Ampeg SVT-6 Pro No Output

[ca7922303]

If you don't have Q5 installed, (short circuit current limiter upper half), I'd lift one end of D28 & D29, which are the coupling diodes to the upper and lower Gate Drive Buses, just to be sure. Then, you can run these tests. Do you have a Variac and power analyzer? Or a series Light Bulb in lieu of that? When I have failures in a power amp stage, and am still in the evaluation stage of checking if my corrections are good, it allows checking for fault currents being drawn from the AC Mains. I have both, and the beauty of a power analyzer following the Variac, with the DUT plugged into the output of the power analyzer (from output of the Variac), if high current is being drawn, you see it immediately. A Series Light Bulb will glow brightly...degree of that has to do with how much current is being drawn.

Yes, have Variac and a power analyzer.

[ca7922303]

I'm not sure what you're measuring. What I want to see is the voltages from the decoupled supply at the top of R20, then that at the base of Q4, then that at the cathode of D32/Q2 Collector, then that at Q2 emitter/R22, then that between R22/R23 (input from front end), then that at Q17 emitter/R23, then A17 collector/D34 Anode, then Q13 Base/D34 cathode/R25, and finally the decoupled supply at the bottom of R25. That is one current string, and allows one to see if there is current flowing in that branch.

The next current branch is again from the top of the decoupled Positive supply at R21, then voltage at emitter of Q4/R21, then that at Q4 collector/D33 anode/R27/Q5 collector, then that at base of Q5/R27/R28, then that at top of bias pot AP1/R28 (bottom), then bottom of AP1/top of R29 (Q5/Q6 collectors), then base of Q6/bottom of R29/top of R30, then that at Q6 collector/Q13 collector/D35 cathode, then that at Q13 emitter/R24, and finally supply at bottom of R24 (which we already got at bottom of R25. That will allow us to see if there is current flowing thru this branch.

It's all about verifying current flow thru these two circuit branches. If there is NO current flowing, we have a problem, and there would NOT be any voltage at TP9 and TP10. I'm suspecting there is an open trace or two. With NO voltage applied, see if you get from the transistor leads to the associated resistor leads/diode leads thru each circuit branch.

Was trying to find earlier and could not locate most of these components on main power board?

[nevetslab]

Was trying to find earlier and could not locate most of these components on main power board?

I just looked at the layout drawing, and found Ampeg DID NOT place the designation numbers on that drawing! RATS! That leaves having to hunt on the PCB's themselves. I hope they didn't place the designation numbers underneath the parts themselves.....usually not, but when space is tight, they do. I'd have to stare at the layout drawing to identify the parts. You could always print out the sections of this layout drawing, then with the schematic, write in the designation numbers for a road map. All part of the chase.

[ca7922303]

I just looked at the layout drawing, and found Ampeg DID NOT place the designation numbers on that drawing! RATS! That leaves having to hunt on the PCB's themselves. I hope they didn't place the designation numbers underneath the parts themselves.....usually not, but when space is tight, they do. I'd have to stare at the layout drawing to identify the parts. You could always print out the sections of this layout drawing, then with the schematic, write in the designation numbers for a road map. All part of the chase.

Component side of power amp board has all the designation numbers to the side of components. I'm able to find some of the ones you've listed but most I cannot find on the power amp board. I will post which ones I can find probably this evening when off work. Thanks for your help.

[ca7922303]

I just looked at the layout drawing, and found Ampeg DID NOT place the designation numbers on that drawing! RATS! That leaves having to hunt on the PCB's themselves. I hope they didn't place the designation numbers underneath the parts themselves.....usually not, but when space is tight, they do. I'd have to stare at the layout drawing to identify the parts. You could always print out the sections of this layout drawing, then with the schematic, write in the designation numbers for a road map. All part of the chase.

Ok, some progress: I can verify that I have good traces between R20 and Q4 base.
Q2 emitter and R22 Cannot get a reading from D32 cathode and Q2 collector
Will post more as I find them.

[ca7922303]

I just looked at the layout drawing, and found Ampeg DID NOT place the designation numbers on that drawing! RATS! That leaves having to hunt on the PCB's themselves. I hope they didn't place the designation numbers underneath the parts themselves.....usually not, but when space is tight, they do. I'd have to stare at the layout drawing to identify the parts. You could always print out the sections of this layout drawing, then with the schematic, write in the designation numbers for a road map. All part of the chase.

I'm guessing there's components under the heat sink on main power amp board I can't identify?

[ca7922303]

I just looked at the layout drawing, and found Ampeg DID NOT place the designation numbers on that drawing! RATS! That leaves having to hunt on the PCB's themselves. I hope they didn't place the designation numbers underneath the parts themselves.....usually not, but when space is tight, they do. I'd have to stare at the layout drawing to identify the parts. You could always print out the sections of this layout drawing, then with the schematic, write in the designation numbers for a road map. All part of the chase.

Yeah I'm seeing the missing components from your list are going to be under the heat sink and the board turned upside down on top of the heat sink. Tried taking 14 screws out of the bottom of heat sink and MOSFETs are stuck to heatsink. Any ideas how to separate without breaking anything? I have a heat gun but wonder if I'll dedolder components if I use it?

[nevetslab]

I had to look at what I did when I killed an SVT6-Pro by selecting the wrong bias pot during service. Monitoring the channel I was looking at, while having inserted the tweaker into the opposite Channel's section, turned it and not seeing any change, nudged it some more, then realized what I had done, but not before seeing the Power Analyzer tell me I screwed the pooch.

