[manywelps]

Fun note, when these are warmed up, all the measurements swing by like 8V, so make sure it's been on for an hour or more before you calibrate it!

 

[crin]

did you do your measurements with the lid closed? i registered around 9 volts difference between open and closed. i suggest using a long thin screw driver and adjustment thru the holes in the closed lid.

 

[manywelps]

Yes, the lid was closed for everything...
 
The screws on the case were what I was using for ground.

 

[core]

Hello,
I have a small problem with my amplifier. Sometimes I can hear power supply noise of 50Hz, in one of the earspeaker. If I touch the metal case, the noise is significantly reduced. I do not feel high voltage when I touch the case.
Also, first time I start the amplifier, I hear couple of clicks, then after a minute or less, the clicks are not there.
Any help is appreciated.

 

[G00dfell0w]

Hello all
 
My first post on here, so I'll keep it brief.
 
I'm helping a friend with some old hifi gear left behind by her husband who died a few years ago. One of the items is a Stax SRM1 Mk2, Professional. There are no headphones with it, so I can't test it. Any advice out there? At the moment I'm not sure whether to keep it, but seeing if it works will be a good start!!
 
Many thanks
 
G

 

[mudihan]

Question regarding the 2pf capacitors, C101, 102, 201, and 202. I am trying to find replacement parts, but they don't go as low as 2pf. What would be the highest value I could use there? It's much easier to find, let's say, 0.01uf/1000v parts, but that's thousands times of the original value. Thanks.

 

[scott16]

I have a broken SRM1 MK2 does anyone repair them at a low price, right now I'm using the SRD-7 and a JVC 45W
per channel Class A Amp and it sounds great!.but I don't want to just throw the SRM1 MK2 out since I have had it since 1984 so does anyone repair them for a fairly low price?

 

[432789]

I recommend the Elna Silmic II 35v 470uf cap for the SRM1 MK2 re-cap, Those caps can sing

 

[Loz_the_guru]

Hi all,

Sorry to bump an old thread, but I was hoping for some advice. I've got an SM-1/Mk2, and i'm thinking of replacing the caps. I opened it up the other day and they look okay, but i'm they're also originals so probably have a fair few hours under there belt at this point.

I've looked at the original capacitor recommendations, and also the points people have made since. I'm hoping to replace the small 35v 470uf cap with this:

https://www.mouser.co.uk/ProductDet...ron/F6h2Q3lkqCRBZTusvJUAxvKPNPyUN3DH3N7Tn1w==

However, the larger 400v 100uf cap is causing me a few more issues. The original recommendation has since expired, so I looked at these two options:

https://www.ebay.co.uk/itm/Nichicon...h=item2cc171fb4c:g:ga4AAOSwstJZSlZQ:rk:2:pf:0

or

https://www.mouser.co.uk/ProductDet...GAEpiMZZMtZ1n0r9vR22TrbYqu733Ay8JPh1A%2boc8w=



Any thoughts on either of those options? I've tried to match as closely as possible the original spec sheet.

Also, thanks to spaceace2014 for leaving such detailed instructions to follow in the first place!

 

[BrintaVett]

I Have an SRM1-MK2 Pro which I have re-capped with Vishay 400V-220u and 35V 470u caps. Tried to dial in bias but my Left channel offset is acting weird. I measure a static voltage no matter which way I turn the pot. It also affects the balance pot for the left channel. I've set that to 0.0V DC as well as I could, left offset then gives me 30V.

Broken offset pot or should I look elsewhere?

 

[legopart]

Exellent thered ! I want to bump it because did not got enoth helf with my amp.
I started some thered here https://www.head-fi.org/threads/need-some-suggestions-to-recap-stax-srm-1.905395/#post-14920378
And even try to got some help from here https://www.head-fi.org/threads/the-entry-level-stax-thread.676272/page-207#post-14895781
Instead of trying to send messages to each one of users that could help, I prefer to sent this message in furom.

the Stax Srm1/mk2

I having the Stax Srm-1 Proffesional
from reading all the post here I got understood that I having the older version of this amp
the Stax Srm1/mk2 Proffesional

I start to recap it but scarred to replace caps to different value

back side

I want to replace the pair of serial capacitors from |( )| form 220uF+220uF to one bipolar capacitor of 100uF (from here)
thanks to kevin gilmore that explain me some information about my amp here https://www.head-fi.org/threads/need-some-suggestions-to-recap-stax-srm-1.905395/#post-14920378
I found that the different between my amp Stax srm1/mk2 proffesional and the Stax srm1/mk2 is only based on this caps and if I short them I will got the same bord as Stax srm1/mk2

Planning to replace the rectifier diodes too, for this one https://www.ebay.com/itm/100Pcs-UF4007-1-Amp-1KV-Ultra-Fast-Diode-FSC-VISHAY-MIC-DO-41-NEW/181846953488?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649
or this https://www.ebay.com/itm/10pcs-ON-1N4007TR-High-Speed-Rectifier-Diode-1A-1000V-with-copper-legs/273026392721?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

Planning to replace all the ceramic capacitors to WIMA MKP 0.1uF one, the rectifire and bias will got the 630v the else will replaced with 400v one (now I see that the schematic wrote 500v one for each ceramic capacitor )

I still cant understand why lots of people here replace the 100uF 400v to 220uF.
I just cant do so.
Stax engineers planned this capacitor by mathematically reasons, higer value of capacitor will store and release its stored electricity much faster and will do it job worst for this board (you will get distortions).

