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Old 29th July 2007, 07:57 PM   #1
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Default Radford STA-15 questions

I just acquired a Radford STA-15 amp. I was wondering whether anyone could help me with some questions:

- Voltage selector. I'm in the US. There is a voltage selector switch, but it's confusingly marked with 3 settings:

110/220
125/235
140/250

I want 110, obviously, not 220... but I can only choose the combination, whatever that means. Does anyone know why both voltages are marked at the same position on the selector switch? Am I safe to plug it in at 110/220 in a 110V mains system?

- Has anyone restored one of these before? Any tips - obvious parts for replacement, etc.? The amp seems to be completely original.

- Does anyone have owner's manual, schematic or service manual for one of these?

Thanks in advance -

Patrick
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Old 29th July 2007, 08:19 PM   #2
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Actually 125 is a better choice - most places have in excess of 120V these days. The 125 / 235 selection is done with an internal soldered jumper. Unless this amp came from Europe, it's probably on the lower setting. You can't hurt it with lower than rated voltage, anyway ,but I'm sure a LOT of US equipment has gotten fried on 230v!

Schematic: http://www.drtube.com/schematics/radford/sta15.gif
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Old 29th July 2007, 08:21 PM   #3
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Ah, great, thanks for the schematic link!

Sorry to be dense, but do I need to go inside and move the jumper then?

I've measured about 117V at my outlet...

Patrick
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Old 29th July 2007, 08:33 PM   #4
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Where was it last used? If US or Canada, it's set for 120V. What plug is on the cord?
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Old 29th July 2007, 09:09 PM   #5
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In the UK. No plug on the cord. I'm sure it's set for European voltage.
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Old 29th July 2007, 11:47 PM   #6
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Yeah, you'll have to change some internal jumper settings to get it to work here.

Remove the connection between PY2 and PY3

Connect PY1 to PY3

Connect PY2 to PY4

The above is probably obvious to you, but in case it wasn't..

IIRC these are hardwired connections that you will have to solder.. (It's been a while since I saw one of these.)

Change the fuse to 3A, medium blow type if possible.

Finally set it to the 125V position on the voltage selector switch, better to slightly under run the amp voltage wise than to significantly over run it.

How old is the unit and are you sure the electrolytics are all copacetic?
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Old 29th July 2007, 11:52 PM   #7
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Thanks Kevin, I'll get to work. Amp is probably early '70s and the 'lytics may well need replacing. I noticed the fuse rating issue too. Thanks!
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Old 30th July 2007, 07:20 PM   #8
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Argh, the label on the power transformer primaries turns out to be gone. It survives on the secondaries side.

On the secondaries, the tap numbers go 13-18 from bottom to top on the left, and 19-24 top to bottom on the right.

Does this order give any clue as to how the primary taps must have been numbered?

Here's a couple of pics:

1. secondaries:

Click the image to open in full size.

1. primaries:

Click the image to open in full size.

anyone have any idea?

Patrick
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Old 30th July 2007, 08:14 PM   #9
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Ah - got it - by tracing the wiring from the voltage selector. I could barely see the jumper between PY2 and PY3 (it's invisible in the photo above) - I've now identified the taps.

Patrick
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Old 30th July 2007, 09:14 PM   #10
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Up and playing music through my Quads. Thanks Tom and Kevin.

Next step: replacing the EL34s, and component check.

Patrick
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