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Old 14th April 2009, 01:01 PM   #1
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Default Newbie wants to rebuild a Stromberg Carlson ASR-433

Hey All,

I've been lurking on the forum mostly for speaker input for a little while now. I'm so impressed with the attitude and amount of abounding knowledge here. With that in mind, I'd like to pick your brains...

Music has always been a huge interest of mine but I only just started to get into the equipment side of things. In the past, I've had some decent stuff, but I decided it was time for a tube amp. I picked up a Stromberg Carlson ASR-433 in seeminly great shape that has 4 Amperex EL84 tubes in it for a wicked good deal and have loved the tube sound so far. I don't think I'll be going back to SS!

I'm ready to see where this vintage amp can go with new caps and whatever you guys would recommend for this type of thing. Although I know little on the subject (trust me, I'm trying to educate myself) I can handle a soldering iron, and know that I am capable of tackling this. If anyone is willing to, I'd really appreciate some help compiling a parts list of things to order. I have a wiring diagram for assistance but these things make me go cross eyed. Please help a newbie out.

<a href="http://s32.photobucket.com/albums/d41/cbates1/?action=view&current=Stromberg_ASR-433_schem.gif" target="_blank"><img src="http://i32.photobucket.com/albums/d41/cbates1/Stromberg_ASR-433_schem.gif" border="0" alt="Photobucket"></a>
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Old 14th April 2009, 01:28 PM   #2
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Default Your link isn't working

First obtain a schematic. The only thing you will probably have to do is replace the electrolytic or can capacitors and maybe a few dielectric caps. If it has any ceramic discs in the signal path then I would replace those also. I would favor the SBC 716P series of capacitor avaliable from Antique Electronic Supply in Tempe, AZ among others. You can also find them on ebone. The Nichicon 105 degree or Panasonic 105 degree radial electrolytic would replace most of the cans. These are avaliable from Mouser and Digi Key among others. If any carbon composition resistors are chared or look flexed then I would replace those, but only if they are off value. Most of the resistors are probably OK. You will need a good meter like a Fluke or Metex or Triplett or Meterman. I would favor a 3- 4.5 digit and a newer model that measures both capacitance and inductance as well as resistance. Good luck and welcome to tube audio! Ray Hughes
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Old 14th April 2009, 03:05 PM   #3
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The coupling caps MUST be changed - they're leaky in most cases. A few are hidden under the shield up front too. I've found a few resistors that have drifted off value too.

Schematic is on the bottom plate, but there's more on my web page: http://www.audiophool.cjb.net/MadeInRoch.html
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Old 15th April 2009, 02:27 AM   #4
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Tom,

Can you post a scan of the schematic? As is frequently the case with units of that time, replacing the source selector switch to make better use of the jack set is something to consider. IMO, a single mag. phono, 3 line level, and a true tape monitor loop is an improvement, given today's signal sources.

With the symmetrical layout, I'm guessing the PSU is SS rectified. Is it by any chance a voltage doubler?
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Old 15th April 2009, 02:32 AM   #5
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Here is link of the photo you tried to link
SCHEMATIC
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Old 15th April 2009, 09:26 AM   #6
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Yeah sorry, I'm not forum savvy. That is the same one, but at least now people can help based on that. Thanks.
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Old 15th April 2009, 09:26 AM   #7
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Thanks for posting that schematic!

Yeah I believe that it is SS rectified. I have no idea what being a voltage doubler means sorry! We have the schematic now so I'm sure you can tell by looking at it or something. Just as a reminder when I say newbie, I really do mean that I have very little electronics knowlege, not that I only have one degree!

When you say that I should replace the source switch for the jacks, what is the advantage that I'd gain? I'm not sure what you're suggesting.
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Old 15th April 2009, 01:49 PM   #8
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Default Things you should know about your amp

I'm impressed. The phono section RIAA EQ is largely passive. I hope you listen to vinyl! There is some gain loss from the output of the first section of the 7025 to the second section due to some insertion loss in the RIAA filter(k2). The rest of the EQ is implmented with the tape(NAB)-LP(RIAA) switch M12. I hope you listen to reel to reel tape!