In this mechanical design, Ampeg didn't help much by using Bergquiest K6 insulator sheets. The long-term thermoset that occurs under time/pressure from the mounting....when you have to pry the devices from the heat sink, with that material now bonded between the MosFET case and heat sink, they're likely to peel open, damaging the insulation. I had to resort to using Greased Mica in the rebuild, as seen in these photos below.

It took thin long-bladed flat screwdrivers to work their way in-between the MosFET's to try and break the thermoset bond of each device in getting them apart.

Now, you CAN measure the parts on the PCB wthout going thru this tedious process, which may result in damage to the insulator. I only go to this depth when there is power device failure, or the components living on the opposite side of the board have failed. Then you have no choice. Welcome to the compromises designers face when packaging constraints force decisions like these, and not providing a good serviceable solution.

[ca7922303]

I had to look at what I did when I killed an SVT6-Pro by selecting the wrong bias pot during service. Monitoring the channel I was looking at, while having inserted the tweaker into the opposite Channel's section, turned it and not seeing any change, nudged it some more, then realized what I had done, but not before seeing the Power Analyzer tell me I screwed the pooch.

In this mechanical design, Ampeg didn't help much by using Bergquiest K6 insulator sheets. The long-term thermoset that occurs under time/pressure from the mounting....when you have to pry the devices from the heat sink, with that material now bonded between the MosFET case and heat sink, they're likely to peel open, damaging the insulation. I had to resort to using Greased Mica in the rebuild, as seen in these photos below.

It took thin long-bladed flat screwdrivers to work their way in-between the MosFET's to try and break the thermoset bond of each device in getting them apart.

Now, you CAN measure the parts on the PCB wthout going thru this tedious process, which may result in damage to the insulator. I only go to this depth when there is power device failure, or the components living on the opposite side of the board have failed. Then you have no choice. Welcome to the compromises designers face when packaging constraints force decisions like these, and not providing a good serviceable solution.

I was able to get board off of top of heat sink so far and have found that R123 and R148 is burnt. Other resistors on this board aren't reading what schematic shows.

[ca7922303]

I was able to get board off of top of heat sink so far and have found that R123 and R148 is burnt. Other resistors on this board aren't reading what schematic shows.

R148 burnt also. Can you tell me a way to check MOSFETs in circuit to see if they're blown on this one? Also 5w resistors are reading 3k ohms except for R86(6k ohms). Do I need to lift a leg on each one of these to check? Thanks.

[ca7922303]

I had to look at what I did when I killed an SVT6-Pro by selecting the wrong bias pot during service. Monitoring the channel I was looking at, while having inserted the tweaker into the opposite Channel's section, turned it and not seeing any change, nudged it some more, then realized what I had done, but not before seeing the Power Analyzer tell me I screwed the pooch.

In this mechanical design, Ampeg didn't help much by using Bergquiest K6 insulator sheets. The long-term thermoset that occurs under time/pressure from the mounting....when you have to pry the devices from the heat sink, with that material now bonded between the MosFET case and heat sink, they're likely to peel open, damaging the insulation. I had to resort to using Greased Mica in the rebuild, as seen in these photos below.

It took thin long-bladed flat screwdrivers to work their way in-between the MosFET's to try and break the thermoset bond of each device in getting them apart.

Now, you CAN measure the parts on the PCB wthout going thru this tedious process, which may result in damage to the insulator. I only go to this depth when there is power device failure, or the components living on the opposite side of the board have failed. Then you have no choice. Welcome to the compromises designers face when packaging constraints force decisions like these, and not providing a good serviceable solution.

Was able to get bottom of heatsink off of main power amp board using needle nose pliers. More burnt resistors (R116). All the 5w resistors are reading (4.7k) R112,138,32-39. R30(15k ohms).

[ca7922303]

Was able to get bottom of heatsink off of main power amp board using needle nose pliers. More burnt resistors (R116). All the 5w resistors are reading (4.7k) R112,138,32-39. R30(15k ohms).

And again, most of these resistors are not reading what schematic shows but ate consistent with nearby resistors so I'm assuming it's because they are in circuit.

[nevetslab]

And again, most of these resistors are not reading what schematic shows but ate consistent with nearby resistors so I'm assuming it's because they are in circuit.

That seems likely. Though R116 being a current limiter source resistor to Q5, which you already stated was bad & removed, have you verified all of the MosFET's are ok? With them out of circuit, I can turn them on and off with my Fluke 8060 in Diode Test Mode (has 1mA current source, and 8V potential behind it. If they have failed, they you normally see short circuit between Source/Drain and Gate. And, when they fail, the 0.47 ohm/5W Source Resistors often burn open. If these are reading 4.7k, then it sounds like they're open circuit, and you're reading something else across those. If they're good, you'll read around 0.47 ohms.

[ca7922303]

And again, most of these resistors are not reading what schematic shows but ate consistent with nearby resistors so I'm assuming it's because they are in circuit.

Have not been able to verify anymore traces per the components list you gave me previously. Could this be due to 3 removed resistors now that we're burnt?

[ca7922303]

That seems likely. Though R116 being a current limiter source resistor to Q5, which you already stated was bad & removed, have you verified all of the MosFET's are ok? With them out of circuit, I can turn them on and off with my Fluke 8060 in Diode Test Mode (has 1mA current source, and 8V potential behind it. If they have failed, they you normally see short circuit between Source/Drain and Gate. And, when they fail, the 0.47 ohm/5W Source Resistors often burn open. If these are reading 4.7k, then it sounds like they're open circuit, and you're reading something else across those. If they're good, you'll read around 0.47 ohms.

Is there a way to check MOSFETs without completely removing all of them?