1176 Rev A burnt R32

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Jay107123

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Joined
Mar 27, 2015
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2
Just finished building the 1176 Rev A.  As soon as I turn it on and increased the input power (nothing connected to the XLR input or output) the R32 resistor smoked.  I have checked all the wiring and it all looks good.  I replaced R32 and it fried again.  The only thing I see wrong is the output transformer winding resistance doesn't match what is shown on the diagram on the hairball website.  It shows 600 ohms Black to Brown and I measure 53 ohms.  Red to blue is 50 ohms instead of 600 and Grey to Violet measures 260 ohms instead of 600.  Red/wht to Blk/wht is only 1 ohm.  Did I get a wrong/bad transformer in my kit (it is marked 5002  1440) or is there another problem?  Any advice would be appreciated.
Thanks all, I am glad you are here,
Jay
 
50Ω looks fine for a 600Ω winding, you are measuring the DC resistance. Impedance is what those 600Ω means at a given frequency, or better expressed what line impedance it's meant for, not the actual impedance of the transformer as a load.

I don't know exactly what's R32 on rev A, I guess the 99Ω one connected to the Q6 emitter through one winding of the transformer, which suggest over current on the output transistor, which suggest greater voltage than expected on Q5, so something going on between R29, R30, CR1. You may try measuring the voltage at the base of Q5 without replacing R32. It should be around 4V, which end in about 0.1W at R32, in which case there shouldn't be a reason to burn it instantly even for a small resistor. If the voltage is much higher than that I would check those resistors and the diode just in case, if not maybe is the output transistor what's wrong from start.

JS
 
Thanks for the reply.  Just in case someone else has the same problem l will report my findings.  I removed Q6 thinking it may be bad  but when I tested it  the gain was right in the middle of the spec.  So I reinstalled it thinking that I would move on to something else.  We I reinstalled it the problem was resolved.  I am guessing that I had a bad solder joint on Q6.  All is good now.  I was able to calibrate the unit and have been running it for about 3 hours now. 
 
It's always a bad solder! That should probably cover most problems, but as you first need to see where it was it's easier to blame a component, then may be a bad solder on that component or the component it self!

Good to know it's working!

JS
 
Hey guys,

I have an original V1 pair of rev A's that are both doing this.  Initially, I think my Q6 was shorting out as I had it too tight to the board.  Figuring those were fried, I removed and installed new ones.  Now, I'm still burning up R32.

I've read something with the output transformer wiring being different in the various revisions?  Is there something more to this that might be causing my problems?

Thanks!
Sig
 
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