marshall 8080 pot problems

[fz911]

hi people, ive got an 8080 and was wondering if anyone can help
i want to replace the dry/eff pot but im not sure what part to buy, i dont have a lot of experience with amps so im completely lost and dont want to kill it
if anyone knows it would be much appreciated
thanx
carl

[oc disorder]

Welcome to the forum fz911.
The pot you require is a linear 22k and its physical size is 16mm.
It has a split fluted shaft for the "push" on type knobs.
Antique Electronic Supply (Tubes and More) have them listed.
POT - 22K LINEAR, MARSHALL, 16mm
R-V22KL-M-16
22K, 16mm, PC mount.
The circuit board has to be removed to get to the underside to unsolder the old one and fit the new one. All the knobs and pot nuts have to come off
and underneath there are little plastic mounts (for want of a better word)
which latch into the board standoffs when pushed in.
Usually I can prise them up with my fingernail and pull out the little receptical.
Carefull they don't flip up and land somewhere in the room where you can't find them.

Before you do this you may find that cleaning the pot with Caig deoxit or similar contact cleaner will fix the problem.
Most valvestates I have seen always seem to have a scratchy/intermittant
dry/eff pot as if its the first port of call for dust !
I usually turn the amp upside down and balance it in the air so I can squirt cleaner into the slot where the connections come out and are soldered to the board. Gravity helps the cleaner run down over the pot track and then I turn it backwards and fowards to help spread the cleaner and wipe the grunge out.

Hope thats some help.

[fz911]

hi oc, thanx for the reply/info, when i first got it there was about 6 scratchy ones, i bought some servisol spray and gave them all a good spraying, ill admit i didnt turn it upside down and let it drain inside the pots but most of them stopped scratchin, one or 2 were very minor, hardly noticeable, its been a while since ive used it now and i cant remember if the eff is one of the scratchy ones, but also during the strip down i noticed the knob on the eff pot was held on with blue tac, its lost half of the shaft
thats the main reason for replacement, once ive done that and i know i can do the job with no problems im gonna go thru the whole amp and replace any more which wont clean up with the spray, ive been reading a bit about cracked solder joints and stuff, im gonna take a good look at everything while im in there and make sure its all ok, im presuming the thing has never seen a clean or repair as the inside had more dust than marjorie dawes, no doubt there will be more problems, its always seemed a little dull to me (although im not a pioneer of amps) i expected more from it.
your run thru guide is excellent, its gave me confidence, i knew sort of what i was doing but it was last year when i last looked at it and used it so my memory of the inside is vague, all i remember was switching it off because of the noise and looking at the broken pot before i put my guitar away
not picked it up since
so yeah, the help is definately appreciated, if you have any tips for things to look for or test while im in there i think it would be a good lesson for me to start learning about it
thanx again
carl

[fz911]

just for reference, is this the same one?
Marshall Amp 22k, Linear Mini PC Pot R-V22KL-M-16 on eBay (end time 25-Dec-09 16:49:01 GMT)
only difference i can see is, the ebay one says 'mini' and there is no writing on the board 'b22k'
thanx
carl

[oc disorder]

Yes it's the same.
The schematic can be found here
http://www.schematicheaven.com/marsh..._8100_8412.pdf
At the top right of the pre-amp diagram you can see VR11 which should be marked on the board.
Usually the zener diodes and the associated resistors ZD1,ZD2,R101 & R102
need resoldering sometimes I had to reinforce with a scrap of wire as the tracks can get damaged by the heat.
Any tall capacitors also may be loose so check their solder connections
while you are there.
The output transistors may need resoldering too but make absolutely sure
that the soldering is neat and there are no shorts.
I'm not familiar with servisol this stuff is about the best I have used
DeoxIT® FaderLube (formerly CaiLube MCL)
DeoxIT® FaderLube
It cleans and leaves a thin coating.
You can be lucky spraying from the top if it goes through the tiny opening
but with the odd obstinate one I have to make an effort to get it inside.
You need a solder sucker or braid to cleanly remove the pot taking care not to damage the tracks.
Check the input jack too - they often come adrift as well as the send return jacks - there is enough info on faulty send return jacks on this forum to write a novel !
Good luck with it.

[fz911]

thanx mate ill let u know how i get on

[spanom]

marshall pots

Hello There!
ive just bought a used marshall valvestate 8080. and seems like i need to change the pots. the Gain on clean chanel is just quiet after the first quarter. and all of them gives this crap noise if you turn it.
i want to change all of them, just i dont know, wich type i need to the different places.. Can you write it down for me?
like
Normal chanel
gain bass middle treble

Boost Chanel
gain bass mid treb. contour volume

and master
effects reverb volume

thx
Spanom

[oc disorder]

Hi Spanom and welcome to the forum.

I would recommend cleaning them first with a suitable pot cleaner.
However if you want to change them the values are in the pdf schematic
posted above (look for VR1 VR2 etc)
I have put the values on the attached diagram and have assumed
VR 1 (VR = variable resistance) is the first pot on the left as you face the chassis and VR 13 is the last on the right.
The first pot is a 220k (220,000 ohms) Linear taper ie 220kB where lin = B.
Log taper or Audio Taper = A
So 1MA is a 1 million ohm (megohm) log pot.
If its an early model they all should be mini 16 mm pots same as discussed above.
The values should be written on them and the circuit board where they are mounted should show VR and the number (1 to 13).

[spanom]

thx man, that will help, the only problem, i dont know how old is the amp.
is that possible its from 94' ?
thanx anyway!

[Enzo]

The age shouldn;t matter fixing it, but what is the serial number?