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GK 400RB Problems

bigdoug1053

bigdoug1053

Jun 23, 2009
46
1
Lakewood, Colorado
www.dougcraftfineart.com
#1
I have a mid-90s GK 400RB, and am experiencing an unusual problem. The amp will unexpectedly shut off output volume - the power and boost channel light are still ON but zero volume. No noise or crackly scatchy sounds - the volume just dies. Powering off and back on it will then work normally. The amp is not thermally hot, fuse is fine, and it's connected to a 4-ohm GK cab with a new speaker cable. I have ruled out my active basses and instrument cables. Could this be a bad pot? :rollno:

Has anyone else had a problem like this?

THANKS!
 
#2
bigdoug1053 said:
I have a mid-90s GK 400RB, and am experiencing an unusual problem. The amp will unexpectedly shut off output volume - the power and boost channel light are still ON but zero volume. No noise or crackly scatchy sounds - the volume just dies. Powering off and back on it will then work normally. The amp is not thermally hot, fuse is fine, and it's connected to a 4-ohm GK cab with a new speaker cable. I have ruled out my active basses and instrument cables. Could this be a bad pot? :rollno:

Has anyone else had a problem like this?

THANKS!
Click to expand...

This has come up a lot for years. It could be the effects circuit. Dirty contacts or damaged or faulty jacks therein.

Take a shielded instrument cable and plug it into the preamp send to the amp in jacks. If it solves the problem that's what it is. If it doesn't it's something else.

I had that problem with my old 400 RB once and I think I was the first one to post about how to fix it here.
 
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#6
I just did a thorough pot and switch cleaning with contact cleaner and sandpapered all the input jacks - just to be sure it's not a pot. I jumped the effects loop with a short patch cord. Then I worked out on several basses for an hour or so - no drop outs at all at a variety of higher volume levels (the cats ran outside!). So, this must have fixed the problem. I had a feeling it was some kind of contact problem.

I will do another follow-up post after my next loud jam session with drummer and guitar players. That's when the overall power and volume will be pushed for an extended period. The improv metal acid test!

What I am REALLY happy about is that it doesn't appear to require service - given that it is already almost 15 years old, it would likely cost as much to fix as it is worth. Since it's my backup/medium level amp (i.e. low priority for spending) that outcome would have been a drag.

Thanks guys! :bassist:
 
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#8
bigdoug1053 said:
I just did a thorough pot and switch cleaning with contact cleaner and sandpapered all the input jacks - just to be sure it's not a pot. I jumped the effects loop with a short patch cord. Then I worked out on several basses for an hour or so - no drop outs at all at a variety of higher volume levels (the cats ran outside!). So, this must have fixed the problem. I had a feeling it was some kind of contact problem.

I will do another follow-up post after my next loud jam session with drummer and guitar players. That's when the overall power and volume will be pushed for an extended period. The improv metal acid test!

What I am REALLY happy about is that it doesn't appear to require service - given that it is already almost 15 years old, it would likely cost as much to fix as it is worth. Since it's my backup/medium level amp (i.e. low priority for spending) that outcome would have been a drag.

Thanks guys! :bassist:
Click to expand...

Well, unfortunately, you may not be done yet. You kind of screwed yourself using sandpaper. Those jacks are made from steel, with either a nickel or chrome plating for corrosion pretection. If the sandpaper took that off, it'll be a short while til that bare stell rusts, and your back to where you were.

good luck.
 
#9
Glad it worked. If you don't need that loop to work as an effects loop, just leave a patch cord in there. If you;'re handy with a soldering iron you could fix it by replacing the four jacks for a fairly low cost.

You might have created a new problem by sanding the contacts. That remains to be seen though.

When that happened to me I was getting frantic because I had another gig that night, and since it was out of warranty I could see big repair costs ahead. I took the cover off, plugged it in at home with a speaker cabinet and played the thing off and on for hours. Couldn't get it to cut out. Then I was tapping inside with the eraser end of a pencil trying to find bad connections when I hit one of the jacks and it cut out. I did it a few more times and got it to cut in and out each time. That's when I had the lightbulb moment and figured what might fix it. Grabbed a patch cord and it worked fine. Took the cord out and it cut out again. So I put the cord back in and there it is to this day. Hasn't had a problem since. The 400 RB is a very good amp.
 
#10
My Backline 210 (600 head) is cutting out like this too. Are you guys saying that plugging a cord into the effects send and return will eliminate the cut outs? After reading this I plugged in and started playing so I could test this and wouldn't ya know it's working fine for now.
 
#11
I had a similar problem with my 400RB. I know not everyone can do this but, I walked mine in to the GK plant in Stockton, CA.

My output volume was severely reduced, all my buttons were crackly, pots were noisy, most of the little blue beauty covers on the knobs were gone and I was just plain bummed out.

Jose (the tech who worked on my unit) gave it the once over, found that I had loose solder joints, dirty pots and buttons, and offered to replace the missing beauty covers for a bargain price of $83.00, tax included (YMMV) - this is the standard price for a bench service/checkup and I think it was a great deal. If you still have output I think its probably worth it to send it in and have them look at it. Turnaround was a little over a week but worth the wait.
 
#12
Rust might be a problem in high humidity areas, but not generally in Colorado. Since I have been playing bass more regularly, I am going to start doing the pot washing and jack reaming regularly on the 400RB and my older 200MB - just as a regular maintenance. If these shut off problems recur with the 400RB - I will likely consider a regular maintenance trip to the local amp guy. There could be still be cold solder connects - I guess that sort of stuff is inevitable if you hold onto gear for a long time.
 
#13
bigdoug1053 said:
Rust might be a problem in high humidity areas, but not generally in Colorado. Since I have been playing bass more regularly, I am going to start doing the pot washing and jack reaming regularly on the 400RB and my older 200MB - just as a regular maintenance. If these shut off problems recur with the 400RB - I will likely consider a regular maintenance trip to the local amp guy. There could be still be cold solder connects - I guess that sort of stuff is inevitable if you hold onto gear for a long time.
Click to expand...

I don't know how inevitable it is or how long a time is considered long. But I used mine for over 10 years, every weekend and while I didn't bash it up or run it at just below meltdown, I wasn't particularly careful with it and I did no maintenance at all. Once in a while I'd wipe the built up bar grunge off the chassis. I don't hear much about these amps being failure prone.

The 800 RB gets all the attention as being a reliable workhorse, but I think the 400 RB does just as well.

You just might not have any more problems for quite some time. It might be due for new power supply caps. That can't hurt.
 
#15
I have had the same problem lately with my late 90's 400RB. It happened twice at my last rehearsal. I had been running a Behringer compressor through the effects loop and immediately unplugged it and went straight into the amp. Then later in the same practice it happened again when I was running through a pedal board into the amp. I checked the fuse and found nothing. Is there a more permanent fix other than jumping the effects loop? I've had this amp since new and have never pushed it and always kept it in a rack case. I'm also checking all cables in the rig as well to try and rule everything out. I have a big gig coming on Saturday and will try the jumper to try and avoid this happening again
 
#17
I have a 1st gen 400RB that I've had to effect two repairs on: The rubber on the power cord degraded to the point that started showing the wires inside (easy fix) and and power transistor went to the great beyond (somewhat less easy fix). GK was good enough to send me a schematic and I got it fixed. I still have it - but I don't use it so much. Good little amp for its time.
 
#18
FYI, old friends; another 400rb from the mid-90s "fixed" using the FX patch!

I physically broke the input volume pot in the cleaning process so I've jumped that internally as always full on. Better than always full off!!
 
#19
Number179 said:
FYI, old friends; another 400rb from the mid-90s "fixed" using the FX patch!

I physically broke the input volume pot in the cleaning process so I've jumped that internally as always full on. Better than always full off!!
Click to expand...
Why not fix it properly???

Inquiring minds want to know how you could have broken a pot while (trying) to clean it?
 
#20
agedhorse said:
Why not fix it properly???

Inquiring minds want to know how you could have broken a pot while (trying) to clean it?
Click to expand...
You know, that was my intent in making that post, but looking at all the previous posters, none of them are around anymore so I didn't bother asking: so the workaround worked...what is the FIX?? I do have a great amp guy, and a 700rbii that needs a onceover too, so I have options.

The volume pot: the silicon in the pot frame cracked between the leads and the body I think because it was brittle and moved around too much during removal and fitting back into the chassis. So I removed it - soldered a jumper so that it is on full and expected that the pot was the root cause to begin with; however the signal still cuts in and out like its dirty or there's a poor contact somewhere in there - which is when I tried the FX jumper "fix".

Whatcha think, @agedhorse ?
 

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