bias a trace elliot ta100r

[MarkusBass]

I measured the current of 1 mosfet, not the entire PA.

I don't know what is the correct value of the current but I would set it to at least 50 mA and verify how hot gets the amp when you provide a signal to the input. If it gets hot, decrease the current slightly. Just calculate: power supply voltage (is it 60Vor less?) multiplied by 50 mA (0.05A) and you get 3W at idle - for me, it seems resonable.

- did you measure the amp with a dummy load (and full power)?
Not sure what you mean here. What am I measuring?

Signal to the input (1V or more), oscilloscope to the dummy load (4 Ohms) and you observe the signal on the output. You can measure the power of the amp before it starts distorting the signal.
You can also check whether the idle current decreases when the amp gets warmer.

Mark

[lowell]

Ok thanks Markus,

Well for now since I have the speakers disconnected, I've monitored the voltages on TR4 and TR6 collectors:

TR4 ranges from -1v to -.375v
TR6 does NOT change when the pot is turned

Does this clue you into something?

[MarkusBass]

Hard to say without some further measurements. Are you sure that the voltage on T4 (C) is negative? It should be positive. And the other should change in similar way. Would it be difficult to desolder the MOSFETs? It would be benefitial to check the amp without the output transistors - without speaker but please note that there is a DC negative feedback loop from the output back to TR3 (R14) so I would put some resistor (e.g. 220R) as a load. Do you have 0V on the output?
If you cannot easily desolder the MOSFETs, I would check voltages on TR6 (compare them with TR5). Maybe TR6 is shorted - this is the easiest one to be replaced (but first do some measurements). In similar amp I had problems with the diodes in the output stage (they testes OK but they were not OK). I hope it's not 2SJ50.

Mark

[lowell]

Markus,

I'll take some measurments first before removing mosfets.

Yes T4 collector is negative.
There is -70mv on the output with NO speakers connected.

TR6
C: -1v
E: -53.5v
B: -52.9

TR5
C: +18v
E: -53.5v
B: -52.9

So if I were alone on this repair I'd probably replace TR6. Voltages seem good except TR6 collector right?

[MarkusBass]

Yes, T4 collector is negative.
There is -70mv on the output with NO speakers connected.

TR6
C: -1v
E: -53.5v
B: -52.9

TR5
C: +18v
E: -53.5v
B: -52.9

So if I were alone on this repair I'd probably replace TR6. Voltages seem good except TR6 collector right?

-1V on TR6(C) seems to be a correct value. I would expect the voltage there to be within -1.5 to -0.5V. Can look at TR4 (I mean measure voltages on it)? This is a current source and it seems to me that it is not working correctly. While being there you can measure voltage drop on D6 (this is also a part of the current source) although it's probably OK because you have +18V on TR5(C). Have to checked other diodes in the output stage - including Zeners?

Mark

[lowell]

Mark,

TR4
E: 53v
C: -400mv
B: 52.7mv

Voltage across D6 is 660mv

Also of note:

590mv across R21.
540mv across R22
~1v across R23

Yes I've checked all the diodes in the PA for shorts/opens.

[MarkusBass]

Mark,

TR4
E: 53v
C: -400mv
B: 52.7mv

Voltage across D6 is 660mv

Also of note:

590mv across R21.
540mv across R22
~1v across R23

Let me quickly do the math:
The V+ rail voltage is: 53V (TR4-E) + 590mV (R21) = 53.6V
The voltage on TR4(B) should be:
53.6V(+Vrail) - (540mV (R22) + 660mV (D6)) = 52.4V so 52.7mV is rather not possible. In this case, instead of measuring the voltage on base, you should measure the voltage between the V+ rail and the base. This will tell you whether TR4 is correctly biased (in order to provide current). And this is most probably: 540mV (R22) + 660mV (D6) = 1.2V, which is OK.
The current provided through the current source:
590mV(R21)/220 Ohms = 2.7mA
The voltage drop on P1 set to 50% (110 Ohms): 2.7mA * 110 = 300mV
The voltage drop on P1 set to 100% (220 Ohms): 2.7mA * 220 = 600mV
I would expect slightly higher voltage - up to at least 1.0V but maybe this is due to some failed component.
Start with replacing TR6 and see whether this changes anything. If not, replace TR4. Measure both transistors when you desolder them.

