Hints on maintaining / improving my Stax SRM-T1S

[sc351cobra]

Kevin

Thanks I saw a picture of them in JimL mod. I was able to get in touch with a fellow I had used for in the past

to repair a Lumley tube amp made in UK. When I told him what the amp was doing his first thought was that it was

a front end problem with the fets that he no longer could get. This did not instill me with a lot of confidence.

I could have him replace the first before going further into the repair and replacement of all the caps. Or will he be

to test them?

 

Grawk

I believe the blue hawaii amp is pro bias only. My SR-x MK3 headphones are normal. There quite old made in about 1978

[kevin gilmore]

front end fets are definitely available.

measure the resistance of the plate resistors first.

put it on the bench and measure the dc output levels after 30 minutes.

[spritzer]

Any amp can be made with any bias so a Blue Hawaii with normal bias is not a problem. 

[sc351cobra]

Just ask for both normal and pro when you order?

 

Didn't realize that was a custom made amp

[plaurids]

20 hours ago, sc351cobra said:

Kevin

Thanks I saw a picture of them in JimL mod. I was able to get in touch with a fellow I had used for in the past

to repair a Lumley tube amp made in UK. When I told him what the amp was doing his first thought was that it was

a front end problem with the fets that he no longer could get. This did not instill me with a lot of confidence.

I could have him replace the first before going further into the repair and replacement of all the caps. Or will he be

to test them?

 

Grawk

I believe the blue hawaii amp is pro bias only. My SR-x MK3 headphones are normal. There quite old made in about 1978

 

Yeah, if you look at the picture of my SRM-T1S's internals (before recap and the CCS mod) in the first post of this thread, these are the eight large pink resistors R27-R30 (x2 each) on the upper left. These are 33 kOhm, 2W power rating each according to the schematic, and are the ones replaced by the CCS array if you do JimL's mod. Spares for these from a good brand (e.g. Vishay) and (say) 1% tolerance cost at e.g. Mouser from USD 0.85 to USD 3.58 each (the latter is the low-noise option, if that matters), so we're talking here about USD 7 - USD 29 in parts (plus shipping).

As for the front-end FETs, if I understand correctly from the schematic, these are the pair of double N-channel 2SK389GR MOSFET's from Toshiba placed at Q1 for each channel, next to the signal input (more precisely, the volume pot). In this case, according to Mouser these are indeed discontinued, but as Kevin said above it may be possible to find them in stock (probably as NOS) elsewhere nonetheless (edit: or, as Pars suggested below, just replace each 2SK389GR with a pair of LSK170 JFETs - a matched quad costs about USD 30-35).

If you ask me, it's worth a shot... maybe trying to replace the plate resistors first (since it's probably cheaper) and from there to these MOSFET's if the problem persists.

@grawk silly me... I guess any e-stat must sound fantastic on the BHSE, of course (provided you can afford the latter)... However, HeadAmp doesn't seem to provide the normal bias option to buyers, only color options for the cabinet and two volume pot options. Since they build it on demand after each order is placed, is it possible to request a normal bias as well? (according to spritzer above, it's technically possible, but the point is whether HeadAmp will do it at the buyer's request or not)

Edited May 5, 2022 by plaurids
Removed spurious link due to Pars's warning below

[Pars]

That Irish supplier you linked to is a guaranteed fake. And it is pretty doubtful that the input FETs are bad to begin with. JFETs aren't particularly static sensitive. Unless they've been abused in some fashion, I don't think these are the problem. The 2SK389s could be replaced by pairs of Linear Systems LSK170s. I'm not sure if LSK389s are available, but certainly not in the Toshiba 7-pin package type.

[plaurids]

1 hour ago, Pars said:

That Irish supplier you linked to is a guaranteed fake. And it is pretty doubtful that the input FETs are bad to begin with. JFETs aren't particularly static sensitive. Unless they've been abused in some fashion, I don't think these are the problem. The 2SK389s could be replaced by pairs of Linear Systems LSK170s. I'm not sure if LSK389s are available, but certainly not in the Toshiba 7-pin package type.

