ElectroVoice Q-66 amp schematic?

[nosaj]

Have you looked at the voltages and compared them with expected values? It's clear that some of them are completely wrong. For example: -83.5V on base of Q123, or -79.1V on base of Q110.
The question is which voltage is the best to look at? I would start with -79.1V on base of Q110. The base is tied with 1k resistor to +15V voltage rail. So such a voltage is almost impossible unless the D105 diode has failed or the R127-R135 resistors are open, or R143 is open. Can you check it (do you have +15V on D105 and R143 is not open)? Of course, failure of Q104, or Q106 can also cause the problems.

Mark

D105 +15.3
D106 -15.3
Q104 2.0 2.2 14.5
Q106 14.67 15.3 78.9

I believe the resistors are not open but I think I need to go back with the magnifier to verify the resistors. I did on the Transistors but have to put it away as storms are moving in.


Thanks for the help
nosaj

[nosaj]

D105 +15.3
D106 -15.3
Q104 2.0 2.2 14.5
Q106 14.67 15.3 78.9

I believe the resistors are not open but I think I need to go back with the magnifier to verify the resistors. I did on the Transistors but have to put it away as storms are moving in.


Thanks for the help
nosaj

They all test in the 4k ohm range. I'll have to pull the board and unsolder a lead on each to be sure.
Based on the voltages of q104 and q106 should I pull them also and test individually?

Thanks,
nosaj

[nosaj]

They all test in the 4k ohm range. I'll have to pull the board and unsolder a lead on each to be sure.
Based on the voltages of q104 and q106 should I pull them also and test individually?

Thanks,
nosaj

Anyone have any other suggestions for me?

Thanks,
nosaj

[The Dude]

There are several of those transistors reverse biased B-E. My hunch is that we're looking for an open transistor. I would check those measuring "oddly" with your diode check function paying attention to basing diagram and look for a junction that "isn't there". Also, if it's possible for you to label voltages on the schematic and repost it, it might be easier to spot something.

[nosaj]

There are several of those transistors reverse biased B-E. My hunch is that we're looking for an open transistor. I would check those measuring "oddly" with your diode check function paying attention to basing diagram and look for a junction that "isn't there". Also, if it's possible for you to label voltages on the schematic and repost it, it might be easier to spot something.

May take me a few days but I'll get them.

Thanks.

nosaj

Would q106 and Q110 be an example of a reverse biased Transistor?

[nosaj]

May take me a few days but I'll get them.

Thanks.

nosaj

Would q106 and Q110 be an example of a reverse biased Transistor?

Here we go i pulled the voltages and transposed them on a schematic. Q115 looks to me to be shorted according to voltages. The ones that have neg and pos values I'm not sure what to make of those.
Really wasn't sure what you meant by a junction that wasn't there when testing.

Thanks so much,
nosaj

[nickb]

From your numbers it looks like (at least) Q116, Q117 and Q118 are bad.

[nosaj]

From your numbers it looks like (at least) Q116, Q117 and Q118 are bad.

Is that because the voltages are so close together?

Thanks,
Jason

[The Dude]

The voltage drop base to emitter on a silicon transistor is the same as a diode (.6 volts or thereabouts). Generally speaking, if it's not and the voltage is higher than a normal junction, either the junction is open or it's reversed biased. Look at the little diode symbol within the transistor diagram to get polarity. A working NPN will have the base more positive than the emitter. A working PNP will have the base more negative than the emitter.

Edit: This is also why most prefer to use the diode check function to check transistors instead of taking resistance measurements.

Edit (again): This might help a bit.

https://www.elexp.com/electrical_mea...ts_part_3.aspx

[nosaj]

The voltage drop base to emitter on a silicon transistor is the same as a diode (.6 volts or thereabouts). Generally speaking, if it's not and the voltage is higher than a normal junction, either the junction is open or it's reversed biased. Look at the little diode symbol within the transistor diagram to get polarity. A working NPN will have the base more positive than the emitter. A working PNP will have the base more negative than the emitter.

Edit: This is also why most prefer to use the diode check function to check transistors instead of taking resistance measurements.

Edit (again): This might help a bit.

https://www.elexp.com/electrical_mea...ts_part_3.aspx

So with q115 all 3 voltages the same would mean a dead transistor?
Thanks,
nosaj

[The Dude]

It would mean it's time to check it with a DVM. It's quite possibly shorted, but it takes only a minute to be sure.

I'd look at the ones nickb pointed out first. The voltage B-E is the correct polarity, yet much more than a typical junction. They are almost certainly open. I have no idea your skill level on solid state devices, so please excuse me if I'm seeming to talk down. That said, the link I posted earlier explains how to check a transistor on diode check about half way down the page. It may well be one of the most useful things you'll learn when working with transistors.

[nosaj]

It would mean it's time to check it with a DVM. It's quite possibly shorted, but it takes only a minute to be sure.

I'd look at the ones nickb pointed out first. The voltage B-E is the correct polarity, yet much more than a typical junction. They are almost certainly open. I have no idea your skill level on solid state devices, so please excuse me if I'm seeming to talk down. That said, the link I posted earlier explains how to check a transistor on diode check about half way down the page. It may well be one of the most useful things you'll learn when working with transistors.

My skill level is always learning. I've printed it out to look over during break and lunch tomorrow. I was just looking at what measurments I posted and thought I ask about but what I thought I understood. I can check them tomorrow with a VOM. I've got a simpson 260 and a fluke 110( I got it buecause I had questionable readings from other meters but it's an electricians model and no diode function test. I also have an 8020 I think it might do a diode test I'll have to check. It maybe this weekend before I can look again. They kickin our arses at the cabinet shop.

nosaj

[nickb]

Is that because the voltages are so close together?

Thanks,
Jason

What The Dude said +
Q116 has about 130V across it's BE junction - it should be 0.7V max
Q118 has over 130V - it should only be about 2V as it's configured as a VBE multiplier
Q117 is a grey area - the voltages are OK but given that the previous two are bad I'd want to test it.

Also check for open resistors in that area.

[nosaj]

What The Dude said +
Q116 has about 130V across it's BE junction - it should be 0.7V max
Q118 has over 130V - it should only be about 2V as it's configured as a VBE multiplier
Q117 is a grey area - the voltages are OK but given that the previous two are bad I'd want to test it.

Also check for open resistors in that area.

Q116,q117, Q118 are all shorted.
R165,167,168,171,170,160,163,172,r156,159 all are not open, I'll have to lift a leg to see if they're in spec or not.

nosaj

Also I need to get a 2sc4793 and a 2sc4370(for another amp) I have a big order at mouser but can't quite come up with something from them that will work.

Thanks,
nosaj

[nosaj]

Can anyone help?
I need to get a 2sc4793 and a 2sc4370(for another amp) I have a big order at mouser but can't quite come up with something from them that will work.

Thanks,
nosaj