Mark

[lowell]

Mark,

Yes there is always a discrepancy when I measure these. It seems everytime I'd take a measurement it'd be slightly different in regards to millivolts. Sometimes a couple hundred millivolts off. I figured this was just my meter being imperfect. So I'm sure your math is correct, and also measuring between the rail and the base as well. I'll report back. I'll need to order these transistors though so it'll be a few days if not a week.

[lowell]

Dang I got the wrong transistor and replaced TR6. Didn't help... ordering TR4 now.

[J M Fahey]

Don't worry too much.
It might ave been worse.
Imagine a Doctor pulling the wrong kidney

[lowell]

Okay, well, replacing TR4 didn't fix it. Still same results on both ends of the bias pot. I tested the original TR4 out of circuit and it tested good. There is 1.1v across R22 and D6 string therefore there is ~52.9v on TR4's base. The voltage rail is 54v.

[lowell]

So Markus I'm stuck here. I'm thinking one of those zeners in the feedback path and/or between the mosfets gate/source junction are causing this problem. How would you suggest troubleshooting this?

Can I lift D8 and D9? And D7/D10? It seems to me that these diodes are here to limit signal and DC biasing will not be affected if they're aren't in circuit.

[lowell]

Okay so I also realized my current measurement was wrong. I found some traces that I had not seen. SO... measuring current of JUST the mosfets I have 0ma on both of them, no matter where the bias pot is. Maybe these "new" mosfets have gone bad or were damaged during or after installation? I get OL on the resistance measurement of the mosfets, except I get 7.5meg from drain/source on the p-channel mosfet, and only 500k from drain/source on the n-channel mosfet. I feel that that seems off.

What do you all think of replacing with these?

IRFP-9240
http://www.mouser.com/Search/Refine....word=IRFP-9240

IRFP-240
http://www.mouser.com/Search/Refine....yword=IRFP-240

[lowell]

Okay well I guess I lost the assistance in this repair. I got these transistors and replaced the ones in the amp again. Now I have these readings:

TR6 C:-3v (previously -1v)
TR4 C: range is -2.3v to -3v on extreme bias pot settings (previously -400mv to -1v)

At this point I'm just throwing shit at the wall and seeing if it sticks. Not the best approach to fixing an amp. I don't really understand why this voltage has changed though. I certainly wouldn't think that changing the MOSFETS would affect the quiescent bias voltage. But again, maybe I don't fully understand how MOSFETS work. These are enhancement mode MOSFETS right? So the gate NEEDS a turn-on voltage. ?

[MarkusBass]

I got these transistors and replaced the ones in the amp again.

Which exactly transistors do you mean? Did you replaced 2SK135 with IRFP? This is not allowed - they are completely different type and one cannot replace the other. The first one is lateral, and the other is vertical. You may read about differences between these two here: http://www12.fairchildsemi.com/an/AN/AN-558.pdf - check that the gate needs a turn on voltage, otherwise MOSFET is not conducting. So you need eg. +/-1.5V on gates (on P1 trimmer).

Now I have these readings:

TR6 C:-3v (previously -1v)
TR4 C: range is -2.3v to -3v on extreme bias pot settings (previously -400mv to -1v)

I'm not sure how to read these measurements. There is a current source and turning the P1 trimmer should vary the voltage between 0 and let's say +/-2V. Does it work in this way, or not?

At this point I'm just throwing shit at the wall and seeing if it sticks. Not the best approach to fixing an amp.

During these few days you could already replace all semiconductors in the amp (without testing them). Alternatively, you can read about differential amp, current source and lateral MOSFETs. My advise is to remove the MOSFETs (whatever MOSFETs you have) and fix the amp without them. You can put 1.5k/10W resistors in place of the MOSFETS (drain-source) - just to be sure that you have 0V on the output. Instead of the speaker connect a resistor - someting like 470 Ohms/5W. Then, using a signal generator, an oscilloscope and a DMM make sure that the amp works. It means that you have nice sinusoidal signal on both ends of the P1 trimmer. And that you can vary the voltage on P1 up to +/-2V (in this case without the input signal).
You can lift the Zener diodes on the output - the are to protect the MOSFETs against to high voltage on the gates. Check each of the and solder them back. Make sure that the amp still works as expected.
Then (or maybe start with this) measure the gate resistors whether they are not open. This is a common failure in such amps.
Separately, you can check each (old) MOSFET with an external power supply and e.g. 16 Ohms resistor. Search for "MOSFET matching circuit". If they are fine, solder them back.

Mark