Stromburg Carlson ASR-120 thoughts

avguytx

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Well I've had my ASR-120 up and playing for 24 hours so far. Ok, not continuously but you know. I recapped it yesterday and rewired the 7199 sockets for 6GH8A's and she's working like a champ and sounds soooooo good on my Klipsch CF-3's. The only issue I have is a slight buzz/hum at idle with the volume down that I need to figure out why. Or maybe it's a normal issue? I'm using an IEC plug but not using the ground lug. I could cut that pin off and reverse it to see if that helps. It could be picking up from other things around it too. It's not too noticeable from my sitting position but I know it's there. I measured the stock resistors and they were all within 5% of spec rating but deliberating on changing them out. I've got 6 orange drop caps in place of the originals. I'm pleased with the sound though and I also replaced the can cap out with new caps in the Magnavox 9304-20 as well. Might drop it in tomorrow for a listen.
 

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I had the same experience as you with an ASR-120 amplifier. It was mainly noticeable in the pause between songs, or a quiet passage in music. Still, it was enough of an irritant that I ended up getting rid of mine. A shame, it had really nice highs, particularly with RCA 6V6GT blackplates.
 
These are the original SC branded 7408's in mine. We're the sourced by someone else? I realize tube amps can be antennas for noise on occasion but if it's possible to lower the noise some, it could be liveable.
 
Tube or solid state, it should be whisper quiet. I also own the 120 and when mine is on with no signal, it is sooooo quiet that you can't even tell if it is on.

Make sure your new capacitors negatives are grounded and/or soldered very solidly to the chassis. Someone else might chime in with some grounding schemes but I just make sure the negatives are either at or real close to the center tap chassis connection.
 
The caps also want to be grounded to the same place as they were originally. Typically with a can cap, you'll have the center tap lead of the power transformer soldered to one of the mounting tabs. Looks like you've got the old can in there still, so I will presume you've got new caps under the hood. Make sure the - leads tie to that center tap lead. If they are mounted to terminal strips, what you can do is tie all the - leads together on the terminal strip, and run a wire from the common - to the original ground point. The idea is you don't want the new capacitors grounded to the chassis in a way that the current from the capacitor - lead has to flow across the chassis to the transformer center tap lead.
 
I want to say I grounded them to the same point the can cap was but now you've got me thiking. Admittedly, there's one section of the can cap (15uf) that's still being used because (stupid me) misread one of the caps. I had a 16uf for the SC amp and a 10uf for the Magnavox and put the 16uf in the Maggie by mistake. Derrr. Maybe that's causing some of it...not sure. What's the best way to drain the caps so I can safely swap them around in both amps?
 
I just short them using a screwdriver. Some people will say that this is bad for the capacitor, that it breaks down the electrolyte or some such thing, but there is no damaging effects from this............unless you do it twenty times a day, 7 days a week, every day on the same capacitor.

Hell, I do it on everything I own and some of my stuff still has the original capacitors in them. Hasn't hurt anything I own. In fact, my electronics engineer friend says that there is no damage being done to the capacitor if you are to do it a few times just to relieve its charge.

Besides, most capacitors in these Magnavox amps (and others) mostly drain pretty much after turn off so you might get a tiny snap if that.

I'm not saying that it is a good thing to do with capacitors, but it won't hurt them to do it a few times.
 
Let it sit 10 minutes or so and check it with a meter. Usually they will self-discharge. if there is any charge remaining, you can just jump it to ground with a 100 ohm or so resistor.
 
That's what I was thinking; a 100w 10ohm resistor if memory serves. I put my Fluke meter on the caps and measured them and they were about a quarter of a volt. I guess I'll do that tomorrow as I was messing with setting up PlexTV to work on the kids Apple tv's so they could access the movies on my medi server. Got that done and they "seem" happy about it. Maybe not as much as me though.
 
I haven't tried those yet in it but will after I swap/replace that last pair of caps between the two. It may be induced through my preamp as it has slight noise in my Carver amp, too. I don't recall hearing any noise in the SC when I was playing it in the garage on the ADS L300C's but they aren't near as efficient as the Klipsch CF-3's. The Carver AC-1 preamp with all the modifications to it is "supposed" to be quieter and also the main 1 preouts are toned down for a less "hot" versus the main 2 outs.
 
