GK 200MB speaker distortion

[JJJack]

My ~1986 GK 200MB combo has let go and now it farts and flaps and isn't the amp it used to be.
I took it to the local music shop and they suggested that it was the speaker surround which is prone to decay.

Has anyone bought a speaker surround kit and fitted it? How was that. There are many many screws to remove.
If you have where did you get it from?
Any websites / companies that may be useful?


Terrible having a non functioning amp and need help!

Cheers

jJJ

[JapanAxe]

What exactly do they mean by 'speaker surround'? The seal?

[JJJack]

I think so!
Apparently the speaker has some kind of foam surrounding which, over time deteriorates and causes a flappy flatulent noise. I think this is the problem but I have found today that I'm not getting this noise with a passive bass but I do with a stingray and a flea bass. I am guessing that the two active basses put a much bigger signal in and this results in the distortion etc. even at low volumes.

I emailed Gk in the USA and they suggested a new speaker was the best bet.

Not quite sure what the way forward is so looking for a bit of advice.

Jjj

[ikay]

Another thread on the same subject here - [url="http://basschat.co.uk/topic/216149-gk-200mb-distortion/"]http://basschat.co.uk/topic/216149-gk-200mb-distortion/[/url]. May be worth messaging bassbod to see if he can advise

[Musky]

If it's the gasket you should be able to use a suitable weather strip.

The surround will be more of a problem but a quick google reveals foam/rubber surrounds are available from overseas, but nothing is immediately obvious in the UK. You could get in touch with Blue Aran who might be able to source them, or Wembley Loudspeakers who are pretty good at repairing almost anything that's wrong with a driver.

[BassBod]

I'm sure Wembley can do it..but you can get a new GK (Eminence) speaker from Thomman for £90 ish. I wanted mine back in service quickly, so that's what I did..and had a great local tech pull the amp apart and fit it. There are 40 odd screws and rubber sealant to deal with, so its not that simple. Then you have to make sure its airtight again to stop any rattles or vibrations.

When I looked into it I found that you really have to use the original spec GK speaker for these amps. Its such a small and peculiar cab design that it needs a very much heavier and stiffer cone than you'd find on other 12" speakers. Luckily the same spec speaker has been used in all the various models up to the present, so you can still get them.

Re-foaming the original speaker is possible, so look into that too - it should be a bit cheaper.

[JJJack]

Thanks everybody.

This is the picture that tells the story, I noticed a dark area at the bottom of the speaker and lo and behold, bits of the surround were easy to dislodge. If I carried on the whole lot would have crumbled away.

I am inclined to order the speaker if it has all the bits I need, so I will contact Thomman.

If I decide to do the job myself, I'll take plenty of photos.

Cheers

jJJ

[BassBod]

Good new is, once its done you've got one of THE nicest little combos ever made…loved mine, and thought it sounded much better than all the later re-designs of the MB series.

All you need is the speaker, some black sealant (like the stuff you'd use around the bath) and a fair bit of determination. Old one out, new one in, but you have to take the box apart to do it. The screws are easy to strip (aluminium case) so don't over tighten..snug is good.

My local tech did it much quicker than I could, checked it over and PAT tested it for a reasonable fee.

Good luck

(also check the nuts that hold the handle on are tight - preferably with locking nuts and sealant. If one ever falls off..its straight into the circuits!)

[JJJack]

Here we are, for those of you that may be interested in the internals of a 200MB, here are some pics.
First two: side views
Pic 3 pre dusting, so much dust and disintegrated surround.
Pic 4 dusted and hoovered
Pic 5 lots of lovely large components!

Plenty of screws to undo (48) but surprisingly easy. The hard bit will be putting it back together without rattles etc. will order speaker tomorrow.

Will also report on progress.

Cheers
JjJ

[BassBod]

Two things amaze me about these amps. First, that they are any use at all as a bass amp, considering the large speaker with no room to breathe, and second that GK ever put such a complicated thing into mainstream production. Its a very different approach to the contemporary "big chipboard box/cheap speaker..that'll do" alternatives.

[JJJack]

Ordered and received speaker the other day, it's now fitted and panels and tops put back on.

No problem with passive basses and a Stingray so far but my Flea bass causes distortion at low volumes (e.g. At levels permitted in the sitting room).
I have got the pre vol lower than the post volume, and I have also rolled all the eq's back.

I seemed to get a big jump in distortion when I put the 1kHz up to 12 o clock as well as when the tone knobs on the guitar are turned up.

Is it possible that the output of the Modulus bass is just too high for the amp? For the info the pickup is Lane Poor and preamp is a Bartolini.

Any suggestions? Feeling quite gutted now.
Btw the speaker was definitely knackered so that repair was probably coincidental.

JJJ

[BassBod]

Check the battery in the Modulus first - then try playing through the effects return socket. It will be a lot quieter, but should be undistorted.

If it overloads the input stage with the preamp gain set below 12 o'clock I'd be looking for a problem. Always worth turning it off, then rotating all controls fully about ten times, and also putting a plug in and out of all the sockets (including effects send and return) a few times - old amps can get grungy inside. Maybe start with the eq controls if they seem to have an influence on the distortion?

If the Stingray is ok I would be checking the Modulus into another amp..just to see.

Good luck

[JJJack]

Bassbod,

Thanks for the tips.
Twiddled and plugged/unplugged.

I am certain battery is new. And I tried two Modulus basses and get same or similar levels of distortion on both.

I did try the Flea through a combo in my local music shop and through a Marshall 10w guitar combo and didn't get the distortion.

Tried Flea bass and stingray through fx return, no distortion, actually louder than plugging into front jack but no tonal adjustments etc possible.

Plugged into the front input and get the distortion with eqs at 12 o clock, with the contour voicing filter in or out. This at both volumes at 2-3.

Any ideas?

JJJ

[BassBod]

Plugging into the effects return should be a lot quieter than plugging into the front and putting the gain half way up. Sounds like something wrong in the preamp circuit - maybe the input gain pot is giving up, and not passing signal though. Worth cleaning it, if you can get into it with some switch cleaner. Other suspect would be input jack socket (and its solder connections). WIth both of those you normally get crackles and volume changes as you jiggle the pot or jack plug. After that..its get the circuit plan and take to a tech.


Can't help thinking its a Modulus thing - is there a trim pot (blue square, with white screw adjuster) in the control cavity? If there is, try turning it down?

Edited January 26, 2014 by BassBod

[tauzero]

Just to say - when I did this on my old one a couple of years ago, I re-sealed the panels by using neoprene tape on the body edges (the panels are originally sealed by using some sort of goop). Did you re-seal it? And have you got the front panel nicely secure? I think I used neoprene tape on the edge of that too to ensure it was damped and unrattly.