1980s GK 200MB combo issue

[ikay]

My old GK 200MB has developed a curious fault - a temporary loss of high-end response when the combo is first turned on. For the first 15 mins or so the upper frequencies are lacking and everything sounds a little muddy. After playing for about 15 mins, the top frequencies come back and everything then sounds fine for the rest of the session.

 

I contacted GK who were very helpful but say it could be any of many things from a leaky capacitor to a dry solder joint. The fact that it rectifies itself when it's warmed up makes me think probably more a leaky cap than a bad connection. Before I send it to an amp tech, I thought I'd post on here in case anyone else has experienced something similar.

 

Wiring schematic for the input stage of the preamp below for ref.

 

Any input welcome. Cheers!

 

 

[basstone]

On 06/01/2022 at 17:31, ikay said:

My old GK 200MB has developed a curious fault - a temporary loss of high-end response when the combo is first turned on. For the first 15 mins or so the upper frequencies are lacking and everything sounds a little muddy. After playing for about 15 mins, the top frequencies come back and everything then sounds fine for the rest of the session.

 

I contacted GK who were very helpful but say it could be any of many things from a leaky capacitor to a dry solder joint. The fact that it rectifies itself when it's warmed up makes me think probably more a leaky cap than a bad connection. Before I send it to an amp tech, I thought I'd post on here in case anyone else has experienced something similar.

 

Wiring schematic for the input stage of the preamp below for ref.

 

Any input welcome. Cheers!

 

 

It's possibly just a poor connection. Are all of the controls and switches working well and scratch free? Same for the connectors. It's also worth trying the jumper cable test between the effect send and return in case there is a poor connection in one of the jack sockets. Also a sharpish bang just with your hand when first switched on might help to confirm if there is a poor connection within the circuit? Next stage is to test to see if the DI output and effect send are also affected to help localise the fault. Pretty much whatever it is will require the amp to be opened up, inspected and then fault diagnosis and repair done. Any experienced engineer should be able to help you get it fixed. Good luck

[ikay]

Thanks basstone. I hadn't thought of giving it a clout, might try that!

[itu]

Likewise there are the footswitch and headphones connectors. Some deoxit might help here. Take a look at the switches (tones, chorus, speakers etc. on both sides), too. When you open the unit, check the speaker connector close to the caps... some deoxit, again.