Ampeg SVT 7 pro power supply issues

[Enzo]

Yamaha has always published really terrific manuals for their stuff. I mean every part listed, mechanical blow up drawings, disassembly instructions, diagnostic procedures, and so on. Nice books and worth the price. The Ampeg docs are nowhere hear, so hopefully...

But all you can do is CALL them and find out.

[jeeptechfred]

I finally got around to working on this again. I did some quick checks for shorts on the rectifiers, 55v power rails and output devices, nothing obvious found. I decided to apply 10v plus minus to the 55v rails and look for inconsistencies between the 2 class D drivers and output devices. I found on Q10 the gate had 7v while Q12 had near 0. I lifted pins 13 and 14 on u300 and found that the voltage stayed the same on the gate. I removed Q10 and found 400 ohms from drain to gate. I replaced the FET and rechecked. I found the voltages on u300 were much better but not quire the same as u17, I suspected u300 was damaged due to failed FET. I have a spare (malfunctioning) board for this same amp. I replaced u300 with one from my spare board. I powered up the amp and found the 55v fRails and power light now function. I put a signal into the amp and checked for an output. I found that the amplifier had an output for a fraction of a second and then would go silent for several seconds... it continues to cycle like this. I am assuming that there is some type of protection circuitry that is coming into play and resetting after several seconds. I found on the CSD pins of the mosfet drivers the voltages drop down to (I canÂ’t recall the actual voltages for certain but I think these are close) .4v the amp comes on for a split second and then go silent, at that point the voltage jumps up to maybe 2v and then slowly begins to fall. I looked at Q33 I found these rising and falling voltages on the collector but I found 0v on both the emitter and base. Does it sound like IÂ’m on the right path? Any suggestions? Thanks

[glebert]

It sounds like you are now up to the common "cutout" problem that is usually related to U2. There are a couple of components that also tend to fail with U2, but I don't remember which ones. If you search on here you should find them.

I ended up replacing U2 a couple of times, eventually putting a heatsink on it, and finally replacing the tube with a solid state "tube". That got it working to the point where it would only cut out with a large vibration/shock, but I could never track down the problem to fix it and sold it on Ebay as a fixer. I was glad to rid of it by then.

[jeeptechfred]

U2 is not the problem. If U2 was having issues I would have problems with all my power supplies. All my power supplies at this point are rocksolid. Thanks for your input

[jeeptechfred]

I did manage to get the amplifier fixed. I ended up finding D 50 for a gate diode And Q 29 which is a driver for the one of the main output FETs shorted. That transistor is not in the schematic. I think the schematic is an early revision. Since then I’ve seen boards without them in and the class D drivers Chip has heat sinks on them. The current revision that I have does not have the heat sinks but has the transistors. I am assuming to take the load off of the class D drivers chips. Once I replaced these components it’s up and working right. Thanks for the help along the way

[Pocoyo]

Greetings, I understand that the last post on this topic was for 2020, I have the same equipment, Ampeg SVT-7 PRO, with the same IC problem U2 (ICE2A165), This one burned in the circuit, Also the D3 and R48 were damaged, the components were replaced, but, When I go to do the tests, By increasing the when
variac little by little with a lamp in series, it has a high current consumption, and if I go above 20VAC, the IC It is damaged, The resistance of the mosfet, Pin 4-5 and Pin 8 go almost short, between 100 or 150 ohms, I have checked almost everything after the transformer y And I can't find shorts, Could someone show me the light at the end of the tunnel, thank you.