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GK 700RB/210 Head temporarily cuts out

Philthy

Philthy

Supporting Member
Jun 12, 2001
825
243
Flanders, NJ
#1
A friend of mine gave me the 700RB head out of his older 700RB/210 combo because he could not figure why it would sporadically cut out and was frustrated with trying to fix it. I said, what the heck, I'll tinker with it. I replaced some bad pots (contour, presence and woofer) and I replaced the effects send and return jacks. All the fuses look good. I thought I had it, until I lifted the amp and it cut out. I'm guessing there may be a loose connection somewhere. I attached a video of it cutting out. I know this could be a needle in a haystack but I thought I'd take a shot here to see if there were any ideas or suggestions before biting the bullet and finding a tech to diagnose it. Thanks!

GK 700rb cuts out
 

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#2
Does it cut out for you? I'd suspect the thermal protection is working properly, or not working correctly—- pretty easy to troubleshoot for a knowledgeable tech.
Replaced 3 bad pots? Those pots see little use, so I'm not sure all 3 (if any) needed to be replaced.
 
#3
Slevage said:
Does it cut out for you? I'd suspect the thermal protection is working properly, or not working correctly—- pretty easy to troubleshoot for a knowledgeable tech.
Replaced 3 bad pots? Those pots see little use, so I'm not sure all 3 (if any) needed to be replaced.
Click to expand...
Yes, it cuts out for me as the video i posted was just from few minutes ago. 2 post were scratchy and still were after cleaning with contact cleaner. 1 pot, the shaft would pull forward so it was broken.
 
#4
Philthy said:
Yes, it cuts out for me as the video i posted was just from few minutes ago. 2 post were scratchy and still were after cleaning with contact cleaner. 1 pot, the shaft would pull forward so it was broken.
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The general rule of thumb is that contact cleaner is bad for pots in the long run. It washes away the lubricant, and pots get scratchy again quicker. You need some lubricant to spray in there, but I can't recall the exact Deoxit product that is often recommended. It is also known as "Fader Lube" when I bought a can years ago from Radio Shack.

I can't say I know much about the internals or whatnot, but it's a known issue among the RB series that the FX loops get corroded. Try a short jumper cable between the in/out of the FX loop (literally just a cable between the two). If that solves it, there's your issue. And that CAN be fixed with contact cleaner, as it's just cleaning the contacts of the jacks.

Between lubing up the pots and cleaning the contacts, I've saved a handful of "broken" amps, and often played them for years or sold them off for a decent margin (considering I got them as "broken", and therefore dirt cheap). Good luck with the amp - I LOVE my 700RB-II!
 
#5
Hope it doesn't now need a new set of pots in addition to whatever the actual cause of the problem was.

Pots damaged by contact cleaner are becoming more common now that internet educated hobbiest techs (and hack techs who shouldn't be anywhere near the inside of an amp) have started relying on "miracle cure-all" sprays in place of diagnostics.

The proper tools are a signal generator, a scope and the knowledge/understanding of exactly how the amp is supposed to work.

Do you know if the problem is with the preamp or power amp yet?
 
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#8
I've got years of training repairing avoinics gear, a pretty good o-scope, and reasonable signal generator. I've come to the conclusion if a good visual inspection doesn't find the problem I won't bother working on modern equipmant. The one caveat is if I care so little about the gear that if in the end I'm willing to strip it of any useable parts and throw the rest away I won't bother.

I've seen "qualified" techs make mistakes that left them on their backs, and units in flames on the bench. Usually when they opted to ignore simple safety procedures. Or when they thought they could repair something that was out of thier area of expertice.... (usually the worst was old CRT TVs).

I only know of one tech on this forum that really knows amps (there could be more) I'd heed his advice.
 
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#10
Peteyboy said:
I used to have that head, and I'm sorry I ever parted with it. It's absolutely worth taking to an authorized tech.
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IF you can find a tech that will work on an amp after a home spun repair attempt.
 
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#13
This smacks of a cracked solder joint. With the amp on, use a WOODEN pencil and gently tap around and on the components. Be judicious - work slowly and methodically. Keep your hands out of the amp when it is powered on and keep other people, potential UFOs, pets, etc away from it while you are working it. I'm betting that you will be able to narrow it down to an area or component that is faulty by observing the amp cutting in / out as you near the fault area. Once you discover the fault area, inspect the solder joints holding the components in that area. Example pic from a Fender Bass 300 I worked on. I see this regularly on equipment tha tis handled alot and sometimes when equipment is not designed for being handled alot.

Example of this technique:
 

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#14
BobKos said:
This smacks of a cracked solder joint. With the amp on, use a WOODEN pencil and gently tap around and on the components. Be judicious - work slowly and methodically. Keep your hands out of the amp when it is powered on and keep other people, potential UFOs, pets, etc away from it while you are working it. I'm betting that you will be able to narrow it down to an area or component that is faulty by observing the amp cutting in / out as you near the fault area. Once you discover the fault area, inspect the solder joints holding the components in that area. Example pic from a Fender Bass 300 I worked on. I see this regularly on equipment tha tis handled alot and sometimes when equipment is not designed for being handled alot.

Example of this technique:
Click to expand...

Right, and when I am talking about following safety protocols, perhaps you are unaware that most pencils are (very) conductive through the graphite-clay center??? At higher voltages, the surprise will be one you remember because totally aren't expecting it.

Again, if you don't understand safety, you probably shouldn't be inside the amp.

The video is exactly what I am referring to, nothing like a YouTube education on how not to do something.

Your picture is a totally different type of PCB, it's a single sided whereas the 700RB-II is double sided and does not suffer from that particular type of defect.
 
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#19
J Wilson said:
I have a friend who is a lineman for a cable company. He is required to check the bare wooden pole for any voltage before he does anything. They're taught that with enough current, any and everything can be a conductor.

Be careful.
Click to expand...

voltage, not current in this instance.

Other than that, intermittent problems can be very frustrating even when you have a signal generator, scope, and audio analyzer due to not being able to consistently replicate the problem.
 
#20
J Wilson said:
I have a friend who is a lineman for a cable company. He is required to check the bare wooden pole for any voltage before he does anything. They're taught that with enough current, any and everything can be a conductor.

Be careful.
Click to expand...

I think there's a song in there somewhere: "I am a lineman for Verizon...".

Re: the gear in question and on a positive note. Stuff that is truly broken does repair itself mid-use. Drop the coin on the bench fee before something gets screwed-up royally.

Riis
 
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