DON'T DO THIS (Connecting DMM probe to upper Driver emitter on SWR 750X, SM900)

[nevetslab]

During bias voltage measurements on SWR SM900's (early versions) and 750x amps during routine preventative maintenance, I made the fatal mistake of connecting my long EZ-Hook DMM probes onto the emitter terminals of the two driver xstrs. The connection to the lower half driver xstr's emitter terminal (Q8 on 750x) is easy enough to get at. The same on the upper half driver xstr's emitter terminal (Q7 on 750x) is not so easily accessed, though I did finally get my probe onto it, with the power off. Powered up, monitored the output DC level of the amp and the DC voltage of the bases of both drivers, typically 1.0V between them, enough to get the outputs close to turning on, but not. If you tweak the bias pot lower than 950mV, it will often drop off quickly, and you'll see a change in the AC mains current. I know....measuring the voltage across the output xstrs' emitter resistors shows the current is starting to flow, and usually set for around 10mV across them. I check that too, but have gotten in the habit of looking at the emitter's of the two drivers (tends to be more accurate in fully complimentary output stages than these quasi-comp stages).

All was fine until I went to disconnect the probe on the upper driver Q7, and while I've done this so many times in the past carefully, making the connections to the device terminals while power is on can be hazardous if your hand/probe slips. My probe must have slipped while disconnecting and removing it from the terminal, contacted both the emitter & collector, saw an arc and flash of light from the insides of the fuse holder, then all was quiet. Fan coasted to a stop. I bought the farm. RATS! What was worse, was a far easier connection for Q7's emitter was right there to the right, the base terminal of the top output xstr....easily accessible, without a nearby hazard begging for trouble. The damage was all six output xstrs failed (Sanken 2SC3264's) & the driver 2SC4382.



Same condition holds true on the early versions of the SWR SM900, as shown in the last photos.

750x Power Amp Rev E Sch.pdf

[drewl]

Doh!
Can't say I haven't done that in the past
But I am more careful now and always remove power before and after measuring.

[nevetslab]

Doh!
Can't say I haven't done that in the past
But I am more careful now and always remove power before and after measuring.

I found with this SWR 750x, it doesn't have power supply ballast resistors, so the supplies stay up around +/- 90V for ages. Its' predecessor, the Bass 750, iDOES at least drain off the supplies.

[Tone Meister]

These threads help me more than you know. Thanks for sharing the ugly side.

[drewl]

I found with this SWR 750x, it doesn't have power supply ballast resistors, so the supplies stay up around +/- 90V for ages. Its' predecessor, the Bass 750, iDOES at least drain off the supplies.

Which is why we drain those supplies with 1K 2 Watt resistor.

[nickb]

I hate it when that happens.


After powering down you often have discharge the main storage caps, the 150V rail 10,0000uF sort of stuff things thru a small resistor. Now, before powering up you always remember to remove it, right? I used to use 16ohm 100w job for this regularly until once I forgot. The phone rang. At the end of the call there was a pop as the resistor finally exploded and a left neat rectangular melted hole in the bench covering.

[nevetslab]

Which is why we drain those supplies with 1K 2 Watt resistor.

Whether in a simple solid state amp, or tube amp.....those lacking this simple addition for safe and sane servicing.......sure saves grief along the way. I've been bitten by older Fender amps's main stacked filter cap array more than once, where they didn't install ballast resistors to even out the DC voltage on each, as well as drain that charge away after powering down.

My discharge probe is a 100 ohm 50W Dale resistor with old DMM probes. Never leave home without it!

[nevetslab]

I hate it when that happens.


After powering down you often have discharge the main storage caps, the 150V rail 10,0000uF sort of stuff things thru a small resistor. Now, before powering up you always remember to remove it, right? I used to use 16ohm 100w job for this regularly until once I forgot. The phone rang. At the end of the call there was a pop as the resistor finally exploded and a neat rectangular melted hole in the bench covering.

Ouch! That was painful! I have to remember to periodically check my discharge probe....a Dale 100ohm/50W power resistor, with the short DMM probes soldered to the ends, heat shrink over the joints. I've had that connection break before, while the heat shrink holds it together, no longer a discharge probe though. Found the results quickly as work on the circuit began!

[Enzo]

Ooh, in the old Behringer powered mixers, disconnect the power amps and the SMPS will hold its 80v rails for almost forever. Don't ask me how I know.

[nickb]

Whether in a simple solid state amp, or tube amp.....those lacking this simple addition for safe and sane servicing.......sure saves grief along the way. I've been bitten by older Fender amps's main stacked filter cap array more than once, where they didn't install ballast resistors to even out the DC voltage on each, as well as drain that charge away after powering down.

My discharge probe is a 100 ohm 50W Dale resistor with old DMM probes. Never leave home without it!

I like the idea of using a DMM probe on one or both ends. I've been using croc clips so I can clip and leave while it does its thing. Maybe add a "discharging" LED too.

[SoulFetish]

I hate it when that happens.


After powering down you often have discharge the main storage caps, the 150V rail 10,0000uF sort of stuff things thru a small resistor. Now, before powering up you always remember to remove it, right? I used to use 16ohm 100w job for this regularly until once I forgot. The phone rang. At the end of the call there was a pop as the resistor finally exploded and a left neat rectangular melted hole in the bench covering.

The last time I accidentally powered up with the bleeder still attached I was fortunate enough that the resistor was rated to handle enough power to survive about 30 seconds or so while I went through the following observations – "I think I smell something burning.. somethings not right. is that smoke?? OH $#!T !!!
I've taken out a couple of fuses from having my probe slip of a solder joint as well. So far I've been fortunate enough that I haven't taken out any output transistors... but it's early yet.

[drewl]

Did that the other week.
"Hmm, why is the high voltage so low.....and what's that burning smell.....DOH!"