Hello all, first let me say I'm honored to be let into your inner sanctum...however, I do need some help from some of the sages. Due to some health problems, four years or so ago I was forced to liquidate most of my stereo, and am just now getting around to rebuilding one. As I've been an audio tech, higher end consumer, pro reinforcement, lots of big and small amps fixed and owned over the years, thought I might take a chance on a highly touted but broken amp. Recently found and bought a blown up Threshold S/500e off Audiogon. It had one channel down, blowing neg rail fuse. Before I go on, let me just say to all the engineers involved with the design of said amp...bravo! I've fixed everything over the years, from Crown, Phase Linear, all of Bobs' stuff, Krell, McIntosh, on and on. The well thought out, logical and exqusitly tight design and manufacture of this amp is just right up there with the best I've ever had to tear up. It really was a joy to tear up. Now that thats been said, I really need to find the set up instructions for the biasing of my now fixed S/500. The one blown channel I totally tore down, removed all TO3's on p and n output banks. One shorted n and two leaky (on my curve tracer) p's. As I had all outputs off board, took beta readings on all good outputs, n's first, took average as all good ones were pretty close to each other, and dug thru my sack of MJ15024's to find a near match and re greased the n's and replaced 'em. Then checked the p's and found two leaky ones that I replaced with MJ15025's that I beta matched as close as I could. Re greased and installed. Checked for obvious problems on driver board, and reassembled entire unit, adding more new grease to output board-external sink interface. Ran her up on the variac, and couldn't believe it didn't work. Since I knew nothing about the mute circuit, I shut it down and went through a bunch of %*)(&&*## chaseing my tail until I decided to just hard start it. Don't know if mute circuit doesn't work right when brought up slowly with variac, or some other stupidity on my part, but when hard started, after about 8 to 10 seconds, bang, sine waves on scope!!!! So this is how I roll, I unpluged amp bleed off filters, unhooked outs and ins and fired it up again to check offset..10mv on channel I fixed, about 150mv on other. 150mv not really ok with me, but ok for burn in. Hooked back up to generator set at 7khz ish, hooked up 8 ohm loads, fired it back up and ran it up to clip, both chns symetrical at clip so I ran her down to just below clip, and then lowered input 3db, and went over to a friends house for about an hour. Both channels not as hot as I expected when I got home, but when I ran it up closer to clip, maybe 1 1/2 db before clip, she did get good and warm pretty quick then. Was able to leave hands on top of heatsinks for about 5 to 7 seconds before I really didn't want to do that anymore. Pretty sure unit is close, bias wise, but would really like to know the proper set up, and any other suggestions you all might have that I missed or just messed up on. Listened to her for a while, Pink Floyd, Momentay Lasp...thru bench speakers, JBL L112's and pretty sweet!! Although I hate cone speakers, these aren't too bad with L-pads properly adjusted. Got a used pair of ML Ascents coming. Would love to have 100% faith in my repair of amp before they get here. Any advise from the "guroos" familiar with this amp would really help. Thanks in advance, Steve
Thanks analog, back when I was a JBL warranty center, I had a guy bring in that pair of L112's one day and wanted me to fix 'em. When I got around to checking them, both woofers had terminal rot of the surround, both mids were open, all L pads were shot, and one of the two tweeters was open!!! A more trashed pair I never saw. Needless to say he declined my estimate and never came to pay the check out fee so...after $ 600 dollars (my cost) of parts I had a "nearly new" pair of L112's. Have used 'em as my bench spks ever since.