Thanks for the warning Pars, and sorry to all for the bad link, I'll delete it in my previous post. Using a pair of LSK170's for each channel would mean that pin 4 in each Q1 PCB pin layout (corresponding to the extra "substrate" lead of LSK389GR) is discarded, I suppose.

Edited May 5, 2022 by plaurids

[grawk]

@justinis here, you can ask him if he'd do a bhse with normal and pro.

 

I think the guy that does moljnir audio is here too, he might be willing.

[plaurids]

4 hours ago, grawk said:

@justinis here, you can ask him if he'd do a bhse with normal and pro.

 

I think the guy that does moljnir audio is here too, he might be willing.

Whoa, I didn't know that, about HeadAmp being around HC... However, as for Mjölnir Audio, I don't remember seeing a Blue Hawaii implementation in the current version of spritzer's catalog...

Edited May 5, 2022 by plaurids

[spritzer]

No, I don't have any BH builds. 

[sc351cobra]

Just wanted to update everyone. I was able to get my amp fixed, My old audio friend check everything and replaced both tube sockets. I have been listening to it for about a week now and it sounds great with my SRX MRK3 phones.

Thanks for all the help

[plaurids]

Hi all,

Against all odds, I've managed to get a pristine SR-007 Mk1 (S/N 71169). Let's see how it performs on my CCS-modded SRM-T1S... I'll update my impressions in the upcoming days.

Edited July 22, 2022 by plaurids

[plaurids]

Ok, first impressions of the SR-007 Mk1 on my CCS-modded SRM-T1S - this post will probably be updated as my assessment of the combo evolves. First of all, the physical impressions:

• First of all, the 007 Mk1 feels slightly lighter on the head than the 007A. I haven't weighed both units on a proper scale to be sure yet, but that's the first thing I noticed.
• The second thing I noticed is that the 007 Mk1 drivers feel farther away from the ears than the 007A's. This matches spritzer's assessment waay back in 2008 that the 007 Mk1's earpads are more angled than the 007A's, despite the 007 Mk1's earpad springs being wider and less angled than the 007A's.
• Most importantly, the earpad seal on the 007 Mk1 is much better than 007A's. As a side effect, though, its Stax farts are loud, more that the 007A's after the blu-tac mod, which makes finding the right fit somewhat annoying. On the other hand, the 007A's earpads are more comfortable and have a more pleasant touch to the skin. This may make 007A a set more suitable for longer listening sessions. I may try later to use the 007A earpads on the 007 Mk1 to see if the increased comfort is worth the ensuing change in sound, this seems doable...
• (edit - July 21st 2022) I found that the clamping of the headband arcs on my head was too loose and couldn't maintain the fit. Maybe the elastic band is also loose? Visually it doesn't seem so... As a result, I kept fiddling with the fit and the ensuing Stax farts were driving me nuts! Fortunately, a gentle additional bend on the arcs has improved the fit considerably - now the set seems to stay at the right position, the arcs no longer touch my head and thus I no longer have to deal with deafening Stax farts so often. Still evaluating whether the arc setup has achieved its optimal shape for my head or not, but results so far are encouraging. The set feels more comfortable now, perhaps I won't need to swap earpads with my 007A. Also considering slightly shortening the elastic band so that the set is kept higher on my head, let's see...

As for the sound impressions... wow. I didn't imagine there would be that much difference. I reckon my findings below have mostly been reported ad nauseam by others on this and other forums already, but again, this is mainly for my own record and I find it convenient to have it all in one place, so I apologize in advance:

