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Old 5th July 2006, 08:27 AM   #1
offwego is offline offwego  United Kingdom
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Unhappy Arc Sp8

Sorry if this is in the wrong place, but this is my first visit to the site!

I have recently been given a rather sad looking SP8 by a friend of the family; it has been sat in stroage for about 10 years and has bit missing like the handles and 2 of the toggle switches (power and monitor...). The missing toggle switches have been bypassed internally and the amp is always on and always selected to input.

After de-dusting the circuit board I switched the amp on gigngerly; there was a very bright glow on the top of some of the tubes and then it settled down with no pops, bangs or smoke. I cycled the amp leaving it on longer each time for a couple of days and then hooked it up through my djing system (quite robust!) to have a listen.

The issue I have is that there is a hum on all inputs that is not affected by the volume control. As this is the first valve product I have encountered I have no idea whether this is a sign of old valves...

The hum is similar to that of a turntable with earthing issues, and then with the 'angry bee' physical vibration of the transformer comin through on top of the soft sounding turntable type hum.

With the power amp switched off you can tell that part of the noise from the speakers is the pysical hum from the transformer, even though the physical noise of the transformer is not that bad.

Basicaly I want to know whether it sounds like a new set of tubes will sort it; I know you can't say for sure, but I need to know if the fault described is a symptom of dead tubes.

Also, has anyone any idea of the different voltages off the transformer? I would quite like to build an off borad power supply to run the amp on.

Thank you!
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Old 5th July 2006, 08:58 AM   #2
EC8010 is offline EC8010  United Kingdom
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It's unlikely to be the valves, much more likely to be power supply electrolytics. Oh, and welcome to the forum. Don't you mean peddler, rather than pedaler? I'm sure you don't spin pedals on your Hi-Fi!
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Old 5th July 2006, 10:01 AM   #3
offwego is offline offwego  United Kingdom
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Funnily enough, I actually hold similar levels of intrest in cycling and hi-fi. I sell hi-fi for a living, but moved into the trade from being a cycle mechanic, and I still build wheels to order.

It wasn't what I ment to type mind...

What would be the next step in locating the problem?
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Old 5th July 2006, 10:30 AM   #4
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by offwego
What would be the next step in locating the problem?
Ah, well that's where it gets trickier. To know for certain what's wrong requires an oscilloscope, and if you had one, you'd already have guessed what the problem was likely to be. Of course, you could always replace the electrolytics anyway (without confirming that they're faulty), but the job will need to be done very carefully to avoid damaging the PCB. And a circuit diagram would be handy, too.
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Old 5th July 2006, 12:59 PM   #5
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Schematics and other information can be found here:

http://www.arcdb.ws/SP8/SP8.html
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Old 5th July 2006, 01:41 PM   #6
EC8010 is offline EC8010  United Kingdom
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Default Whatever weren't they thinking of?

Thanks for the diagram link. OK, so once we've worked our way past the flim-flam, the phono stage is a the standard RCA circuit with a cathode follower added to be able to drive the feedback network. The line stage is the same gain stage with an awful lot of feedback wrapped around it.

The HT supply is a standard series regulator with a colossal amount of open-loop gain achieved by cascading half a TL072 with a 12AT7 gain stage to drive the series pass valve (12BH7). The HT supply will either work perfectly or expire in short order - I don't see any half measures there. My guess is that if the hum is due to a power supply fault, then it's C14 and C15 (100uF, 450V).

Otherwise, the hum is likely to be a corroded earth bond from the HT 0V to the chassis.
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Old 5th July 2006, 09:09 PM   #7
offwego is offline offwego  United Kingdom
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Corrosion seems likely now you mention it...I'll try cleaning it all up and seeing what happens.

What caps do people think best for the power supply?

Thanks for all the help with this; if I can get it up and running from the advice off this forum I'll paypal a wedge of cash in place of what I'd pay to someone else! (Although considering my level of practical experience it might be a big ask!)
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Old 5th July 2006, 09:49 PM   #8
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Check for corrosion while you are chaning out the caps. C14 & 15 definitely need to go. I would also change out the other electrolytics, C16, 12 & 35. They are all suspect and you might be buying more than caps if they really go. New caps might also clear up the mechanical hum of the mains xformer. It just might be working overtime to feed leaky caps. A sign the end might be near.

With such a busy power supply, I can't see the cap mfgr being critical. Recently I notice that I am replacing modern Xicons, so I avoid them. Any other should be fine. Sprague, Nichicon, UCC, Illinois, Panasonic, whatever. Your biggest issue will be to make sure the replacements mechanically fit. Look at the existing caps, noting size and lead arrangement. Then hit the catalogs and purchase accordingly. Get the same F rating, same or better voltage rating, similar size and leads coming out the same way. You will cuss a storm trying to replace a snap mount with an axial leaded cap, or trying to replace with a cap physically twice as big.

Bypassing the toggle switches is pretty ham-fisted. Bypassing a power switch is true cave-manish. I would look to clean that up too and keep an eye out for other irregularities. Make sure the fuse isn't bypassed, for example.
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Old 5th July 2006, 10:00 PM   #9
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by ethermion
Bypassing a power switch is true cave-manish. I would look to clean that up too and keep an eye out for other irregularities. Make sure the fuse isn't bypassed, for example.
And that the incoming mains earth hasn't been removed...
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Old 6th July 2006, 06:41 AM   #10
offwego is offline offwego  United Kingdom
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Quote:
Originally posted by ethermion
Bypassing the toggle switches is pretty ham-fisted. Bypassing a power switch is true cave-manish. I would look to clean that up too and keep an eye out for other irregularities. Make sure the fuse isn't bypassed, for example.
Hi there, thanks for that.

As far as I can tell the toggle switches are the only irregularities; I thinks someone has snapped the actual toggles off and then bypassed them to avoide buying new ones!

Right then, lets see what we can do with this thing then...

I'll let you know how I get on!



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