About the S/500 bias, I assume
bigeyes: ) your taking that measurement across the .3ohm emitter resistor of the rear xistor of the top bank of each channel...the one with the thermistor in parallel with it? Also, I can't tell you how many different search permiations of "S/500 bias" I made before starting this thread. Several different ways...no joy. What search perameters would you have tried? Maybe I can learn and just find the stuff without bothering anyone? Anyway, thinking about pulling diff pair on channel w/150mv offset and trying to lower that a bit. Any other suggestions? Thanks, Steve
About the S/500 bias, I assume
bigeyes: ) your taking that measurement across the .3ohm emitter resistor of the rear xistor of the top bank of each channel...the one with the thermistor in parallel with it? Also, I can't tell you how many different search permiations of "S/500 bias" I made before starting this thread. Several different ways...no joy. What search perameters would you have tried? Maybe I can learn and just find the stuff without bothering anyone? Anyway, thinking about pulling diff pair on channel w/150mv offset and trying to lower that a bit. Any other suggestions? Thanks, Steve
Hey ANADIGIT, thanks for the offer of help to repair. I ALREADY have amp up and running, so what I really need is the bias set up procedure and any suggestions for lowering DC offset of one channel, it's not terrible, but I think 150mv is a bit high. If your S550 has similar bias set up procedure, please let me know what that is. I know that the "rule of thumb" would be to buy a big bag of 15024 and 25's, beta match enough of 'em to fill all the holes in the offending channel, then go about adjusting, but I'm cheap!!! I did match replacement outputs as close to good ones as I could, within 8% or so at worst, and like I said, amp now works. Just need help "tweekin" it. Feel free to add any suggestions and I will gladly read all, and like I said earlier, if someone has better search directions so that I can find of this info on my own, I'll gladly read them so as not to bother people on stuff already delt with in this forum. I'm new, so please help me learn how to properly use this wonderful source of knowledge and experience. Thanks in advance, Steve
Go here Stevie, and there's some circuits for you too. Remember the search tool is your friend.
http://www.diyaudio.com/forums/showthread.php?postid=1016700#post1016700
Cheers George
http://www.diyaudio.com/forums/showthread.php?postid=1016700#post1016700
Cheers George
Congrautlations Steve, it looks like you have it under control.
Myself, I tend to bias so that the sinks are 50 to 55 deg C after a
couple hours, which pretty much fits your 7 second hand holding.
You can get a little lower distortion biasing to spec, but I prefer it
a little higher, presuming that the spec doesn't get you to the 55 deg
point.
Myself, I tend to bias so that the sinks are 50 to 55 deg C after a
couple hours, which pretty much fits your 7 second hand holding.
You can get a little lower distortion biasing to spec, but I prefer it
a little higher, presuming that the spec doesn't get you to the 55 deg
point.
Hey George and Nelson, thanks for the info. Will pull back off S/500 today and figure out which of the two blue trim pots does the bias and which is the dc offset and get to tweekin'. All that gushie stuff I said about the engineers of this I now take some of it back. Come on, pull the back to get at trim pots? Am I missing something here? Some trick I can't see to get a screw driver into those trim pots? Will take dc mv readings across a few emitter resistors along the top bank of each channel and set it cold for about 150mv. Will then do offset to 0 and recheck bias in a couple hours for around 100 mv or so, recheck dc offset, then see how long I can hold on to the top of the heat sinks. I did notice in my very unscientific "hands on the heat sink" test that the sinks closest to the front panel were noticeably cooler than those toward the rear of the amp. I attributed this anomally to the fact that the front panel is a lot more metal than the rear panel of the amp, and is connected to the front heat sink by four large diameter and fairly long screws, thus acting as yet more heat sink mass, removing a significant amount of heat from the forward sinks. Sound possible or am I just drunk again? Anybody ever notice this phenomenon before on S/500? I really appriciate all the advise from those "who have been there done that" on this amp. Very eager to get out my dvd of "Hell Freezes Over" tour with the Eagles and hear this amp through the ML's. It's no big deal to me however, the front panel "trim piece" that hides all the screws and has the printing on it, well, mine has a couple of fairly deep gouges in it. Obviously just cosmetic, however, I fear that these tiny imperfections may "phycoacousticly" inhibit my full and complete audio nirvana experience when I get system going. Would be happy to discuss price for used, but pretty, front trim piece with anyone who might have a salvage amp or even NOS front panel trim piece just layin' around. This S/500 has had the balanced input mod done to it in the past, has the Threshold sticker stating "e" mod done on top of amp so I would be open to either trim piece...S/500 or S/500e, I assume 
they are interchangeable? Just shoot me a reply if anyone wants to get rid of one. Is this proper place to even discuss this? Remember, newbie here. Thanks again for all your help. Any news re: Threshold on line schematic web page? See, I have done some reading here. Stevie
they are interchangeable? Just shoot me a reply if anyone wants to get rid of one. Is this proper place to even discuss this? Remember, newbie here. Thanks again for all your help. Any news re: Threshold on line schematic web page? See, I have done some reading here. Stevie
Steve,
Please post the "which pot does what" answer here when you figure it out. I have searched here and asked the question before but was never successful in finding the answer. The schematics I have seen show only one pot. I have an S-150 that barely runs above room temperature, so I would like to bias it a little higher.
