Altec 1593B bias issue - diyAudio
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Old 10th November 2014, 03:40 AM   #1
mazur is offline mazur  United States
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Default Altec 1593B bias issue

Hi.

I'm having a real strange issue with my Altec 1593B. I have 2 amps, one sounds great but the other has no bias on the power transistors at all. At very low volumes the sound is distorted, as expected, but as the volume is turned up the transistors are biased and starts to sound better and at loud volume sounds fine, since the bias gets to the operating point.
I have attached the schematic. Here is what I've done so far:

- both circuit boards (the part on the schematic surrounded by the dotted lines) work on the good amp. But neither work on the bad amp. So it's definitely not the board.

- with no signal, on the bad amp, the base to ground voltage of Q1 and Q3 are around .5V as expected from the circuit. But the emitter to ground voltage which should be around 30mV is zero.

- I tested all transistors and they are all fine. Still, I replaced all transistors with a new set and still the same. I also replaced the zener CR5 for the 15V line and the diodes CR2 and CR3.

- all collector voltages are fine at 31V, the OPT looks OK and it's probably OK since it plays fine at loud volumes. The stabistor measure 1.3V as expected. The power resistors measure fine and are not open.

- I re-soldered all connections from the circuit board connector and the connections on the power transistors.

Don't know what I'm missing here. Any help is appreciated.

Thanks.
Elias
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Old 10th November 2014, 03:45 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by mazur View Post
Hi.

I'm having a real strange issue with my Altec 1593B. I have 2 amps, one sounds great but the other has no bias on the power transistors at all.

-
How about the double diode CR1? This may be bad, or its connection to the circuit. If both transistors of the pair are no bias, it must be something that affects
both sides of the circuit the same, symmetrically.

Last edited by rayma; 10th November 2014 at 03:51 AM.
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Old 10th November 2014, 03:13 PM   #3
mazur is offline mazur  United States
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Quote:
Originally Posted by rayma View Post
How about the double diode CR1? This may be bad, or its connection to the circuit. If both transistors of the pair are no bias, it must be something that affects
both sides of the circuit the same, symmetrically.
I thought about that as well, but the voltage drop on the CR1 is 1.3V as shown in the schematic and the voltage in the connector is 1.3V as well. Also, the forward bias voltage out of the board is .5V on both sides. I checked continuity and it looks fine.

What is really strange is that the voltage on both bases are .5V as per schematic, but the emitter in on 0V.

Thanks
Elias
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Old 10th November 2014, 03:31 PM   #4
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Have you checked the bias voltage resistors, R124/125 and R7?
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Old 10th November 2014, 03:34 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by mazur View Post
I thought about that as well, but the voltage drop on the CR1 is 1.3V as shown in the schematic and the voltage in the connector is 1.3V as well. Also, the forward bias voltage out of the board is .5V on both sides. I checked continuity and it looks fine.

What is really strange is that the voltage on both bases are .5V as per schematic, but the emitter in on 0V.

Thanks
Elias
The 0.5V is applied as a bias voltage, so it's not the result of current flowing in the output transistors.
Can you ohm the connections from the emitters of Q1,2,3,4 to ground? It seems like there's an open connection there.

Last edited by rayma; 10th November 2014 at 03:51 PM.
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Old 10th November 2014, 04:38 PM   #6
mazur is offline mazur  United States
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Quote:
Originally Posted by JonSnell Electronic View Post
Have you checked the bias voltage resistors, R124/125 and R7?
Yes. R124/125 is in the circuit board so I know it works, also because the bias is reaching the base of Q1 and Q3. I checked R7 and it tests fine.
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