ARC SP8 - restored
Thanks to Frank who emailed me the schematics, and Steiner, who provided the link, I have managed to get the amp back into shape.
The amp is more than 20 years old ( bought in 1981 ) , it died after 10 years of use, and was left in a warehouse until recently ( I went to the dark side and went silicon for a decade and now the prodigal son is back ).
The first step of restoration was cosmetic. There were 20 years of muck to clean up. Loads of contact cleaner, autosol, WD40, fine emery paper, soap water and hours of dedication and cussing later, the amp almost look presentable. Now to feed some juice into it again after so many years.
I went for the switch very fast on turn as the heater glowed extremely bright on turn on - I thought the heater supply must have been way too high. As it turns out, it was not. The tubes in the amp were perculiar - they glowed extremely bright for a short duration, and then behaved normally. I replace all the tubes and that effect was gone. It must be the stock tubes that behave in this manner. Wonder what brand they are - look interesting.
Alas - no joy. Still does not work after all these years. In the next few post, I will described what I fixed. Hopefully this info will be useful to others who may be restoring this same amp.
Thanks to Frank who emailed me the schematics, and Steiner, who provided the link, I have managed to get the amp back into shape.
The amp is more than 20 years old ( bought in 1981 ) , it died after 10 years of use, and was left in a warehouse until recently ( I went to the dark side and went silicon for a decade and now the prodigal son is back ).
The first step of restoration was cosmetic. There were 20 years of muck to clean up. Loads of contact cleaner, autosol, WD40, fine emery paper, soap water and hours of dedication and cussing later, the amp almost look presentable. Now to feed some juice into it again after so many years.
I went for the switch very fast on turn as the heater glowed extremely bright on turn on - I thought the heater supply must have been way too high. As it turns out, it was not. The tubes in the amp were perculiar - they glowed extremely bright for a short duration, and then behaved normally. I replace all the tubes and that effect was gone. It must be the stock tubes that behave in this manner. Wonder what brand they are - look interesting.
Alas - no joy. Still does not work after all these years. In the next few post, I will described what I fixed. Hopefully this info will be useful to others who may be restoring this same amp.
First pass - comapring the schematic with the physical amp, there are a few minor differences. I guess my amp is a different revision from the schematic.
There were two power transitors that were rusted to the point that I cannot see the part number any more, but a measurement of the voltages shows that they are working, bu the voltages are off a bit.
First off- replace all tubes:
4 pieces 12AX7 replaced by EH 12AX7
12AT7, 12BH7 - replaced 12AT7, 12BH7 EH
AMperex 6DJ8 replaced with Shuguang 6N11 Mil grade.
All heater voltages looks ok, tubes glow OK, b+ off but OK, but no sound. Traced the problem to a leaky tant cap at the 555 output delay circuitry. The b+ voltage problem was due to a 24V zener used to drop 24V to power a 741 opamp regulating the b+. It was a 1W part when it should be 3W. It was shorted. I replaced it with a 5W 24V. Everything looks fine now and now the amp starts to produce sound. OK 90% there now.
There were two power transitors that were rusted to the point that I cannot see the part number any more, but a measurement of the voltages shows that they are working, bu the voltages are off a bit.
First off- replace all tubes:
4 pieces 12AX7 replaced by EH 12AX7
12AT7, 12BH7 - replaced 12AT7, 12BH7 EH
AMperex 6DJ8 replaced with Shuguang 6N11 Mil grade.
All heater voltages looks ok, tubes glow OK, b+ off but OK, but no sound. Traced the problem to a leaky tant cap at the 555 output delay circuitry. The b+ voltage problem was due to a 24V zener used to drop 24V to power a 741 opamp regulating the b+. It was a 1W part when it should be 3W. It was shorted. I replaced it with a 5W 24V. Everything looks fine now and now the amp starts to produce sound. OK 90% there now.
After listening to the amp for a couple of hours, I was disappointed. It sounded aweful. I know I used cheap tubes but they are not that bad.
On phono connected to a Luxman turntable with Supex 900, the hum was louder than the music.
Something else is wrong with the amp.
I spend the next few days trying to understand the convoluted b+ regulation scheme.
First thing - 20+ year old electrolytic caps cannot be a good thing. Since the 24V heater is also used to derive the reference for the phono b+, then all electrolytics for the 24V and the b+ were replace.
The huge 24V supply cap 1500u + 1500uf were replaced by two small Elna low ESR 4700uF caps. The three huge 200uF 400V caps were replace with Philips 220uF 450V caps. I add Nichicon Muse 47u caps across all zeners to reduce the zener noise. In addition, I added a Nichicon muse across each heater on the phono stage. Now - some of the ARC magic returned. But it still do not sound right yet. The hum with the phono inout disappeared. But the noise floor still appears high. Something else is not right.....
On phono connected to a Luxman turntable with Supex 900, the hum was louder than the music.
Something else is wrong with the amp.
I spend the next few days trying to understand the convoluted b+ regulation scheme.
First thing - 20+ year old electrolytic caps cannot be a good thing. Since the 24V heater is also used to derive the reference for the phono b+, then all electrolytics for the 24V and the b+ were replace.
