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 Forum index » DIY Hardware and Software
where should I be seeing 36V start in this circuit?
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rjd2



Joined: Sep 02, 2007
Posts: 235
Location: philly

PostPosted: Wed Jun 12, 2013 9:14 am    Post subject: where should I be seeing 36V start in this circuit? Reply with quote  Mark this post and the followings unread

Hi folks, I'm hoping you can give me some insight into reading this schem:

http://wikigear.org/wiki/images/f/fd/M441m-schematic3_assembled.jpg

particularly, this: in the upper right hand corner, you see a network. it starts with CR7, and ends with R49/R50. so 48VDC comes into CR7. and out of R49/R50, one SHOULD be seeing 36V.

My question: at what point in that network should I be seeing 36V start? I'm getting about 43.5V, and can't deduce what component is not working properly. Thanks for any help.
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elektrouwe



Joined: May 27, 2012
Posts: 46
Location: Germany

PostPosted: Wed Jun 12, 2013 2:36 pm    Post subject: Re: where should I be seeing 36V start in this circuit? Reply with quote  Mark this post and the followings unread

rjd2 wrote:
at what point in that network should I be seeing 36V start?...

there is a voltage divider : R57, R56. Voltage over R56 should be
about 37V.
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rjd2



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PostPosted: Wed Jun 12, 2013 6:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

AHA! thanks so much for this. super helpful. i focused on that area, and found that R56 and R57 don't read their proper values when in circuit. i know for a fact that they should. i now need to try to figure out what could be pulling them down, particularly the 47K R. it reads about 12K in circuit, and the 12K reads about 9k in circuit. these old traces are starting to wear thin with the desoldering....
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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Jun 12, 2013 8:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Be warned, resistances in circuit don't necessarily read what you expect, because things end up being in parallel through various paths etc.
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elektrouwe



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PostPosted: Wed Jun 12, 2013 11:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Elmegil is right. measuring Rs @ power off state needs always the right interpretation. I would suggest you measure the base to ground voltages
@Q12,Q11 first. @ Q11 shou should have the 37V from the voltage divider.
@ Q12 same minus 0,7V.

Last edited by elektrouwe on Thu Jun 13, 2013 4:50 am; edited 1 time in total
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rjd2



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PostPosted: Thu Jun 13, 2013 4:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Yea, I know that a resistor will not always read it's value in circuit. i do, however, have 2 of these preamps, a perfectly working one, and the one im fixing. on the good one, R56 and R57 read their actual values, 47K and 12K, so i know it SHOULD.

i'm a bit stumped, now. i tried lifting all of the legs of the components that are in line with ground and R56, and it just lowers the resistance readings. that 47k R needs to read 47K. my only thought now is that something in that network of transistors is off. i replaced all of those components, though. the only change was that the 2N5137 is an obsolete item-replaced it with a BC337, and checked the B-C-E legs. something is off there...
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elmegil



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PostPosted: Thu Jun 13, 2013 5:42 am    Post subject: Reply with quote  Mark this post and the followings unread

rjd2 wrote:
Yea, I know that a resistor will not always read it's value in circuit. i do, however, have 2 of these preamps, a perfectly working one, and the one im fixing. on the good one, R56 and R57 read their actual values, 47K and 12K, so i know it SHOULD.


Fair enough Smile
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rjd2



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PostPosted: Thu Jun 13, 2013 8:12 am    Post subject: Reply with quote  Mark this post and the followings unread

OK-making progress. one quick follow-up question:

Q11 was blown. when i got the unit, it was blown. so something downstream of it is likely causing it to repeatedly blow. i happen to have several of that trans here, but the traces are pretty much toast from all the replacing. is there something(ferrite bead?) i could put just north of it that would work as a fuse of sorts?

thanks for the help!!
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elektrouwe



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PostPosted: Thu Jun 13, 2013 8:43 am    Post subject: Reply with quote  Mark this post and the followings unread

rjd2 wrote:
Q11 was blown.

That was very likely Smile If possible use a lab power supply and disconnect
Q11s emitter from the circuit. If you don't have one, use a 33R power resistor or at least a 12V,someW lamp as fuse. Because you could measure
43V there is no "hard" short circuit. Measure and post the current to get an idea.
Hope the opamps are not damaged: their absolute max. voltage rating is 36V!
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rjd2



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PostPosted: Fri Jun 14, 2013 5:03 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks so much, everybody. very helpful.

something is pulling impedance measurements low in the front section of this. from CR7 thru R56, im not reading what i should across almost any of the components.

is there a valid way for me to test the capacitors in the rest of the unit without powering it on? im taking a new approach: i've got the good one and the sick one open side by side, and im not going to power the sick one up til the impedance readings match. thanks!
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