• The soundstage definitely feels wider on the 007 Mk1 than on the 007A. This is particularly noticeable in live tracks such as Blue Öyster Cult's "Veteran of the Psychic Wars" from "Extraterrestrial Live" or opera tracks such as Sir Georg Solti's "Don Giovanni, a cenar teco m'invitasti" from Mozart's "Don Giovanni". This corroborates my impression above about the drivers feeling farther away from the ears on the former.
• Instrument separation and fine sound details are more evident on the 007 Mk1, with better resolved low-volume information. Put differently, the 007A sounds darker (more details on that later). For instance, unlike with the 007A, with the 007 Mk1 I can clearly hear the intro's bass line on Metallica's "Master of Puppets" (the DCC 24K Gold remaster, of course - the standard master is obscenely compressed, as most of Metallica's tracks, a trend that got even worse in more recent albums. OK, end of off-topic rant). In the same spirit, I hear a better defined bass line on Rage Against the Machine's "Bullet in the Head" with the 007 Mk1.
• Imaging also feels slightly more precise on the 007 Mk1, take e.g. La Venexiana's "Si ch'io vorrei morire" and "Io mi son giovinetta" from Monteverdi's 4th Book of Madrigals, or even Bill Evans's version of his "Waltz for Debby" from the album of the same name.
• All of the above probably have to do mostly with the driver position and the far superior earpad seal on the 007 Mk1. One thing that's probably due to something else, though (i.e. tuning?), is the frequency response: even after the blu-tac mod, the 007A retains a slight damping in the upper mids (don't get me wrong, it gets much worse without blu-tac) which still bothers me in some tracks, especially ones with some degree of compression. I've tried to correct this with EQ, but that caused other problems that made the ensuing sound less natural to me, so I gave up that route. This problem is completely absent on the 007 Mk1 - to a fault: (edit - July 22nd 2022) after listening to it for a somewhat longer period, it comes out in fact as a bit shouty in the high mids / lower treble. This alone makes the 007 Mk1 a bit more tiring for longer listening sessions than the more relaxed 007A, even if the latter's characteristic comes at the above price.
• Bass still feels slightly stronger on the 007A than on the 007 Mk1, despite the blu-tac mod on the former (which controls the stock 007A's wonky bass response) and the better earpad seal on the latter, but better resolved on the 007 Mk1.

On the other hand, regarding timbral quality the 007 Mk1 and the 007A seem to me so far unsurprisingly quite close to each other, and so does their transient response. You can "taste" the texture and materials of acoustic instruments (particularly guitar, bass and percussion) in a similar way with both units. That's top-notch e-stat speed for you. The 007 Mk1 feels very slightly more resolving, though, but that may be just me - I need more time listening and comparing both. To sum it up so far, it seems that both sets have essentially the same drivers, but with slight differences in tuning. In any case, these are rather minor differences if compared to the former listed above.

Overall, now I get the hype about the 007 Mk1. It's definitely a superior headphone to the 007A in most ways that matter. I'm still evaluating the effect of the 007 Mk1's earpad seal and materials on the long-term comfort; it may be the case that the 007A ends up being more comfortable for long listening sessions - or not. As pointed above, swapping earpads may also settle this issue.

Some claim that the latest (i.e. currently being sold new) 007 Mk2 / 007A is close enough to (maybe even slightly better than?) the 007 Mk1, with many of its earlier versions' problems having been solved, to the point it may make no sense anymore to overpay for a 007 Mk1 (see, on the other hand, how spritzer's opinion on this has evolved compared to the previous two links, to quote one example - wondering here what made him bump the 007 Mk1 above the current 007 Mk2 / 007A in his personal preference between 2018 and 2021 🤔 ), but mine was still a bargain compared to a brand-new 007 Mk2 / 007A, at least here in Brazil (especially considering it came with some Mk1 spare parts and the standard flight case - by the way, this is what the actual carbon fiber case for the very early 007 Mk1's looks like, completely different from mine) and I think I got somewhat lucky with the serial number, so I guess that's a win? 😁

