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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by galaxiex on Today at 11:47:17 AM »
Pin 3 of the TDA2040 is connected to the heat sink tab.

If the supply is single ended, pin 3 can be grounded, according to the data sheet.

https://www.st.com/resource/en/datasheet/cd00000130.pdf

If using a split supply, pin 3 and the heat sink tab MUST NOT be grounded.
2
Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on Today at 09:51:33 AM »
None of the photos of this amp that I've found (three or four examples) have any wafer under the IC, just some thermal compound. So something else must have gone wrong.
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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on Today at 09:07:49 AM »
The thermal compound grease is NOT an insulator.  There should be a mica or silicone wafer between the power amp IC and the heat sink.   You are shorting the power supply to ground.

There is/was no wafer -- should I add one? Is that the solution here to prevent the board from grounding on the chassis? Or would this be happening due to a component failure?
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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by Enzo on June 15, 2019, 08:39:53 PM »
The thermal compound grease is NOT an insulator.  There should be a mica or silicone wafer between the power amp IC and the heat sink.   You are shorting the power supply to ground.
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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on June 15, 2019, 05:39:18 PM »
Yeah it seems like if I ground the board to the chassis anywhere then I lose signal.
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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on June 15, 2019, 02:24:33 PM »
It seems the power transistor is grounding through the heat sink to the chassis and that causes the signal to cut out. So by taking the board off everything worked, putting it back the transistor grounds and the signal cuts out.

I guess previously there was enough heat sink compound to prevent the short, and when it got hot enough it conducted, when I replaced the board the first time I must have put it on more secure and therefore made the short permanent (until I took the board off again).

It seems, by the layout that it's suppose to ground here anyway. Could they really have been depending on the heat sink compound to prevent this?

Unless the TDA2040 has an internal short.
7
Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on June 15, 2019, 01:32:35 PM »
Took the board off, pulled the solder off R49 again, cleans the area (some brown residue), soldered it back again, took off C31 and it read fine. Re-flowed some questionable looking pads and it's working again. Have to try it for an extended period once my  8-month-old is up from his nap ;)
8
Amplifier Discussion / Re: Fender Acoustasonic Jr. Squeal
« Last post by g1 on June 15, 2019, 01:20:58 PM »
Try again how you did it with the electric guitar plugged in, but set all the guitar controls (and reverb) to zero.  Still squeal?
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Amplifier Discussion / Re: Peavy Rage 158 no sound after 1-5 mins
« Last post by substatica on June 15, 2019, 11:25:51 AM »
I can't get any signal out of it now, seems like I left it on long enough for something to fry for good. All of the voltages checked against the schematic seem within reason 3.2 VDC where it says 3, 26.8 VDC where it says 25, 30.7 VDC where it says 27. Nothing too out of whack.

One odd thing is when I touch D7 it reads 13.8 VDC, but I get a big pop and radio starts coming through.
10
Amplifier Discussion / Re: Fender Acoustasonic Jr. Squeal
« Last post by phatt on June 15, 2019, 09:07:04 AM »
OK Obviously a Electrical problem and not acoustic FBack.

Ok the reverb was faulty and you fixed it but if you are not familiar with reverb circuits there are a few thing that can catch you out.
1/ check the tank leads for correct in/out. The owner may have tried to fix it and messed up the plug connections. So don't assume it was correct when you got it.
(some rev circuit will squeal if plugs are reversed)
2/ check your work and make sure you did not inadvertently ground the wiring on the drive end of the tank.

Note that the driver circuit (U9A) output is not grounded,, while (U9B) the Pickup circuit IS Grounded.  If the leads have been reversed it will likely squeal like a pig.

The Ground lifted output of U9A has to be reflected in the tank connections as well.
If you follow the ground connections in the tank you will (should) find that the tank Case is only grounded on the Pickup end.

If unsure how to establish correct in/out then rule of thumb is the drive transducer will be the lower resistance. Ball park readings would be ~30-Ohms for drive Tx and ~200-Ohms for PU Tx.
Only the Pu Tx is grounded to reverb tank if both Tx's are grounded to case then you may have  wired it incorrectly when you fixed it.
Phil.
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