?Testing Transistors?
- Thread starter rswojo
- Start date
Ylli
Super Member
Cycle the switches a few times. Function Switch, Input Selector, Monitor Switch, Mode Switch. Ideally we would want to clean these, but for now a little cycling should get some signal through them if that is the problem.
If the above doesn't help,
This is the 'Preamp' board - Equalizer Amp and Flat Amp
Try injecting hum' into the circled points. Won't be real loud - in fact you may just hear a 'clicking' as you make contact with the point. Left and right both work here?
If the above doesn't help,
This is the 'Preamp' board - Equalizer Amp and Flat Amp
Try injecting hum' into the circled points. Won't be real loud - in fact you may just hear a 'clicking' as you make contact with the point. Left and right both work here?
However if something is out on power amplifier board speaker will have no power to react to screwdriver.
Not saying that is where the problem is, just that poking that leg of the cap wouldn't tell us anything if power amp was out
Last edited:
Ylli
Super Member
Is this probing at C614 and C714? Both channels should sound the same.
It looks like everything except the AUX and Tape input goes through the EQ Amp. The eq networks are switched in as appropriate. Is the collector of Q701 actually zero?
Next point to try to hum inject is shown in the pic. I'd like you to be able to say both channels give you some audible hum and they are the same level. If there is not enough gain between there and the output to hear the hum, we may have to steal a bit of audio from somewhere else to inject.
Last edited:
No, just ear to speakers at full volume, no input. At full volume left speaker is very slight hum (noise floor?). Right speaker very slight slow static.
Yes collector of Q701 is 0V.
Hum inject at R637. Hum at left channel. Right channel silent.
Ylli
Super Member
It's possible the speaker select switch is dirty. Try cycling that a few times. If you have some headphones, you could try those, the 'phones bypass the speaker switch. If that does is not helpful, then...
Looks like we have a power amplifier to trouble shoot. Don't know how accessible they are, but you might want to start by checking the device voltages as shown in the schematic. I could walk you through it step by step, but you seem to have a handle on how to do it. Note discrepancies between what is listed on the schematic and what you see.
Am I keeping you busy enough?
Looks like we have a power amplifier to trouble shoot. Don't know how accessible they are, but you might want to start by checking the device voltages as shown in the schematic. I could walk you through it step by step, but you seem to have a handle on how to do it. Note discrepancies between what is listed on the schematic and what you see.
What do you see on the base and emitter of Q701? How about collector/base/emitter of Q702. I don't see any failure mode that could make the collector of Q701 zero except if the transistor had a collector-emitter short. Leave it in circuit and (with power off) use the diode mode of your meter to check collector to emitter - swap the meter leads and do it again.
Am I keeping you busy enough?
Diode test: 1.06, OL I expected something like 0.6 or 0.7.
Transistor readings on pre amp board. First column is actual. Second column is what is expected from schematic.
Q701
E=0.004 E=0.1
B=0.5 B=0.55
C=2.28 C=3.2
Q702
E=1.66 E=2.5
B=2.28 B=3.1
C=1.8 C=15.5
Q703
E=0.3 E=0.5
B=0.7 B=1.1
C=2.29 C=3.3
Q704
E=0.3 E=2.8
B=2.28 B=3.3
C=12.0 C=33.5
Q705
E=1.12 E=2.2
B=1.75 B=2.8
C=3.76 C=13.5
Looks like some large discrepancies here.
The collectors have a plastic sleeve covering the leg. On Q701 there was a little, loose piece of plastic inside the plastic sleeve that was sliding down and blocking my probe. It almost looked like the collector leg was broken off, but I poked around and that little piece of plastic came out.
That power amp problem cropped up during our testing. I know that when output transistors blow they take out things near them. If I shorted/grounded something on the MPX board I could understand that causing a problem on the adjacent pre amp board but all the way into the power amp board?
I have been checking things with the speakers and when I have doubts I plug in the headphones.
The right channel problem led to a little confusion with the MPX problem apparently.
Expect many more questions, I am getting a tuition free education here and I am taking advantage of that.
Thank you very much.
I will check the devices on the power amp board tomorrow afternoon. I've had enough for today.
I'm the one that instigated this busyness. I'm getting what I deserve but I feel bad keeping you so busy.
