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A2801 Soundcraftsmen's seemingly rare power amp

The rarest model of the series is actually the RA5504. All of the RA550_ series are the same MOSFET amp as well. The RA5504 says it's a 420 watt DJ amp and also the SUPER-POWER-MASTER. lol
I have one. I will take photos and post later. It's the same as the A2502, except no speaker switches. I think I have had one or more of every one of these MOSFET amps at some point.
 
The rarest Soundcraftsmen amp has to be the DDX410. It is described as a "digital switch mode stereo amplifier" It uses MOSFET and has the same specs for power and noise as the PCR amps at 205W per channel and says it weighed 13 lbs. That is 7 lbs less than the PCR800. Makes me wonder if it is class D or a switched power supply, or both.
 
I instantly and dramatically popped one of the two 200 watt 8 ohm dummy loads I use, when I fed the A2801 an overly high input from my signal generator while checking for an intermittent issue. One of the volt meter leads had come off and I cranked on the level knob thinking nothing was changing. Those resisters are mounted to large aluminum heat sinks, regardless, I typically wouldn't go anywhere near the 140 watt rated output level with them.
 
I believe many models that shared the same design, had additional mosfets for the higher power ones. I know the A2801 has a place for a third pair that is empty on the left and right channel heatsinks.
 
That looks practically new, great work. I know you put a lot of work into these. I have fully recapped two pre amps and the one A2801, all for my own use. The DX3000 was the first one I bought new with the A2801 in 1988, the DX4000 was more recent as an upgrade. I've also replaced all the carbon film resistors with hand matched metal film, and all four ICs with new production ones, also added the snubbing circuit that I have mentioned before for transformer ringing. The photo also shows a crude RF shield I made for around the transformer, the snubber circuit is on the vertical turret board. Everything is dead quiet and sounds great.
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Here's how the 2801 arrived. Lately, everything coming and going seems to be dropped hard. I could have made the paint look decent, but now the metal will show it was bent and more paint cracked.
 

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Not easy to get straight. In order to get it flat, it would need to be worked a bit and the paint would be further destroyed. I will bend it back and touch it up, but it will likely be noticeable.
 
I have plans for this amp to go with a budget MTX Soundcraftsmen MX1550 preamp/mixer. It also had bent ears that were strightened before I got it. And can easily be seen. So it will be one banged up budget system with a lot of sound for the money. If I kept it, I would be tempted to stuff another transformer in there and make it a dual mono-block. I have an extra transformer for these. My guess is clipping would be up in the 225-250 watt per channel 8 ohm. Although it would likely need some fans added.
 
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I worked on the A2801 today. Removed the cover, handles, and face-plate and painted. The cover and handles were striped down to metal. and the face-plate was pounded flat and had the edges painted and blended in and spots touched up. During testing I found slightly higher distortion on one channel. .03% vs .01%. Also lower clipping on the same channel. I found a bad N-channel MOSFET. Replaced and retested.
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An amazing transformation of another old soldier. That is an unfortunate type of failure as it would go un-noticed by someone using it. I had hoped they didn't degrade, rather just failed out right. Do you source new MOSFETs, modern subs or scavenge old ones? I know there were still subs awhile back, but the form factor has lost much support and may become extinct. Man I wish I lived close enough to come hang out at your workshop. I would love to hear some of those and even see the tests. I was an electronics mechanic for Army air defense systems for forty years, so I can even be useful at times, also how I got into SoundCraftsmen stuff while in Germany.
 
Thank you. I installed a used MOSFET in this one, but I have put new MOSFETs from Profusion / Exicon in amps. That's why I have good used ones. And it would go unnoticed. I wasn't even sure if I was wasting my time investigating the distortion until found that. It was shorted from gate to emitter. So all the real work was being done by the remaining good one. And it would even hit rated power that way, but less than the other channel. You would certainly be welcome at the shop, but it can often be boring and tedious as it was on this one.
 
This one also had the chassis ground resistor burned open. Not sure why, must have been what they had it hooked up to. And I'm working on an RP22215R connected to a DX4000, and they both seem to have a chassis ground connection that's not listed in the schematic. The 2215 schematic doesn't have a earth ground, but if it does in real life, the cord is connected to chassis and there is a chassis to circuit ground wire that goes from the face-plate to the board. The 4000 also seems to have a ground connection that is not shown in the schematic. I will investigate that further when I have time.
 
I forgot to mention that I also replaced all electrolytic caps except the power caps, and the input differential and current source transistors.
 
If the DC offset was good then they shouldn't really need replacing. Mine had one good channel and one getting high, so replaced them all. They both wound up the same as the original good channel originally measured, 35 millivolts I believe. I use a trick I learned working with military equipment, where I put a dab of thermal past on one of the two transistor bodies and then bind them physically together so they are thermally bonded. If one starts to get hot it will warm the other as well preventing thermal run away. Of course they need to physically close enough and you want non conductive thermal paste.
 
It wasn't good, but I would have done it anyway. I upgrade from the L01s to A29s and 2N5551. Plus I have gotten burned before by trying to save a penny by not putting them in, and then came back with noise and low gain in the current sources. I don't trust them, and the only way to check them is to remove them. And I'm not putting them back in at that point. Same deal with the caps, I think they were all good yet, but for how long.... The only reason I didn't replace the power caps is because this is a budget amp for my friends garage. But it may happen in the future.
 
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