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Old 16th February 2017, 07:34 PM   #1
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Default help this mikawa amplifier, cheap but weird

2 days ago, i come to my relative house with my mom
after some family conversation, i see her amplifier on the table
i ask her is this good or not
she answer me, this is almost doomed
so i take it down to my home for repair
you know, asian people habit always help their relatives and neighbors without payment

so after open this thing i found something weird...
this amplifier is a cheap china product but have toroid, something rare today in my country
after some test, i found this sound is not good
repairing its tone control testing with my amplifier and its good
but testing amplifier circuit, i can't say anything

so this is my plan...
i will change every transistor into good one
also with cheap transistor too but good ones
my idea is changing final transistor to TIP41 and TIP42, driver with c945 and a733
is it possible or not???
i put a result of LTSpice and schematic from LTSpice too

amplifier brand is Mikawa AV100
toroid is 80VA

regards
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Old 16th February 2017, 07:37 PM   #2
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While the transistors are out you can check the resistors to see if any are open circuit.
If you have an LCR meter check the capacitors as well.
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Old 16th February 2017, 07:44 PM   #3
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Quote:
Originally Posted by nigelwright7557 View Post
While the transistors are out you can check the resistors to see if any are open circuit.
If you have an LCR meter check the capacitors as well.
have been checked transistors
they are almost doomed
need help for this one cause there is no one sell this type again near my home town
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Old 16th February 2017, 07:46 PM   #4
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Quote:
Originally Posted by fandi bin stress View Post
have been checked transistors
they are almost doomed
need help for this one cause there is no one sell this type again near my home town
Yes I know.
But while transistors are out check other discrete components.
Sometimes a faulty transistor can burn out resistors too.
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Old 16th February 2017, 07:51 PM   #5
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Quote:
Originally Posted by nigelwright7557 View Post
Yes I know.
But while transistors are out check other discrete components.
Sometimes a faulty transistor can burn out resistors too.
all resistors are normal
also after first test and changing caps(same value) they are normal

Last edited by fandi bin stress; 16th February 2017 at 07:58 PM.
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Old 17th February 2017, 02:00 AM   #6
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Your schematic is misleading on the transistors used in the amplifier. From what I can see in the pictures, all of the to92 types are 2n5551 and 2n5401 and the outputs being 2sc2073/a940 pairs. The green one appears to be one that has been replaced previously. If the rail voltages are actually +/- 28 then it would be a sure thing those outputs would go bad before long. The tip41/42 pair should work ok as long as you use the A or better voltage rated ones, and if the 2n5551/5401's are not damaged I would leave them be.
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Old 17th February 2017, 07:29 AM   #7
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Never wondered on the sim results in the 1st picture? How does show do if you swap R7's right leg from rR0/R11 to R12/R13?

Best regards!
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Old 17th February 2017, 08:17 AM   #8
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Yes there should be a connection from the junction of r10/11 to the junction of r12/13.
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Old 17th February 2017, 08:27 AM   #9
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Quote:
Originally Posted by jerluwoo View Post
Yes there should be a connection from the junction of r10/11 to the junction of r12/13.
Well...
Like you said...
This thing not only make me confused...
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Old 17th February 2017, 08:33 AM   #10
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Quote:
Originally Posted by jerluwoo View Post
Your schematic is misleading on the transistors used in the amplifier. From what I can see in the pictures, all of the to92 types are 2n5551 and 2n5401 and the outputs being 2sc2073/a940 pairs. The green one appears to be one that has been replaced previously. If the rail voltages are actually +/- 28 then it would be a sure thing those outputs would go bad before long. The tip41/42 pair should work ok as long as you use the A or better voltage rated ones, and if the 2n5551/5401's are not damaged I would leave them be.
That's one thing i have been thinking out last night...
On the PCB 5551 and 5401 but on the transistor 2N6520 and 2N6517
But there is no sign on the solder area those transistor have been repaired or changed before
But i am just found those A940 C2073 have been changed before

On the sim i use different final transistor cause i have no model for C2073 and A940
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