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The Headcase Stax thread


thrice

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Can I post request for help on my SRM-1 Mk1 here over from the other thread (Spritz has already given a bit of guidance somewhat). I can't figure out what the heck is wrong with this dead left channel. Got measurements all up on a pic once I get the get-go.

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No problem posting it here but the basics are simple enough.  Do a diode test on all the transistors to sort out obvious shorts and then compare the voltages between the channels. 

 

People were pretty impressed by the one channel they could hear. :P  When I got home I felt like an idiot for not utilizing the volume balance on the T1...it's been too long since I've used a Stax amp.  Now that I have that sorted out, they sound great.  I plan to clean them up, open them, and restore them so they work as new again.  Next meet should be interesting...maybe I'll connect it to the Blue Hawaii >:D

 

It is a very nice headphone, lacking in most areas but it just sounds right despite the shortcomings.  I really have to get that really old one I found shipped here as it might be a 150V version...

 

It's through your research that people including me and Milos have esoteric 'stats in the first place :D Lord knows I have way too many vintage 'stats as it is, albeit a miniscule fraction of the amount you've collected over the years. Been meaning to find some new pads for my gutted ESP6 with ESP950 cable and it's been over a year at the very least. Laziness trumps all.

 

 

Hope you figure out how to fix the channel imbalance! And if you do use the BH, don't forget to decrease the bias to 180-200V. I'm not sure if the SR-1 can handle any higher, deferring to Birgir on this one. What are your thoughts on sound, compared to other 'stats and in general?

 

The BH is already set to 230V so it's all good.  :)

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Cool. This convo just made me finally buy the Pro4aa pads. Years after I got the ESP6 I'll finally be able to listen to it!

 

Also, quick q, is Kuboten still the best/cheapest place to order Stax earpads? I will be buying in moderate bulk if I do order, to make shipping from Japan more worth it.

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It's through your research that people including me and Milos have esoteric 'stats in the first place :D Lord knows I have way too many vintage 'stats as it is, albeit a miniscule fraction of the amount you've collected over the years. Been meaning to find some new pads for my gutted ESP6 with ESP950 cable and it's been over a year at the very least. Laziness trumps all.

 

 

Hope you figure out how to fix the channel imbalance! And if you do use the BH, don't forget to decrease the bias to 180-200V. I'm not sure if the SR-1 can handle any higher, deferring to Birgir on this one. What are your thoughts on sound, compared to other 'stats and in general?

As Birgir said, my BH has a normal bias jack on it which seems to work fine without arcing the drivers.  I can't offer much in the way of impressions yet because I can only listen to them for a small amount of time before the humidity in the cups rises and the imbalance gets more severe - at least that's what I think is happening.  I will say they sound a bit rolled off in the bass, a little bright, but they still sound very good.  Impact is really surprising.  I'll try to get some measurements up later.

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Ffs...did the diode test (twice and many combinations following a few guides of measuring the right combos of Case+Emitter+Base etc....

What a headfuck this amp has been and due to my inexperience with troubleshooting these sort of things.

All the 4 2SC and 2SD Sankens all emit about the same numbers throughout the diode tests to each other in pairings.

Then I measured the 6 x 3 legged TO-92 black silicon enclosed 2SK68A transistors near the balance/offset trimpots (as seen above). Two of them measure less than the other 4. 1 of them measures 134 and 78 respectively, the other measures 108 and 78 respectively. All the other's emit numbers around 500+ and 700+.

Then I measured the section with a lot of ceramic film caps and again black transistor looking shit next to the 2 big cap section (left hand corner section in the above photo). Measured every diode in the amp and no sign of shorted diodes with correct readings from having the appropriate probes on the anode and cathode sides. Back to what looks like the bias section, all the transistors measure over 700-1800 range and lower or higher when I reverse the probes of my DMM to measure the same points again.

Gonna upload the rough measurements I took down on a gay diagram I drew soon.

post-4170-0-31140800-1374843805_thumb.jp

Edited by DefQon
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Sigh of relief...