The capacitors you need to worry about are;C60, C61, C62, C63, C64, C65. If these electrolytics are leaky..... these are swell hum producers! You should check your layout to see if an axial or a radial ( both leads coming out of the bottom) capacitor will work best; probably an axial (a lead coming out of each end). Coupling caps C67,C71, C87, C91, C75, C76,C96, C97,C82, C103,C84, C105.

The most important cap to replace is C57, C58, C59A,B,C,D. If this is leaking then the consequences could be LETHAL! These are probably aluminum cans with all four caps in one can for C59. The B+ power supply is a voltage doubler. I would replace the rectifiers M6 and M7 with modern hexfreds. The filaments are all AC which worries me if you are listening to vinyl or reel to reel tape and will be using the 1st high gain preamp. If you are then I would consider an after market 6.3 vdc regulator to power the filaments of V14 and V19 and possibly V15 and V20.

You will also have to replace the short in the RIAA input plug with a 47K resistor shunt to ground in parallel with the recommended capacitor (generally 100pf) depending on the magnetic cartridge you choose if you are going to listen to vinyl.

The output stage operates CATHODE BIAS so there is no bias adjustment. This means V17, V18 and V22, V23 need to be matched to each other's transconductance as they manufacture their own bias. Good luck! Ray
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Old 15th April 2009, 03:07 PM   #9
limono is offline limono  United States
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Tom
Thanks for the link to your collection and schematics
I have S-C 21 amp to restore. I was going to modify it but I think I'll keep it as is ,just won't use Mics pre and install some Psu choke. Its is a reall beauty with copper plated chassis.It says on the back sticker that it's licensed by Bell and Western Electric for PA use only.
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Old 15th April 2009, 07:06 PM   #10
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Default Re: Things you should know about your amp

Quote:
Originally posted by grhughes
I'm impressed. The phono section RIAA EQ is largely passive. I hope you listen to vinyl! There is some gain loss from the output of the first section of the 7025 to the second section due to some insertion loss in the RIAA filter(k2). The rest of the EQ is implmented with the tape(NAB)-LP(RIAA) switch M12. I hope you listen to reel to reel tape!

The capacitors you need to worry about are;C60, C61, C62, C63, C64, C65. If these electrolytics are leaky..... these are swell hum producers! You should check your layout to see if an axial or a radial ( both leads coming out of the bottom) capacitor will work best; probably an axial (a lead coming out of each end). Coupling caps C67,C71, C87, C91, C75, C76,C96, C97,C82, C103,C84, C105.

The most important cap to replace is C57, C58, C59A,B,C,D. If this is leaking then the consequences could be LETHAL! These are probably aluminum cans with all four caps in one can for C59. The B+ power supply is a voltage doubler. I would replace the rectifiers M6 and M7 with modern hexfreds. The filaments are all AC which worries me if you are listening to vinyl or reel to reel tape and will be using the 1st high gain preamp. If you are then I would consider an after market 6.3 vdc regulator to power the filaments of V14 and V19 and possibly V15 and V20.

You will also have to replace the short in the RIAA input plug with a 47K resistor shunt to ground in parallel with the recommended capacitor (generally 100pf) depending on the magnetic cartridge you choose if you are going to listen to vinyl.

The output stage operates CATHODE BIAS so there is no bias adjustment. This means V17, V18 and V22, V23 need to be matched to each other's transconductance as they manufacture their own bias. Good luck! Ray

Thanks for a very helpful post. While I don't fully understand everything you're suggesting right now, but I'm sure I will soon. It seems like I picked up a decent amp so I'm glad about that. I will definitely be listening to vinyl once I get a decent TT setup but that's a whole can of worms there. I don't listen to reel to reel tape and being that I live in a cramped apartment, I just don't have room for several formats!

Just so you know, nearly all the caps in the amp are axial so I'll be looking for ones like that to replace them. I think for the aluminum can cap group, should just get radial caps and group them up, or should I try to hide them under the chassis? I've heard of people gutting the old can and stuffing them in there. Sounds like a lot of work but It'd be nice to not have a gaping hole in the chassis. This amp has a clean layout IMO.

I'll start shopping for caps. Can anyone tell me the correct way to link to an offsite picture? I'd like to show pictures when I make some progress.
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