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Threshold S/500 bias set up anyone?
Threshold S/500 bias set up anyone?
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Old 28th July 2009, 06:52 AM   #1
stevieg123 is offline stevieg123  United States
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Default Threshold S/500 bias set up anyone?

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
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Old 28th July 2009, 08:49 AM   #2
analog_sa is offline analog_sa  Europe
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Hi Steve

I believe this has been discussed a few times but from what i remember: cold bias 145mV, hot bias 100mV.

The L112 were my first "serious" speakers when i was at uni. Sweet memories.
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Old 28th July 2009, 02:56 PM   #3
stevieg123 is offline stevieg123  United States
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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 ( ) 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
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Old 28th July 2009, 05:50 PM   #4
ANADIGIT is offline ANADIGIT  Europe
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Default No problem !

I have Threshol S/550E on hand and cloned it already !
I learned all of its schematic and trouble shooting, please you take a photo of your Amp ?

I can suggest you to repair step by step.

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Old 28th July 2009, 06:34 PM   #5
stevieg123 is offline stevieg123  United States
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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
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Old 28th July 2009, 10:50 PM   #6
georgehifi is offline georgehifi  Australia
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Go here Stevie, and there's some circuits for you too. Remember the search tool is your friend.

Cheers George
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Old 28th July 2009, 11:57 PM   #7
Nelson Pass is offline Nelson Pass  United States
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Threshold S/500 bias set up anyone?
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

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Old 29th July 2009, 03:00 PM   #8
stevieg123 is offline stevieg123  United States
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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
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Old 30th July 2009, 05:00 AM   #9
MashBill is offline MashBill  United States
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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.

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Old 30th July 2009, 06:31 AM   #10
stevieg123 is offline stevieg123  United States
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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
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