My friend has a pair of SA-1's and they exhibit the same heatsink characteristics you mention. The front section feels cooler than the rear. I have never measured it, but it is easily detectable to the touch.
Bill
Please post the "which pot does what" answer here when you figure it out. I have searched here and asked the question before but was never successful in finding the answer. The schematics I have seen show only one pot. I have an S-150 that barely runs above room temperature, so I would like to bias it a little higher.
My friend has a pair of SA-1's and they exhibit the same heatsink characteristics you mention. The front section feels cooler than the rear. I have never measured it, but it is easily detectable to the touch.
Bill
Hey Bill, still trying to decide how to best just get to those pots to adjust them. I've fixed a number of amps over the years, and when I run into a deal like this where I just have to guess at it, it's no big deal to hook up meter to monitor bias and note what it is in both channels, across emitter resistors. That way if the first pot I twist is the bias adjust pot, the bias will change. If the pot I twist doesn't change bias on meter, then it's the probably the offset adjust. I then put bias back to spec, or to where I want it, then move meter to output jacks and twist the other pot, hopefully the offset pot for 0. The deal with this amp is that there is no room to get at the pots with a screwdriver. Also, I haven't been feeling very good last couple of days. May have to make a screwdriver tomorrow and break the factory seals still on the trim pots. Even with back loose from chassis, the mountings for the rail fuse holders, input and output jacks, are right behind where I need to get. Maybe 2 1/2 inches and several wires to get around. Wish me luck, will update thread when I get some answers. Thanks Stevie
Ok, I know I blew it.
Although I did manage to get this amp up and runnin' by myself, I was under the impression that it was pretty much the way my old boss taught me, Stevie, he said, it's just an amplifier, find the bad parts and replace 'em. This has served me well most of my repair tech career. However, I was reminded of an experience long ago shortly after starting my tech life that I had with a McIntosh MC2300. One was brought into the store for service, one channel out, by a local bar that used it for low end on their system. This was my first Mac, so I decided to go to local dealer and pick the brain of their tech. He saw me comin a mile away and boy did he have a laugh at my expence. Either he thought his advise would eventually lead to me to bringing the amp to him to fix, ('cause ya know, he was the authourized repair tech after all) or he was a sadistic ba----d and just wanted to mess with me. Anyway, I wanted to order replacement parts from Mac for it, so I thought I'd just go over there to dealership and borrow a manual and write a parts order for them to sell me. He said that was just silly, "It's just an amp", "go over to Radio Supply (local parts house) and just buy ECG substitues for 'em. They'll work just fine!!" Well, you can guess, maybe, how much time and how many subsitute parts I went through before I realized the gain on some of the replacement driver transistors was soooo much higher than the specs for the original, that the amp was going into high freq oscilation and blowing up, again, before I got it half way up to power on the variac after replaceing all those parts....more than once, that's all I say. Guess what? I ordered all the parts from Mac, replaced the crap I had put in, and lowe and behold, it worked.
