JCM 800 2204 Self oscillation problem

[Rockstar Okan]

Hi Guys;

I have a Marshall JCM 800 2204 with horizontal inputs (pcb mounted pots and jacks). I changed the original transformers with Hammond 1750N OT and 290GX PT. Please watch the video and help me for this issue. My amp self oscillating!

it was much worse before i changed the grid stoppers 5,6K to 220K and reduce negative feedback. This version is much better.

What can i do? Please help me. I have digital multimeter and scope. I guess i read 11KHZ as oscillation frequency.

Thanks a lot.

Self Oscillation Okan - YouTube

[Chuck H]

Is this your amp or are you repairing it for someone? It would help a lot to know the history. Did the amp always do this or did it start after some repair or modification?

[Rockstar Okan]

I am amateur for this applications and i have a small workshop room t my home. I am repairing it for my friend. But someone had tried to fix the amp before me. And he messed up. I changed the transformers completely by a new ones.

[Enzo]

Since you replaced the output tranformer, perhaps you have the primary leads reversed?

DOn't reduce the negative feedback, disconnect it entirely. Now what does the amp do? Removing the NFB removes the phase requirement and would stop hard-wired feedback oscillation.

[Rockstar Okan]

Since you replaced the output tranformer, perhaps you have the primary leads reversed?

DOn't reduce the negative feedback, disconnect it entirely. Now what does the amp do? Removing the NFB removes the phase requirement and would stop hard-wired feedback oscillation.

https://www.tube-town.net/ttstore/im...ges/2205_1.jpg

I swapped OT primers (blue and red) and the oscillation stopped.

[g1]

Now that you have stopped the oscillation, you can go back to the stock values for the feedback and gridstoppers.