SVT 4 Pro won’t power on

[realgtrhero]

My bass player had a friend donate him a SVT 4 Pro because it didn’t work and he had no use for it anymore. I gave it my best Backline Tech and basic understanding of electronics shot but, couldn’t figure it out. Any advice would be greatly appreciated.

Thanks
Tim

[Enzo]

Start at the start. Does it show ANY sign of life? ANy light?

Your primary wiring is simple. A mains plug and cord, a circuit breaker and a power switch all wired to the transformer primary. HAve you tried a different power cord? When the unit is plugged into the wall, does the convenience outlet work? It appears the transformer primary are white and black and plugged onto posts J19 and J21. With the thing plugged in and switch ON, is there 120vAC between those two posts? Are any of the transformer secondaries producing voltage?

[nevetslab]

Welcome to the Forum!

There are many of us here that know this amp well. I have all of the documentation for it, own one myself, which had to be restored, as it was a gift from one of our members....just had to pay for shipping from the East Coast to the West Coast, then find the time to get it up on the bench, and go thru it to see what the patient told the doctor, and what the doctor found, so to speak.

These are not what I could call Service-Friendly. Somewhat of a challenge just to take one apart, so I'd suggest your Backline Tech become a member of www.music-electronics-form.com to be able to jump in to learn what he can from our experienced techs and engineers here. The front end PCB has lethal voltages on it to run the preamp tubes, and the main amplifier assembly to which the two heat sinks are attached has AC Mains Voltage on it to run the chassis-mounted power xfmr. It's a system of sub-assemblies....Low Voltage power supply assy, High Voltage Power Supplies to run the MosFET outut stages and the preamp Tubes. Clumsy once pulled apart. If the MosFET output transistors have failures on the two channels, those parts are matched devices, and expensive to replace if you buy them from Ampeg. Yamaha now owns Ampeg, and I don't know if you can purchase parts from them anymore without being an authorized service center. Some of us who service the Ampeg MosFET-equipped parts purchase them in bulk quantity and then go thru a processes to batch and match them from a measured distribution of what the tests reveal

There's a wealth of information on servicing this amp on our Forum. I've composed a good number of posts over the years, as has Enzo and many others. Very good reading material available using our search engine within our archives.

Here's some images of what's inside the covers: