Ampeg SVT-6 Pro No Output

[ca7922303]

Just opened this up and find R147 burnt, some corrosion on components and jumpers, 2008 SLM 6 pro, no output. Anyone have any experience with these?

[Jon Snell]

Check Q5 and surrounding control components also check C50.

[vintagekiki]

also check D28

[vintagekiki]

The amplifier works in the bridge mode, Channel 1 (+) Half and Channel 1 (-) Half.
Compare (check) elements in limiter circuits, (Q5, Q6, Q23, Q24)

[nevetslab]

I work on these as often as they come in, or those in our rental inventory develop problems. As you've found already, these are a PITA to service. And, you need access to the bottom PCB assy to get both amplifier halves verified, biased and checked out. As you've already powered this up, under what conditions? You don't need a load to service them for operational state...only need a load to verify both halves will drive loudspeaker current into a cabinet.

Do you have any way to monitor AC Mains current? My bench is fitted with a Variac and a power analyzer, which yields adjusted line voltage, AC line current and wattage consumption, so I can spot fault current right away bringing conventional amplifiers up on the variac. Without that, a light bulb limiter would be advisable.

When you powered this up, did you hear the relays click in? If not, you may have one or more shorted MosFET's in the output stage. Often with that, the Source resistors will open as will the Gate resistors for any fully-shorted MosFET. For R147 resistor in the positive limiter circuit on the upper half drive amp to overheat, that output would need to be held negative for some time, , and C50 would need to be shorted...if this was a DC condition that caused it to burn. That resistor is on the sense current buss that drives the positive current limiter xstr to pull drive current away from the positive half MosFET's. So, it being an AC signal, I'd guess LF, and I'd expect similar stress on R151....it's partner in crime on the negative current limiter circuit.

I'd check all of the MosFET's on this lower amplifier stage, as well as all of the gate and Source Resistors to see that all are 47 ohm (gate) and 0.47 ohm (Source Resistors). Likewise, check as mentioned by Jon Snell and vintagekiki, all of the components in the limiter circuits.

If all those check ok, we need to see if the relays will pull in. If there's significant DC offset on the output buss of either amplifier stages, it's associated relay will not pull in. The relays are controlled by xstrs Q15 and Q17 for the two amp halves. +60VDC on one side of each relay, then there's a 1.5k 5W resistor in the collector circuit of the control xstrs.

In working on these, this is how I've set them up:



In these images shown probing the Source terminals, I was adjusting bias for a nominal 25mV across the source resistors and the output buss. I had tack-soldered a jumper on the output buss to hang my Black voltage probe on, using the 200mVDC range on the meter. The MosFET's in Ampeg SVT xPRO series (3PRO, 4PRO, 5PRO, 6PRO) use matched IRFP-240P and IRFP-9240P MosFETs. I buy them in bulk, and screen them/match them in sets so I get predictable results. It's not unusual to find an amp that's run for years to now have idle bias Source voltages between 7mV and 50mV in an output stage....obviously no longer matched. Load current sharing in those cases is no longer even.



The lower output stage Source & Gate terminals are only accessible when the full amp PCB assembly is lifted out of the chassis. Xfmr comes out too, so you can power up the amp. It is a clumsy and electrically dangerous test setup, so extreme care needs to be made. I normally have replaced the 15A rear panel circuit breaker with a 4A breaker, or a similar lower-current in-line fuse holder for protection. And, of course non-conductive surfaces for the amp assembly to be set onto.

Ampeg's documentation isn't that good, plus they didn't produce a parts placement drawing to aid in navigating the circuit board trying to find what you see on the schematic. I made the mistake once adjusting the wrong bias trim pot while monitoring the upper half output stage. I was turning that or the lowr half, and....YES, there's enough range on the bias pots to turn the MosFETs on hard enough to kill them, as I found out the hard way.

[ca7922303]

also check D28

D28(0.53v) diode range

[ca7922303]

Check Q5 and surrounding control components also check C50.

C50(.06 nf) cap range

[ca7922303]

Check Q5 and surrounding control components also check C50.

Q5(0.78m) ohms range

[ca7922303]

The amplifier works in the bridge mode, Channel 1 (+) Half and Channel 1 (-) Half.
Compare (check) elements in limiter circuits, (Q5, Q6, Q23, Q24)

Q6(0.55k) ohms range

[ca7922303]

I work on these as often as they come in, or those in our rental inventory develop problems. As you've found already, these are a PITA to service. And, you need access to the bottom PCB assy to get both amplifier halves verified, biased and checked out. As you've already powered this up, under what conditions? You don't need a load to service them for operational state...only need a load to verify both halves will drive loudspeaker current into a cabinet.

Do you have any way to monitor AC Mains current? My bench is fitted with a Variac and a power analyzer, which yields adjusted line voltage, AC line current and wattage consumption, so I can spot fault current right away bringing conventional amplifiers up on the variac. Without that, a light bulb limiter would be advisable.