As for the role of the CCS-modded SRM-T1S proper in all this, my impression is that the CCS mod together with the 007 Mk1's efficient earpad seal are usually enough to make the SRM-T1S able to drive these cans in the volumes I typically use (10-11 o'clock, depending on the source), which is good since it'll be quite a while until I can get another, more powerful energizer (if I can get it at all), which may help in a few, more compressed tracks for which I need to raise the volume to noon or higher and hence start hitting the 6CG7's limitations on the SRM-T1S's design. On the other hand (edit - July 22nd 2022), the aforementioned shoutiness of the 007 Mk1 indicates that I may be listening to it at a bit too high a volume, but otherwise I lose resolution and clarity with both 007's. This should be compared to the 9-10 o'clock optimal volume setting of the SRM-T1S with the Lambda SR-L407 and hence signals that the 007's are still underpowered by the SRM-T1S, even after the CCS mod. I may get some minor additional improvements once I change tubes to (say) some Japanese Raytheons (I have a few lying around waiting for their turn), but right now I don't feel compelled enough to do it. I've given up for the time being the idea of rewiring the tube sockets and moving to brand-new JJ ECC99 tubes, based on spritzer's recent advice on the matter.

(as both a side note and a cautionary tale, I noticed an improvement in sound quality and an increase in bass response on the 007A, a few weeks before getting the 007 Mk1, after I changed my glasses - the new frame is much less obtrusive, thus affecting far less the earpad seal. However, I didn't realize the new frame was the "culprit" until the 007 Mk1 arrived, which made me understand more viscerally the importance of a good earpad seal for sound quality in headphones)

Edited August 8, 2022 by plaurids
Added links and updated impressions, added update on headband arc clamp issue

[padam]

Your description regarding Mk1 vs Mk2 is spot-on. I would add that the narrower sound of the Mk2 adds a slightly better sense of height in my opinion.
Personally, I found that the tighter arc assembly makes the Mk2 seal better (and farting much more after the mod), requiring way less bending, but if the seal and positioning are good enough, then it does not change the sound any further. I tried pad or spring swapping, and I found it is best to leave everything how it was.

What I don't agree about the CCS T1S, which I used to have, still have a stock SRM-T1W, nice amps with a nice sound signature (doing a good job of "plugging in" some of that upper-mid dip or to draw a nicely sized stage), but they are just inadequate to drive these really well, there is a whole lot more to gain with regards to dynamics, transients, clarity and transparency and probably some more minor things that I haven't observed yet. Plus, opening up the bass region means it becomes much easier to listen at a lower volume. (Higher volume was never a reason why I wished for more powerful amplifiers, in case of a 007, it has more to do with softness, congestion and rounded-off edges at both ends)
I currently use a KGSSHV Carbon and I was really surprised that the fight has been reversed. Up to this point, the Mk1 used to come out on top every time, being cleaner, smoother, more airy, etc.. But this amp has more of an analytical take on things coupled with smoothness and refinement, and the slightly rougher, livelier character of the Mk2 comes out as the winner, keeping a good balance between liveliness and smoothness. The Mk1 still has its advantages, but it requires careful matching (probably an even more exquisite amp with some nice tubes) because in this setup it actually sounds bland and lifeless (and the staging isn't as nicely laid out as with others - minor point).
So in the end, debating between versions might not matter that much, as long as the rest is nicely set up to it.

[plaurids]

19 hours ago, padam said:

Your description regarding Mk1 vs Mk2 is spot-on. I would add that the narrower sound of the Mk2 adds a slightly better sense of height in my opinion.
Personally, I found that the tighter arc assembly makes the Mk2 seal better (and farting much more after the mod), requiring way less bending, but if the seal and positioning are good enough, then it does not change the sound any further. I tried pad or spring swapping, and I found it is best to leave everything how it was.

You may be right regarding sense of height, I'll pay closer attention to that in future comparisons and update here after that.

Also... Your Mk2's seal is better than your Mk1's? Interesting... My 007A got farty after the blu-tac mod, but nowhere as farty (or as loud doing that) as my 007 Mk1. I guess I got lucky with my 007A in that regard - its long-term comfort is flawless, I basically put it on my head and forget it's there.

