Ampeg SVT-6 Pro No Output

[ca7922303]

A guy from Poland with ten feet comes to mind.

What do you make of the readings in posts 27 & 29?

[ca7922303]

I'm not sure if you have the service manual doc's, so I'm posting them here:

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The Power Amp PCB Schematic shows both the two output stage amplifier assemblies, as well as the AC Mains connection to the main power amp PCB assy, with wire colors and designation numbers. When I disassemble these and any like it, where there are quick disconnects and PCB designation numbers, I write those onto the connectors with a Sharpie so I don't loose track of where stuff goes. I see you did take photos, which is always wise! I looked at the photos I posted of both the SVT5-Pro and SVT6-Pro thread I had posted previously, and while I didn't see using any extender wires, I made myself a collection of male-female insulated 1/4" quick disconnects for when I need to remove, say, an SVT4-Pro from the chassis, where there, the power xfmr and mains wiring is NOT long enough to reach in my setup, I gain the distance using those. I'd have to go back to my service notes from when I took those photos to get further details. As I recall, the problem was associated with the power amp section, so I didn't need the preamp, apart from it needing to be powered. I inserted signal thru the Power Amp Input jack, which is on the rear panel, joined with the Preamp Output jack, and that gets disconnected when you patch into the power amp input jack.

What do you make of the readings in posts 27 & 29?

[nevetslab]

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.

[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.

I was measuring components, without voltage, that were mentioned in the previous posts, diode test for "D"s and resistance forward and reverse for "Q"s.

[nevetslab]

Please follow what I've outlined. You do NOT have a functional driver stage nor bias, so we need to find where the fault is. Does this make sense or not??

[ca7922303]

Please follow what I've outlined. You do NOT have a functional driver stage nor bias, so we need to find where the fault is. Does this make sense or not??

Makes sense, I'll try to keep up with you, your light years ahead of my experience level.

[ca7922303]

Getting back on this finally. Noticing what looks like a two(2) fuse holder in photo. Is a fuse missing here?

[ca7922303]

So, I'm trying to set this up again for testing and I'm not sure if what I've got matches up with photos previously posted. I can't make out everything in photo.

[ca7922303]

Please follow what I've outlined. You do NOT have a functional driver stage nor bias, so we need to find where the fault is. Does this make sense or not??

Do you have any clearer photos of board lugs to wires? I've got my photos from disassembly, but not sure from yours what wires I don't have to connect for testing, for example power on/off switch. I don't see it anywhere in your photos? Thanks.

[ca7922303]

Please follow what I've outlined. You do NOT have a functional driver stage nor bias, so we need to find where the fault is. Does this make sense or not??

Got power switch and fuse connected to board. I'm guessing convenience outlet does not have to be connected?

[nevetslab]

Getting back on this finally. Noticing what looks like a two(2) fuse holder in photo. Is a fuse missing here?

I had to open the PWA 1000Watt Bass Amp PCB Layout drawing from an earlier post in this thread to check if there's only one or two fuse holders in this location shown in your photo. I only see the one fuse...a 3AG size fuse loaded into the fuse clips.

[nevetslab]

Do you have any clearer photos of board lugs to wires? I've got my photos from disassembly, but not sure from yours what wires I don't have to connect for testing, for example power on/off switch. I don't see it anywhere in your photos? Thanks.

The wiring of the Power Xfmr Primary is shown in the Power Amp PCB Schematic, second schematic of that sequence, where the wire colors are called out on the xfmr drawing, and the male terminal connections are called out in the drawing. That drawing shows 120VAC Primary wiring. If you're at a different mains voltage, you'll have to make those adjustments. I don't believe Ampeg gave a connection chart for all Mains Voltage wirings.

[nevetslab]

Got power switch and fuse connected to board. I'm guessing convenience outlet does not have to be connected?

That's correct. The Convenience Outlet isn't required here.

[ca7922303]

That's correct. The Convenience Outlet isn't required here.

Appreciate it. Just ordered (Q5) today from digikey. Since I've got this wired now , is it possible to run voltage tests without Q5 installed?

[nevetslab]

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.