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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 13th Aug 2018, 7:46 pm   #1
Glennymac
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Default Sobell SG679 upgrade advice sought.

Hi there,

I've recently acquired this seemingly looked after and well stored, though it hasn't been used for many years, stereogram. My plan is to replace the turntable with a modern unit to play my record collection of 33 1/3 and 45 rpm vinyl, fit an aux-in or Bluetooth unit for internet radio, and attach a CD player.

I've pulled the chassis out and given it a thorough visual inspection and apart from a few swollen/split capacitors (which will be replaced) everything looks great. I've tested the mains and output transformers and they seem to be in order.

I've purchased the service sheet but after studying I still can't seem to locate "that capacitor" and am still unsure exactly what kind of amplifier it uses as most of the sheet is taken up with how to tune the thing.

So, essentially, I'm seeking advice in a few areas:

1. The grid coupling capacitor... can anyone advise with this? I won't post the service sheet on here but am happy to share it with anyone privately who could help me locate it.

2. Connecting aux-in/Bluetooth... these would be modern line level inputs. I was wondering if i could just use the rear tape input, or would there be impedance matching issues?

3. Ditto the modern turntable... would i also need a preamp since i believe the modern decks are lower output than than the old 78 units.

4. Ditto the CD player...

I've looked through the forums and can't find anyone with this specific set or similar plans so forgive me if I'm asking questions that have already been answered. All advice/input greatly appreciated, thanks
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Old 13th Aug 2018, 8:42 pm   #2
crackle
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Default Re: Sobell SG679 upgrade advice sought.

Hi Glen
I see you got the service data from Paul's web site as I recommended.
You can post extracts from the service data on here to help explain what you are referring to.

The audio coupling capacitors, that some confusingly call that capacitor, are C42 and C49 on the ERT service sheet. It connects from V6A anode to V6B grid.

Mike

Last edited by crackle; 13th Aug 2018 at 8:49 pm.
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Old 13th Aug 2018, 9:42 pm   #3
Edward Huggins
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Default Re: Sobell SG679 upgrade advice sought.

What kind of new turntable do you intend to use? If it is one with a Moving Magnet cartridge then you will need a pre-amp. Bearing in mind that the amplifier (per channel) is just a single ECL86 valve driving an 8" x 5" speaker, I'm not quite sure what quality of sound you are expecting to get out of this?
Forgive me, but it's a bit like putting high octane petrol into an Austin A35 and expecting fantastic performance....
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Old 14th Aug 2018, 7:52 am   #4
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Default

Hi Mike, ahaa I see the world of ancient radios is a small one! Thanks for the advice, it makes perfect sense now, I was looking for caps coupled directly to the ground line from the grid.

Edward, thanks for the input (no pun intended). As for your question... because I want a challenge, because I looked in the back and thought "ooh valves! This will be fun", because I want to give new life to something old and beautiful, because I'm a nerd, because it will make one of my friends very happy as the unit used to belong to her dad, because I'm a massive nerd, because who wouldn't want an Austin A35 That spits flames and can't go round corners, because it will nicely complement my other stereo that is also beautiful but also very loud, and finally, just because. I understand your question and hope this answers it.

Glenn

Hi Mike, thank you, that makes perfect sense now. I was looking for caps coupling the grid directly to the ground line. Small world eh!

Edward, thanks for that input (no pun intended). As for your question, I understand what you mean but I'd like a challenge, might learn something, and it will make a friend of mine very happy as the unit was her father's. Also who wouldn't love an A35 on big shiny alloys that spits flames?

Cheers guys.
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Old 14th Aug 2018, 8:51 am   #5
Glennymac
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Default Re: Sobell SG679 upgrade advice sought.

Apologies for the odd reply there... I wrote a post, "lost" it, so wrote another, and now they are conjoined. This is not an expression of madness. The madness is perfectly expressed in what I'm trying to achieve with my stereogram.
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Old 14th Aug 2018, 9:42 am   #6
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Default Re: Sobell SG679 upgrade advice sought.

Re:- a suitable turntable.

About 18 months ago I bought an Audio Technica AT LT 120 USB Turntable: basically for playing 78 rpm shellac discs. It has a standard interchangeable head shell and spare shells are available. Speeds selectable are 33 1/3 rd 45 or 78 rpm. There is a choice of outputs; direct from cartridge / line level RIAA corrected and USB for computer connection.

