Otari MTR-90 MK III burn in ( burnin' )

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digitall2000

Well-known member
Joined
Jun 22, 2013
Messages
74
Location
central california
hello

and thanks in advance for any suggestions and advice
attached is the burn problem pics
as i remember this card still still had output (not 100% on that)
the burn is in the active balanced output section
junction Q127-128 - R185-186 - SW101 BAL/UNBAL OUTPUT
from there on to the out put connector

1. any ideas on what could cause such a burn?

2. is there a way to test the output transistors in circuit?

i have a small transistor, diode,inductor, blah . . .  tester,
if i try to test the transistor as just that, all three legs, it fails
if i try to test it as a diode, just two of the legs, it tests as
different combinations of single and double diodes

that is all in circuit

not sure if this is the proper way to go about posting this
someone clue me in if it is not

thanks
lance
 

Attachments

  • OUTPUT SCHEM.jpg
    OUTPUT SCHEM.jpg
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Not burnt by shortcircuit - they're short-circuit-protected by the 20 Ohm/4W total resistors at their output.

My guess is that it could maybe have been connected to an input with P48 present?

Jakob E.

 
thanks guys

plimousse  --  have you ever seen this happen before?

P48 = phantom power?
i didn't think that phantom would supply enough current
seems to me it would take quite a lot of current to burn that
board so badly

lance
 
digitall2000 said:
.....
P48 = phantom power?
Yes, 48V Phantom Power

digitall2000 said:
i didn't think that phantom would supply enough current
seems to me it would take quite a lot of current to burn that board so badly
That was my first thought too, especially because the transistors are rated for 160V and second the 2x10 Ohm/2W resistors (R185 - R188) which got so hot ARE the mentioned "current protection" resistors (20 Ohm/4W).
But since R183 and the track to Q126 (2SA1011) appear to be burnt too. there is a very slight possibility that phantom-power destroyed Q126 first.

 
digitall2000 said:
1. any ideas on what could cause such a burn?
If for some reason there is a short and the level is pumped up, the output amp is capable of delivering close to 5W, which is enough to cook the pair of 2W resistors. The way the solder joints and the PCB look seems to support that explanation. I would think these two resistors have survived (maybe value has drifted).
 

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