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Having opened up a client’s older SWR SM900 amp, the 2-pair quasi-comp output stage variety (four 2SC3265 Sanken MT200 pwr xstrs) and having the larger single EI power xfmr, I found the rear heat sink assy (right channel) had significant burn damage throughout the module. After initial clean up with alcohol, then removal of the power xstrs/PCB from the heat sink, and then removal of all the failed xstrs & burnt resistors, I scrubbed the PCB further to reveal major charring of the PCB on the upper half of the output stage, both sides of the board, and only a few small signal xstrs remaining unharmed, while surprisingly the 0.1 ohm/5W emitter resistors of the output stage were intact. The damage was severe enough to motivate me to see if I still had one remaining power amp module in a boneyard SM400 chassis. I did, though that module had an earlier PCB layout than the pair in this SM900. Mainly what was missing on the right ch module removed from the SM400 was the turn-on/fast-off circuit that runs the front end of the amp circuit, and was easy enough to extract, part by part from the other module that had been cannibalized before to yield what was needed for a replacement module.
I powered that module up outside the chassis with an external dual-tracking lab supply to verify basic operation and to set the bias. Then, transferred it to the chassis and installed it. The other amp module (left ch) had a bulging 1000uF/35V buss cap on the fan circuit, which I replaced. I also decided to install the 300 ohm/5W slow speed fan resistor on that left ch module. When I powered up the amp, I found the DC fan was just pulsing, trying to turn on, but not getting there. The voltage at the fan was about 5VDC. I removed the 300 ohm resistor, replacing it with a 100 ohm/5W resistor, and tried again. Still had the same pulsing behavior on the fan at start-up. Reaching in with EZ-hook jumper cable, I shorted the TO-220-case Thermal Switch, to see if the fan would run, and what that voltage was. 11.3VDC, and very low velocity. THIS is FULL SPEED???!!
I removed the left ch module to see what the fan was. A Mechatronics 24VDC fan (didn’t record the model #). I looked to see what the AC voltage was feeding this half-wave rectifier circuit on the module. 23.6VAC, with the other wire to the fan circuit tied to chassis ground. I don’t have the power xfmr schematic, but looking at the wires leaving the secondary, there’s the power amp’s HV bipolar winding with C/T (red-red-red/yel), a HV winding (vio-vio) to run the tube plate voltage, a heater winding (grn-grn) and a red/wht lead, which I assume to be a tap off of the power amp supply winding, used to run this half-wave rectifier circuit for the fan.
I tried adding an additional 1000uF cap following the 100 ohm resistor to charge that fan voltage up a bit more. It was barely enough to get the fan started, but got it past the pulsing behavior I had without it. Time to break out the scope probe and current probe to see what’s happening.
Before doing that, I went back to that boneyard SM400 chassis to extract it’s fan, which was a Nidec TA-300DC 24VDC fan. I unsoldered the chassis’ 24V fan and connected this Nidec 24V fan. And, then connected the scope probe at the rectifier/filter stage, and the current probe across the AC tap running this circuit. I left the additional 1000uF cap in place, as it did help, though increased the initial inrush current.
That inrush current was found to be in the 8-11Amp peak current, dropping immediately to around 4A and lower, as the filter(s) charged up. I haven’t looked at the current behavior of 1N4002-4008 class diodes, but, with that additional 1000uF cap, we’re getting a lot of current pulsing thru that single half-wave rectifier. This ‘stock’ circuit is a joke!
Even with a second cap, and a lower value ‘slow’ speed resistor added, the thermal switch shorting that out to put the fan into ‘high speed’……is only a 2VDC change. With the fan then running at 12VDC, maybe 12.3VDC, hardly anywhere near the velocity needed or what a 24VDC fan COULD provide in this amp. As neither of these heat sinks have any high temp cutouts to shut them down, I suspect it is this pitiful fan circuit that allowed the full flameout / melt-down of the one amp module. I did verify the TO-220 style thermal switch works, having activated it with that new GJ-8018LCD Heat gun / small nozzle shown in the set-up photo.
I thought about maybe using a 12VDC fan instead of the 24VDC fan, but…..looking at the start-up characteristics the fan sees for the first couple seconds, AND, I’d be pulling twice the current thru that diode, I’m not sure that’s a good solution, though at least it would get the proper CFM flowing to cool the modules. Maybe cobbling together a voltage-doubler circuit to run the 24VDC fan? Not without that circuit creating rating issues on the filter caps and the 24VDC fan, until everything stabilizes, no doubt. So, what IS the solution? Put it back to stock, close the lid and send it back, as these amps have been running that way for years, haven’t they? Sometimes ya never want to know the details.