The cartridge supplied is a reasonable AT95 for 33 / 45 vinyl. I use an ortofon mono 78 specific cartridge for shellac discs.

I think the price was £250.
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Old 14th Aug 2018, 10:18 am   #7
Edward Huggins
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Default Re: Sobell SG679 upgrade advice sought.

Thanks for the reply to my Post #3. I guess that most of us here are about Restoration and Repair, rather than what you may plan to do.
However there are all manner of attempts these days by well intentioned folk to "Upgrade" or "Re-Purpose" these classic old valve Radiograms into something they were never meant to be, so I guess I should not over-react.....so keep us posted.
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Old 14th Aug 2018, 1:51 pm   #8
Glennymac
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Default Re: Sobell SG679 upgrade advice sought.

Rontech - thanks for the information. I won't be spending that much, I'm just seeking something that will work.

Edward - if this was a valuable or particularly rare example I wouldn't dare do what I'm thinking of doing. I suppose it may seem sacrilege or folly to some, but to me it is a challenge and a way-in to this world. If successful I hope to perform further modification, specifically building a power amplifier section that can drive my Mordaunt Shorts, or working out a way to put my guitar through it.

Also Edward - before doing any of this i want to get the thing working!
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Old 14th Aug 2018, 3:14 pm   #9
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Default Re: Sobell SG679 upgrade advice sought.

A stereogram (if it really is stereo) should be fitted with a multi-speed turntable already. Stereo was actually invented first, but stereo records became mainstream later than microgrooves.

A machine that only played 78s but was capable of doing so in stereo would be exceedingly rare -- an engineering prototype, most probably, or a or small production run, and probably should be kept as original as possible.
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Old 14th Aug 2018, 3:25 pm   #10
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Default Re: Sobell SG679 upgrade advice sought.

This appears to be it: https://www.radiomuseum.org/r/sobell_sg679.html The original record deck seems to have been a Balfour Princess 4 speed deck with a ceramic stereo cartridge: https://www.vintage-radio.net/forum/...ad.php?t=62388

The existing ECL86 amps will drive your MS speakers quite well, though there's not a lot of point in hooking up hifi speakers to a 60s stereogram. It isn't intended to be hifi though it should sound very pleasant if working properly.
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Old 14th Aug 2018, 4:31 pm   #11
Glennymac
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Default Re: Sobell SG679 upgrade advice sought.

Julie, Paul, you are both right. I've been so engrossed in understanding and checking the internals I'd barely looked at the record player section, but now I see what I thought was a simple on/off switch is actually the speed selector with all the numbers worn off! This simplifies the plan greatly.

All i need to do now is figure out how to get an aux-in. There is a 5 pin din socket on the back for tape input/output and I'm hoping i can use this so as to keep the internals as original as possible, and use an external switch to change between Bluetooth and CD player. Any advice greatly appreciated, and thank you all so much for taking the time to talk about this
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Old 14th Aug 2018, 4:52 pm   #12
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Default Re: Sobell SG679 upgrade advice sought.

You'll need a 3.5mm (stereo) jack plug to your device and a 5 pin DIN plug to the DIN socket at the rear of the Sobell. You can buy these ready wired. The input sensitivity on the Sobell (should) be something like 250mV into 1 megohm.
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Old 14th Aug 2018, 5:02 pm   #13
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Default Re: Sobell SG679 upgrade advice sought.

Yes, the tape input should work very well with a CD player or other line level source, though the sensitivity may be a bit low if using a headphone source like an MP3 player or phone. To clarfify Edward's post, you need a lead with a 5 pin DIN plug on one end and suitable connectors on the other. The most versatile solution is to buy a DIN to phono adaptor, which will have the DIN plug connected to 4 standard RCA phono sockets. You can then connect a phono-to-phono or phono-to-3.5mm cable to the adaptor as required.

https://en.wikipedia.org/wiki/DIN_connector
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Old 14th Aug 2018, 8:37 pm   #14
Glennymac
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Default Re: Sobell SG679 upgrade advice sought.

Thank you Edward and Paul. I have a 5 pin din plug kicking about in one of those boxes of bits that never get thrown away in case they're useful one day (finally, some justification!) and have many spare Phono and jack leads so will be able to make up a connector.
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Old 14th Aug 2018, 9:37 pm   #15
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Default Re: Sobell SG679 upgrade advice sought.

That's fine if you have all the necessary bits.
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