Hope for your help.
Thanks.

 

[kevin gilmore]

more info here including corrected schematics and cascade current source

https://www.head-fi.org/threads/nee...-recap-stax-srm-1.905395/page-2#post-14923141

 

[goodvibes]

Exellent thered ! I want to bump it because did not got enoth helf with my amp.
I started some thered here https://www.head-fi.org/threads/need-some-suggestions-to-recap-stax-srm-1.905395/#post-14920378
And even try to got some help from here https://www.head-fi.org/threads/the-entry-level-stax-thread.676272/page-207#post-14895781
Instead of trying to send messages to each one of users that could help, I prefer to sent this message in furom.

the Stax Srm1/mk2

I having the Stax Srm-1 Proffesional
from reading all the post here I got understood that I having the older version of this amp
the Stax Srm1/mk2 Proffesional

I start to recap it but scarred to replace caps to different value

back side

I want to replace the pair of serial capacitors from |( )| form 220uF+220uF to one bipolar capacitor of 100uF (from here)
thanks to kevin gilmore that explain me some information about my amp here https://www.head-fi.org/threads/need-some-suggestions-to-recap-stax-srm-1.905395/#post-14920378
I found that the different between my amp Stax srm1/mk2 proffesional and the Stax srm1/mk2 is only based on this caps and if I short them I will got the same bord as Stax srm1/mk2

Planning to replace the rectifier diodes too, for this one https://www.ebay.com/itm/100Pcs-UF4007-1-Amp-1KV-Ultra-Fast-Diode-FSC-VISHAY-MIC-DO-41-NEW/181846953488?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649
or this https://www.ebay.com/itm/10pcs-ON-1N4007TR-High-Speed-Rectifier-Diode-1A-1000V-with-copper-legs/273026392721?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

Planning to replace all the ceramic capacitors to WIMA MKP 0.1uF one, the rectifire and bias will got the 630v the else will replaced with 400v one (now I see that the schematic wrote 500v one for each ceramic capacitor )

I still cant understand why lots of people here replace the 100uF 400v to 220uF.
I just cant do so.
Stax engineers planned this capacitor by mathematically reasons, higer value of capacitor will store and release its stored electricity much faster and will do it job worst for this board (you will get distortions).

Hope for your help.
Thanks.

The theory for the larger caps is that this was what fit at the time but really, they could have just made a slightly larger case if that was an issue. When I recapped I used 150uf for that reason. I didn't want to go that far out of original spec. The original value would have been used for 2 reasons. It's sufficient (which doesn't mean you can't use more or wouldn't be better) and/or that value had good charge refresh timing with the transformer size used. I've found going too large can make amps lose some PRaT. In this circuit, I don't think there's any detriment to going 220uf but I still didn't want to overdo it so went with those 150uf and it's clearly snappy enough. More important to use 105* caps. Long life spec won't hurt.

I have 2 of these that didn't originally come with Pro Bias that I just finished recapping. I populated the pro circuit on one to allow it to drive any Stax phone, that I will sell, and I'll keep the other as I'm a big fan of the original low bias Lambda and SRX mk3. I can always get pro bias from that one as well if needed.

@spritzer

Have you ever done a dual bias on the low bias earphones to separate right and left channels from any minor crosstalk that may occur from a single bias supply for both channels? Can't be much and minor crosstalk adds center but since they have separate R + L bias pins (unlike the pro bias models) and the 2nd bias circuit could easily be populated that way...

​

​

 

[TheDoogooder]

Alright the operation is complete and the patient lives. I used the capacitors mentioned in the previous two posts.
I ordered them from mouser and it cost about $25 shipped.

The four power supply capacitors removed. as seen from the top back.



HV Capacitors.
The new capacitor is on the right. It is significantly smaller in both height and diameter.
I had to bend the pins outward to get them to fit on the board but that was easy and caused no issues that I can see.



The four new power supply capacitors installed in the SRM-1/MK-2
If you look closely you can see the much larger white outlines where the old capacitors used to rest



The next step is setting the balance and offset. I'm letting the unit warm up for a couple of hours so it can be done right.

Here is what spritzer has said on the subject in the past:

Post A:
There should be two pots per channel, one marked balance and the other one marked offset. To adjust them you need a multimeter set to DC volts and you insert the probes into the Stax socket. To adjust the balance you put probes between the + and - outputs for that channel and adjust for 0VDC. Now to adjust the offset remove the probe from the - output and connect it to ground (i.e. anywhere on the chassis, including the grounding post in the back) and adjust for 0VDC. Now repeat for the other channel. While you can do this with the amp cold it is best to do it when it is running at its regular operating temperature as heat is a factor here. The Stax pinout can be found all over the place.

Post B:
To set the DC offset you first set the meter to 100V or more DC. Then you put the red and black probes in the+ and - sockets for each channel and adjust the balance pot for that channel (which is marked on the PCB) for 0VDC. Once that is done then you remove the black probe from the - output and connect it to ground. Now you adjust the offset pot for 0VDC as well.

Stax Pinout: (Found somewhere on Head-Fi)

Hello,
I have been lurking for a while and want to thank all the knowledgeable people for their very useful info.
I have recently bought an SRM-T1 and replaced the valves.
The bias process was easy to follow.
Getting the values close to zero however was like masturbating a butterfly
Is that normal and has anyone changed the pots to modern multi turn trimpots?
Curious
Matthew

 

[oneguy]