SM-900 (Post 1997, Complete).pdf
The attached SM900 Schematic shows both this 2-pair quasi-comp amp module as well as the next generation fully-comp 4-pair output stage modules, with a fan circuit there that DOES work, though still using a 24VDC van, still won't get to full 24VDC potential, that I recall. I don't have one on the bench to verfy it. In that model, there are TWO fans, each using that variable-speed fan circuit and a thermistor to control it.
I powered that module up outside the chassis with an external dual-tracking lab supply to verify basic operation and to set the bias. Then, transferred it to the chassis and installed it. The other amp module (left ch) had a bulging 1000uF/35V buss cap on the fan circuit, which I replaced. I also decided to install the 300 ohm/5W slow speed fan resistor on that left ch module. When I powered up the amp, I found the DC fan was just pulsing, trying to turn on, but not getting there. The voltage at the fan was about 5VDC. I removed the 300 ohm resistor, replacing it with a 100 ohm/5W resistor, and tried again. Still had the same pulsing behavior on the fan at start-up. Reaching in with EZ-hook jumper cable, I shorted the TO-220-case Thermal Switch, to see if the fan would run, and what that voltage was. 11.3VDC, and very low velocity. THIS is FULL SPEED???!!
I removed the left ch module to see what the fan was. A Mechatronics 24VDC fan (didn’t record the model #). I looked to see what the AC voltage was feeding this half-wave rectifier circuit on the module. 23.6VAC, with the other wire to the fan circuit tied to chassis ground. I don’t have the power xfmr schematic, but looking at the wires leaving the secondary, there’s the power amp’s HV bipolar winding with C/T (red-red-red/yel), a HV winding (vio-vio) to run the tube plate voltage, a heater winding (grn-grn) and a red/wht lead, which I assume to be a tap off of the power amp supply winding, used to run this half-wave rectifier circuit for the fan.
I tried adding an additional 1000uF cap following the 100 ohm resistor to charge that fan voltage up a bit more. It was barely enough to get the fan started, but got it past the pulsing behavior I had without it. Time to break out the scope probe and current probe to see what’s happening.
Before doing that, I went back to that boneyard SM400 chassis to extract it’s fan, which was a Nidec TA-300DC 24VDC fan. I unsoldered the chassis’ 24V fan and connected this Nidec 24V fan. And, then connected the scope probe at the rectifier/filter stage, and the current probe across the AC tap running this circuit. I left the additional 1000uF cap in place, as it did help, though increased the initial inrush current.
That inrush current was found to be in the 8-11Amp peak current, dropping immediately to around 4A and lower, as the filter(s) charged up. I haven’t looked at the current behavior of 1N4002-4008 class diodes, but, with that additional 1000uF cap, we’re getting a lot of current pulsing thru that single half-wave rectifier. This ‘stock’ circuit is a joke!
Even with a second cap, and a lower value ‘slow’ speed resistor added, the thermal switch shorting that out to put the fan into ‘high speed’……is only a 2VDC change. With the fan then running at 12VDC, maybe 12.3VDC, hardly anywhere near the velocity needed or what a 24VDC fan COULD provide in this amp. As neither of these heat sinks have any high temp cutouts to shut them down, I suspect it is this pitiful fan circuit that allowed the full flameout / melt-down of the one amp module. I did verify the TO-220 style thermal switch works, having activated it with that new GJ-8018LCD Heat gun / small nozzle shown in the set-up photo.
I thought about maybe using a 12VDC fan instead of the 24VDC fan, but…..looking at the start-up characteristics the fan sees for the first couple seconds, AND, I’d be pulling twice the current thru that diode, I’m not sure that’s a good solution, though at least it would get the proper CFM flowing to cool the modules. Maybe cobbling together a voltage-doubler circuit to run the 24VDC fan? Not without that circuit creating rating issues on the filter caps and the 24VDC fan, until everything stabilizes, no doubt. So, what IS the solution? Put it back to stock, close the lid and send it back, as these amps have been running that way for years, haven’t they? Sometimes ya never want to know the details.
SM-900 (Post 1997, Complete).pdf
The attached SM900 Schematic shows both this 2-pair quasi-comp amp module as well as the next generation fully-comp 4-pair output stage modules, with a fan circuit there that DOES work, though still using a 24VDC van, still won't get to full 24VDC potential, that I recall. I don't have one on the bench to verfy it. In that model, there are TWO fans, each using that variable-speed fan circuit and a thermistor to control it.
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