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The SRM727 thread


kevin gilmore
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17 hours ago, arnaud said:

Sorry for the revival from the dead but I am finally contemplating doing the feedback mod to my 727 amp while the bhse is dead (bad luck, second time some shit happens with mine, one of the iec block fuses blows up right after the delayed relay switch since yesterday...).

So about the 150k resistors, 1/4W suffices?

cheers,

arnaud

What's going on with your BHSE? Twice now? Is it the amp? Justin's work is usually bullet-proof. Or is there something else going on?

Edited by Audiojunkie
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hopefully just the fuse. i did undersize them a little, and found they blow one in every few hundred tries. enough that i occasionally get an email, tell the person to check the fuses, and replace. that usually does it. i did increase them 0.5A on the latest batch to try and prevent that.

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Arnaud, I just used 1/4 watt RN60 resistors like Birgir had originally shared.  I don't recall if he states it in this way anywhere else but the stock resistors are glued onto the PCB and the leads are tightly spaced, I think I ended up desoldering the original resistors with a pump or braid and then snipped the resistors off the board with a flush cutter.  After that is a piece of cake.

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Thanks Mr.Sneis for the tips on desoldering, I was not keen on byuing a heat gun type soldering station just for this so the pump and cutter will do I hope ;-). Will probably stop by akihabara today to grab resistors instead of the week time wait to score them online.

In regards to the BHSE, initial problem at reception back was with a tiny tiny piece of wire that managed to short the HV line to the tubes I recall.

Then, a few months back or so, the IEC block gave up the gost (after a few hundred hours of use) in a bizarre way. One of the poles seemed to have overheated to the point I no longer had continuity from end to end. It was fixed by changing the iec input block (a 5min job) and the doctor advised to visually inspect the socket periodically. I was and am still using hospital grade power cords (standard stuff, not fancy garden hoses) so Justin said it could have been a faultly iec block from the start.

Today, another few hundred hours laters (call me neerdy but I use this neat app to log the hours I put into the tubes I roll ;-) and hence total airtime on the bhse...), one of the fuses in the IEC socket keeps blowing up after the delayed relay kicks in. I bought some standard fuses at hardware store of same spec and visual aspect as original ones (i.e. 2A / 250V ) but same thing happens (with main amp connected or without).

Justin mentioned I need slow blow fuses but the hardware store did not have 2kinds, just amp ratings and I took highest value justin recommended... I will also search for fuses at akiba today...

Otherwise, I popped the hood on both the PS and amp, nothing seems out of place (to me eye though so doesn't mean squat lol). Justin found the brisk of copper in the images I sent him before, I was so impressed since it was so frigging tiny.

Cheers,

arnaud

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I was told on head-fi that putting sorbothane in my BHSE would help, should I trust this opinion ? ...

Thanks Birgir and all, sorry for the ever noob questions, got some slow blow fuses on the way (2A rating for now) from amazon...

Will keep you posted, without smoke and smell if possible.

arnaud

 

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Well, so much drama for just this... Turns out it was the fast blow fuses that were the issue... It's singing again with the 2A slow blow!

Dropped xf2s, adjusted offsets, plugged the 009 and ahhhh, good to be back :-).

I have the HD800s of a friend here on loan for a week or so, it's very very spacious and layered but I prefer the 009 voicing and bass quality out of the bhse.

Who said the stats had no bass ;-). HD800s are definitely an improvement on the HD800 but I think they're still a bit too hot for my ears.

Thanks everyone for the help, I still need to get on the 727 mod...

arnaud

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On 28 Apr 2016 at 1:46 AM, arnaud said:

Sorry for the revival from the dead but I am finally contemplating doing the feedback mod to my 727 amp while the bhse is dead (bad luck, second time some shit happens with mine, one of the iec block fuses blows up right after the delayed relay switch since yesterday...).

So about the 150k resistors, 1/4W suffices?

cheers,

arnaud

Hi Arnaud

I am sorry to hear your BHSE has gone wrong again. Will you sent it back to Justin this time, as I think last time you replaced the IEC yourself?

I am glad you seem ok now with the slow blow. Wonder of your mains wavers around out of spec? I live in a modern apartment and mine did so I got a PS Audio P10. I can set it now at rock solid 220v and 0.1 distortion. Right out of the wall it can be anywhere between 220v - 258v and distortion of 2.8% on busy periods of the day. 

Edited by astrostar59
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I think I will mod my 727A. I open the amp but my board are a bit different, I think it's only aesthetic. On Spritzer's picture, the boards are flocked "PB-301" on mine it's "PB-302" is there any difference ?

 

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Hmm why on earth are they using jumpers on a double sided board?  Seriously!!

The feedback is still identical though.  Two 150K resistors in series so just remove the bottom one and connect the new resistor between the top pad and the empty socket connection. 

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jumpers could be superior to trying to route the pcb on 2 layers. i'm sure there are many boards that we made that were worse because we felt we were doing something wrong if we used a jumper. before people started moving to SMD parts there was even the anti vias group

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Maybe in some rare cases that might be the case but I've never run into that even with my super cramped PCB's.  Filaments being the most obvious example. 

That's clearly not the case here as we could do this layout in our sleep with a complete ground plane. 

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I forgot to thank you.
I don't have 150k resistance in stock, anyway I need to order some components from mouser, I will do the mod on my return from vacation. 

Do you have any idea why the amp has a second version ? 

Edited by charlo89
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  • 1 month later...

So how violently will an SRM-727a explode if I were to plug it into my healthy 123V USA outlet? Stax is getting really mean these days, my newly acquired 727a is the same revision as charlo89's and the transformer doesn't even have the 120V windings cut anymore, they simply are not even there on this new transformer.

I have a step down transformer on order but I'm impatient and foolish. What's the damage if I do it?

727a_transformer.jpg

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Don't do it, just don't......! :o

Even if you get away with it this time, it may embolden you to try it when the consequences may be considerably more severe.

Good practice and habits should never be sacrificed on the altar of expedience, unless you know what you're doing or you don't mind the fallout.

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