentiometer

glassman said:
If they are really dirty, yes...but it is almost always accompanied by a scratchy sound when the control is turned.

A cracked/cold solder joint is the usual culprit when the condition comes and goes. If you have a permanent volume drop, and have lost some gain too, there is a 33uF/450v electrolytic on the main board that has a tendency to dry up and short. It won't blow a fuse but it will drop the volume WAY down.
Click to expand...

Hi bro,

I guess really dirty should be the word.
But no scratchy sound when turned. Strange.
It is not permanent loss, thanks for the additional info.
So I don't think its the electrolytic.

Anyway, behold ... the interior of my chassis and the Master Volume is VR10 (1MA) !!!

10082010528.jpg


10082010529.jpg


10082010531.jpg


Do I just buy them at mouser or something and just solder it in? Just afraid of the height mismatch. Any better ideas?

Thanks guys. Been great ...
 
Re: JCM800, 4010 potentiometer

Update ... the darn pins are bent having fulcrum on the board.
This makes life worst ...
Anyone experienced in this?
I don't know if using a test pen will skew the thing out.

10082010534.jpg


sucks ... just leave it ...

=(
 
Re: JCM800, 4010 potentiometer

I doubt if the pot is bad. I'd bet that simply re-wetting solder joints will cure the problem. And while your at it, raise those new electrolytics a mm or so off the board and shoot a dab of silicone underneath them...they look tight enough that eventually thermal expansion and contraction is gonna loosen those joints and make them fail. Plus, the ones seen in the photos are your bias circuit filtering; if they lose contact, your amp will run away on you.
 
Re: JCM800, 4010 potentiometer

Thanks for the info bro.

Channel 2 pots are toasted. Confirmed, turn and no sound change, which means, the treble or bass or mids didn't increase at all. Bad joint/contact? not sure ... its been awhile since I use them.

I will just prolly resolder them as what you suggested.
 
Re: JCM800, 4010 potentiometer

Hmmm, I can't say anything from this far :)

"Re-wetting solder joints" as Glassman said, seems like a sensible idea.
If you look carefully you are sometimes able to see a micro crack in a solder when you move the pot/jack.
Take a shot with those EQ pots from channel 2?

We don't have to remind you to watch out for big capacitors, maybe even discharge them with a resistor?
 
Re: JCM800, 4010 potentiometer

cronnin said:
Hmmm, I can't say anything from this far :)

"Re-wetting solder joints" as Glassman said, seems like a sensible idea.
If you look carefully you are sometimes able to see a micro crack in a solder when you move the pot/jack.
Take a shot with those EQ pots from channel 2?

We don't have to remind you to watch out for big capacitors, maybe even discharge them with a resistor?
Click to expand...

Hey bro

Thanks for the info also
Yep, will ensure full discharge before doing anything.

Will just desolder and re solder those.
Hopefully it works
 
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