Ampeg SVT 7 pro power supply issues

[jeeptechfred]

SVT7PRO MAIN BOARD & PSU 0032543-XX SCHEMATIC.pdfI am working on an Ampeg svt pro 7. A tech I know gave up on this after replacing U2, the 2a165 with a 2a265. I’m not sure it was even bad.
When I started on it I found the 5 and 15v supplies “pumping” 0-4v as if it was. Faulting out most of the time. Voltages on U8 optocouple would support that. I found that the supply that feeds pin15 of U5 was also on and off. For a short while the voltages came on and I noticed pin 16 of U5 (vref) was not at 5 volts even though the supply seemed ok at that time. I did not measure a short to ground on pin16 so I replaced U5. Now the voltages created by U2 are solid, but I do not have the main 55v rails. I can see some sort of switching at Q1 and Q2 but they are different from each other. The +55 rail measures 7v and the -55 measures zero. The rectifiers test ok.
Pin 10 of the U5 measures 3.5v, the data sheet makes it sound (to me but what do I know?) like that should shut it down but that also appears to be the voltage that turns on relay 2A. I believe that needs to be on to work. Any help understanding where to test from here and any theory or ideas that might help me get to the bottom of my issues would be appreciated. Thanks, Fred

[glebert]

Replacing the 2a165 with the 2a265 is the basic "Rev H" rework IIRC for a problem some had where the amp would go in and out of operation. Just google "SVT7 Pro cutting out." There are a couple other components that tend to fail when that problem happens, if you use the search function you should find a few threads talking about it.

I hate that amp. It makes me feel dumb. It seems like it has more ways to fail that it has to work, which should make me feel smart. But no, just dumb.

[Enzo]

An SMPS that won't come up to voltage and cycles, is trying to start but shutting down. First suspects are loads on the outputs of the SMPS. You are reading low voltage supplies, but even shorted outputs on the power amps can load the supply. The whole SMPS will be affected. next the SMPS itself can be bad, I always start by just checking all the secondary side rectifier diodes for shorted.

[jeeptechfred]

Thanks, the low volt supply was cycling until I replaced the U5 Ka5325, now it’s not. The high voltage rectifiers checked ok but I have not yet check the output devices for the power amp. That will be my next step thanks. Any thoughts I what should expect to see on pin 10 of U5 in a normally operating circuit? What provides the over current feedback on the high voltage supply?
Thanks again!

[glebert]

Thanks, the low volt supply was cycling until I replaced the U5 Ka5325, now it’s not. The high voltage rectifiers checked ok but I have not yet check the output devices for the power amp. That will be my next step thanks. Any thoughts I what should expect to see on pin 10 of U5 in a normally operating circuit? What provides the over current feedback on the high voltage supply?
Thanks again!

I believe the functionality of the KA3525a is the same as the SG3525. This datasheet (https://www.onsemi.com/pub/Collateral/SG3525A-D.PDF) has a better description of the shutdown (pin 10) and soft start and compensation pins than the KA3525 datasheet, but I will admit it is still kind of confusing. In general logic high on pin 10 will cause it to shutdown, but it will try to start up again if you hold it high, or something. Like I said, this amp makes me feel dumb. I get the concepts of SMPS and class D, but the implentations make my head hurt.

Also, which side of T1 are we calling primary and which is secondary? U5 is driving the switching, right? So is that side "primary?" I get in the habit of calling the side closest to wall AC "primary" but it doesn't seem right here.

[Enzo]

The primary is the side where something is put IN, the secondary is where something it taken OUT. The schematic is drawn in conventional fashion, and all three transformers are shown with primary on the left side.

And really, if we think about it, the drive side of each is still closer to the mains, they just have other stuff in between them and the wall socket.


COnsider other transformers: low-Z line transformers for low Z outputs, or reverb pan drive transformer, or transformer phase splitters. In all I think primary and secondary are pretty clear.


Note: all three primaries NOT on left as explained below, sorry.

