burning resistors-why?

[dale116dot7]

If you don't have a variac, I would go through the circuit - each part at a time, and measure the base to emitter voltage of every transistor, measure the resistance of every resistor, and measure the forward and reverse voltage of every diode. You may have to desolder some connections to get good readings.

A classic three-stage amplifier (which this is one) needs to have its DC stuff all worked out first or it won't work right. Let's isolate stuff first.

Remove Q3, Q4, Q5, and Q6 from the circuit.
Solder a 220 ohm, 1 watt resistor across D2. Connect a 220 ohm, 1 watt resistor across D1. Remove all loads - the 8 ohm resistor, speaker, anything.

Connect a voltmeter to the output of the amplifier. Turn on the amplifier. Do you get 0 volts? Yes? then the input stage is either working or something isn't connected quite right. Measure the voltage on the collector of Q2. You should see a voltage between positive supply voltage and the output voltage (0). In addition, you should see:

0 volts at the base of Q1 (and Q1A)
0.7 volts below the positive supply voltage at the collector of Q1.
-0.7 volts at the emitter of Q1 (and Q2).

The output appeared to be at the right voltage, but it is possible for a subtle failure to cause a problem, so I've found it useful to make sure the input isn't causing a wierd problem that's being masked elsewhere.

Oh yea, there's always a possibility of oscillation. Do you have access to a 'scope?

 

[mich]

Remove Q3, Q4, Q5, and Q6 from the circuit.
Solder a 220 ohm, 1 watt resistor across D2. Connect a 220 ohm, 1 watt resistor across D1. Remove all loads - the 8 ohm resistor, speaker, anything.

-done (with 180 ohm across D1-didnt have 220)

now im measuring 19mV on the output.

2.4V on collector of Q2
-44 mV on base of Q1
-30 mV on base of Q1A
33 V(!) on collector of Q1
-0.6V on emiter of Q1(and Q1A)

i have a software scope on the pc.......

 

[BYacey]

Did you put a 180 across D1 AND D2? If so, what is your voltage at the junction of D1 and D2? It should be sitting within 20mv of zero. Double check R20 and R21. Lift one leg of each resistor from the circuit and measure them.

 

[mich]

Did you put a 180 across D1 AND D2?

i have 220 ohm across D1 and 270 ohm across D2,
and im measuring 71 mV on D1-D2 junction.(with only Q1 and Q2 in the circuit)

 

[BYacey]

These two resistors should really be the same value for accurate testing.

 

[mich]

i have the same reading with 2 x 270 ohm......what am i doing wrong?

 

[BYacey]

No, you're not doing anything wrong. Once the other transistors are in place the drive current should pull it closer to 0 volts, and the negative feedback will take care of the rest. Does your amp have the switch or jumper in place on the bottom right of the schematic? I guess next thing to do is put back Q3 and Q4, and measure the speaker ouput with respect to ground (remove the 270 ohm resistors) . You should still have 0 volts and be able to measure equal voltage across R15 and R16. Also measure across R17 and R18, this voltage will be very low, probably around 1 to 2 mv depending where the bias control is set for. Make sure C9 isn't leaky or shorted.

 

[mich]

o.k!-
Q3 and Q4 are in the circuit,
i've got 24mV on the output .
same voltage across R16+R15 (0.97V)
same voltage across R17+R18 (0.6mV)
AND NOTHING IS BURNING.....

i guess its time to put in the big transistors?
(i dont have the P.A switch- i do have the jumper)

 

[BYacey]

If you juggle the value of R12 around a little you should be able to get it balanced at zero. 24mv is still a little much for offset. Once you get around 5mv or less, then you can put the outputs in (after testing to make sure they aren't damaged). Then measure the speaker output, again do not connect a load yet at this point. If it is still balanced within 5 to 10mv of zero, connect a 200 ohm resistor across the output terminals instead of a speaker. Apply a sine wave signal to the input and scope across the speaker + output and ground the scope lead to chassis ground, NOT the speaker minus terminal. You should have a nice sinewave from zero to clipping. Note if the rails clip at the same time and check for crossover notch at very low output, around .25 volts RMS. This will show if there is enough bias on the outputs.

 

[mich]

thanks bill! ill get on with that-and ill tell u what i came up with.

 

[mich]

now i have 0.3 mV on the output from + to - of the speaker.(with the output transistors in the circuit)

i dont have a scope,so i conected to the preamp and speaker ,and everything looks o.k-but there is only little amplification going on.i have a clean sound(no oscillation of any kind)-i checked ALL the transistors again(with mark's simple silly tester-great thing!), and they are all fine.wich leads me to think that maybe the replacement driver transistors that i have are not 100% compatible?(i have nte185 for mje172,and bd785 for mje182)?