I'm still experimenting with bending the arcs on my 007 Mk1, until I manage to keep the set at the right position. Since the farts are so loud, it gets really annoying to keep adjusting the fit, but I do appreciate both the fact that the seal on my 007 Mk1 is so good and the augmented soundstage due to the angle on the earpads. That's why I'm still hesitating to swap earpads, also because the 007 Mk1's earpads are reportedly more difficult to reassemble than the 007A's due to the wider springs.

Quote

What I don't agree about the CCS T1S, which I used to have, still have a stock SRM-T1W, nice amps with a nice sound signature (doing a good job of "plugging in" some of that upper-mid dip or to draw a nicely sized stage), but they are just inadequate to drive these really well, there is a whole lot more to gain with regards to dynamics, transients, clarity and transparency and probably some more minor things that I haven't observed yet. Plus, opening up the bass region means it becomes much easier to listen at a lower volume. (Higher volume was never a reason why I wished for more powerful amplifiers, in case of a 007, it has more to do with softness, congestion and rounded-off edges at both ends)

I currently use a KGSSHV Carbon and I was really surprised that the fight has been reversed. Up to this point, the Mk1 used to come out on top every time, being cleaner, smoother, more airy, etc.. But this amp has more of an analytical take on things coupled with smoothness and refinement, and the slightly rougher, livelier character of the Mk2 comes out as the winner, keeping a good balance between liveliness and smoothness. The Mk1 still has its advantages, but it requires careful matching (probably an even more exquisite amp with some nice tubes) because in this setup it actually sounds bland and lifeless (and the staging isn't as nicely laid out as with others - minor point).

So in the end, debating between versions might not matter that much, as long as the rest is nicely set up to it.

A couple of interesting points you made above. Indeed with my CCS-modded SRM-T1S both my 007A and my 007 Mk1 get too "dark" at lower volumes (9 o'clock and lower), whereas my older Lambda keeps sounding just fine at 9 o'clock. The problem rises to 10-11 o'clock with more compressed tracks, and is somewhat worse with the 007A. This is the clearest indication that the SRM-T1S is still somewhat underpowering these sets even after the CCS mod, but I concur that this energizer is completely inadequate for driving a 007 (of any generation) without this mod.

I also have the impression, after listening for a somewhat longer period, that the 007 Mk1 is in fact a tad too shouty in the high mids / lower treble at the "optimal" volume setting of 10-11 o'clock as compared to the more (sometimes a bit too much) relaxed 007A - yet another sign of underpowering. With a more powerful amp, I could probably listen to them at lower volumes (with ensuing greater sonic comfort, in the case of the 007 Mk1) without losing resolution or clarity.

Which brings me to the point of which would be the ideal match. Much has been said about how well the 007 Mk1 pairs with the BHSE (that was, by the way, the combo used by my 007 Mk1's former owner); among all the expected improvements, I reckon this combo probably doesn't sound as shouty. The fact that the 007 Mk2 sounds better to you with the Carbon than the Mk1 makes sense since it's a SS circuit, however warm it may be as far as SS goes (thanks to the SiC output FETs). However, the BHSE is far beyond my budget right now (and I don't even have enough room for it due to its separate power supply), as is the Carbon. That's why the KGST actually sits higher on my wish list than the BHSE, since I still crave the warmth of the tube sound but don't want to get a mortgage and/or remodel my office for its sake. For the time being, though, I'll have to make do with what I have, so I'm trying to make the most out of it.

Edited July 22, 2022 by plaurids

[padam]

Yes the Mk2 seals way better for me and easier fitting. Maybe at some point I will need more cutting on the slightly stretched headpad on the Mk1, but the slight looseness of the Mk1 makes it disappear on the head a bit better. But as I said, as long as the seal is not broken, less or more farts don't make an audible difference to me.

And I think the drivers are not quite the same and sound different (when Spritzer completely modded the Mk2 to be like the Mk1, it still remained different). Personally I think the Mk1 driver has a brighter, more evenly distributed, polite and airy, open sound, while the Mk2 is more focused, lively and upfront. The Mk2.0 can be a bit shouty in the mids, but on the Carbon I don't hear it anymore.

