Triad C7-X in Stereomour II

[amritdesai]

Can I replace the 270ohm resistors in the PS with c7x chokes? Is this as simple as this sounds?

I was playing around last night, removed the 270 resistors 44L/45L and 41L/42L and attached the leads of the each choke to these terminals - one of the leads from choke A to 44L the other to 45L. For choke B 41L/42L respectively. Because I have a tall base i was able to mount them perpendicular to and below the OT's.

I turned on the amp, it lit up for a second and blew a fuse.

I'm going to buy some fuses at radioshack and have spare 270 ohm resistors to revert to stock but i'd like to the try to make this work.

Any suggestions?

Thank you!

 

[Paul Joppa]

There are no 270 ohm resistors in Stereomour II. I think you have replaced the 270K ohm resistors, which serve to bleed charge from the PSU electrolytic caps when the amp is turned off. This means a nearly short circuit across the power supply - no wonder the fuse blew.

The old Stereomour used 270 ohm resistors (in the form of 360 ohms in parallel with 1200 ohms) in a CRC filter. Stereomour II uses four 130 ohm resistors in a CRCRC filter, they are potential locations to replace with chokes.

You said "...perpendicular to and below the OT's." You didn't explicitly say the coil axis was perpendicular, so I mention it just in case there was some confusion.

 

[amritdesai]

Wow :-[

I completely mis interpreted this condition - yikes. I will revert to stock and stop messing with this thing.

Thank you for your help!

 

[Paul Joppa]

Definitely the first thing to do is revert to stock, in order to be sure there are no problems remaining. (I don't expect any, but better safe than sorry!)

I will correct my previous post to clarify that the original Stereomour used 360 ohms in parallel with 1200 ohms, to make approximately 270 ohms.

 

[amritdesai]

I have soldered back in place new 270K ohm resistors. New 1.6a fast fuse continues to blow when I turn on the amp 

Resistance measurements are good. Perhaps the UF4007 rectifier diodes were affected?

Thanks for you help.

 

[amritdesai]

Installed new UF4007 rectifier diodes and tested all of the 130 ohm resistors - no issues here. Also new 270k resistors between terminals L 41 and 42, L 44 and 45. new 1.6amp 250v fast blow. Connections are solid.

Don't know where to go from here - 4 fuses later.

Any assistance is greatly appreciated.

 

[Bonzo]

Just a doubt: the fuse shoul be slow blow...shouldn't it?

 

[amritdesai]

Good question. The unit came with a fast
blow but other users have been okay using also slow blow.

 

[lth1]

What does the parts list in your manual say the fuse should be?  I have never built a BH kit that doesn't use a slo-blo fuse.

 

[Natural Sound]

What does the parts list in your manual say the fuse should be?  I have never built a BH kit that doesn't use a slo-blo fuse.

The Stereomour II manual I have only specifies a 1.5 A fuse. No mention of fast or slow blow. It looks like some users are having an issue with a spike in inrush current when charging the caps on startup, others are not. Maybe it has to do with variation in mains voltage and component tolerance stack up. But I'm just guessing.

 

[amritdesai]

I was about to say the same thing about the manual. I could have sworn the original fuse was fast blow but my memory, judgment and decision making is questionable based on recent events 

Thanks for the feedback guys!

I'll try a slow blow and go from there

 

[amritdesai]

The slow blow fuse fixed the issue! Really appreciate the help!

Now what to do with these extra chokes...

Thanks again.

 

[lth1]

If the 130 ohm resisters are installed in series (= 260 ohms) then the chokes can be installed in the CRCRC filter.  I do not have a manual for the Stereomour II and I can not tell from the picture in the products page, so this may not be correct.

 

[Paul Joppa]

If the 130 ohm resisters are installed in series (= 260 ohms) then the chokes can be installed in the CRCRC filter.  I do not have a manual for the Stereomour II and I can not tell from the picture in the products page, so this may not be correct.

They are in series, but there's a capacitor from the CT to ground.

 

[chard]

c/choke/c/10 ohm resistor/c      total 280 ohms instead of designed 260 ohms. Hardly any difference. 

 

[Paul Joppa]

The 10 ohms will pretty much eliminate any effect of that stage. Look up PSUD on the Duncan Amps site; it's an excellent learning tool, practically a video game for this community!

 

[chard]

Thanks Paul, for the information.

 

[Jamier]

Would a Hammond 156L (135 ohm / 5H / 400V /  75ma) be a good
choice for replacement of the last 130 ohm resistor? Or the first
130?

 

[Paul Joppa]

Looks good to me.

Try this: install the choke(s) without connecting them to anything at first. Read the AC voltage between the two choke wires - this will be the induced hum from the power transformer magnetic field. If it's very small then you can put the choke in place of the last 130 ohm resistor. If it's large then you'll want it in place of the first 130 ohm resistor. I would guess that 10mVrms is "small". Of course it's not terribly difficult to try it both ways, instead of taking a guess from some dude on the internet (me, in this case).  :^)

 

[mcandmar]

I love how you come out with these brilliant ideas, yet they sound so obvious when you say them