 

Finally found the fucking culprit (and learnt a few things along the way too). 

 

Swapped the Sanken transistors and my original assumption of 2 pairs per channel aligned with where the L and R signal input pads were (traced through the back of the circuit) was right. So swapped the two Sanken pairs around to see if the bad side will emit sound and the right side produce nothing. Nope, nada, still no sound on the Left channel.

 

So I look at the circuit hard again with a bit Heineken in the system I thought hard to myself wtf is happening. So I look hard at the bank of 6 2SK68A jfets (3 for each channel), desolder them each and do a diode test, properly this time. Two of jfets from the Left side gave me weird numbers and didn't match with the right bank of jfets. So how about the old fashion way to confirm my findings, desolder everything and swap the jfets around, this should mean no sound on right channel this time and sound from the left. Resoldered...opened another 6 pack.....

 

Powered the bitch on and VOILA! No sound on right channel, but sound on left channel. Measured everything again to make sure no odd jump in voltage or so. Now time to find some 2Sk68A jfets which are discontinued but ceitron.com ($0.49 a pop) has some and so does ebay ($7 a pop). Think I will finish the rest of the slab early tonight coz am one happy chap. 

 

Cheers Birgir.

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Well that's certainly one way to find the problem... :P may want to replace the old electrolytics while you're at it. And above all, please find better beer than Heineken.

 

n3rdling: thanks for the description! Impact is indeed surprising, and I bet the mids are glorious. That humidity issue actually sounds similar to when I listen to my ET1000 too long, especially with the circumaural pads I'm using from another 'phone. I'm going to try with the original ET1000 pads, which are much more closed off, and see if the same problem occurs. Haven't listened to the bugger (along with most of my other 'phones) in years, so might as well. Please let me/us know if you figure out how to solve the problem.

Edited by mypasswordis
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Yep going to buy a lot of spare k68a's just to keep everything in line incase one goes kaputz again. 

 

Btw, for beer I only drink Heineken, Corona and Carlsberg (and one more local made beer) and there ain't much beer choices down under.

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XXXX is pronounced fourex.

we have a running joke down here that the Queenslanders didn't introduce daylight saving time there for decades because they were worried the extra hour of sunlight would fade their curtains faster.

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XXXX is pronounced fourex.

we have a running joke down here that the Queenslanders didn't introduce daylight saving time there for decades because they were worried the extra hour of sunlight would fade their curtains faster.

 

What does that say for those folks out in Perth who still don't have daylight savings!  ;D

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So update, Pro4AA pads don't fit the ESP6. :( They're also terrible quality so I would've had to find something else anyway, there's a reason Koss sells their pads for $5 shipped.

 

Listening to the ESP6 with O2 pads, they're actually very good, slightly sibilant and too echoey but that's expected since all I did was gut out the internal PCBs and thereby converting them to normal dipole operation. Will play around with damping when I listen to them again a few years from now.

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I never bothered with new pads on them as they were always a torture device.  :laugh:  The ESP6 is also a slightly different shape from the ESP9 so perhaps the Pro4AA pads only fit them? 

 

It's fun to have some sets that are almost never used.  My SR-X Mk1 is that way, can't even remember when I powered it up last... 

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Haha yeah, thankfully my head is barely small enough that the headband doesn't vicegrip my head into oblivion, and of course taking out the PCBs also helps a lot. Don't know why Koss thought clamping 2lbs of headphone to peoples' heads would be acceptable. The HF reviewer in my sig would probably shit his pants putting them on.

 

You could be right about the ESP9, maybe I should eventually get a pair. I did some further online research yesterday and found a pic of an ESP6 with what looks to be ESP950 pads. Koss earpads just aren't very good quality (which is very much expected for $5) so I think down the line I'll just have to pick up more sets of O2 pads or maybe look for some aftermarket pads since paying twice as much for the earpads as the headphones is a little bit crazy. :P

 

Ooh, between the SR-X Mk1 and the SR-1, the rarity level is off the charts. Hope you someday manage to finally snag a SR-2, and let us know how it sounds.

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