Ampeg SVT 3 pro.

[catstrat]

Does anyone have the output section schematic for an Ampeg SVT 3 pro. Or can someone steer me in the right direction? Thanks

[g1]

http://music-electronics-forum.com/a...g-svt3-pro.zip

[catstrat]

SVT 3 pro

Thanks G-one. I found one on Schematics unlimited. The SVT on my bench has a 60 cycle hum that's originating in the output section. I went ahead and replaced the 80 volt filter caps as well has the 450 volt caps. The hum is still there and it responds to turning the bias adjusting pot.When i set the bias to pull .8 on the variac meter, the hum is pretty loud. Even with the 2 tubes removed from the output section, the hum is still there. I ran a signal into the power amp in jack and nothing but hum. I'm losing signal somewhere between the ax7 tube and the au7 tube. It tells me if i have an open resistor in the tube circuit then that might be the problem. Any other suggestions out there. Thanks.

[catstrat]

svt 3 pro

I found and replaced an open r5 which is a grid bleed resistor, or that's what i call it. The hum is still there. It's a 60 cycle hum which i was told is always a ground issue but will someone let me know for sure. The other thing is the O/P heatsink is getting so hot to the point of tripping the relay and it trips it in about a minute and a half. Another thing i was told is that a transistor in itself does not cause a hum. Is this correct? It passes no AC signal at all. Could this be a bad o/p transistor? I'm getting a great signal from the preamp. I can put a signal into the power amp in jack and nothing. It seems to be losing it somewhere before it reaches the 12aux tube. I touched up all the solder joints. Could it be a bad Q 2 bias servo transistor because it does seem to be a bias issue. When the heatsink overheats it isn't concentrated in one area of the heatsink. The heatsink is hot all over. I'm pretty convinced it's a bias problem, but i've i've been wrong many tmes before. So any one who can give me some advice, i have open ears.

[catstrat]

SVT 3 pro

Hi all. I still have some questions that i was hoping someone would answer for me. Is it true a transistor on its own does not produce any hum? It merely amplifies it. The next question, can you test a solid state amp without hooking it up to a dummy load or a speaker cabinet? I'm looking for some clarification because i'm not sure the information i was given by other people is correct. Being a tube amp guy, these are mysteries of solid state that i need to sort out. I figure they're no better people to ask than you guys. The SVT 3 pro on my bench has a bad hum with severely over heating heatsink. The hum is originating in the output section. Thanks everyone.

[Enzo]

Transistors on their own do not produce hum, any more than a 6L6 all by itself could. The transistor, just like the tube, handles currents and voltages. If there is hum on a signal entering a transistor, it will be there when it leaves too.

Now, if an output transistor shorts E-C, that can wind up putting a power rail right on the speaker. That will result in a loud hum, but the hum is the poor speaker loading down the power supply and the hum is ripple. SO the bad transistor can cause hum to happen that way, but makes none of its own.


Tube amps MUST have a load when operating. Solid state amps do not need a load, in fact it is pretty important to NOT put a load on them until we know they are stable.

[catstrat]

svt

Thanks Enzo. You're tops. There is so much mis information out there i wanted to hear from guys like you, because you know. Thanks for helping a guy learn more about the transistor side. So the problem with the SVT i have now could very well be an output transistor shorted emitter to collector. As i said after only a minute and a half the heatsink is too hot to touch and will kick the protection circuit. I'll pull the transistors and check with a meter to see if one of them has a problem. Again, thanks so much Enzo.

[mozwell]

As a general rule, even if one output transistor is shorted, i would replace the whole lot, on both positive & negative sides. The faulty transistor will have "stressed" the others, and while they may still work, they may not be reliable.
Also check the driver transistors, in this case Q3 and Q4, also the bias transistor Q2, and current limit transistors Q5 & Q6 are all ok
Another check is the source resistors, R32 to R39, do they all measure 0.47 ohm
Install the reliability modification
do NOT connect a speaker until the very end of the repair process.

Remove the power transistors & power on the unit. Measure the power supply rails
+100V on D2 cathode or TP13
-100V on D3 anode or TP14
+65V on TP9
-65V on TP10
-16V on TP12
+16V on TP11
All good, then measure TP5, it should be close to 0V, measure TP4 and the emitter of Q2, are these approx 4V above & below what you measure for TP5

All good, then we can turn the power off, make sure the power supplies have all discharged & reinstall the output transistors, make sure if they were insulated before that the new ones are still insulated. Measure each one from heat sink to each of the transistor pins with a meter on ohms range, you should get many Meg ohms.

All good, then power up through a light bulb limiter, and measure all the power supplies, and the other voltages as above. the bulb should flash when you turn on, then go dim.