Transistor readings on pre amp board. First column is actual. Second column is what is expected from schematic.
Q701
E=0.004 E=0.1
B=0.5 B=0.55
C=2.28 C=3.2
Q702
E=1.66 E=2.5
B=2.28 B=3.1
C=1.8 C=15.5
Q703
E=0.3 E=0.5
B=0.7 B=1.1
C=2.29 C=3.3
Q704
E=0.3 E=2.8
B=2.28 B=3.3
C=12.0 C=33.5
Q705
E=1.12 E=2.2
B=1.75 B=2.8
C=3.76 C=13.5
Looks like some large discrepancies here.
The collectors have a plastic sleeve covering the leg. On Q701 there was a little, loose piece of plastic inside the plastic sleeve that was sliding down and blocking my probe. It almost looked like the collector leg was broken off, but I poked around and that little piece of plastic came out.
That power amp problem cropped up during our testing. I know that when output transistors blow they take out things near them. If I shorted/grounded something on the MPX board I could understand that causing a problem on the adjacent pre amp board but all the way into the power amp board?
I have been checking things with the speakers and when I have doubts I plug in the headphones.
The right channel problem led to a little confusion with the MPX problem apparently.
Expect many more questions, I am getting a tuition free education here and I am taking advantage of that.
Thank you very much.
I will check the devices on the power amp board tomorrow afternoon. I've had enough for today.
I'm the one that instigated this busyness. I'm getting what I deserve but I feel bad keeping you so busy.
Nope.
Ylli
Super Member
Everything is low. I'm wondering about that 33.5V power buss. The voltage at the collector of Q704 has only a 39 ohm resistor to the buss, so should be very close to the actual buss voltage. 12 is not close to 33.5.
The main power supply generates +80 volts. It is fused with F801. That 80 volts goes to the protection board where it connects to the collector of Q802. Q802 is configured as a 'Capacitance Multiplier' and exits the emitter of Q802 at +78 volts. That 78 volts is used by the power amps. In addition, that 78 volts passes through R801, a 2.7K 2W wirewound resistor and drops to +33.5 for use by the preamps.
Things to check:
F801. Be sure it is not blown.
Find Q802 on the protection board. You should measure 80 volts on the collector and about 78 volts on the emitter. If that is OK, locate R801. Looks like it is chassis mounted. Measure the voltage on each side of it. Then, with power off, measure its resistance (in circuit is OK), Reverse the meter leads and measure in the other direction. [we measure in both directions to be sure a transistor junction is not messing with our measurement]
The main power supply generates +80 volts. It is fused with F801. That 80 volts goes to the protection board where it connects to the collector of Q802. Q802 is configured as a 'Capacitance Multiplier' and exits the emitter of Q802 at +78 volts. That 78 volts is used by the power amps. In addition, that 78 volts passes through R801, a 2.7K 2W wirewound resistor and drops to +33.5 for use by the preamps.
Things to check:
F801. Be sure it is not blown.
Find Q802 on the protection board. You should measure 80 volts on the collector and about 78 volts on the emitter. If that is OK, locate R801. Looks like it is chassis mounted. Measure the voltage on each side of it. Then, with power off, measure its resistance (in circuit is OK), Reverse the meter leads and measure in the other direction. [we measure in both directions to be sure a transistor junction is not messing with our measurement]
Ylli
Super Member
If the emitter voltage is only 36.4, don't bother with R801.
No, I don't think protection. They just decided to put this series pass transistor on the 'Protection' board.
B - E, then reverse leads and measure again
B - C, then reverse leads and measure again
C - E, then reverse leads and measure again.
If this transistor looks good, then we will need to look at R805 and C814.
No, I don't think protection. They just decided to put this series pass transistor on the 'Protection' board.
This sure looks like a defective Q802. Probably need to pull it and test it with your meter on diode range. Remember 6 readings:
B - E, then reverse leads and measure again
B - C, then reverse leads and measure again
C - E, then reverse leads and measure again.
If this transistor looks good, then we will need to look at R805 and C814.
Ylli
Super Member
With the base and emitter voltages the same, I expect to see a B-E short show up in the test data. Been staring at the schematic trying to figure out what may have caused this device to fail (assumed it actually did fail and we are not chasing a false lead). Don't see anything downstream that could have cascaded to take out Q802.
