Nate Dort wrote:X-POST from the what are you buying? thread:Just grabbed a Trace Elliot V-Type 1x15 Combo for $100 from a pawnshop near me. Seems like they go for $400-$500 used normally, based on old talkbass posts. Tube preamp, 250 W solid-state power amp.T̶h̶e̶ ̶i̶n̶t̶e̶r̶n̶e̶t̶ ̶t̶e̶l̶l̶s̶ ̶m̶e̶ ̶t̶h̶a̶t̶ ̶t̶h̶i̶s̶ ̶t̶h̶i̶n̶g̶ ̶a̶p̶p̶a̶r̶e̶n̶t̶l̶y̶ ̶h̶a̶s̶ ̶t̶h̶e̶ ̶i̶n̶f̶a̶m̶o̶u̶s̶ ̶̶B̶i̶p̶o̶l̶a̶r̶ ̶B̶e̶a̶r̶̶ ̶p̶o̶w̶e̶r̶ ̶a̶m̶p̶ ̶s̶e̶c̶t̶i̶o̶n̶,̶ ̶c̶o̶m̶p̶l̶e̶t̶e̶ ̶w̶i̶t̶h̶ ̶t̶h̶e̶ ̶̶M̶a̶k̶e̶ ̶i̶t̶ ̶s̶o̶,̶ ̶n̶u̶m̶b̶e̶r̶ ̶o̶n̶e̶̶ ̶t̶e̶x̶t̶ ̶o̶n̶ ̶t̶h̶e̶ ̶P̶C̶B̶.̶I actually opened it up last night and found that it has a 300W MOSFET-based power stage, designed by Clive Button (according to the PCB), who also did a bunch of the Ashdown stuff after Trace Elliot was shuttered. This is supposed to be a replacement for the unreliable Bipolar Bear, though I think this board has its own set of problems.I had to replace the TIP31C fan driver transistor (the one attached to the large black heatsink). The heat-sink mounting tabs through the PCB had broken, so the transistor legs were the only physical support keeping this giant heatsink upright. I was able to get enough of the old tabs through the PBC to solder it again, then I reinforced it with silicone caulk on the top of the PCB around the base of the heatsink.The output wires were zip-tied to the power transformer output wires, which were also resting against this heatsink. I re-routed all the internal wiring in a way that made more sense.What was the deal with that amp anyway? What's the deal with that Bipolar Bear circuit? I brought home a similar rescue, a V-type bass head. It's Bipolar. I was playing it at a moderate loud volume at rehearsal for about 5 min (through a 1x15 cab/8 ohm) and the output signal all of a sudden dropped a whole lot. The amp was working/sounding normal, but after the volume dropped it the signal became way more fuzzy/distorted, no matter what the gain/vol setting. Sounded kinda like an SVT that I had that needed to be re-biased, fuzzy and weak.I popped it open and the tubes lit up, and were not loose, none of the caps I could see seemed to be leaking, nothing looked smoked/charred, not sure what else to check out here. I don't have a tube tester.What could I be missing?

The fact that it's still passing audio is a good sign. Usually if an output transistor fails short, it will just blow fuses on power-up.Does that thing have a preamp output? Try running the preamp out into another power amp to isolate which section of the circuit is at fault.

If it were mine, I'd get out the scope and start signal tracing through the power amp board with a function generator feeding a 1k tone through it. Might also be worth just reflowing every solder point on the power amp board.

Nate Dort wrote:The fact that it's still passing audio is a good sign. Usually if an output transistor fails short, it will just blow fuses on power-up.Does that thing have a preamp output? Try running the preamp out into another power amp to isolate which section of the circuit is at fault.I just tested the line output and it seemed to pass signal alright, the gain knob, etc all worked, the master vol didn't do anything - might be how the line out works on this amp(?).What next?

Nate Dort wrote:If it were mine, I'd get out the scope and start signal tracing through the power amp board with a function generator feeding a 1k tone through it. Might also be worth just reflowing every solder point on the power amp board.Time to take it to a pro, unless you want to fix somebody else's problem

Sunn Concert Slave (silverface) cont.:Recapped and replaced the burned resistor. Turns out this PCB already had the "stability mods" installed from the factory, so no need to do that. Fired it up on the variac with a power meter in-line to make sure nothing was going up in smoke, then tested it at half and full power output to make sure it would remain stable.Sunn Beta Lead:Another of FM garthplinko's. This one wouldn't pass audio until you turned up the master and blasted it with signal. I immediately suspected a cold solder joint at or post the master volume pot. I was sort-of right. It was an oxidized connection at the master output switching 1/4 jack on the back panel, found during a chopstick test. Some contact cleaner and a few swipes with emery cloth on all the switching jacks and we're back in business.The spring reverb tank was toast. The laminations on one of the pickups were broken and rattling around inside the tank. I replaced it with a new 3-spring long-decay MOD tank.

Acoustic 270:The last of FM garthplinko's amps that he dropped off a while back.Almost no output, lots of low frequency noise and rumble, almost like pink noise.I ran the Monitor Out into another power amp and it had the same symptoms, which tells me that the problem is on the preamp board.Recapped that board and resoldered a few iffy connections, along with cleaning all the oxidation off the card-edge connectors. No change.The noise responds to the graphic EQ, but not the tone controls. If I turn up the reverb level and bang on the tank, I get full output from the tank, so the problem has to be somewhere in the tone control amplifier. I'm suspecting a bad transistor and/or a bad tantalum coupling cap.EDIT: Yep, I was right. C118 was failed short, like tantalum caps typically do.

Acoustic 270 (cont.):Recapped the other two PCBs. The gigantic computer-grade caps seem fine, so I'm going to leave them alone. There's a lot of local filter capacitance on those PCBs anyway, so they're doing most of the critical work.Fired it up, and it was still distorting slightly, like background fuzz at all volumes. Looking at the schematic again, it turns out that you need a footswitch to completely shut off the built-in fuzz circuit. The footswitch controls a set of FET switches which routes the clean/fuzz signal.I'm just glad that the power amp section works properly. Those old RCA 40409 & 40410 transistors are getting pricey.These Acoustics are notorious for requiring the RCA 2N3055H power transistors as well (modern 2N3055s don't hold up), which are getting harder to find. The ones in this amp are fine.

weezy wrote:What was the deal with that amp anyway? What's the deal with that Bipolar Bear circuit? I brought home a similar rescue, a V-type bass head. It's Bipolar. I was playing it at a moderate loud volume at rehearsal for about 5 min (through a 1x15 cab/8 ohm) and the output signal all of a sudden dropped a whole lot. The amp was working/sounding normal, but after the volume dropped it the signal became way more fuzzy/distorted, no matter what the gain/vol setting. Sounded kinda like an SVT that I had that needed to be re-biased, fuzzy and weak.I popped it open and the tubes lit up, and were not loose, none of the caps I could see seemed to be leaking, nothing looked smoked/charred, not sure what else to check out here. I don't have a tube tester.What could I be missing?Well it turns out this thing had some iffy solder joints, so dude shotgun soldered it for me, and we're now back in business. The tech said the root cause might be because the board seems to have a little flex in it, i.e. not supported all that well underneath.I recovered the enclosure in the meantime with the Duratex.

Thermal imaging camera?I gotta step my game up...