[jeeptechfred]

Thanks for your information. At this point I need to get back at it and check my output devices. If the smoothing caps for the high voltage rails had higher than normal ESR would that prevent this supply from coming up?

[glebert]

The primary is the side where something is put IN, the secondary is where something it taken OUT. The schematic is drawn in conventional fashion, and all three transformers are shown with primary on the left side.

And really, if we think about it, the drive side of each is still closer to the mains, they just have other stuff in between them and the wall socket.

What is the IN to the left side of T1? It looks to me like on the right side the OUTs of U5 are driving in to T1, on the left side it is driving the gates of Q1 and Q2, which then chop (switch) the DC voltage which is rectified directly from the AC mains. What am I missing?

[Enzo]

You are missing nothing, I didn't look close enough and was wrong. I saw the four rectifiers and looked no closer, But they are really tapping off U5 output to make a small supply for relay2 and U5 itself.

But really, U5 drives T1, which in turn is driving Q1, Q2, who ultimately drive the primary of T2, which actually is on the left.

U5 powers itself once it is running, by that small supply. But to get started, there are the two 220k resistors way left, R232 and R84. They get U2 running. That starts T3, which makes the low voltages, 300v, and other. Once T3 kicks, it makes a small supply via T3A to keep U2 running. But T3A also makes another small supply via D13 to start U5.

So, a trickle charge starts U2, which starts T3. T3 takes over powering itself and sending a turn on supply to U5. U5 then drives Q1,2 to drive T2, which makes the main 55v rails for the amp. T3 makes all the auxiliary voltages, so the main rails won't come up until the other voltages are all present.

[glebert]

Thanks for your information. At this point I need to get back at it and check my output devices. If the smoothing caps for the high voltage rails had higher than normal ESR would that prevent this supply from coming up?

By high voltage do you mean the +/-55V? Just want to differentiate from the initial rectified DC which should be something like 160V (I think). I am curious what U7 is doing. Some sort of power good protection circuit? They actually have test points on the inputs to it, it must be important!

[Enzo]

Shorted or super leaky filter cap on the 55v rails might shut it down, but higher than normal ESR? Not likely.

The mains voltage is rectified up front. The 120/240 switch decides if the supply doubles or not. In 120v mode it charges each cap to about 160-170vDC, and they are stacked so we get 330vDC. In 240v land they open the switch, and the 240vAC is rectified directly for the same result.

A word about safety. Note a lot of the drive side parts refer to HGND for their grounds. H as in High VOltage Ground. This is NOT earth ground or chassis ground. DO NOT hook your scope probe ground to it. LOok top left center at the main rectifier bridge. Note HGND is the left corner of that bridge. it is the -170v terminal. HGND is -170v from chassis ground.

At the moment I am too fuzzy to 'splain U7, but it is a reset circuit. I believe the two test points by it are there to briefly short together to reset the SMPS. As in triggers the reset/power-up sequence. It is a plain old 555 timer chip.

All I have is the schematic, but I think the test procedure is in the whole manual.

[jeeptechfred]

Yes, I’m referring to the 55v rails. I know that u7 controls relay rel2-a for sure and the out of it is also connected to the shutdown of U5. Test points 1 and 2 both had what appeared to be correct voltages on them based on the math I did (voltage divider circuit). I’ve been too busy so far this week to get back to it yet. When I do I certainly have some stuff to check. Thanks for your thoughts guys!

[glebert]

All I have is the schematic, but I think the test procedure is in the whole manual.

I wonder if Yamaha will be more friendly with sharing repair manuals than whoever owned Ampeg last. I was basically told to get lost when I asked about getting support for an SVT7 Pro a couple of years ago.

[jeeptechfred]

It’s nice to hear your explanation, that’s basically how I had it figured (-:

[g1]

I wonder if Yamaha will be more friendly with sharing repair manuals than whoever owned Ampeg last.

As far as I know you always had to purchase manuals from Yamaha.