Ampeg SVT-6 Pro No Output

[ca7922303]

When you say about 3VDC on all the busses, is this just referring to +3VDC on the positive side Gate Busses and -3VDC on the negative side busses, same thing on the Current Sense Busses (where the 2.2k resistors feed to the current limiter clamps) and 3VDC on the output buss? Or just 3VDC on the Gate Busses, no polarity implied? 0VDC on the output buss that the MosFET's are feeding thru their Source resistors? You realize there IS a difference in what you said.

Also, what does 100/500/OL mean? Have you been able to turn a MosFET on and off yet with your DMM in Diode Test mode? Have you looked at it's manual to find out if it even CAN turn on a MosFET, which requires around 4VDC or -4VDC to turn it on or off?

I'm not willing to tell you all is fine, go ahead and with power applied, and NO DC Offset on the Output (sitting around 0V or within mV of that in either direction), and then, WHILE MONITORING Source Resistors, slowly turn up the Bias pot to start getting readings across the source resistors. As you have ten MosFET's that you have to monitor ON EACH OUTPUT STAGE, and the readings will no doubt vary wildly, as you DO NOT have matched MosFETs installed, its a critical step.

(+3vdc) positive gate busses (-3vdc) negative gate busses
(+3vdc) positive sense busses
(-3vdc) negative sense busses

[ca7922303]

One clue you get when you've turned a MosFET on....you have the Drain and Source in conduction. You can remove the probes that turned it on, and it remains in that state until you reverse the probes and apply the opposite polarity gate voltage. Then that 'ON' State is gone. That's when in one direction, you can measure the protection diode junction, and in the opposite direction, it measures open circuit.

You still are not stating WHAT the DC Voltage source is of your DMM behind it's 1mA Constant Current Source. You can either read it in your instruction manual, or measure it with your second DMM in Voltage mode. Let's NOT Assume. Let's KNOW definitively.

Looks like the 2nd dmm I used on MOSFET test is reading(2.8v) and the 1st one(1.5v). Had one in diode test mode and the other in volts to test.

[nevetslab]

(+3vdc) positive gate busses (-3vdc) negative gate busses
(+3vdc) positive sense busses
(-3vdc) negative sense busses

Reading the schematic of the Power Amp section, the only way you're going to hve +/- 3V on both the Positive and Negative Sense Busses (measuring across the 0.47 ohm Source resistors with the 2.2k resistors, and feeding the bases of Q5 and Q6 is that both Q5 and Q6 are now blown (no B-E junctions). And for the sense busses to be at 3V, suggests all of your 0.47 ohm Source resistors are now open. And if you have +- 3V on the gate buses, that indicates you have Gate-Source Shorts.

Your DMM cannot turn on MosFET's, which is why you can't test them for function...you can only test for Shorts.

When you finished installing all new parts in the output stages, did you leave the Bias pots where they were when you started, or did you turn the pots fully CCW to prevent applying gate voltage?

Your problems with this amp just keep getting worse. I suggest you pack it up and take it to somebody who can repair it. I'm sorry, but you're just not qualified to tackle something like this, and you're burning up money trying.

[ca7922303]

Reading the schematic of the Power Amp section, the only way you're going to hve +/- 3V on both the Positive and Negative Sense Busses (measuring across the 0.47 ohm Source resistors with the 2.2k resistors, and feeding the bases of Q5 and Q6 is that both Q5 and Q6 are now blown (no B-E junctions). And for the sense busses to be at 3V, suggests all of your 0.47 ohm Source resistors are now open. And if you have +- 3V on the gate buses, that indicates you have Gate-Source Shorts.

Your DMM cannot turn on MosFET's, which is why you can't test them for function...you can only test for Shorts.

When you finished installing all new parts in the output stages, did you leave the Bias pots where they were when you started, or did you turn the pots fully CCW to prevent applying gate voltage?

Your problems with this amp just keep getting worse. I suggest you pack it up and take it to somebody who can repair it. I'm sorry, but you're just not qualified to tackle something like this, and you're burning up money trying.

Both bias pots where turned back to full CCW after I reinstalled all the resistors on both boards. I tested all resistors before reinstalling them and they were all correct. I will pull them again and recheck. Will check Q5/and Q6 also. Appreciate all the help. I'll continue posting as I get it fixed on my own. Maybe a miracle will happen. Thanks again.

[ca7922303]

Q5 and Q6 not shorted, so will put this one back together and see what I got.