This is my first Mac all over again, only a little different. It's a Threshold, and I'm not familiar with them at all. Closest I ever got was fixin' a Nak PA-7 a few years ago. Anyway, I'm ready to eat my crow now, medium well please, and humblly ask again for any help you guys, who I know are here, are willing to share with an old fart who really doesn't know crap on this subject. Forgive me, this was an amp for me, first step in rebuilding my stereo that I was forced to sell over 4 years ago. NO SYSTEM FOR FOUR YEARS!!! You are amoung the only people on earth who really understand how anxious I really am to do this, and do it right. Thanks for being patient with me. Stevie
Although I did manage to get this amp up and runnin' by myself, I was under the impression that it was pretty much the way my old boss taught me, Stevie, he said, it's just an amplifier, find the bad parts and replace 'em. This has served me well most of my repair tech career. However, I was reminded of an experience long ago shortly after starting my tech life that I had with a McIntosh MC2300. One was brought into the store for service, one channel out, by a local bar that used it for low end on their system. This was my first Mac, so I decided to go to local dealer and pick the brain of their tech. He saw me comin a mile away and boy did he have a laugh at my expence. Either he thought his advise would eventually lead to me to bringing the amp to him to fix, ('cause ya know, he was the authourized repair tech after all) or he was a sadistic ba----d and just wanted to mess with me. Anyway, I wanted to order replacement parts from Mac for it, so I thought I'd just go over there to dealership and borrow a manual and write a parts order for them to sell me. He said that was just silly, "It's just an amp", "go over to Radio Supply (local parts house) and just buy ECG substitues for 'em. They'll work just fine!!" Well, you can guess, maybe, how much time and how many subsitute parts I went through before I realized the gain on some of the replacement driver transistors was soooo much higher than the specs for the original, that the amp was going into high freq oscilation and blowing up, again, before I got it half way up to power on the variac after replaceing all those parts....more than once, that's all I say. Guess what? I ordered all the parts from Mac, replaced the crap I had put in, and lowe and behold, it worked.
This is my first Mac all over again, only a little different. It's a Threshold, and I'm not familiar with them at all. Closest I ever got was fixin' a Nak PA-7 a few years ago. Anyway, I'm ready to eat my crow now, medium well please, and humblly ask again for any help you guys, who I know are here, are willing to share with an old fart who really doesn't know crap on this subject. Forgive me, this was an amp for me, first step in rebuilding my stereo that I was forced to sell over 4 years ago. NO SYSTEM FOR FOUR YEARS!!! You are amoung the only people on earth who really understand how anxious I really am to do this, and do it right. Thanks for being patient with me. Stevie
Anadigit, that looks like it could be the original! You did that board? Good job from the looks of it, great work! So are you building a compete S/550 from scratch? That's a hell of a tall order. Good luck. Do you have pics of entire amp, I saw the output section pic in your last reply, did you make that too?
Stevie
Stevie
Hi,
No, Geoge your schematic that's not S/550E, it's only S/550.
Take a look at OPTO of them !
Now I have original S/550E on hand, too
I took a photo from scratch S/550E and my Threshold Fet 10/E , I'm never build with a large or huge heatsink (it's only my plan)
Thanks
Anadigit
No, Geoge your schematic that's not S/550E, it's only S/550.
Take a look at OPTO of them !
Now I have original S/550E on hand, too
I took a photo from scratch S/550E and my Threshold Fet 10/E , I'm never build with a large or huge heatsink (it's only my plan)
Thanks
Anadigit
Attachments
Hey George and Anadigit
Thanks George, front end schematic tells me a lot about what to look at. Would really like to converse with someone who has experience setting up one of these, bias, and offset. They just really don't seem stable enough for me to believe that there is not something wrong still. Maybe I'm wrong, hope I'm not, but willing to be wrong and get schooled. Again, thanks for taking time to send me prints. Stevie
Thanks George, front end schematic tells me a lot about what to look at. Would really like to converse with someone who has experience setting up one of these, bias, and offset. They just really don't seem stable enough for me to believe that there is not something wrong still. Maybe I'm wrong, hope I'm not, but willing to be wrong and get schooled. Again, thanks for taking time to send me prints. Stevie
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