• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Audio Research SP-10 help needed

Hi

I'm restoring a rather badly treated SP-10. Pls don't ask me why :)

I need a good detailed picture of the underside of the board of the audio chassis. Many wires are disconnected and many have been replaced and it looks like they are going to different places :-(

The power supply is blown, so my question is: can I work on it without the audio chassis connected? Or with it connected but with tubes out?

Thanks for help in advance

M
 
Hi

I'm restoring a rather badly treated SP-10. Pls don't ask me why :)

I need a good detailed picture of the underside of the board of the audio chassis. Many wires are disconnected and many have been replaced and it looks like they are going to different places :-(

The power supply is blown, so my question is: can I work on it without the audio chassis connected? Or with it connected but with tubes out?

Thanks for help in advance

M

Mark
First determine if you have the MK1 or MK2 version. You can work on the power supply without it being connected as long as the power switches are on the power-supply chassis. Keep in mind your voltages will be a bit higher because you don't have all the tubes pulling current to bring down the rail voltages.
 
not a problem

Thank you

It's an early Mk I

M

There is a MK1 schematic you can download and basically the ARC units are famous for electrolytics and Zeners failing..They don't usually short but they develop leakage as well as the occasional regulator that shorts from collector to emitter. The SP10 is pretty straight forward..I have rebuilt 2 of them in 15 years but still have a vivid memory of it.I like it better than the SP11 which i have heard but never worked on one. The wires you see going to different places may be the same electrical point as originally so you have to really eye that down. Keep in mind ARC doesn't label their components so it's rather difficult to follow at times...
 
Last edited:
Hello and thank you again, that is very useful information. I need a picture of the underside because previous owner has modified it a lot, and all of his work, including the parts (AXON caps on audio output, for example), looks very mediocre. I'd like to return it to the original state. I think that may be this will be a bit too much to chew for me, I'm not an experienced electronics guy... I bought this one from a repair facility, where it was left by the owner who decided not to pay (!). I have downloaded the schematics. The repair man told me that a couple of regulators went, and they were missing. As a lot of other parts :) I have located pretty much most of them, including the new caps, but what scares me most is the huge amount of IC chips and other solid state devices.

As far as the SP-11 goes, I had it and did not like it. My preamp of choice, and I've lived with it for the last 20 or so ys is the MFA Lumi. I got this SP-10 because I have heard so much about it but could never afford one. So far this looks like a pretty involved project.
 

mr2racer

Member
2009-07-09 12:53 am
I would start by buying an original SP10 manual directly from Audio Research. They are very complete and aren't that expensive. Also they are very helpful with information. Tonight I replaced the power transistor in my SP3A-1. The original Delco unit is very rare so I substituted an ON MJ15022G. Big mistake. I bought ARC's replacement MJ8504 Motorola.
 
Yesterday fired it up for the first time, on Variac and a bulb current limiter. It didn't blow up, that's good news, the bulb didn't even light up at all. Today it did light up, and I measured the current. It started out innocently enough, but as I got closer to 100 v ac it started drawing around 1.2 amps. Too much, imho. Have to look for a short somewhere...?
 

mr2racer

Member
2009-07-09 12:53 am
Seems if you had a short you would have blown the main fuse? You have a 1 amp quick blow fuse for the HV and a 1.5 amp slow blow. Change the 1.5 amp for a quick blow and power up the amp. If the fuses don't blow its unlikely you have a short.
 
Last edited:
True that

Seems if you had a short you would have blown the main fuse? You have a 1 amp quick blow fuse for the HV and a 1.5 amp slow blow. Change the 1.5 amp for a quick blow and power up the amp. If the fuses don't blow its unlikely you have a short.

Thank you.

At first I put in a 3/4 amp main fuse, and it did blow srangely enough at TURN-OFF (?). Today I put in a 1 amp, and it didn't blow when it was drawing 1.2 amps. Pardon my luck of knowledge of this unit, but doesn't 1.2 amps is a lot? My Luminescence diesn't draw more than 800 mA. But it does not have the six large filament regulators either...

Another problem it seems to have is that both lv and hv regulators are not regulating, because the supply voltage diesn't reach the specified high figures. At least it didn't yesterday. Perhaps the current limiting bulb was sucking too much juice out. I will not get to it for two weeks, have to go to see my mom in FL post-op...

And I still don't know if the chips have to be replaced. And auto-mute doesn't seem to work either. Lots of work still :-(
 
Hi

Can anybody explain to me the symptoms of the IC chips failing in the SP-10? The capacitance multiplier circuit: if the chip fails, what happens? The HV regulator, what happens if its chip fails?

Thx

The current sources won't supply any current to VT1,2,5,6,7,8 --- too bad they didn't have DN2540 Depletion MOSFETs.

I think that the current sources for the preamp would best be described as a "mishegoss".
 

mr2racer

Member
2009-07-09 12:53 am
jackinnj is right if the HT voltage regulator transistor fails no HT no sound. (Been there fixed that.) I had to replace the Delco transistor in my SP3A1 for that reason. I bought what Digikey said was the closest transistor they had. Sounded awful, really annoying high frequencies. Bought what ARC sold and it is now fine. If you have any doubts about the parts just replace them they're old. I would strongly recommend buying them directly from ARC. First they have very high quality parts. Second they are proven to work as well as the originals. And the price, although higher than aftermarket, is not so great as you might think. Buy them all at one time because shipping is a big part of the cost if you don't.
 
Thank you o gurus :). So since Ihave high voltages running to all my tubes I can assume that the IC chips with their associated transistors are fine. Im trying to avoid like plaque replacing the chips, thats a big pain in butt. I have to figure out now why it doesnt achieve the specked power supply voltages and draws a bit too much current. I think theres a leak somewhere.

As to original parts, I used whatever I could find original or close. I have used 5 Motorola MC7815 and one Tesla, just to see if it will work out. It works fine.

Thx

M