All good, power up without the bulb limiter & check all the voltages again.
You MUST have very close to 0VDC on TP5, do not connect a speaker until all voltages are correct.

[catstrat]

SVT

Thanks. As it stands right now the plan is to focus on the o/p transistors. I was told if you have one bad one replace all of them plus drivers and predrivers. I can turn the bias pot to where the bias point should be and the hum is really bad. Of course when i turn it the other way the hums dies down. I know on a tube amp if the bias gets too hot that can cause a hum. That's what got me thinking this may be a bias issue. A bias that's too hot will cause the O/P trans to overheat plus the heatsink. But just like a tube amp, a bad tube can cause it and i'm sure a bad out put transistor can cause it to. Again, transistors are not my strong point. Thanks everyone.

[catstrat]

SVT

I did some in circuit tests with a meter this morning on the o/p trans. I know you really can't tell much by testing in circuit with an ohm meter but all of the output transistors measured about the same with an ohmmeter. I didn't find one that stuck out. But these are igfets in this amp and i was told you can't test an igfet like you can a bjt. Is this true or just another piece of misinformation? The other symptom i have, you can hear the cooling fan motor through the speakers. Could all of this be just a bad ground somewhere? After all, it does have a 60 cycle hum. I'm going to go back and look at the solder side of the board closely again. Maybe i just missed something. Thanks.

[52 Bill]

I did some in circuit tests with a meter this morning on the o/p trans. I know you really can't tell much by testing in circuit with an ohm meter but all of the output transistors measured about the same with an ohmmeter. I didn't find one that stuck out.

The problem is that the outputs are paralleled so if one is shorted, they will all show the same short. What is more important is to learn is how to tell if the reading you are getting is right or wrong.

But these are igfets in this amp and i was told you can't test an igfet like you can a bjt. Is this true or just another piece of misinformation?

Well they are different devices types, but both still can be tested with the diode test of a basic meter. You just have to look for different readings.

The other symptom i have, you can hear the cooling fan motor through the speakers. Could all of this be just a bad ground somewhere? After all, it does have a 60 cycle hum. I'm going to go back and look at the solder side of the board closely again. Maybe i just missed something. Thanks.

Anything is possible. Did you ever check/post all of the power supply voltages? Do they all come close to the correct values?

[mozwell]

Output devices are mosfets. easy to measure with a multimeter using the ohms range & diode ranges
From left to right the pins are Gate - Drain - Source
With the transistors IN CIRCUIT, POWER OFF & NO SPEAKER ATTACHED, On ohms range, measure between G & S, you should see > 1k ohm, measure between D & S, you should see > 1k, check these with the probes both ways
On diode range, between G & S you should see > 1V one way & >1V the other way, measure between D & S, you should see >1V one way & approx 0.6V the other way.
Check all the resistors, zener diodes, diodes & transistors in the power amp with your meter.
Basically we a re checking that nothing reads 0 ohm or 0V (on diode range).

If all of these are good, then power up & measure ALL the power supplies, refer post # 8, you may just be missing one power supply and that may be the only issue here.....

[catstrat]

SVT

Thanks everyone. That's useful info. All the o/p transistors all measured about 1 ohm 1 way and 6 ohms the other way with my meter on the 20 meg range. It is possible then that one of them is shorted giving false readings on all of them. Since i have the board out anyway, i'll pull each transistor out and check it that way. I know i checked the voltages early on but i made a classic mistake i always make, i forgot to write them down. With the 4th this weekend, i'm going the enjoy the holiday with my family and get back to you next week. Thanks for the help and everyone have a nice 4th, except if you're downunder. Just enjoy your weekend.

[J M Fahey]

All the o/p transistors all measured about 1 ohm 1 way and 6 ohms the other way with my meter on the 20 meg range.

HOW can you even *see* 1 or 6 ohms readings in the 20 Megohm scale?
Impossible.

[catstrat]

SVT

Well, that's what my meter was telling me. Whether or not i believe it is another question. I thought it kind of odd to. But again, i'm fairly new to solid state stuff and i still have a lot to learn. Once again i was always told when you measure a diode or a transistor ,set your meter to the highest ohms range you can. Is this not the correct way,or am i a victim of more mis information? I do know if i try to measure an emitter resistor that's .47 ohms, i have better luck using the meters diode check function and if it beeps then at least the resistor is not open. Seems like i remember some saying you can rig up a test circuit by using a 100k resistor or so in series with the resistor you checking and it will give you a more accurate reading, but maybe someone can set me straight on this as well. At least i'm getting better with solid state stuff and i don't run from it anymore. But i have a lot left to learn. I'm usually successful on probably 85 per cent of the transistor equipment i get in, so that's not too bad i guess. Thanks everyone.