EDEN EC10 50watt solid state Motorboating

[Arbutt]

Hello all
I have an Eden EC10 50 watt 10" bass combo. Reported fault is motorboating.
On testing it was difficult to get it to motorboat. The only way at first seems to be if you plug a guitar lead in and touch the tip with your thumb, On testing with bass , If you have the bass control up and play at high volume , You might get a tiny bit of motorboating
I can't find a schematic output stage is LM1875
Replaced 2 x 4700Uf caps, Still the same
Replaced the LT Caps
I decided to piggy back 2 more 1000Uf 35v on to the filter caps , Still the same
any thoughts
Thanks in advance

[Jon Snell]

A schematic would be helpfull.

[The Dude]

It sounds to me like the amp is going into oscillation. I'd look at the zobel network, first. Make sure components are good and well connected. Since you don't have a schematic, here's a datasheet that includes a typical circuit.

https://www.ti.com/lit/ds/symlink/lm1875.pdf

I'd also check the cap C2 (first drawing) from the datasheet if your amp has that cap and is a similar circuit. Obviously the board designation wouldn't be the same, but I'm betting if you trace out the circuit, it's very similar.

[Arbutt]

A schematic would be helpfull.

Yes i know, but none seem to be available, I have searched the net, Nothing

[Arbutt]

It sounds to me like the amp is going into oscillation. I'd look at the zobel network, first. Make sure components are good and well connected. Since you don't have a schematic, here's a datasheet that includes a typical circuit.

https://www.ti.com/lit/ds/symlink/lm1875.pdf

I'd also check the cap C2 (first drawing) from the datasheet if your amp has that cap and is a similar circuit. Obviously the board designation wouldn't be the same, but I'm betting if you trace out the circuit, it's very similar.

Thanks Dude. I will check out and change all caps in the lm1875 circuit ..

[Arbutt]

I have found the fault, Believe it or not a ceramic cap on the 1st op amp stage.
Hit it with Freezer spray and it motorboated, Replace the cap. Amp is perfect
Many thanks for all your help