Sears/Silvertone Amp Modification Questions

T

tlowc34

New Member
I have a 1959/1960 Silvertone Model 2075 Console Stereo that has been working well aside from the turntable drive motor, scratchy volume pot, dial & indicator lights that are burned out, and poor bass response. I'm competent in electrical repair (following directions and the advice of others) and have the necessary tools to do the work. I studied mechanical engineering in school and did great with the early EE courses when we were just building wheatstone bridges and playing with breadboards. But, when we got into mesh analysis and higher level subjects I kinda just survived it.

The plan for the scratchy pot is Deoxit D5 with the amp out of the cabinet to be sure that stuff doesn't go everywhere and cause any problems.

Light bulbs are easy (TS47). I've just got to find a reputable seller where I don't get burned on shipping $3 in bulbs for $10. Might bundle this in with a couple other items.

To handle the issues with the turntable, I'm going to swap the original unit for a Garrard Model 50 that fits into the cabinet. I have properly reconditioned the Syntronic changer (with the help of Gary at The Voice of Music. I also rebuilt the motor 3 times, but it keeps seizing, I'm going to hang on to it, in case I ever figure it out.

Swapping in the Garrard Model 50 has some benefits, including: a removable headshell (for easier cartridge swaps), adjustable tonearm tracking weight, a 4 pole motor, heavier platter, and swappable spindle (if I want to run a stub spindle for the use of a record weight). Plus, I got it for $25 with a Sonotone 9TA which I'm anxious to compare to the Electro-Voice 26.

The Syntronic changer connects to the amp with a proprietary looking 4 pin connector that mates with a pigtail that is hardwired inside the amp. The Garrard has standard RCA connectors. My plan is to mount Syntronic BPJR03AUX and BPJR02AUX RCA jacks to the amp chassis to accommodate connection through a high quality RCA cable, with the accompanying phono ground attached to the chassis as well.

But, I'm getting the "while I'm here" bug and want to know about doing some preventative maintenance, circuit simplifications, and making potential upgrades. Schematic attached. This is where the questions start ranked in increasing order of "while I'm here" silliness:

1. Do I replace the electrolytic caps while I'm in here, including the bundled filter caps? This thing is 63 years old and it still has the original Silvertone tubes - I doubt it has ever had service inside the chassis, nothing looks to be bulged, leaking or dusty, but the filter caps are trapped inside that aluminum cylinder with no way to inspect. Is this affecting bass response? What is or maybe a better question - where should I look? I'm putting a Mouser order together any way so...
2. Desoldering the speaker wires from the transformer to remove the amp from the cabinet is kinda annoying. Can I put female 1/4 tab quick connects on the ends of the speaker wires? It looks like they would slide over the tabs on the transformers perfectly. Any downside?
3. How can I completely remove the remote circuitry? It is currently just blocked off by the shorting plug. I have never seen one of these "remotes" anywhere (ebay, internet, in person, in a catalog). It looks to accomplish only two things (if I'm reading correctly): reject the record and adjust L/R channel balance. Sounds like a gimmick that is just introducing noise into the circuit by creating another loop. Do I just cap the blue wire from the transformer, and connect the wires from the L/R channels to ground? Goal is to limit chances to introduce noise and open up some space for other planned items (see below).
4. Can I swap the tape jack for a 3.5MM panel mount stereo jack in its place as an aux input? I don't know what signal level the amp is expecting from this input as I have no experience with Reel to Reel tape units.
5. One advantage to swapping to the RCA phono inputs is that I could potentially run a moving magnet cartridge (the model 50 can handle it, as it was delivered in MKII form with MM carts) by wiring a preamp in between the changer and the amp. But that got me thinking, do I add a switchable 12AX7 preamp in the chassis? What would that look like? Worth it?

Sorry for the long post, wanted to give all of the context. Thanks in advance.

-Todd
 

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Replacing the electrolytic capacitors will go a long way for dependability.
If there are any multi cap cans, usually in the power supply, it or they can he replaced with individual caps underneath the chassis if there is room, or restuffling the cap can with new individual caps. Of course you could spend more and get a new cap can. As for the bass, that could just be the console speakers. These old tube consoles tended to use speakers with small Alinco magnets with small voice coils along with stiff paper surrounds. Maybe disconnect them and connect a pair of better speakers and see if the bass is still lacking. Some folks here have pulled the amplifiers out of beat up consoles and modified them to run stand alone. These are usually 6V6 or EL-84/6BQ5 push pull amps similar to what you have minus the tone controls. One thing many do is raise the coupling cap values. In one of the amp I modified, I went from .047uf to .1uf. Doubling the value of those caps bumped the low end up some. But if the speakers just can't produce the lower end to begin with that won't help.
 
Those coupling caps also should just be replaced if they are a paper type. Paper caps are notoriously unreliable. Any sort of film cap will do just fine, I use a lot of Illinois MPW types

I'd be careful about messing with cap values in this. The tone controls are very oddly done here, and I suspect messing with the cap values will screw that up. The "normal" way is to put them in front of the power amp, and the power amp is just straight amplification. The normal odd way is to put it in the feedback loop of the amp so it monkeys around with the response that way. These are just right in the middle of the amp with the feedback loop around it. That would tend to want to correct for anything the tone controls are doing, so I'm curious how well that even works.
 
gadget73 said:
Those coupling caps also should just be replaced if they are a paper type. Paper caps are notoriously unreliable. Any sort of film cap will do just fine, I use a lot of Illinois MPW types

I'd be careful about messing with cap values in this. The tone controls are very oddly done here, and I suspect messing with the cap values will screw that up. The "normal" way is to put them in front of the power amp, and the power amp is just straight amplification. The normal odd way is to put it in the feedback loop of the amp so it monkeys around with the response that way. These are just right in the middle of the amp with the feedback loop around it. That would tend to want to correct for anything the tone controls are doing, so I'm curious how well that even works.
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In all honesty, it doesn't work well. In order to get any bass I have to take that knob almost all the way to the max. But then the treble gets "muddy" so I have to bring it up to compensate.
 
not surprised at all. Were this mine I'd probably try to re-design those controls, or just eliminate them but ideally it needs some measurement gear to figure out what its currently doing and how to best correct for it.
 
gadget73 said:
not surprised at all. Were this mine I'd probably try to re-design those controls, or just eliminate them but ideally it needs some measurement gear to figure out what its currently doing and how to best correct for it.
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This is probably a loaded question, but is there an ideal layout/schematic that I could build the amp to using the existing transformers and tubes that would incorporate a better control configuration? I'm ok completely redoing all of the wiring, resistors, pots, and capacitors. But unfortunately, I lack the appropriate experience to design one of my own.
 

Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

Geeze, Dave, after this it seems we need to get you a pair of Akai M8 amps to dissect and rebuild. :thmbsp: -D
audiokarma.org

would be a solid starting point, but its probably going to need a few tweaks at least. Does the Sams by chance tell you what the output transformers are? The values in the feedback loop for that amp I linked to are set up for an 8 ohm transformer, but if yours is something else it needs a tweak. May also need to change that 470k to ground at the input to something higher if you are keeping the ceramic phono cart. Those need a fairly high impedance to work into. If you are using a mag cart and a phono preamp then the 470k is fine.

Probably will have to fiddle with resistors in the power supply to get the right voltage but its easier to drop than to add. Can also skip the choke if you want. It should be adequately filtered without it.

tone controls really need another tube to do it in a decent way. Its possible to do it in the feedback loop but I'd have to do some experimenting to figure that out, and ideally it should be tested in the actual amp for stability and whatnot just to make sure it doesn't cause things to go stupid.
 
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