Ampeg SVT-400T Thump/DC On Power On

[kdawg]

Hi all - I have an Ampeg SVT-400T on the bench, works OK but has a large, almost subwoofer damaging initial thump every time it's powered up. I'm not too familiar with these older solid state Ampegs, I know the V4B tube era and the SVT Pros. I don't see a relay or mute circuit, would this be a cap filtering kind of thing on this amp or is there another area prone to causing that large DC swing? Thanks,

-kdawg

Ampeg SVT400T.PDF

[kdawg]

Anyone? I haven't checked caps yet because it has to be completely disassembled. There are 6 3300uf / 80v filter caps and a couple 1000uf / 35v for the 15v supply. I did check the bias circuit, seemed to be fairly close to 300mv across the power resistors.

I did find better schematics here: Prowess Amplifiers - Ampeg Schematics

Thanks
-kdawg

[MarkusBass]

I did check the bias circuit, seemed to be fairly close to 300mv across the power resistors.

And do you mean by power resistors? I'm asking because this is class B amp and you have to know how it works. Do you have this 300mV on the schematic? What voltages do you have on TP6 and TP7?

Mark

[kdawg]

I went by schematic instructions, it says across R31/R131 or R32/R132 to measure 300mv. Could bias affect the turn on thump? I'm not sure how may volts DC on the speaker jacks would be a normal turn on spike on an amp like this.

-kdawg

[MarkusBass]

Sometime people consider R33-36 to be power resistors (that's why I'm asking). So the bias seems to be OK. I suggest to disconnect the power amp input and turn the amp on to see whether this is a problem related to the power amp or rather to the preamp.

Mark

[Enzo]

Perhaps we can make sure we are all on the same schematic? It sounds to me like we have people thinking tubes and solid state at the same time here?

[kdawg]

Good point Enzo - I am using pages 1 and 2 of:

Prowess Amplifiers - Ampeg - Schematics - SVT400T pt1of2

Solid state only, no tubes in sight on this amp. The bias is for the mosfets. I did try plugging in a dummy plug into "power amp in", bypassing preamp - no change. I will also try pulling the ribbon cable just in case, but I think it's either main power rail or MOSFET related.

-kdawg

[MarkusBass]

KDAWG,

Please note that this amp does not contain either tubes, nor MOSFETs. It uses bipolar transistors in the output stage. So what are the voltages on TP6 and TP7? Does the thump changes when you switch the amp stereo/mono?

Mark

[kdawg]

Sorry, yes they are bipolar transistors.

I have more info:
* Thump is only when turning off, not on... I thought it was both before but I was wrong.
* TP6/7 are 1.59 V and -1.56 V
* Mono/Slave mode makes no difference. Using bridged output makes thump twice as bad than A or B channels.

I'm not sure what is considered normal for this amp, I know putting a 4 ohm speaker cab on bridged output feels like it could be damaging. I'm using a 16 ohm load right now for testing and its not as severe.

[MarkusBass]

The schematic says 1.45V forTP6/7. If you try to make the voltage lower, does it change much the thump? Does it depend on limiter switch?

Mark

[J M Fahey]

Short answer, thump is normal, won't damage your speakers.
It's not worse than the bass player slapping or tapping *once* at full power.
If bias is as specified and amp seems to run normal, it's fine.

[kdawg]

Well I completely misdiagnosed this one - turns out it IS the preamp, when I had plugged in dummy plug into Power Amp IN jacks I didn't realize I had it in stereo mode, so I had to have two jacks plugged in to actually test that correctly.

So bottom line is when preamp is disconnected, no turn off thump. I did find a bent Q1 transistor that looked to be shorting to itself, and when I checked the AP-1 limiter bias it was only about -4.5V and it says it should be -8.5V on the schematic so I reset. No change in thump after straightening that transistor out and resetting limiter bias.

I can see the thump start all the way from IC1 pin 1 and 2 (first opamp from input jack). DC spike both of those pins, more on pin 1 than 2. Caps C5 and C6 (22uf/50v) measure OK.

I don't have a J112 transistor to try replacing the shorted out one, would removing it be OK to see if that is causing an issue?

Thanks

[J M Fahey]

J112 transistor to try replacing the shorted out one,

Why do you think J112 is shorted?
It's a FET and with no bias it should read around 100 or 200 ohms, drain to source, both ways.
As of having turnoff thump on input pins of the first Op Amp. straight at the jack connection, speaks (to me) of a very poor grounding system.
Maybe it's bad, maybe it was born so.
For what it's worth, I once repaired an SVT400 amplifier which was chock full of bad/terrible ground connections; the chassis was very rusted, so much that one piece of it did not make contact with the next one to which it was bolted to.
I had to fully dissassemble it, scratch all rust and reassemble it as-is, only using some oil or grease as a temporary protection, because, as usual, my customer was in a hurry and just *HAD" to use that very amp (the last one in the world?) two days later.(maybe the Humanity would dissappear if he didn't do so)
Oh well.
My advice was to empty the chassis and send it to be paint-peeled, and have it either phosphatized and re-painted or galvanized, which would have solved those ground problems for good.
He chose to buy one of my tube preamps instead, to drive a QSC900 and 2 Ampeg 8x10", so all ended well