MTX TA7801 repair - diyAudio
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Old 21st March 2008, 05:33 AM   #1
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Default MTX TA7801 repair

I was given a MTX TA7801 to try to repair, so far it looks like the positive 12 V regulator is toasted, R145, R146 are open, and C118 is pregnant, I can't for the life of me read what part numbers Q106, Q107, and Q110 are, does anyone have information on these amps?
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Old 21st March 2008, 05:55 AM   #2
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MTX7801 folder. Read the \mtx7801notes.htm file.

The previous tech probably powered it up without having the regulator transistor clamped to the sink. Unfortunately, I did the same thing. These are some of the notes:

When working on an amplifier that you're not intimately familiar with, you need to have the transistors mounted to the heatsink. Case in point... When troubleshooting this amp I unclamped the heatsink mounted transistors so that I could remove the heatsink (to locate the traces for the thermistors). Before re-clamping the transistors, I powered up the amp for a couple of seconds. In that time (less than 5 seconds), one of the regulators overheated and the series pass transistor of the regulator (FET191) failed. When it did, it did a huge amount of damage. It blew out two transistors, one resistor and one zener diode in the regulator circuit. It blew out both op-amps in the class D driver stage. It blew two surface mount electrolytic capacitors, it destroyed both fans and the transistors that drive the fans. Because I didn't follow the rules I set for others (clamp transistors when troubleshooting), more damage was done and more time was unnecessarily spent repairing the damage.


END NOTES

I don't know if yours has this much damage but it's probably close to the same condition.

Q106 and 110 are MMBTA42s (marked 1D). Q107 is probably an MMBT5401 (marked 2L).
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Old 21st March 2008, 03:17 PM   #3
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Well it was still warranty sealed when I received it so there was no previous tech inside it. I think the failure is due to bad battery connections and someone must have dropped the amp for a cap in the PWM IC to break loose and a cap in the level shifter for the high side output fets to also break loose.

I'll pull the surface mount transistors today and test them as well as the 15V Zener.

Thanks Perry, I found the files and pictures, this amp had oxidation so I could no longer read the part numbers in the regulator section, but now I can see what the values should be from your pictures.
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Old 31st March 2008, 09:30 PM   #4
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How do I figure out what capacitor C118 is?

It say's:
10
25s
5G2

And a cap in the level-shifter for high side output fets:
10
KFK
5K2

I have the parts for the 15V+ regulator except those 2 caps.

I replaced C145 in the PWM driver IC with a 100uF 25V cap, I hope thats correct.
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Old 31st March 2008, 11:32 PM   #5
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C118 is a 10uf 25v. I'm not sure what the S signifies.

I pulled a KFK capacitor out of a junk amp and checked it. The dielectric broke down over ~130v so it's very likely a 100v capacitor.

100uf 25v was correct for C145.
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Old 9th April 2008, 12:04 AM   #6
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I took a break from studying tonight and fixed the 15v+ regulator, so I was able to do some tests.

The HCPL-3120/J312's check out except pin 2 and 3, they don't read open they read 18 ohms on both chips with red on 3, and black on 2.
On diode check, im supposed to get 1.4 volts, but both chips read 0.13v with the red on 2, and black on 3 like your diagram.

On the protection circuit comparators the the numbers are off a bit, in your picture the bottom voltages are as follows:

(The 2nd set of numbers are your measurements)
Pin# / Yours
1= 5.96v / 6.71v
2=0.002v / 0.004v
3=11.12v / 13.26v **
4=4.76v / 4.60v
5=3.44v / 3.47v
6=0.56v / 0.85v
7=3.44v / 3.47v
8=3.37v / 4.02v
9=5.04v / 5.08v
10=5.04v / 5.08v
11=6.75v / 8.04v **
12=0.001v / 0.004v
13=4.77v /4.78v
14=4.77v / 4.78v

Top chip
1=0.115v / 0.140v
2=1.8v / 1.8v
3=11.12v / 13.26v **
4=1.9v / 1.9v
5=1.9v / 1.9v
6=1.7v / 1.7v
7=0.919v / 0.626v
8=2.54v / 2.54v
9=4.5v / 5.2v
10=0.08v / 0.075v
11=1.73v / 1.738v
12-0.001v / 0.004v
13=4.91v / 6.0v **
14=0.667v / 0.676v


And the 494 measurements
Pin#
1=2.3v
2=2.45v
3=0.061v
4=0.347v
5=1.51v
6=3.23v
7=0.001v
8=10.37v
9=0.001v
10=0.001v
11=10.36v
12=11.16v
13=9.96v
14=4.96v
15=4.96v
16=0.001v


I still have no audio output, if the 494 measurements look ok to you, then i'll start looking at the output fets I guess?
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Old 9th April 2008, 12:24 AM   #7
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2 and 3 have to be tied together as the amp powers up or it will pull EXCESSIVE current. After the mute delay, the jfet that's shorting them together is switched off and that will remove the low resistance between 2 and 3.

The 3120s have to be checked out of the board. Leakage in the board through other components makes it impossible to get definitive readings unless you pull them. Don't pull them yet.

The voltage on the comparators and the 494 looks OK. For each pair of inputs on the comparators, look at the relative voltage on the inputs. If x is higher than y on both yours and the reference amp, the voltage is OK. For some of the inputs, the power supply voltage will make a difference.

Is it producing rail voltage?

Do you have ~20v DC between pins 5 and 8 of each of the 3120s?
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Old 9th April 2008, 12:48 AM   #8
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I am getting a rail voltage of +82V and -83V, and my readings between the 5 and 8 pins of the 3120's are 15.5V on the left one and 18V on the right one.

I plugged in the "prizm effects" board to see if the amp is turning on but im not getting any light, but I did test the pins in the connector and im getting about 1V on 3 of the pins, and the 4th is ground.
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Old 9th April 2008, 07:21 AM   #9
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Are both the positive and negative 15v regulators working?

Are you getting ±15v to the op-amps on the LED/crossover board?

Does the amp ever drop out of muting (~1/5v across pins 2 and 3 of the 3120s?

Is there any DC across the positive and negative speaker terminals?
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Old 9th April 2008, 09:00 PM   #10
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Both +-15V regulators are working, and I did test +-15V~ on the opamps on the crossover board.

On the 3120's I am only getting 0.5VDC across 2/3 on both chips, with and without a input signal.

There is DC across the output (speaker) terminals, 7.46VDC.
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