MTX Thunder 895 - looking to Fix it.... finally

I'm looking to fix this amp. I've been able to identify that Resistor R820 has failed. (See Picture)

I've contacted MTX and they advised that it's a 10 ohm, 1%, 1/10th W resistor. Unfortunately for me, I can only get 5% locally (I can get 10 for $1, so I'll get them and measure each to find the closest to 10 ohm I can find and use it).

Now this I think is the power stage for the Pre-Amp on this amp. I don't understand how this failed though and what else may have failed with it.

I do know that the silver cap next to R610 may also have failed as if the amp is powered long enough (~20min) it gets extremely warm. The 3 large electrolytic caps in the photo also get a little warm.

Any suggestions on how to diagnose what other components may have failed? I'll have 10 of the 10ohm resistors, I guess I could replace it and go from there, but I expect whatever took it out in the first place will take it out again before I've had time to go through that area of the amp.

I've already asked MTX for a copy of the schematic, but they won't give it to me, told me they don't have access to it. They just have the parts list. I don't buy it... but ok.

The next challenge is getting the small surface mount components without disturbing the others. I only have conventional soldering irons and not a heating station (though I'm not apposed to buying one if there are well priced options out there that someone may suggest)

I found 2 other threads here from other people that repaired their amps, but unfortunately for me, I have a different issue.

Thanks in advance

Vin
 

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Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
The 10 ohm resistors are typically used between the regulated power supplies and the various ICs on those supplies. They allow a bit better filtering from the cap across the power supply terminals for those ICs. They don't need to be 1% although what you intended to do would have worked if they did need to be 1%.

Do you have approximately ±18v on the power supply pins of the op-amps?

You don't need a hot-air station to work with SMD components.

You can find hot-air stations for $65 on ebay if you want one. I have one but generally only use it for heatshrink tubing and working with massive components that my iron can't heat alone (terminal blocks, mainly).
 
On the bottom side of R810 (closest to the yellow Cap) I'm getting 47.5v DC, after R820 I get nothing (failed Open).

Thinking that one pair is left and the other is right and also +/-;
R710 = +47.5V on both sides
R821 = -47.5V on only the right side (I think it's failed open too)
R526 = -47.5V on both sides

I can't identify the D410-A06 regulator (I think it's a regulator), I tried looking that number on on Digikey and Fairchild's wedsite and it doesn't come up with any hits.

Vin
 
The first cap I found hot is a silver unit, I thought it was the one next to R160, but it's the unit 2 more caps down (201/10/50.. 10uf/50v?) I'm only reading 1.8v on the + side.

The large cap next to R714 got quite hot, the voltage readings I got from it don't make any sense to me. On the Positive leg I got +2v and on the negative leg I got -16v? This is with my meter on the negative leg of the power supply.

The source for R820 is from the center pin (Drain) of FET 901 [part#IRFP240, Both FET 901 and 902 are the same part number]. The overall voltages on the mosfets also don't make sense to me either.

FET901
Source = -44v
Drain = +48v
Gate = -44v

FET902
Source = -45v
Drain = -45v
Gate = -33v

Troubleshooting this blind is proving difficult, which I had a schematic to follow.

Vin
 

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Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
It's easiest to determine the voltage across a component by placing the meter probes directly on the leads of that component (not ground).

You can't do anything other than checking components with your meter until you replace the 10 ohm resistor and get all power supplies up and running.

If you want an MTX diagram, you'll likely have to make your own. In car audio repair, it's generally difficult to find diagrams for amps from American companies.
 
Good new first, replaced the two failed 10ohm resistors (temp installed 10 ohm 1/8th watt as I had them). The power across both FET's normalized to +/-35V which made more sense to me.

Powered up the amp back in it's frame and hooked up a few 4ohm speakers.

Volume was present but low, seemed like the gain on any channel didn't affect the output and wasn't doing anything but there was sound at least.

The Bad News... then after about a minute of probing around the repair. all the resistors around Q906/Q931 went up in smoke. As that happened I noticed my power supply output current jumped from 2.5a to 9a. and loaded down the voltage from 13.5v down to 8v.

Incoming power FET's 1001/1002 were too hot to touch. heat circuit limiter next to FET1001 didn't shutdown the amp either.

I think I might call this amp toast. Component count on what's visually failed is what I call TMTC (Too Many To Count). I can get an amp used for less that it would take to fix this one.

Thank you for all the help Perry, I very much appreciate it.

Vin
 
Yes, it's a sandwich setup. FET sits on a pad to the frame and a top bar of aluminum is clamped on top of the FET holding it down.

When I was doing the initial power testing I kept my Fluke 52II on them and kept watch on all the FETs as I had it out of the HS frame.

After re-installing the amp as I found it I put the meter aside and trusted the onboard thermister.

Vin
 
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I agree the FETs are 75339p's (75a/55v). I scanned that part of the circuit when I started and there was nothing there that led me to believe there was anything wrong there at all. I saw the 2 posts here and others from other forums where that seemed like they were the typical weak point. Those and R1013/1014 seem to be common failures.

I'm guessing theirs failed open and mine failed to a short.

With the board not in the frame powered prior to fixing R820/821 they warmed up a bit but I could still touch them, but nothing like they were after the board was re-installed after the repair.

Vin
 

tybu

Member
2016-04-01 11:38 pm
mtx audio 895 repair r918,919 and 934

Hello to all. I am new in this page, someone can say to me the values of r919, r918 and r934 or put the photo of these resistances, forgive my Englishman that I am translating with google. thank you very much
 

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