Ok thanks for the explanation. Both pins 3 and 5 are the same as pins 1 and 7 of U101. When you asked that the amp needs to be powered on, I think that your assuming that these readings I am giving you look like its not turned on? Obviously I know the amp needs to be turned on, and I think I just found the problem (lets see if I say this correctly) Pin 1 of HP01 (the 5 pin connector that attaches to the main board) does NOT have 12volts, I checked my other amp and it has 12 volts. This is probably like the 1st thing your suppose to check and I had honestly no idea, this is completely out of my comfort zone. As I mentioned before there was some corrosion on that connector, my guess is that the pin has completely corroded away? Can I jumper that pin with 12volts real quick to see if the issue is resolved?
Thanks Perry
I didn't think to tell you to check the voltage since you stated that another board worked perfectly in the amp.
If you don't have positive and negative voltage on pins 4 and 11 of the TL074s, that has to be resolved first.
You can check continuity of the pin with your meter.
If you don't have positive and negative voltage on pins 4 and 11 of the TL074s, that has to be resolved first.
You can check continuity of the pin with your meter.
Ok, I think I got all excited for nothing. I put in the other board, and the amp works, I checked the continuity of the 5 pins and they are all good. I put back in the original board and now I have 12volts at that pin. I then checked pins 4 of both TL074's, they each have 12volts, and ground at pin 11.
Do you have the same voltage as is driven into the RCA jacks on both terminals of C101 and C107?
Using the pin numbering in post 20 for U101, re-check the AC voltage on pins 8 and 14 with the gain control and the crossover controls fully clockwise.
Rechecked pins 8 & 14 of u101 and it was pretty much the same as yesterday, both pins showed .072 (AC)
Is U101 hotter than U102?
Can you plug another board in with the same setting to confirm that the voltage on U101 pins 8 and 14 should be significantly greater than pins 1 and 7?
Can you plug another board in with the same setting to confirm that the voltage on U101 pins 8 and 14 should be significantly greater than pins 1 and 7?
No, they feel about the same, both feel about room temp or so. I plugged in the other board. RCA voltage .54 Pins 1 & 7 show .175 & Pins 7 & 14 show 5.1 (AC)
With no power applied and the black probe on the GND pin of the preamp board, what is the resistance to pin 13 of U101?
What is the resistance between pins 13 and 14 of U101 with the gain control fully clockwise and fully counterclockwise?
Set it to the fully clockwise position after you measure.
What is the resistance between pins 13 and 14 of U101 with the gain control fully clockwise and fully counterclockwise?
Set it to the fully clockwise position after you measure.
I checked both the bad board and the good board, they are both changing resistance and read essentially the same. (no power applied)
Pin 13 to Ground pin .678k (ohm)
Pin 13 to 14 of U101 (gain on low) .022k
Pin 13 to 14 of U101 (gain on high) 17.7k
It appears that the gain components around that section of the op-amp are OK, If you have positive and negative supply voltage, I would think that the op-amp (U101) is defective.
Well thank you for your help, I do appreciate you taking the time walking me though this it has been very helpful and informative. So should I buy a tl074c chip and have somebody with experience and the correct tools install the chip? or is it something that I maybe able to do?
thanks
Jeff
I don't know how much experience you have soldering or what tools you have to replace it. It's not difficult.
You should understand that there is a chance that the IC is OK but from the information in this thread, it appears defective.
You should understand that there is a chance that the IC is OK but from the information in this thread, it appears defective.
I have little sound coming from my amp, I've replicated the 50 ohm pontentiometer for the gain and xover now they work great but still not loud like it's supposed to be. you mention something about c123 opening. Enlighten me on that. would I have to replace that or what? thanks.
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