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STAX SRM-007t Transformer Help


chocolates
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Hi all, I was advised by some head-fi members to inquire about the technical aspects of this amplifier here. Do let me know if I'm failing to follow any rules; just trying to avoid any catastrophes!

Here are the photos of my amp: https://imgur.com/a/sshHA7B

The amp is an 8FQ7 SRM-007t rated for 100v 50/60hz outlets. I have a step down transformer (PowerBright) rated for 120v to 100v, and my outlets are, I believe, 120v at 60hz (American). The transformer works fine with my Japanese SRM-252S, but the fuse has blown 3 times now when turning on the 007t (always the 2nd time it turns on; the first time seems to be working). I've tried debugging this for a bit, hence the 3 blown fuses, but I'm not too sure why since the 007t is rated for 100v in and the transformer is rated for 100v out. Birgir kindly helped me debug this somewhat and believes that the most likely culprit is a short somewhere on the board, but I'm fairly inexperienced with more complex circuit boards and having investigated every individual trace on the board could not find anything that looked like a traditional short. If I'm being honest, the board looks relatively pristine if a bit dusty, which I'm trying to clean myself; could anyone help me figure this problem out?

The fuses are 1 Amp / 250V 5x20mm, unbranded. I've since bought a handful of extras from a different brand but haven't yet tried them with the 007t. The tubes look to be vintage GE 8FQ7 / 8CG7 tubes, which are apparently the original tubes the amplifier came with, so I am fairly certain that they're not the problem, but I might have to rebias them as it's quite old. There are 3 red lights in the case and the power button is red; I'm not too sure if the lights indicate anything out of the ordinary.

Thanks in advance!

E: As far as I can tell, the wiring is for 100v; the fuses are in the 1,3,6 positions. I'm not entirely sure about the colored wires coming from the transformer inside the amplifier, though.

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you better take a real good picture of the voltage selector block. there are many different kinds.

if blue and purple are jumpered together then its 120v

if brown and green are wired together its 100v

white should be wired to grey

the correct fuse for 100v should be 1.5 amp

 

 

 

Edited by kevin gilmore
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thanks all for the help!

4 hours ago, n_maher said:

I was thinking this and one does look a little domed in some of the pictures.

which one? i'm afraid i've little experience with caps; the big brown ones seem like they're doming a little bit.

3 hours ago, kevin gilmore said:

you better take a real good picture of the voltage selector block. there are many different kinds.

if blue and purple are jumpered together then its 120v

if brown and green are wired together its 100v

white should be wired to grey

the correct fuse for 100v should be 1.5 amp

 

 

 

looks like they're all hooked up to the voltage selector, and the ones with fuses in them - 1 and 3 - are wired to brown and green, so i guess i just need to change the fuses? that'll be a good deal easier to work with 😅

 

are there any recommendations for cap replacements? i've done through hole soldering before but never done any sort of replacement for electronics like this; looks like i've got 400v/220uf big brown caps and a handful of small 35v 10uf. i can't quite make out the two middle caps, but i'll find out once i get them off the board i guess never mind, they're 35v 470uf   

Edited by chocolates
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Here you go chocolate

Regarding Voltage input to the 007t

The power supply transformer has 7 tap on the input side:

Yellow= common (winding 1)
1 White=same as yellow (winding 1)
2 Green=100v tap (winding 1)
3 Purple=120v tap (winding 1)
4 Black=common (winding 2)
5 Brown=100v (winding 2)
6 Blue=120v (winding 2)

Six jumper bar for voltage change number 1 thru 6
1 thru 4 are to select:1=100v 2=120v 3=100v 4=120v 5=220/240 6=100v/120v
So for 100v you install one bar each for 1,3,6.
For 120v you install one bar each for 2,4,6. You will need to do this for your 120v
For 220v/240v you install on bar each 3,5.

Unscrew the two screw at the back

They remove the sStax plate covering the transformer

Carefully remove the board and change the desired voltage.

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Edited by Mach3
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  • 4 weeks later...

Remove both black jumpers, move the left hand white wire to the empty spot above the blue wire (so from brown to blue) and finally, put in a new jumper between gray and purple. 

Also, make sure the gray and blue wires actually go inside the transformer and aren't cut off.  Either measure continuity between the next wires or remove the blue PCB on top of the transformer for a visual inspection. 

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4 minutes ago, spritzer said:

Remove both black jumpers, move the left hand white wire to the empty spot above the blue wire (so from brown to blue) and finally, put in a new jumper between gray and purple. 

Also, make sure the gray and blue wires actually go inside the transformer and aren't cut off.  Either measure continuity between the next wires or remove the blue PCB on top of the transformer for a visual inspection. 

Thank you so much Birgir for a fast response! The wiring changes suggested are for 230 or 240v?

I have verified that both blue and gray wires go inside the transformer. Blue and Gray wires

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I did the same to mine about 2 years ago to my SRM-600 I think.

It had the cut wire to the 240v tap on the transformer, Spritzer help me with this.

Then gave me the correct information to sort the jumper for the voltage selection

. Should look like this.

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Edited by Mach3
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This is the 240V setting.  If you wouldn't move the white wire the transformer would be set to 220V which isn't a great idea these days. 

That picture of the wires doesn't tell us anything as the windings entering the transformer are not visible.  Just pull off the PCB or use a volt meter to measure between blue and brown and then green and purple.  Should be a few ohms but if you have nothing... then the windings have been cut.  

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7 hours ago, spritzer said:

This is the 240V setting.  If you wouldn't move the white wire the transformer would be set to 220V which isn't a great idea these days. 

That picture of the wires doesn't tell us anything as the windings entering the transformer are not visible.  Just pull off the PCB or use a volt meter to measure between blue and brown and then green and purple.  Should be a few ohms but if you have nothing... then the windings have been cut.  

This makes things pretty lucid for me. Thanks much@spritzer!

I shall update here once I am done with the mod. Make take some time as my schedule is fully packed.

Cheers and Happy Holidays everyone!

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On 12/24/2020 at 3:38 PM, Mach3 said:

See that last post (blue binding) they cut the cable.

Check if you're has the same issue. If not all good.

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Hi, @Mach3 @spritzer. Exactly the same issue with me. Purple and Blue wires are not connected to transformer windings.

What are the next steps? Please share them over pm. Is it even possible. Thanks again for the support this awesome forum is extending.

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19 hours ago, spritzer said:

If there is enough copper there to solder a wire to, then it can be fixed but I've come across some units which were beyond hope. 

Mission accomplished!!! Pics for your viewing pleasure. Job may not be neat but serves the purpose really well. 🙂

Album - Rewiring to 240V step wise

Many thanks to @spritzer and special mention to @Mach3!!!

 

 

 

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  • 10 months later...

Hello! Got a quick question. I have a Stax SRM007t (mk1). Older version with pin jumpers.
 

It’s currently got the jumpers set for 100V (1,3 and 6). I think for 240V you use two jumpers on 3 and 5 only and don’t use a third pin but wanted to check in case that’s for 220v (As I really need 240V for sure as my electricity is near to 250V).

Any advice would be much appreciated. 

 

 

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