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challenging, as they have been glued down with just gobs and gobs of glue. And, more fun
extracting all the solder out of their PCB pads & plate-thrus in the process.
Ive changed to MJL21193 (PNP) and MJL21194 (NPN) f rom the 2SA1943 & 2SC5200, as
they have better SOA and current capacity.
In the HV power supply area, its not uncommon to find the 1/2 W zeners D3, D30
(1N5245) and 1W zeners D2 & D38 shorted, and resist ors R1 (2.2k 5W) & R65 (3.3k 5W)
open and having applied enough heat to the PCB over time for the board to be discolored.
Theres always current flowing thru these 5W resistors, and yes, theyre always hot to the
touch. Filter caps C3 & C34 (100uF/25V) are sometim es bad. I use 105 deg C rated caps for
them, usually higher voltage as well for higher charge current rating & lower Z). You also
want to check the gate resistors R3, R64, bias resistors R2 & R63 and the high current
clamp diodes D4, D32 and the remainder of the high speed diodes D5, D11, D30 & D37 &
steering diodes D10 & D33. Ive never seen those large axial lead diodes fail, but always
good to check. I have had broken leads on D4 or D32 occur while getting the PCB up off the
power plate before.
The opto-isolated gate driver IC HCPL-3100 rarely fails. But, its a critical part to the HV
supply, and there are no direct replacements. I find them on ebay from IC sources in Asia
from time to time. Its a discontinued part, so at some point, all the Mackie SWA 1501s,
1801s, SA1232s and SA1532s will be in the bone y ard for lack of this part. Its a good
system, though a bit thin in the area of having enough silicone in the power path to
withstand the current demands of loudspeaker system in use.
The special caps Mackie had made to order C1 & C3 have four leads on them-¦.and all the
HV power supply current is flowing thru the leads on the fuse side of the caps. Ive found
one with a broken lead before, but was able to reco ver, as there was enough stubble
underneath on the cap to wrap in a new lead and re- install them.
The insulator material between the aluminum heat sp readers and the heat sink of the power
plate is another headache. Thermoset takes place between the power semiconductors and
the material, so when you have gotten all the hardware off the power plate, the power
devices are still glued into place from this long-term action, and the insulation usually
peels away, still adhered to the heat spreader of t he devices. You cant just install new
devices and tighten them down now. You no longer ha ve full insulation.
I usually install cut-to-fit Bergquist K10 insulator material or greased mica, after cleaning off
the original material. I havent identified who the mfgr is of that green thermal insulator
compound. Using greased mica, you have to be sure t o align the holes with that of the
xstrs.
In this system, the front end signal processing board gets its supply voltage from the
bipolar 18V supplies. I have had failures on the bipolar 12V supply on that board. The MUTE
signal comes from it, so I normally power up the LF amp without the HV fuses F1 & F4 in
place, the connector J1 unplugged, and a jumper acr oss R2 on the AC Mains PCB so I can
bring up the power amp on the variac, watching the AC Power Analyzer for any fault
current, monitoring output of the amp at J4-1 (or D11, D30), while watching LEDs D9 &
D12. Usually both these LEDs will come on at the same time. If not, there will be a DC
offset at J4-1, but no fault current (unless I missed something). Normally, by the time Im
up to 80VAC or greater, both LEDs are lit, the pow er relay has clicked in, and theres little
to no DC level offset at the output.
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If I had any failures on the signal processing board, I normally power it up from an external
bipolar supply during surgery. Once cured, it goes back onto the power panel, and I can
plug it back in. I verify I have a functional amplifier before I ever couple in the HV supplies.
Prior to doing that, Ive powered down and then dis charged the HV supplies before ever
installing the fuses F1 & F4.
When powering the complete system back up, still ha ving the in-rush resistor R2 bypassed,
I have scope probes on the steering diodes D10 & D33, which allows me to watch the
voltages come up to their idle point at around +/- 18V. For AC signal, I use pink noise
passing thru a 1/3 oct. filter, usually 80Hz to start. The random pink noise thru the LF 1/3
oct filter produces variable amplitude sine waves, that can very in magnitude over 10dB
from min to max, so its a very useful test signal for observing the switched behavior of the
HV system. My main output is monitored on the scope along with the positive and negative
peaks, coming from +/- 95VDC. If all is well, the baseline of the HV system remains at +/-
18V, and ONLY the signal peaks pass on up to the su pply limits. Once I have that, Ill
connect a known working 4 ohm woofer, and run it. I f the woofer in the SWA 1801 has been
verified to be functional, I'll then connect it. 40Hz & 50Hz 1/3 oct pink will produce
thunderous acoustic level if all is well.
At that point, Ill power down, remove the clip lea d around R2 on the AC Mains board, and
check the system powers up and down reliably, and t hen burn in the amp under drive,
usually at 50Hz or 63Hz. Shakes the walls in the shop for sure.
