GK 400RB Problems

[agedhorse]

I’ve never seen any silicon in a pot or frame. It’s a standard part, have your amp guy fix it right before you create bigger problems. He can then diagnose the other issues too.

There’s a rule of thumb when working on amps… don’t break things worse, it always increases the cost of fixing it right later.

 

[Number179]

PCB material, not silicon, sorry. I guess PCB is made of fiberglass.

 

[fdeck]

I think I know the material you're talking about. It looks like the older type of PCB material that was beige or brown, called "paper phenolic." It's still used as the substrate for the resistive film of some pots. Those are also the pots that are open on the bottom, inviting application of sprays.

Those pots can be fragile if there's any trauma between the body of the pot and the circuit board. Enough force / torque can crack the phenolic, which invariably renders the resistive material either intermittent or open. It's one of the areas where getting an amp apart can be dicey. They're usually robust enough once everything's bolted together, although a really hard whack that flexes the front panel can break pots.

 

[agedhorse]

What exactly did you break? The PCB or the resistive element inside the pot?

I'm going to suggest that you talk with your amp guy before you end up with a bigger mess.

 

[Number179]

Cracked right here:

I think it was weak because there must have been stress on it while tightened down to the front panel instead of sitting without tension - for 25 years. I'm running it by my amp tech today for him to do his science. He'll fix the problem itself and most importantly provide an explanation for the actual cause of the problem.

Good new is, the 2 bottles of Deoxit I bought for $41 was more money than I spent on the amp. But this thing sounds awesome. I liked the 700rbii when I got it a couple weeks earlier - again for like a killer deal. But the 400rb does the GK sound a step beyond the 700rbii.

 

[agedhorse]

2 bottles??? That would last me 10 years in a shop that repairs over a hundred amps a year. Less is more, and only the tiniest amount possible is the best approach.

Just for reference, be sure to tell your tech that you tried to self repair the amp with “miracle cure-all spray”, it’s very possible that you ruined the entire set of pots and the damage you think is a broken pot is actually related to the internal element (or residue all over the PCB) getting damaged from the DeOxit.

 

[cfortner]

A problem could be to get exactly fitting pots with center detent.

 

[Number179]

2 bottles??? That would last me 10 years in a shop that repairs over a hundred amps a year. Less is more, and only the tiniest amount possible is the best approach.

Just for reference, be sure to tell your tech that you tried to self repair the amp with “miracle cure-all spray”, it’s very possible that you ruined the entire set of pots and the damage you think is a broken pot is actually related to the internal element (or residue all over the PCB) getting damaged from the DeOxit.

One of D5, one of F5.

So, the pots being currently fully functional might be an indication that they are ruined? Not following the logic here…also that the FX patch and the jumper on the input volume have stabilized the original problem 100%..(stabilized, not fixed).

 

[agedhorse]

One of D5, one of F5.

So, the pots being currently fully functional might be an indication that they are ruined? Not following the logic here…also that the FX patch and the jumper on the input volume have stabilized the original problem 100%..(stabilized, not fixed).

F5 isn’t a great choice, but one or the other is preferable.

Just because they appear functional now doesn’t mean that they haven’t been damaged and are deteriorating. In a professional service environment, we don’t want to see amps back in the shop because of a poor quality repair attempt.

Once a tech services something, they become married to it because of the service warranty. You do not want a tech that doesn’t warranty their work.

That’s why you are not following the logic.

 

[Number179]

F5 isn’t a great choice, but one or the other is preferable.

Just because they appear functional now doesn’t mean that they haven’t been damaged and are deteriorating. In a professional service environment, we don’t want to see amps back in the shop because of a poor quality repair attempt.

Once a tech services something, they become married to it because of the service warranty. You do not want a tech that doesn’t warranty their work.

That’s why you are not following the logic.

Yes, I know, but no. They don't appear to be functional - they ARE functional. There isn't an in between. Could they spontaneously combust tomorrow due to something an amateur tech did to it (including myself or anyone previous to me) - maybe. But as of now - the work I have done to it IS functional. It is exactly functional. And the fun I've had taking it apart to tinker has been substantive. Now, onto the 700rbii with scratchy pots

 

[agedhorse]

Yes, I know, but no. They don't appear to be functional - they ARE functional. There isn't an in between. Could they spontaneously combust tomorrow due to something an amateur tech did to it (including myself or anyone previous to me) - maybe. But as of now - the work I have done to it IS functional. It is exactly functional. And the fun I've had taking it apart to tinker has been substantive. Now, onto the 700rbii with scratchy pots

There is absolutely an in between.

Sorry to expend the effort to educate somebody who doesn't know what they don't know.

 

[Number179]

"take it to a tech" ain't very educational. Anyway - it's no big deal. I know you're the best in the business, and I have the highest regard and respect for your contributions, knowledge, experience, artistry, sincerely. I don't yet own any of your designs, although I can't wait until I do. But "take it to a tech" is not educational.

 

[agedhorse]

"take it to a tech" ain't very educational. Anyway - it's no big deal. I know you're the best in the business, and I have the highest regard and respect for your contributions, knowledge, experience, artistry, sincerely. I don't yet own any of your designs, although I can't wait until I do. But "take it to a tech" is not educational.

It's probably the most educational AND important information in my replies.