[ca7922303]

Tight fit. Will test it tomorrow, bias pots at 10:00 and see what happens

[ca7922303]

[nevetslab]

Tight fit. Will test it tomorrow, bias pots at 10:00 and see what happens

I would not recommend pre-setting the bias pots. I would start from full CCW on one of the two output modules, and advance the bias pot while monitoring one each N-Ch and P-Ch with your two DMM.s across the Source resistor. You may want to tack-solder test points so you can easily hang your hook-probes and quickly move them from device to device, seeking the first ones that start conducting, and then see what the 'spread' in current is thru the Source resistors. You don't want to go over 25mA per device if you can help it. 11.8mV across 0.47 ohms is 25mA, .23.5mV across 0.47 ohms is 50mA, so watch it.

[ca7922303]

I would not recommend pre-setting the bias pots. I would start from full CCW on one of the two output modules, and advance the bias pot while monitoring one each N-Ch and P-Ch with your two DMM.s across the Source resistor. You may want to tack-solder test points so you can easily hang your hook-probes and quickly move them from device to device, seeking the first ones that start conducting, and then see what the 'spread' in current is thru the Source resistors. You don't want to go over 25mA per device if you can help it. 11.8mV across 0.47 ohms is 25mA, .23.5mV across 0.47 ohms is 50mA, so watch it.

Got a maximum of (7mv) on 9-source resistors, 1@(11mv) with bias pot @ 1:00. That one is a 240 which I have one more spare. Appreciate your help.

[ca7922303]

Wondering why all 5w cement resistors are reading in or out of circuit(.47 ohms), gate resistors (47 ohms) in or out, but only 3 sense resistors read(2.2k ohms).the other 17 read correct out of circuit, but in circuit read (.44k-.73k ohms)?

[ca7922303]

I would not recommend pre-setting the bias pots. I would start from full CCW on one of the two output modules, and advance the bias pot while monitoring one each N-Ch and P-Ch with your two DMM.s across the Source resistor. You may want to tack-solder test points so you can easily hang your hook-probes and quickly move them from device to device, seeking the first ones that start conducting, and then see what the 'spread' in current is thru the Source resistors. You don't want to go over 25mA per device if you can help it. 11.8mV across 0.47 ohms is 25mA, .23.5mV across 0.47 ohms is 50mA, so watch it.

Would (+65v/-65v) reading on all drains relative to ground be correct? I'd like to think that maybe the mosfets and associated resistors are about correct and can maybe move on to why there is no signal passing to the speaker output?

[nevetslab]

Reading +/- 65VDC on the Drains IS correct. No idea why your Current Sense resistors 2.2k are not reading correct while in circuit. Good that you're reading 0.47 ohms on the Source Resistors, as well as the 47 ohm gate resistors. Are you seeing output signal on the Output busses ahead of the output relay? If there is any DC offset on the output busses, that will prevent the relays from closing. You could also have issues with your Relay control circuit.

[ca7922303]

Reading +/- 65VDC on the Drains IS correct. No idea why your Current Sense resistors 2.2k are not reading correct while in circuit. Good that you're reading 0.47 ohms on the Source Resistors, as well as the 47 ohm gate resistors. Are you seeing output signal on the Output busses ahead of the output relay? If there is any DC offset on the output busses, that will prevent the relays from closing. You could also have issues with your Relay control circuit.

Hate to admit it but I'm unable to identify the output busses ahead of the output relay or the relay control circuit. Would you mind pointing me in the right direction? Hate to ask. Thanks.

[nevetslab]

You're kidding, of course. You've been staring at that buss all along. What is at the other end of the 0.47 ohm source resistors...one end soldered to the MosFET Source terminals? It's the output buss! That output buss is also your '0V' reference when looking at the voltage drop on any/all of the Source resistors to determine what voltage, and then calculate Source Current is for each stage. For measuring DC level of the output buss, which IS ahead of the output relay, then you DO need System Ground to measure that.

[ca7922303]

You're kidding, of course. You've been staring at that buss all along. What is at the other end of the 0.47 ohm source resistors...one end soldered to the MosFET Source terminals? It's the output buss! That output buss is also your '0V' reference when looking at the voltage drop on any/all of the Source resistors to determine what voltage, and then calculate Source Current is for each stage. For measuring DC level of the output buss, which IS ahead of the output relay, then you DO need System Ground to measure that.

Misunderstood your terminology I guess. So is there a relay control circuit I'm not aware of?