Infinity 255a Repair Help

[Therum]

Hello Guys,

I have recently been repairing a Infinity 255a. I am having the pleasure of repairing this amp behind someone whom made an "attempt" to repair it. The sad part about it is that this person "repairs" amps on ebay We will skip to the issues that I have. I have very low volume out of 3 of 4 channels and the sub channel. 1 channel works without issue. I also noticed that the crossovers did not work and almost acted as a gain but not really. It led me to believe that the opamps or transistors or both on the crossover/gain boards were bad. I found that the - reg was bad and was reading 18-19v. Replaced the zeners and transistors in both regs so now both regs are within .1v of each other. I have already removed all of the boards (That was fun) I started probing a few transistors and found 1 that appears to be totally open (Q1 2sc3859). I think I am just gonna replace all of the transistors and opamps on the boards to save headaches (I think) I have the service manual and according to it the smd transistors are 2SC3859, 2SA1781, and 2SC4936. I cant find any of them. In attempting to find subs for 2SC3859 and its listed as being a transistor with bias resistance (Mute?). Not sure what to look for in a sub for that one. I found BCV72 for a replacement for the 1781 tell me what you think. I cant find anything for the 4936.

THanks

[Perry Babin]

It's probably not a good idea to randomly replace parts, especially when the originals are not available. The subs may not be compatible and that could cause hours of unnecessary troubleshooting.

If the inverting and non-inverting inputs of each op-amp match within ~0.01v, the op-amps is almost certainly not defective.

ChipQuik makes removing the boards VERY easy with absolutely no damage to the main board.

What page of the service manual is that open transistor on?

[Therum]

The transistor is on p18, upper left diagram, q1 is located near the lower right corner of the card. Initially I was hoping to find the originals but that didnt happen. ;( Im gonna have to use some IDE cable so I can probe the boards while connected to the amp. One thing that I forgot to mention is that I saw clipping at a very low voltage which was the reason for the low output.

[Perry Babin]

That appears to be a DTC114.

[Therum]

Checked both crossover boards and they are good. The signal is clean coming out of those. I have only checked one of the pre amp boards and there is a noise riding on the wave (90hz crossover set to flat/bypass gain pot full counter clockwise)I tried it both with the probe grounded to gnd and center tap gnd. Get the same results. I also found a noise /"error". it only effected the the rising GND-V+ and the falling side V- - GND of the wave. I found a oddly clipped signal on pin 5-6 on the op amp (IC1). I also noticed that the op amp at IC3 and IC2 get REALLY hot. Im gonna check more tomorrow to see what else I can find.