The caps also want to be grounded to the same place as they were originally. Typically with a can cap, you'll have the center tap lead of the power transformer soldered to one of the mounting tabs. Looks like you've got the old can in there still, so I will presume you've got new caps under the hood. Make sure the - leads tie to that center tap lead. If they are mounted to terminal strips, what you can do is tie all the - leads together on the terminal strip, and run a wire from the common - to the original ground point. The idea is you don't want the new capacitors grounded to the chassis in a way that the current from the capacitor - lead has to flow across the chassis to the transformer center tap lead.

The caps were/are all grounded to the same place they were before which is one of the lugs on the can cap. The center tap lead from the transformer was tied into the terminal strip where the "death cap" is and no caps were tied there (silver wire). Should I attach a ground wire from where the caps are grounded to that same spot on the terminal strip? (See attached picture)
 

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Are you still using a section of the twistlock cap. It looks like it in the picture unless I'm not seeing it right. BTW, I doubt if that speaker wire insulation is rated for 120v. (be careful)
 
Yes, one section is still being used and is getting changed out in a little bit. I had put a cap intended for the SC but went on the Magnavox by mistake. I've got to swap those out. Yeah, I need to change that power wire out. The day it was worked on a couple years back, my buddy used that wire when I wasn't around. I've been questioning that every time I look underneath. I think I've got some 120v rated wire here left over from a larger 12/2 extension cord that was shortened.
 
Did the 6bl8 tubes have the same buzz at idle

Just popped the 6BL8's in and it has the same noise/hum in it. But, wow, does the SC have more output at the same volume setting on preamp! Maybe it's just different input sensitivity but still seems to have more guts. Also, the ASR-120 in comparison sounds better to my ears...the upper end is better, the low end is broader/deeper, and just overall clarity is a step up. I played "I Can't Tell You Why" from the Eagles "Hell Freezes Over" and the detail was most noticeable towards the end with the synthesizers.

I'm also thinking new tube sockets might be a good thing to swap out as the just don't seem tight enough in the sockets and make scratchy noises with any slight movement of the tubes. On the 6BL8's for sure. Some of the pin leads underneath just seem loose. Maybe a benefit to swap for new/better ones. D maybe new wiring at that point. Hmmmm...
 
If the B+ center tap on the transformer grounds to somewhere other than the can cap, I'd tie the capacitor ground leads to that point instead and see if that shuts it up. Or, maybe given that its connected right next to the AC mains wire, I might move the center tap wire to the can cap lugs so you're not quite so close to the AC power line. That wouldn't make it hum, I just don't like to mix the AC mains with the internal power supply wiring any more than necessary. Physical segregation is a good thing.
 
Would that be the B+ tied in one one side of that "death cap"? That's where I see one lead coming from the transformer to ground. So tie the cap grounds where they are across to that same location or totally disconnect the cap grounds and tie them to that spot?
 
If thats the center tap of the high voltage winding, then yes. Its actually B-, but either way its the center tap of the HV winding, and appears to be the red/yellow wire from the transformer.

http://www.audiophool.com/Misc/S-C_Amp.gif
http://www.audiophool.com/Schem_A/S-C_ASR-120_schem.gif

Its a bit unconventional for that to not be tied to the case of the can cap. They did that to keep current from flowing through the chassis and causing hum. S-C definitely had the engineering background to know that kind of thing. However you want to tie the cap grounds to the transformer center tap lead is your call, but I'd make both points the same, wherever it happens to be. You can even use one of those empty terminals on the end of the strip to tie all the cap leads together, and run a short jumper wire from that point to the center tap if that makes it lay out cleaner.
 
I guess I had a brain cloud and didn't see the red/yellow lead from the transformer tied in on one of the cap can legs. But it's tied to one, two other caps tied to another, and two to another which is how I tied them back in upon replacing them. How about a "bus bar" connector between the three? Dat work?
 
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