MNATS 1176A finished but blowing fuses

[kent]

Hi All

I have recently put together my MNATS 1176 rev A with transformers and transistors from Hairball.  It looks good but I am blowing fuses and it has me stumped.  Here's the symptoms:

The first time I fired it up and started calibrating everything was going fine. Set the Q bias and started setting the meter when I got interrupted and had to turn it off and finish later.  The next time I tried to turn it on it popped a fuse.  I tried it again a couple more times and blew those fuses as well.  Went out and bought more fuses, blew a couple of those and then the next time it stayed on so I checked my voltages both off of the power supply and directly off the power transformer.  Those measurements all looked fen once again.  Then I blew a few more fuses.  Even with disconnecting the transformer from the board and having no load at all it blew a fuse.  I removed the power switch so just plugging it in would power it up wondering if the switch was bad and shorting internally .  Another blown fuse.  After resoldering and double checking all my connections again which appeared ok I turned it on and it stayed on.  However I noticed this time when I sent a test tone to start the calibration process again the meter slowly but surely started reading lower and lower level until after an hour or so it settled at some arbitrary level.  At least now it would turn on and off without blowing fuses.  So I checked the power supply again and everything looked fine.  I started doing the calibration and I could get no meter deflection while adjusting the Q bias.  When looking at the voltage directly off the Q bias wiper with my meter it was reading a nicely varied couple of volts range.  So this morning I was going to go back at it and trouble shoot some more but - it started blowing fuses again.

So after that long winded post - does anyone have ideas on where to start checking next?  Is it possible that the power transformer is suspect?  That seems unlikely to me.  Thanks.

 

[RuudNL]

The inrush current of a torrodial transformer is high, much higher than the steady average current.
Also you will have to charge the capacitors in the powersupply, causing extra current during turn on.
For this reason, you will need a 'slow' fuse.
What is the transformer and the fuse you are using?

 

[kent]

The transformer is the one from Hairball - so successfully used by many others:
http://www.hairballaudio.com/shop/product_info.php?cPath=33&products_id=37

I double and triple checked the transformer polarity and it seems fine.  It wouldn't have worked at all even intermittently if my polarity was messed up.  I would think it must be something downstream.  Perhaps a filter cap issue since there was that weird slow VU meter drop issue?

The fuses are slow blow:
http://www.mouser.com/Search/ProductDetail.aspx?R=0034.3111virtualkey69300000virtualkey693-0034.3111

In fact all the parts are from the MNATs parts list:
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=026c56e86f

 

[Harpo]

http://mnats.net/1176_reva-d_hairball_wiring_power.html
scroll down for fuse rating.
The 250mA/T looks like a parts value for european 230VAC mains for this type of mains transformer.
You'll need a 500mA/T for the same type of mains transformer when operating with local US mains voltage.

 

[kent]

If you look at the Rev F schematic it shows 1/8 amp for 220-240V and 1/4 amp for 110-120V.
http://www.gyraf.dk/gy_pd/1176/1176sch.gif

Again for the Rev A it has 1/4 amp at 115V.
http://mnats.net/files/1176_125.pdf

That said, I did a no-no and tried a 1/2 amp fuse (I ran out of 1/4 amp) and it blew as well.  I am sure I have something wrong but the intermittent nature of the problem is frustrating.

 

[RuudNL]

Does it also blow fuses with the secundary winding disconnected from the circuit?

(I would first suspect the rectifier diodes and/or the buffer capacitor.)

 

[PRR]

I have seen teeny 120V transformers which would not stay lit until I upped the fuse to half-Amp.

Obviously they were not really taking 60 Watts of power. Musta been a lot of reactive energy.

If the PT is not precious, just put a half-Amp in there. Keep a sharp eye for heat smoke and stink, the first minutes and the first hour.

 

[Harpo]

If you look at the Rev F schematic it shows 1/8 amp for 220-240V and 1/4 amp for 110-120V.
http://www.gyraf.dk/gy_pd/1176/1176sch.gif

...for a 16-11754 center tapped mains transformer (T3).

Again for the Rev A it has 1/4 amp at 115V.
http://mnats.net/files/1176_125.pdf

...for a UA-861A center tapped mains transformer (T3).

That said, I did a no-no and tried a 1/2 amp fuse (I ran out of 1/4 amp) and it blew as well.  I am sure I have something wrong but the intermittent nature of the problem is frustrating.

a 0.5A fuse with 'T' fusing characteristic (for time lag/slow blow) for your 30VA Avel Lindberg Y236106 dual 25VAC secondaries mains transformer ?
The UniversalAudio mains transformers have a lower VA rating than the AvelLindberg part, thus require a lower fuse rating than your mains transformer.

 

[kent]

Guys,

You are correct about the half amp.  After digging around in the fuse supply at work it looks like they were not slow blow like the box was labeled (in sharpie).  I picked up some half amp slow blow and so far so good.  Now if I can figure out why my Q bias isn't setting right - but that will be another post if I can't figure it out.

BTW - it's good to see your name up on the forum, PRR.  It's been a long time since I was fooling around with this stuff (kids and whatnot) but I have fond memories of working on the poor man's 12AU7 Vari-Mu with you back on the old forum.  Cheers!