Hi,
my new vintage amp got two problems:
Sometimes on one channel the output relais does not switch on.
If the channel goes on, the sound is distorted if the volume is more than room level.
So far I have compared DC voltages on the small board that is attached to each heatsink.
There are TO220 Transistors, and one leg has 0,7V each transistor on both pcbs, but another leg got -0,5 on each leg on the working channel but it is also 0,7V on each leg on the the defective channel.
Does that point to a bias problem located on the main driver board ?
To me it looks like distortion starts when bias current is exceeded by signal current, just a guess...
There is offset 80mV both channels.
I did not measure anything in condition when relais did not switch after pushing power button.
Both heatsinks stay cold.
Any fast & useful hints ?
Thanks, Bernhard
my new vintage amp got two problems:
Sometimes on one channel the output relais does not switch on.
If the channel goes on, the sound is distorted if the volume is more than room level.
So far I have compared DC voltages on the small board that is attached to each heatsink.
There are TO220 Transistors, and one leg has 0,7V each transistor on both pcbs, but another leg got -0,5 on each leg on the working channel but it is also 0,7V on each leg on the the defective channel.
Does that point to a bias problem located on the main driver board ?
To me it looks like distortion starts when bias current is exceeded by signal current, just a guess...
There is offset 80mV both channels.
I did not measure anything in condition when relais did not switch after pushing power button.
Both heatsinks stay cold.
Any fast & useful hints ?
Thanks, Bernhard
Bernhard said:
Bernhard,
The oscillations stop at the point where the other half takes over. Could it be that the bias is too low, that it is in class B instead of class AB? The oscillations could trigger the protection circuit and/or show up as offset on the DC meter. The difference in Vbe also points that way.
You verify the correct bias setting by measuring between 30 and 70 mV across two Re's in series, like R140-143, or R141-1444, or R142-145, other channel: R240-243, R241-244, R242-245.
You can measure that on each power stage sub board: put the amp front of you so that the sub-board looks like a 3-legged bridge, the "legs" connecting to the three individual power transistor sub-sub boards. The bias test points are the top left two pads of the "bridge", where no wires are connected.
Jan Didden
To adjust the bias, turn RV202 (R-channel) or RV102 (left channel).
Bernhard said:
.... meaning somebody "improved" the amp after it left the factory. Can you adjust the good channel to 30-70 mV?
Jan Didden
Edit: With this info I think that one power transistor on the bad channel is (partially) shorted, the one that measured the lower Vbe. Can you isolate that sub-sub board, take off the wires to it, and see it the rest now recovers? Don't forget to turn the bad channel bias to min first!
janneman said:
No, it is turned to max. by factory.
On the main boards, there are transistors with heatsinks, two each.
On the collector of the inner transistor of the good channel I measure -64V.
On the bad channel it is also -64V but when distortion starts, it breaks down to 0,2V. The 100 ohm resistor seems to be ok.
+64V on the other transistor is ok.
Bernhard said:
Not anymore, maybe that was an error of me, but in the beginning I measured 0,2V also without signal.
But the emitter of this transistor got 0,7V while it is -0,5V on the good channel.
On that leg the ac sine signal has spikes while it is a clean sine on the good channel .
Spike:
Attachments
Bernhard said:
That would be Q130-131 (230-23), the bias current sources. The +/-64 are the main supplies for the VFETs. There seems to be a problem there, then. Check the 64v supplies: C301 (dual 22.000uF/71V), (C401), they have parallel caps of 2.2uF/100V, C305-306 (405-406). See if those supplies stay up with some (dummy) load. If they stay up, there may be a problem in the wiring to the boards.
But I find it strange that both bias pots are " factory" set to max. Are you sure they have not been fiddles with? You didn't buy this amp new, did you?
Jan Didden
More educated guesses...
Could be Q130 is shot. That 100 ohms series resistor, does that get hot? If yes, looks like Q130 is gone. If no, the resistor may be bad.
But that +0.7 on Q131 seems to say that Q130 is shorting, and that drives Q131 to absorb a high current and that drives the Ve up.
Jan Didden
Could be Q130 is shot. That 100 ohms series resistor, does that get hot? If yes, looks like Q130 is gone. If no, the resistor may be bad.
But that +0.7 on Q131 seems to say that Q130 is shorting, and that drives Q131 to absorb a high current and that drives the Ve up.
Jan Didden
janneman said:
The voltage on emitter of Q130 is ok = 0,7V , on both ends of R135 is same 0,732V so Q131 is dead , E-C does not conduct and sometimes get short ? Also R136 measures 250ohm while the others measure about 130ohm.
If 2SA835 is dead, is there an easy-to-get replacement ?
The heatsinks of the transistors of the good channel feel like nothing when I touch them while on the bad channel both feel
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