Help needed with Yorkville Sonax 750-G instrument power amp

Thread Starter

chuck_b

Joined Nov 17, 2022
7
I have a yorkville sound Sonax 750-g cooking Q6 40361 driver transistor it drives the output transistor 2n3055, I have checked and double checked all the resistors on the board all in spec all the caps have been replaced, supply voltage +36 -36 dc the mz2361 replaced with two 1n4148 in series, I don't know what is causing Q8 to burn up, any help will be appreciated
 

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Dodgydave

Joined Jun 22, 2012
10,583
R34 is 22 ohms but R31 is 47 ohms, this doesn't look right to me..
What is the voltage at the output of the speakers?
 
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Thread Starter

chuck_b

Joined Nov 17, 2022
7
R84 is 22 ohms but R31 is 47 ohms, this doesn't look right to me..
What is the voltage at the output of the speakers?
the old amp had not been touched since it was built the schematic values are the values on the board, the only voltage discrepancy is at MZ2361 -.6 v is low -.3v and the +.6v is +1.2v the MZ 2361 is obsolete and unobtainable research on the web indicates it can be replaced with 2 1N4148 diodes in series but the voltages remain unchanged. the Q8 40361 also obsolete replaced with a NTE 128 as the recommended replacement, scratching my head
 

Ian0

Joined Aug 7, 2020
6,659
I have a yorkville sound Sonax 750-g cooking Q6 40361 driver transistor it drives the output transistor 2n3055, I have checked and double checked all the resistors on the board all in spec all the caps have been replaced, supply voltage +36 -36 dc the mz2361 replaced with two 1n4148 in series, I don't know what is causing Q8 to burn up, any help will be appreciated
Any sign of overheating on R33, R34 or R35?
 

Thread Starter

chuck_b

Joined Nov 17, 2022
7
Any sign of overheating on R33, R34 or R35?
when the transistor Q8 overheats and fails r34 22Ω goes up in smoke, R33 and R34 and 35 do not get warm but I can only power up for about 30 seconds before Q8 gets too hot to touch, I have burnt out two replacements nte 128 not fast enough powering down

thanks for helping out
 

yellowfish

Joined Feb 9, 2009
41
Burnt R34 indicates bad Q9. A variac would help. +/- 36V supply, that's 72V rail to rail. 2N3055 are rated only for 60V. I would install higher voltage output transistors, like the MJ15001 rated for 140V @15A
 

Thread Starter

chuck_b

Joined Nov 17, 2022
7
Burnt R34 indicates bad Q9. A variac would help. +/- 36V supply, that's 72V rail to rail. 2N3055 are rated only for 60V. I would install higher voltage output transistors, like the MJ15001 rated for 140V @15A
thanks, I will look into that replacement transistor, I have new 2n3055 arriving Monday or Tuesday
 

Ian0

Joined Aug 7, 2020
6,659
If the Vceo values for 2N3055 and MJ15015 were closer together I'd suspect that they were tested for Vceo and the ones that passed are marked as MJ15015 and those that only made 60V are marked as 2N3055 and sold cheaper. However, as they are 60V and 120V, I expect that the MJ15015 is made with higher resistivity silicon.
A while ago I cut the lids of several power transistors of this type, and MJ15015 and 2N3055 were indistinguishable.
 
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