 

[BYacey]

.3mv is good. What is your voltage across R18 and R19 now? it should be around a few mv, probably not much more due to the small value of the emitter resistors.

This thing could possibly be in high frequency oscillation but you wouldn't hear it, the outputs will heat up and the only way to really tell is to check with a scope. Measure the AC voltage on the output with out any input signal. Some meters will respond to AC frequencies well past the audio band. If it's below 100Khz. you might read something if it is oscillating.

 

[PRR]

> 220 ohm, 1 watt resistor across D2. Connect a 220 ohm, 1 watt resistor across D1.

D1 D2 are the protection diodes. A very odd place to put 220 ohm resistors. And will kill the gain, which matches the new reports. The amp will not work normally that way.

> im measuring 19mV on the output.

That's not bad. Actually I'd expect at least that much. Trying to trim this low-cost design to zero offset is futile. Yes, a tenth of a volt DC is acceptable. Less would be good, but that requires a better circuit than just 7 transistors.

Are you very sure about the pin-out of the old and new transistors? Some of your results could be due to that.

> have 290mV across R16 and 230mV across R15.

And I said "near 0.6V". Nevermind. It is OK to be less. These voltages are strongly affected by the R14 bias pot. If they are less than 0.5V, transistors Q5 Q6 will idle cold, but come to life with signal. Many amps work this way. When it is working 99.9% but you think it has a grating quality at low levels, we can trim R14 to make the voltage across R17 R18 about 5mV-10mV (no higher!).

 

[BYacey]

[quote author="PRR"]> 220 ohm, 1 watt resistor across D2. Connect a 220 ohm, 1 watt resistor across D1.

D1 D2 are the protection diodes. A very odd place to put 220 ohm resistors. And will kill the gain, which matches the new reports. The amp will not work normally that way.

> im measuring 19mV on the output.

That's not bad. Actually I'd expect at least that much. Trying to trim this low-cost design to zero offset is futile. Yes, a tenth of a volt DC is acceptable. Less would be good, but that requires a better circuit than just 7 transistors.

Are you very sure about the pin-out of the old and new transistors? Some of your results could be due to that.

> have 290mV across R16 and 230mV across R15.

And I said "near 0.6V". Nevermind. It is OK to be less. These voltages are strongly affected by the R14 bias pot. If they are less than 0.5V, transistors Q5 Q6 will idle cold, but come to life with signal. Many amps work this way. When it is working 99.9% but you think it has a grating quality at low levels, we can trim R14 to make the voltage across R17 R18 about 5mV-10mV (no higher!).[/quote] The resistors were just in place for testing while the drivers and outputs were removed, not to be there permanently.

 

[mich]

This thing could possibly be in high frequency oscillation but you wouldn't hear it, the outputs will heat up and the only way to really tell is to check with a scope. Measure the AC voltage on the output with out any input signal. Some meters will respond to AC frequencies well past the audio band. If it's below 100Khz. you might read something if it is oscillating

couldnt find any oscillation with the voltmeter.....BUT i found why the output was so low- it had a 470 ohm resistor soldered on the negative output
cable(that goes to the speaker) - it seemed like it should have been there,but i dont see it anywhere on the schematic...so i took it out.

so now it works!
but im still not sure that i finished....
i have about 8V AC on the output(measuring from + to - of the speaker with the preamp volume at 10,and a strum of a chord from a guitar)
and it doesent sound like 120 Watt to me..... also the output transistors get very hot in their hitsinks.(put my finger on the hitsink and hear "tsssss"........)- dont know if its normal.....

 

[BYacey]

If the outputs are hot, either it's over-biased - check DC voltage across R17 and R18. At idle you should only have a few millivolts here. Try reducing this voltage and see if the outputs cool off. If not, there most likely is some ultrasonic noise heating up the outputs. Without a load, if the DC voltage on the output is less than 50mv and the transistors run cool until you connect the load, HF oscillation is the most likely culprit. What about when the input jack is shorted, does it run cool then?

 

[mich]

now im almost sure its some oscillation.....

after 2-3 hours of playing,i've burned another pair of output transistors......

or maybe the heatsinks i have aren't big enough?
does these "look like" they will do the job?:

ive got 10mV on R17,R18 and the output is very close to zero.

thanks.

 

[BYacey]

10mv across R17 and R18 is too much. That's quite a bit of current at idle. I would see if you can reduce it to about 1 or 2 mv. Those emitter resistors are pretty low in value. Also, make sure the diodes in the bias circuit are in contact with the heatsink, otherwise you will experience thermal runaway. At idle without any signal for at least 15 minutes, the heatsinks should be just slightly warm to the touch.

 

[mich]

thanks bill.ill do that.