400RB blowin fuses

Power supply caps, power supply rectifiers...... Take it in and get an estimate, that's a great old amp!

 

keep us posted if you can, i just bought one of these

 

I had the same problem with my 400RB-IV. I contacted GK and they sent me a schematic for a simple mod that local tech guy applied. Three years later the amp is working without any problems.
My advice - contact GK, they are really helpful.

Raz

 

Hi.

+100 for contacting GK. I have owned only one of their great products and they were super helpful in getting the information I'll need if I ever find the parts to fix that damn thing .

I've heard that there's been some changes since then, but I was very pleased with their CS.

And a great + for reprting the cause and the remedy too. Too many threads like this are just "left hanging". The archieve building thing is at least as important for me as it is to get one individual unit fixed.

Regards
Sam

 

Sorry to necromance this thread but do you still have that schematic? GK isn’t getting back to me.

 

This looks promising

 

Most service guys can look after an RB 400. They were built like tanks.

 

Fixed it for you

 

Thanks but I was referring to the "fix" that razboinikoff alluded to, I already have the schematics. I am able to initiate any repair, just don't have the funds to get a Variac and a Oscilloscope right now. I would like to know what this fix is before I go spend hundreds with a tech.

 

Immediately blowing fuses indicated a catastrophic fault. There’s no “easy fix”, test equipment and some troubleshooting experience is going to be necessary to really fix your amp.

 

Thanks for your help. To help anyone else looking at this thread having the same issue, I took it apart and the control board was really dusty (where the knobs are connected). I did notice some cut leads floating around on top of it which may have caused the issue. I cleaned it up and then turned it on with a new fuse and speaker disconnected and everything was fine.

Now this is a combo so there is the speaker cable coming out of the back of the cabinet that then plugs into the amp. I checked connectivity between tip and sleeve and noticed that there was some weird readings. I had resistance then I didn't as I moved the wire around. This leads me to believe that there is a possible short at the speaker terminals, or that the wire is faulty. Once I plugged it in and there was resistance being shown on the tip and sleeve of the speaker, there was no more issue. Now i just have to disassemble the cabinet and check the connections.

If you have a 400RB MK III combo, this might be the issue you're having, since the speaker cable is pretty loose in there and the connection might have been damaged over time. It also might help to just go a once over on the soldering work, as some residual leads and solder were left on these boards.

 

Cut leads floating around??? Could this have been from a previous repair attempt?

 

It was a few pieces semi-stuck in residual flux, it could have come in from somewhere else as well. There was even a jumper wire to fix a PCB printing error, which I can only assume was fixed in a following revision. There was nothing I observed that was indicative of a botched or sloppy repair as I've seen stuff like this in some other factory builds.

I also ordered two new power supply caps; even if they are in spec, it's been 20+ years so I might as well while I have it apart. I also contacted GK about the fan protection circuit fix for the III, which I will decline to post here because it depends on a certain board revision and I don't want anyone attempting it without verifying with GK, so if you're not sure, GK will help you out, just contact them.

 

If the original caps are still working properly you might want to leave them alone. 20 years for this era and chemistry cap is less than 1/2 the useful life IME.