When you powered this up, did you hear the relays click in? If not, you may have one or more shorted MosFET's in the output stage. Often with that, the Source resistors will open as will the Gate resistors for any fully-shorted MosFET. For R147 resistor in the positive limiter circuit on the upper half drive amp to overheat, that output would need to be held negative for some time, , and C50 would need to be shorted...if this was a DC condition that caused it to burn. That resistor is on the sense current buss that drives the positive current limiter xstr to pull drive current away from the positive half MosFET's. So, it being an AC signal, I'd guess LF, and I'd expect similar stress on R151....it's partner in crime on the negative current limiter circuit.

I'd check all of the MosFET's on this lower amplifier stage, as well as all of the gate and Source Resistors to see that all are 47 ohm (gate) and 0.47 ohm (Source Resistors). Likewise, check as mentioned by Jon Snell and vintagekiki, all of the components in the limiter circuits.

If all those check ok, we need to see if the relays will pull in. If there's significant DC offset on the output buss of either amplifier stages, it's associated relay will not pull in. The relays are controlled by xstrs Q15 and Q17 for the two amp halves. +60VDC on one side of each relay, then there's a 1.5k 5W resistor in the collector circuit of the control xstrs.

In working on these, this is how I've set them up:



In these images shown probing the Source terminals, I was adjusting bias for a nominal 25mV across the source resistors and the output buss. I had tack-soldered a jumper on the output buss to hang my Black voltage probe on, using the 200mVDC range on the meter. The MosFET's in Ampeg SVT xPRO series (3PRO, 4PRO, 5PRO, 6PRO) use matched IRFP-240P and IRFP-9240P MosFETs. I buy them in bulk, and screen them/match them in sets so I get predictable results. It's not unusual to find an amp that's run for years to now have idle bias Source voltages between 7mV and 50mV in an output stage....obviously no longer matched. Load current sharing in those cases is no longer even.



The lower output stage Source & Gate terminals are only accessible when the full amp PCB assembly is lifted out of the chassis. Xfmr comes out too, so you can power up the amp. It is a clumsy and electrically dangerous test setup, so extreme care needs to be made. I normally have replaced the 15A rear panel circuit breaker with a 4A breaker, or a similar lower-current in-line fuse holder for protection. And, of course non-conductive surfaces for the amp assembly to be set onto.

Ampeg's documentation isn't that good, plus they didn't produce a parts placement drawing to aid in navigating the circuit board trying to find what you see on the schematic. I made the mistake once adjusting the wrong bias trim pot while monitoring the upper half output stage. I was turning that or the lowr half, and....YES, there's enough range on the bias pots to turn the MosFETs on hard enough to kill them, as I found out the hard way.

R151(100) ohms range, haven't removed anything at this point. Took some measurements that I could get to on top of board. Could not find Q23,24 on top of board. Will get to those if I have to remove board.

[nevetslab]

C50(.06 nf) cap range

Remove c50 to test....take it out of circuit.

[ca7922303]

Remove c50 to test....take it out of circuit.

C50 removed(0.31 n f)

[nevetslab]

Should be in the neighborhood of 10uF. Try reading a new cap in that range to see how your meter responds. 31nF (0.031uF) is way low. If your meter is ok, replace that cap. We're still wondering why R147 was overheating. And, why you have no output. Have you checked the MosFET's, source resistors, gate resistors, as well as teh 13V zener diodes D4, D5, D6, D7 & D33, D34, D38, D41. Also Q3, Q4 in the upper amp, and Q19, Q18 same location in the lower amp. We'll have to wait to check the two tubes (per stage).

Electrically, you'll need to verify power supply potentials. For some reason, Ampeg didn't go a good job with completing their product documentation, so there's no Voltage chart as you would find in the other SVTx-Pro amps, no silk screen image imposed over the PCB layout for a 'road map' in finding where C50 and R147, for example, are located. I seem to recall they have the designation numbers within the rectangular box for all resistors and diodes, so they'd be obscured from view, just to make life easier.

[ca7922303]

Should be in the neighborhood of 10uF. Try reading a new cap in that range to see how your meter responds. 31nF (0.031uF) is way low. If your meter is ok, replace that cap. We're still wondering why R147 was overheating. And, why you have no output. Have you checked the MosFET's, source resistors, gate resistors, as well as teh 13V zener diodes D4, D5, D6, D7 & D33, D34, D38, D41. Also Q3, Q4 in the upper amp, and Q19, Q18 same location in the lower amp. We'll have to wait to check the two tubes (per stage).

Electrically, you'll need to verify power supply potentials. For some reason, Ampeg didn't go a good job with completing their product documentation, so there's no Voltage chart as you would find in the other SVTx-Pro amps, no silk screen image imposed over the PCB layout for a 'road map' in finding where C50 and R147, for example, are located. I seem to recall they have the designation numbers within the rectangular box for all resistors and diodes, so they'd be obscured from view, just to make life easier.

Auto ranging dmm is good, so I'm looking to order 10 if 50v 5% C50. Everything I'm seeing is 20% tolerance. Will those be ok in pro 6?

[nevetslab]

Oh sure....not critical. Where are you at in checking components on the power amp PCB assy? Any new clues?

[ca7922303]

Oh sure....not critical. Where are you at in checking components on the power amp PCB assy? Any new clues?

Working alot of construction job overtime right now, sorry I'm slow at getting measurements posted, but will get to them. Thanks for your help.