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Old 24th November 2009, 05:30 AM   #1
tomat is offline tomat  Indonesia
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Unhappy Help with this amp

a few years ago my friend built me this hybrid amp,now he move to other country and i lost contact with him.
this amp now damage,it has distort sound,such like clipping on every volume.
after i turn off the amp,i measure the dc offset on one channel,it show -50vdc,after the relay click on.
somebody can help me please..
here is the schematic for input stage (tube) & output stage

lot of thanks for the help
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Old 24th November 2009, 05:39 AM   #2
tomat is offline tomat  Indonesia
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transistors information:
q42= a970
q58= c2240
q43= d669
q57= b649
q44= 2n6488 (not mje 3055)
q56= 2n6491 (not mje 2955)
q45= c3281 ,4 pcs each channel
q52= a1302 ,4 pcs each channel

the emitter resistor for q45 and q52 are 0.47ohm,they are not bypass such on pic

the supply are +/- 55vdc
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Old 24th November 2009, 05:42 AM   #3
jleaman is offline jleaman  Belgium
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Quote:
Originally Posted by tomat View Post
transistors information:
q42= a970
q58= c2240
q43= d669
q57= b649
q44= 2n6488 (not mje 3055)
q56= 2n6491 (not mje 2955)
q45= c3281 ,4 pcs each channel
q52= a1302 ,4 pcs each channel

the emitter resistor for q45 and q52 are 0.47ohm,they are not bypass such on pic

the supply are +/- 55vdc
q45= c3281 ,4 pcs each channel
q52= a1302 ,4 pcs each channel, toshiba's

So are you saying there is 55V on the speaker output ?
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Old 24th November 2009, 06:22 AM   #4
tomat is offline tomat  Indonesia
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yes, -50,... vdc on the speaker output ,when the relay turn on,but when the relay still turn off,there is no significant dc on output speaker,and that just on one channel,other channel is normal,but the sound is distort,like clipping.
thanks
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Old 24th November 2009, 06:27 AM   #5
mos57 is offline mos57  Italy
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Quote:
Originally Posted by tomat View Post
transistors information:
q42= a970
q58= c2240
q43= d669
q57= b649
q44= 2n6488 (not mje 3055)
q56= 2n6491 (not mje 2955)
q45= c3281 ,4 pcs each channel
q52= a1302 ,4 pcs each channel

the emitter resistor for q45 and q52 are 0.47ohm,they are not bypass such on pic

the supply are +/- 55vdc
.

You verify before the fuse on negative and positive rail. If it is Ok, then it miss the bias in the upper half. If fuse is broken, then you have a transistors short circuit in the relative side.

Francesco.
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Old 24th November 2009, 06:32 AM   #6
Mooly is offline Mooly  United Kingdom
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Just quickly looked at this.
If it only gives DC offset when relay closes it must be that 0.22mfd cap on the valve output.
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Old 24th November 2009, 06:35 AM   #7
tomat is offline tomat  Indonesia
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Quote:
Originally Posted by mos57 View Post
.

You verify before the fuse on negative and positive rail. If it is Ok, then it miss the bias in the upper half. If fuse is broken, then you have a transistors short circuit in the relative side.

Francesco.

thank you,all transistors were check and they are normal,the fuse also ok,and i don't know where to set the bias for that amp,there is no bias pot as you see on schematic

cheers
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Old 24th November 2009, 06:37 AM   #8
mos57 is offline mos57  Italy
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Quote:
Originally Posted by tomat View Post
yes, -50,... vdc on the speaker output ,when the relay turn on,but when the relay still turn off,there is no significant dc on output speaker,and that just on one channel,other channel is normal,but the sound is distort,like clipping.
thanks
Yes, i not be seen your answer before: if you have dc offset only with relay on, then you have the 0,22 cap in short.

Francesco.
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Old 24th November 2009, 06:41 AM   #9
tomat is offline tomat  Indonesia
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Quote:
Originally Posted by Mooly View Post
Just quickly looked at this.
If it only gives DC offset when relay closes it must be that 0.22mfd cap on the valve output.
thank you very much,i was ever try to swap the cap 0,22uf for left channel and right channel,but the problem is still on same channel,the tubes was swap also for left and right,the result is the same
uuufffhhh... very headache
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Old 24th November 2009, 07:03 AM   #10
Mooly is offline Mooly  United Kingdom
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Are you saying you have swapped the cap and it's still faulty ?

If you only get an offset when the relay at the output of the valve closes then either the cap must be leaky or there is some strange wiring fault/short etc.

It's not possible for it to be anything else.
Remove the cap and see if the offset problem remains when the relay closes.

Extremely obscure is the posibility that the valve stage is oscillating, you would need a scope to check... or remove the valve.
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