Marshall JVM 210h
Well this one took some unexpected turns.
It came in with a new output transformer because the customer suspected it had blown... but also was unsure that the new one had been installed correctly. Great.
Turns out the new, and original, output transformers were totally fine! BUT there was a series of other problems.
I first discovered the blown fuse FS6 which is for the HT to the anodes of the tubes. This then led to finding blown power tubes AND a mechanically failed 16 ohm speaker output jack.
Did i discover that as quickly as I should have? Of course not!
Here is how it all went down...
I verified that the output transformer was good, and that the new one was hooked up correctly, tested the tubes, took resistance and voltage measurements but there was still no sound. I found a blown fuse for the HT voltage but nothing changed when i replaced it with a good one. I then replaced all tubes and there was STILL no sound until i plugged directly into my speaker cabinet (bypassing my attenuator) using a different speaker output. Everything was suddenly good! I then replaced the new tubes with the old, one by one, starting with the preamp tubes and found that all preamp tubes were good, but the power tubes were bad. SO, we have a mechanically failed speaker output that caused the power tubes to fail and take out the HT fuse with them. The amp works fine now with new power tubes, a bias adjustment, and using one of the OTHER speaker outputs. I don't have the speaker jack in stock so the customer is returning with the amp once one arrives.
These are the jacks that are used for the speaker outputs on this model of amp. It took more time to find a replacement jack than it did to figure out that the jack was the cause of the issues.