The limitation regarding drive is dynamics. Just not a very well defined sound and the dynamics are flat. It's not like it's bad, I've been listening like that for a decade I think and it was still enjoyable to me as I didn't hear it any better. When I heard the KGSSHV I was surprised, with the Carbon I sort of new what I can expect in some aspects so the appreciation was slower-hitting, but I was surprised for a different reason. (not enjoying the SR-007 Mk1 or the SR-Omega as much as I imagined, while other phones are just plain better than they were before)

I think there will be more used BHSEs over the market as more T2s come into fruition. For the 007 in general, even though I have not auditioned it yet, I have spoken to several people (including the original Carbon owner, who does have a BHSE and several copies of the 007) and I am quite confident I would gravitate towards the BHSE over the Carbon and I would choose the Mk1 version. The midrange is probably going to be more natural and the stage will open up. But with the Carbon I have much more appreciation for the Mk2's charm.

Edited July 22, 2022 by padam

[plaurids]

On 7/22/2022 at 4:08 AM, padam said:

Yes the Mk2 seals way better for me and easier fitting. Maybe at some point I will need more cutting on the slightly stretched headpad on the Mk1, but the slight looseness of the Mk1 makes it disappear on the head a bit better. But as I said, as long as the seal is not broken, less or more farts don't make an audible difference to me.

Indeed the amount of farts has no impact on the sound once the seal is established (man, this sentence sounds so wrong out of context... 🤪 ), the problem is that my 007 Mk1's farts are borderline painful to the ear and will creep in whenever I have to reposition the unit on my head, which is quite often if the set is loose and cannot maintain its fit.

On 7/22/2022 at 4:08 AM, padam said:

And I think the drivers are not quite the same and sound different (when Spritzer completely modded the Mk2 to be like the Mk1, it still remained different). Personally I think the Mk1 driver has a brighter, more evenly distributed, polite and airy, open sound, while the Mk2 is more focused, lively and upfront. The Mk2.0 can be a bit shouty in the mids, but on the Carbon I don't hear it anymore.

Wouldn't at least a large part of it amount to tuning? Technically I don't know what needs to be done in an e-stat driver such as these to change tuning, apart from the obvious (and infamous) port on the 007 Mk2 / 007A. What's objectively true is that the measured frequency responses of the 007 Mk1 and the 007 Mk2 with the blu-tac mod are still different, which shows that the port alone cannot account for all differences in that regard. Granted that the measured 007 Mk2 unit in the link above is the latest (2.9) version and hence different from my 007A (which, by the serial number, is likely from the first production run), but you get the gist. It's possible that the earpads' differences in geometry may also have an impact on tuning, but I believe it's minor (I think it mostly affects the soundstage and the imaging, supposing all seals are equally good). If I get the nerve to swap the earpads, I'll be able to somewhat settle this question (at least in my case).

On 7/22/2022 at 4:08 AM, padam said:

The limitation regarding drive is dynamics. Just not a very well defined sound and the dynamics are flat. It's not like it's bad, I've been listening like that for a decade I think and it was still enjoyable to me as I didn't hear it any better. When I heard the KGSSHV I was surprised, with the Carbon I sort of new what I can expect in some aspects so the appreciation was slower-hitting, but I was surprised for a different reason. (not enjoying the SR-007 Mk1 or the SR-Omega as much as I imagined, while other phones are just plain better than they were before)

I've noticed a considerable upgrade in dynamics after the CCS mod on the SRM-T1S, but I do realize there are still some shackles binding my 007's with this energizer. I don't really consider using the 007 with a volume higher than 10 o'clock something normal - for instance, a Lambda SR-L407 paired with the SRM-T1S sounds fine with one hour less volume (as it should, since the latter was indeed designed to be used with the former) -, and the aforementioned limitations on both the 007A and the 007 Mk1 are evidence of that.

