SVT CL 1997 slm pre out power in issues

[xjbear]

Weird issue hoping someones dealt with before. Got an svt needed the normal run through and fixing of sockets and solder joints. After about 5 minutes either with or without signal, a small hiss/hum kicks in just enough to light both bias light red. Plug a jumper into pre/power jacks it goes away. Plug just into power amp in goes away, plug into the slave it goes away.
Cleaned jacks, reflowed joints, checked all shunts and all is perfect until something I guess warms up in the circuit after standby. Im thinking the tlo72 opamp buffering the pre out but that would have no bearing on the power amp out jack.
This is the model/schem with slave, preout, power amp in, and transformer coupled di. No tuner out, loop, or footswitch. All else is perfect. New screens, cathode resistors, power tubes, biases perfectly and sounds amazing with a jumper in those jacks

[xjbear]

The correct schematic is page 3 in this pdf

https://www.dropbox.com/s/4mm1zn3c6x...atics.pdf?dl=0

[Jazz P Bass]

How about simply posting the schematic?
(for those who do not have a 'Drop Box' account)

It sounds like a bad jack.

[xjbear]

Yeah ill attempt to add the schematic. It should be publically downloadable though. Jacks are fine.

[xjbear]

page 3
SVT CL alt power amp schematics.pdf

[g1]

Can't see anything that would be common to all those jacks like that. Perhaps it is just the "thump" when you plug into any of those jacks that is correcting the fault.
Try powering up from cold with the jumper already installed. Does the amp now run fault free for an extended period of time?

[J M Fahey]

Agree and add:
1) what does Mr Scope say is happening at the speaker output?
Any hum/hiss you hear must also be visible.

2) measure voltage across R35 ... 40 before and after hiss/hum appears.

I guess that bias voltage jumps (down) so bias current jumps (up) , and so does sensitivity/gain.

Hum/hiss is always present but local gain at tube stage makes it audible/inaudible.

Just a theory, needs confirmation or rejection.

[xjbear]

Havent got it back apart yet and new scope yet either. Ive been slacking. Its not a bias issue at all, I meant to say its about the same signal youd get from lifting the ground on the input jack shunt with the volume down completely. It presents itself completely like the power amp out jack is bad but its not.
This one has been around the world so its possible its seen some bad slaving accidents. Im going to getback in and check the components to ground off that opamp and etc.
I was hoping it was a common failure in these and save some troubleshooting

[xjbear]

*it doesnt go into fault mode from this. the signal path just opens up somewhere or gets pulled down to ground.

[xjbear]

Still havent opened it back up yet but have used it all weekend for testing pedal builds and repairs and everything is perfect with a jumper in it. Damn gremlins. Almost caught up enough to get in it tonight

[MarkusBass]

Plug a jumper into pre/power jacks it goes away. Plug just into power amp in goes away, plug into the slave it goes away.

Doesn't it mean that the ground is missing in the power amp (not contacting correctly)? And, as far as I remember, the ground connection is provided by one of mounting screws. Make a test by connecting grounds between the preamp and the power amp.

Mark

[nevetslab]

Solder Fractures

With the described problem NOT occurring when patched into the Power Amp input jack, I'd be looking close at all your solder joints of the preamp's pots. The fact that it also goes away when you've patched around the Normal contact using Pre Out to Pwr In....that says the problem is NOT in the preamp. But, as you indicated this beasty has traveled the globe for some duration of it's life, I'd be sure ALL the hardware is totally tightened down.

Yes, three is ONE critical screw AND its' mating standoff which is the main Grounding path to chassis. All too often, I find both screws mating to the standoffs loose. It's the left-corner screw, output side of the main PCB that is the critical one (when viewed from rear, chassis upside down showing main PCB.

The 4mm spaced square pin headers used for power supply and secondary voltage interface between power and preamp boards, as well as the two-pin header on the output board...those are also problematic with solder fractures over time and vibration. Don't forget to closely inspect the main power tube PCB. I've had issues on that board as well, though not as much as the preamp and main board.

These amps perpuate solder fractures from their sheer weight & mass, using their panel-mounted parts' solder terminals as PCB support, main PCB, output PCB, AC Mains PCB, preamp PCB. The pair of Bias Pots on the rear panel are one of the 'PCB Supports' for the main board. Having fractures on those joints makes for some fun.

Maybe it's just me, but when I find solder fractures, I de-solder them completely, THEN re-solder so it's all fresh solder rather than re-flowing existing solder.

[xjbear]

Yes I do the same with newer amps. Suck out the shit new solder and replace it with standard old solder.

The fix was v2b was bad in the preamp, ehx pre's causing issues again. When the jacks are jumped the preout engages the tlo72 buffer and was providing enough drive to pull the signal thru to the next stage. As I tested and traced the board then finally with signal everything was til that cathode out of v2b. Jump loop and signal increased. Swapped for a good pre and all is well.