The huge 24V supply cap 1500u + 1500uf were replaced by two small Elna low ESR 4700uF caps. The three huge 200uF 400V caps were replace with Philips 220uF 450V caps. I add Nichicon Muse 47u caps across all zeners to reduce the zener noise. In addition, I added a Nichicon muse across each heater on the phono stage. Now - some of the ARC magic returned. But it still do not sound right yet. The hum with the phono inout disappeared. But the noise floor still appears high. Something else is not right.....
In the schematics, there is a 24V zener at the cathode of the 12AT7. measuring on my amp, it reads 10.15 volts. This is perculiar. In my expereicnes with zeners, they are either shorted, or open.
Desoldering the part out and examining under the microscope, it reads 1N47x0. x is a messed up illegible smudge. It must be a 1N4740 10V zener. The naughty guys at ARC must have misloaded the zener, or they use a cheap low wattage 1W part than a higher wattage higher voltage part. Maybe high wattage zeners cost a bomb 20 years ago. A 10V zener means that the regulating opamp will ouput 2V, with a supply of the op amp at 0 to 24V. Not much headroom I must say. With a 24V zener, it should be 16V, as per the schematic. So if I want to bias the opamp at the middle, I will need a 20V zener. I have a 5W 22V zener on hand and hence in it goes.
Voila - the ARC magic is really back. Noise floor drop significantly.
The amo is undergoing burn in right now.
Next I plan to replace as much of the semiconductors as I can with more modern parts. All the contacts of the semiconductors are corroded. The 741s ( TL071 in the schematics ) will be upgraded to TL071s.
1N4006 heater bridge to be replaced with 1n5822 Schockey, and the b+1N4007 ( they are now a black mess of corroded contacts and blacken body with no marking left on them ) will be replace with soft recovery diodes.
All in all a very worthwhile project for me. It goes well with my home brew and still under testing SET amp ( 6J5G, VT25, 845 ).
Hope my journey and expereince would be useful for anyone else with a desire for such a restoration projects.
Desoldering the part out and examining under the microscope, it reads 1N47x0. x is a messed up illegible smudge. It must be a 1N4740 10V zener. The naughty guys at ARC must have misloaded the zener, or they use a cheap low wattage 1W part than a higher wattage higher voltage part. Maybe high wattage zeners cost a bomb 20 years ago. A 10V zener means that the regulating opamp will ouput 2V, with a supply of the op amp at 0 to 24V. Not much headroom I must say. With a 24V zener, it should be 16V, as per the schematic. So if I want to bias the opamp at the middle, I will need a 20V zener. I have a 5W 22V zener on hand and hence in it goes.
Voila - the ARC magic is really back. Noise floor drop significantly.
The amo is undergoing burn in right now.
Next I plan to replace as much of the semiconductors as I can with more modern parts. All the contacts of the semiconductors are corroded. The 741s ( TL071 in the schematics ) will be upgraded to TL071s.
1N4006 heater bridge to be replaced with 1n5822 Schockey, and the b+1N4007 ( they are now a black mess of corroded contacts and blacken body with no marking left on them ) will be replace with soft recovery diodes.
All in all a very worthwhile project for me. It goes well with my home brew and still under testing SET amp ( 6J5G, VT25, 845 ).
Hope my journey and expereince would be useful for anyone else with a desire for such a restoration projects.
Hi,
Allow me to seriously doubt ARC put TFK valves in their stock units...
I can think of only one US Hi-Fi manufacturer that actually did that but that was way before ARC even existed anyway.
BTW, there's nothing wrong with that tube's heater glowing brightly at start-up.
That's just a tube predating the controlled heater technology...or a "cheapo" East-European one a la RFT, Polam, Telam etc...
Occasionally you'd find it on more recent ones as well but that's just because it escaped QC and had it's heater sticking out of the cathode sleeve too much.
Cheers,
Allow me to seriously doubt ARC put TFK valves in their stock units...
I can think of only one US Hi-Fi manufacturer that actually did that but that was way before ARC even existed anyway.
BTW, there's nothing wrong with that tube's heater glowing brightly at start-up.
That's just a tube predating the controlled heater technology...or a "cheapo" East-European one a la RFT, Polam, Telam etc...
Occasionally you'd find it on more recent ones as well but that's just because it escaped QC and had it's heater sticking out of the cathode sleeve too much.
Cheers,
The stock 12AX7 has no markings on them. Could be bulk purchase OEM. They ALL behave like that. Like no other tubes I had seen. So it is not a one off QC problem. They could be the real Mc Coy - better keep them instead of downgrading them into the misc tube junk pile.
The stock 6DJ8 are Amperex with Made in Hungary markings. The marking smudges. I have compared them to my Philips 6922, 6h23 and 6N11 and these Amperex outperform everyone of them by a margin. Even after so many years!
The stock 6DJ8 are Amperex with Made in Hungary markings. The marking smudges. I have compared them to my Philips 6922, 6h23 and 6N11 and these Amperex outperform everyone of them by a margin. Even after so many years!
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