On 7/22/2022 at 4:08 AM, padam said:

I think there will be more used BHSEs over the market as more T2s come into fruition. For the 007 in general, even though I have not auditioned it yet, I have spoken to several people (including the original Carbon owner, who does have a BHSE and several copies of the 007) and I am quite confident I would gravitate towards the BHSE over the Carbon and I would choose the Mk1 version. The midrange is probably going to be more natural and the stage will open up. But with the Carbon I have much more appreciation for the Mk2's charm.

Let's see... But I don't think these will lose too much value even if that happens, because the demand is high despite the BHSE's price. Problem for me is, these Mjölnir Audio / HeadAmp energizers never show up in the used audio market here in Brazil, and our custom taxes are no joke. Again, I'd be happy with a used KGST already... 😔

Edited July 25, 2022 by plaurids

[plaurids]

Ok, a short update on my impressions about the 007 Mk1 after roughly two months of brain burn-in. As before, this post will be updated again in a few days, this time because my 007A got a bit of mold on the earpads and needed a rather thorough cleaning. Sitting now inside a sealed plastic bag with a couple of bags of silica gel for a few days to remove any excess humidity that may still be lingering, so any further comparisons will have to wait.

• The fit / Stax fart issue has largely been solved. Bending the arcs in the headband assembly a bit further helped a lot, and I finally found the optimal angle for the earpads so that the drivers stay in place comfortably and with a good seal. This somehow largely reduced the Stax fart problem, not only (albeit mainly) because I no longer keep fiddling with the fit, but also because the farts are no longer so loud in the current optimal angle for the earpads. For reference, one needs to position the lateral sewings on the earpads facing front but very slightly downwards (at least for my head and ears).
• Most of my initial impressions on the 007 Mk1 still stand, and now I like it even more. Bass on the 007 Mk1 is actually very impressive, all the more because now the the bass details sank in. The optimal positioning of the earpads must also have helped somehow. As for the aforementioned shoutiness in the upper mids, there is something funny happening - either I got used to it or it got a bit damped somehow compared to two months ago. It feels more pleasant but I find myself pushing the volume of the energizer a tiny tad up in some tracks. In retrospect, since it has been more than a year since I did the CCS mod on my SRM-T1S, it may be time for checking for tube bias drift. I may even take the opportunity and start tube rolling, let's see...

[plaurids]

Some bad news...

First, a short prologue: the need to raise the volume on my CCS-modded SRM-T1S has increased in the last couple of weeks, to a point it could no longer be ignored. This meant that either the tube bias drifted and needed to be adjusted, or the (remember, stock) tubes were reaching EOL. Thus, today I decided to try a rebias session to see if the tubes could hold it together - if not, tube rolling is in order. First differential (+/-) measurements indicated a bias drift of 1-2 volts that couldn't be pulled down enough by pot adjustments, so I decided to proceed to changing the tubes. After a while with the energizer turned off, I decided just out of curiosity to see how the DC offset was measuring... and then it started.

Both channels are now measuring DC offset between 150 and 300 volts, with a differential measurement of over 600 volts (!) Curiously, both channels seem to be measuring the same at similar pot positions.

My previous experience with the CCS module installment suggests a short circuit is happening somewhere, but this time on both channels. As a first sanity check, I've checked if some power supply electrolytic cap (among the ones I'd replaced) or some of the polypropylene blue caps (far less likely, but just in case) were shorted, neither of which was the case. I've also tried replacing tubes, to no avail.

Tomorrow I'll go through the PCB again with a good light to check for potential short points, but I'm getting worried some component has gone bad and needs to be replaced. Any ideas about what could affect both channels equally in the aforementioned manner?

[spritzer]

Check the resistors in the power supply, they are known to go bad. 

[plaurids]

1 hour ago, spritzer said:

Check the resistors in the power supply, they are known to go bad. 

Thanks spritzer - do you mean the big green ones (R31-32, 15kOhm, 5W) and/or the big blue ones (R33-34, 15kOhm, 3W) around the electrolytic caps C9-10 in my PCB? I'll detach them from the PCB and measure them tomorrow.

