Avalon 737sp burned resistors

[Whoops]

Now I have replaced these four transistors but it still burns r18 when power is switched on.  Please help what to do next..

Transistors .....

See Pucho advise

On circuit card # 5600-7372 The value of r18 is 3r48.

Check q8 for failure as well it should be a bd139. Also check q5,q6,q7 as well.
.

 

[rajuz]

Yes, that's excatly what I did. Replaced q5,q6,q7, q8 and r18. I just checked these removed transistors with multimeter and they were ok.

 

[Whoops]

Yes, that's excatly what I did. Replaced q5,q6,q7, q8 and r18. I just checked these removed transistors with multimeter and they were ok.

Check all the other resistors in that circuit to see if any of them is open.
Maybe some  other resistor is shot but cant be seen from the outside

 

[pucho812]

Now I have replaced these four transistors but it still burns r18 when power is switched on.  Please help what to do next..

I never said to replace the transistors. Why replace a perfectly good/working transistor.  Check/test those.  You may also look into q12.  I don't have schematics on hand but it also is tied to r18  in path...    If you are not familiar with how to test transistors,  Better get a handle on that before continuing.  replacing parts  that are not a failed part is a waste of money.

The circuit is not as complicated as  it seems, like any circuit it's a little harder when you do not have a schematic but even still can work it out without the power on.  Burning resistors is all due to current draw, something has shorted out or has gone highΩ causing r18 to go.  If you really want to see where r18 connects to, use a continuity meter and test continuity on all the traces. That can be done with the power off. In fact  test the card out of circuit passively, same goes for transistors, resistors, and caps.

 

[rajuz]

I found the problem, it was capacitor c13. I replaced it and now my avalon is working again .
By the way, these resistors r18 and r19 are not 3r48 in my device. For some reason they are 10 ohms. There may be different versions of these boards.
There is still one thing that makes me a little worried. The phantom power is quite high, it is 52,9 volts measured in the end  of the microphone cable.
Many thanks for help!

 

[CJ]

52 volts is ok, will probably come down with mic plugged on,  ( if the mic uses phantom)

 

[pucho812]

I found the problem, it was capacitor c13. I replaced it and now my avalon is working again .
By the way, these resistors r18 and r19 are not 3r48 in my device. For some reason they are 10 ohms. There may be different versions of these boards.
There is still one thing that makes me a little worried. The phantom power is quite high, it is 52,9 volts measured in the end  of the microphone cable.
Many thanks for help!

Yes in older revs they are 10K... the 3.48 was a revision  but I forget when it was implemented...  as cj says your 52 volts is o.k. without a mic, when I mic is connected using phantom it will drop.

If the resistor fries,  replacing it is the last step,  making sure other components are working is first. I'll have to go back and check schematics as to c13 and it's location in the mix...  off the top of my head it is an electro in parallel on the path running to ground 100uF at 50V. But I could be wrong...  so don't quote me yet...

 

[tonija]

that would be fantastic!!!    if it could be.
If you dont have a camera I only need the value or color codes of the 4 burned resistors really, R49, R37, R19 and R18...

Thanks!

Arnau
R1819=3.31 ohms  R20 R32=22.1ohms

 

[CrisNich]

We would like to thank the maintainers of this site, always with wonderful technical resources that help all of us, electronics and audio technicians. I will leave my contribution, recently it was necessary for me to perform maintenance on one of these Avalon737sp and also with the burning of these transistors and resistors of the input differential preamp board.

 

[map2112]

Looks like the zener diode (ZD2) is not soldered well. Not sure where it is in the circuit but that could be related to an over-voltage condition if used as a voltage reference

 

[CrisNich]

Looks like the zener diode (ZD2) is not soldered well. Not sure where it is in the circuit but that could be related to an over-voltage condition if used as a voltage reference

Really, you´re right, but I´v have see and is ok. Btw, thanks for advice.

 

[Atalantos]

Greetings to everyone. I just joined the forum because of the same problem of our
737 (R18 is fried).

A big thanks to everyone for contributing and especially to ChrisNich for the component's values. Now I have enough info to attempt a repair.

 

[CrisNich]

Youre welcome! Good luck!
Advice: transistors at this board, replace them all.

 

[Atalantos]

Hello again, I just wanted to give an update of my repair, in case it might help
someone. I discovered that although the C9 had some cosmetic damage from the burned R18 (which are next to each other), it was the C13 that was damaged (shorted) and caused the R18 to fry. After replacing it everything works again. Thanks again for all the help.