We were asked to have a look at an Ampeg B200R bass amp. This is a Chinese built amp and is known to have a few challenges. Faults include badly fitting chassis in the case resulting in the chassis being damaged and known to fail when the speaker is switched from ” no horns” to “horns” jack socket.
This failure mode is caused by the panel which is mounted in the chassis with the speaker cross over network. Due to a mis-understanding between the Chinese production unit and Ampeg in the USA, the with horns socket shorts out the output of the amp. The fix is to remove conductive material on the printed circuit board from around one of the “with horns” jack sockets so that the short to earth no longer happens. I will upload a picture here.
This one was dead on arrival. Powering up but no sound whatsoever.
This Ampeg amp has one valve an ECC82 right on the input to add some warmth to the otherwise fully solid state amp. It was replaced with the same make and model as the existing valve showed as very worn on our valve testers .We replaced it with and Electro Harmonix EH 12AU7.
Here is a picture of the chassis,and there is a lot of cables to unplug to get the boards out ! The bend on the top of the picture is not camera distortion. The chassis was bent and a corner torn when the Chinese were battering it into the chassis
There are two key boards in this amp. One is the signal path pre-amp board with the controls mounted which is all analog with the usual op-amps. the other board is the power board with driver transistors and the four main power output MOSFET transistors. You can see three of the four replaced MOSFET transistors in the bottom right of the picture below.
Injecting a signal into the input showed that the signal path board was working perfectly..BUT,
There is one buffer dual op amp on the power board which was working perfectly. Everything after that point was blown. We replaced zener diodes, signal diodes, driver transistors and MOSFET transistors. The amp was now back to its old self and working again. Here is a picture of most of the parts that were replaced.
When we tried the speaker in the cabinet that was completely distorting and clearly the speaker is wrecked. The bad news is that the Ampeg cabinet is built so that there is no way to get into the cabinet that I find unless you wreck the cabinet in the process.
So does anyone know how to get into an Ampeg B200R speaker cabinet without destroying it ? Please email to firstname.lastname@example.org
Otherwise this Ampeg may need to be made into a head cabinet.