Supposing one or more of them is the culprit, which kind / brand do you recommend? The SRM-T1 schematic states these should be metal oxide resistors (although R31-32 seem wirewound visually), which are usually available (e.g. at Mouser) with 5% tolerance. Is this enough? Are metal film resistors with the same specs OK for this as well? Or should I go for wirewound resistors to get a tighter 1% tolerance instead?

Edited October 5, 2022 by plaurids

[spritzer]

It really doesn't matter, 5% is plenty good so just go with what ever fits. 

[plaurids]

The plot thickens...

I confess I'd never paid attention to the ratings of R31-34 as compared to the SRM-T1 schematic... To my surprise, they are actually rated as follows:

• R31-32: 8.2kOhm, 5% tolerance (code letter J);
• R33-34: 8.2kOhm, 5% tolerance (4 code strips - grey, red, red, golden),

and not 15kOhm as in the schematic. No idea about their actual power ratings. As for the measured resistances (detached from the circuit, of course):

• R31-32: 8.15kOhm;
• R33-34: 8.0-8.1kOhm.

This raises a bunch of new questions:

1. Why the difference in the ratings as compared to the SRM-T1 schematic? Did Stax intend to make this change in the SRM-T1S circuit? The R31-34 resistors in this CCS-modded SRM-T1S look exactly the same as mine, including their ratings.
2. The R31-34 resistors measured within 5% of their ratings, which means they are (probably) OK (for their intended use). If so, why were the bias voltages normal when I installed the CCS module about a year ago, but now no longer so? This means that R31-34 are probably not the culprit, regardless of the difference in their ratings as compared to the SRM-T1 schematic. If so, what is actually causing the problem then?
3. Since R31-34 are subject to a fair amount of power stress, it may be advisable to replace them anyway due to their age. Should I keep their ratings, or go for 15kOhm resistors as in the SRM-T1 schematic?

I'll try to check again for short points in the next few days...

Edited October 5, 2022 by plaurids

[spritzer]

My schematic shows 8.2K for that position so not sure what schematic you have. 

Also check R44 and R45 if the voltages look a bit low. 

[plaurids]

On 10/6/2022 at 3:03 AM, spritzer said:

My schematic shows 8.2K for that position so not sure what schematic you have. 

Also check R44 and R45 if the voltages look a bit low. 

Hmm... The one I have is this low-quality PDF scan of the original SRM-T1 schematic, the only one I could find (the same as this one). It's reproduced with higher quality (but less info on the components) in JimL's AudioXpress article on his CCS mod (particularly, there R31-34 are rated at 15kOhm as well). Couldn't find one for the actual SRM-T1S (would love to, really), so it seems this is something that has really changed in the SRM-T1S circuit. Or maybe the schematic I have is for the first-generation SRM-T1...?

Just to be sure in case I actually decide to replace R31-34, may I assume the power ratings are still the same (i.e. 5W for R31-32 and 3W for R33-34)?

Regarding R44-45 (the small 150 Ohm resistors right next to the big C11-14 caps), I'm not sure about what you mean... Which voltage measurements should I watch out for being low?

Anyway, I'll have a look at these resistors as well... That may take a few days, though, for I have quite a bit of work to catch up to... Thanks!

Update (October 6th '22): In due time, R44-45 are measuring both 150 Ohms as they should, so they're not the culprit either. I also couldn't find any short points yet. I'll try to partially remove the CCS module to see if something in its PCB is shorted, as it happened a year ago. If there's nothing happening there, I'm out of ideas...

Update (October 10th '22): no shorts on the CCS module's PCB, all measurements therein OK. Checked the safety output resistors, also OK. No apparent shorts elsewhere on the PCB. I'll try and replace R31-34 anyway, also because one of R34's leads was fragile and broke when I was putting these resistors back, but now I have no idea what could be the cause of the problem... I could really use an updated schematic for the SRM-T1S, any hints?

Edited October 10, 2022 by plaurids