MTX TA7801 repair

[intensem1rider]

With jumpers in place, I get 0.047V DC, and 85.3 kHz across the speaker terminals.

I get 1.75 k ohms across pin 1 of the input and pin 2 of the servo both ways.

And I get 1.71 k ohms across pin 3 of the servo and pin 7 of the input both ways.

 

[Perry Babin]

83Khz? That wouldn't be audible.

Does it produce audio now?

 

[intensem1rider]

Humm, I didn't even think of that, I will check when I get back in town Sunday and let you know!

 

[intensem1rider]

It produces audio with the jumpers in place, but with the X-over board im still getting DC and no music, probably because my test speaker is maxxed out from the DC.

 

[Perry Babin]

If it's producing audio with the jumpers, everything on the main board is probably OK.

It's possible that all of the op-amps on the preamp board need to be replaced. Plug the preamp board in and see if any of the op-amps are running hotter than the others. Change the ones that are running hotter than the others first. If none are running hot, find the ones that have significantly different voltage between the two input pins for each of the 4 op-amps on each IC. I think the 3 amongst the pots and switches are used for audio. I think the other two near the corner of the board are for the display (but I'm not sure).

Look up the datasheet for a TL074. It will give you the pin configuration.

 

[intensem1rider]

Yeah, theres a few with some very wacky numbers, which case do I need? The SOIC-14?

 

[Perry Babin]

Mouser:
595-TL074CD

You have to be careful when ordering SMD ICs. There's an SOP and an SOIC. I've seen both designated as SO from time to time. The SOP is wider and won't generally fit on the pads. The SOIC package is the most commonly used package for op-amps.

Have you checked to see if any had too much offset between inputs or if there were any running hot?

 

[intensem1rider]

None of them were running hotter than the others, but on 3 of them I wasn't getting positive VCC, or negative VCC.

 

[intensem1rider]

Well, i replaced the op-amps, still no go. I get better numbers than before but on 3 of them im not getting +VCC

Op-amp#1
Pin 1: -10.8
Pin 2: -10.74
Pin 3: 1.01
Pin 4: -11.28 **
Pin 5: 2.51
Pin 6: -10.68
Pin 7: -10.89
Pin 8: -10.81
Pin 9: -10.65
Pin 10: 3.87
Pin 11: -15.01
Pin 12: -10.64
Pin 13: 2.88
Pin 14: -13.69

Op-amp#2
Pin 1: -10.76
Pin 2: -10.62
Pin 3: 3.69
Pin 4: -11.21 **
Pin 5: -0.001
Pin 6: -0.125
Pin 7: -10.33
Pin 8: -10.27
Pin 9: -0.128
Pin 10: -0.001
Pin 11: -15.02
Pin 12: -0.002
Pin 13: -0.148
Pin 14: -10.38

Op-amp#3
Pin 1: 0.001
Pin 2: -0.001
Pin 3: -0.002
Pin 4: 16.00
Pin 5: -0.001
Pin 6: -0.003
Pin 7: -0.003
Pin 8: -0.003
Pin 9: -0.001
Pin 10: -0.001
Pin 11: -15.18
Pin 12: -0.002
Pin 13: -0.001
Pin 14: -0.001

Op-amp#4
Pin 1: -13.61
Pin 2: -11.86
Pin 3: -13.16
Pin 4: 15.9
Pin 5: -13.59
Pin 6: -4.08
Pin 7: 4.80
Pin 8: 9.07
Pin 9: 0
Pin 10: -0.001
Pin 11: -15.05
Pin 12: -0.002
Pin 13: -0.002
Pin 14: 0

Op-amp#5
Pin 1: -13.12
Pin 2: -13.11
Pin 3: -10.6
Pin 4: -13.88 **
Pin 5: -6.54
Pin 6: -13.11
Pin 7: -13.15
Pin 8: -13.20
Pin 9: -13.20
Pin 10: -0.001
Pin 11: -15.22
Pin 12: -6.53
Pin 13: -11.69
Pin 14: -13.18

 

[Perry Babin]

Check all of the 10 ohm resistors (marked 10R0). There are probably a few open ones.

 

[intensem1rider]

Yup, there were 2 open, and that fixed it

Thanks Perry!

 

[intensem1rider]

Well, the +15V regulator fried on it again, I wonder what could have caused this?

 

[Perry Babin]

I can't tell you what caused it but I think you should add some sort of protection to the circuit to shut down the amp if the voltage goes above ~18v.

You've already been through this once so you may not need help getting it repaired. Before you reinstall it, let me know and we'll do something that will help protect the amp.

 

[intensem1rider]

I have the parts to repair the voltage regulator. What else do you have in mind to protect the amp?

 

[Perry Babin]

There are many ways to do it. The simplest would be to insert a fuse (2 amp?) in series with the regulator transistor. Then you could parallel an 18v 5w Zener across its outputs. If the transistor shorts and the voltage goes above 18v, the fuse will blow. This should protect most of the components on the 15v rail.

 

[intensem1rider]

Where specifically should I be placing the fuse? I'm just trying to figure out what type of package I should get the fuse in.

 

[Perry Babin]

I'd insert it between the board and the drain (or source) of the transistor. A pico-fuse with leads (similar in size to a 1/4w resistor) should fit easily.

 

[intensem1rider]

Ok, so basically cut the trace and insert the fuse. Sounds good, I'll pick up some pico fuses.

Thanks!

 

[Perry Babin]

I wouldn't cut the trace. Place heatshrink on the transistor leg (so it can't short to the clamp) and bend it so it extends from under the clamp. Solder one end of the fuse to the transistor's leg. The other end of the fuse will go into the through-hole.

After you get it working (including having the fuse in the circuit), use a non-conductive probe and push on various areas of the board to see if you can find any intermittent problems that are causing this to happen. At first, do it with the preamp board out of the circuit. If you don't find any problems with the preamp board out, reconnect it and try to find an intermittent problem with it in the circuit. Look closely at the board (with a lighted magnifying glass) to see if you can find anything that looks like it could be a problem. Sometimes a small ball of solder in just the wrong place can cause problem.

 

[luka]

Hi

You could use 15v regulator like 7815...but it wasn't explained why all this happened again, problem is still present somewhere(supply?)