[gitarretyp]

I posted this over at the asylum but have gotten no responses. Hopefully someone over here can help.

I have a PS Audio HCA-2 that i modded a few months ago (replaced a few caps in the signal path and power supply and replaced the internal speaker cables). This afternoon while dusting my power condition (which the amp is not plugged into, so i think this may just be a coincidence), the right channel swelled immensely in volume and then fell to a level lower than normal and sounds distorted/farty. The channel also pops lightly when the amp comes out of stand-by. I've checked the obvious things: made sure not a blown speaker, double checked solder joints, looked for burnt components but can not identify any problems. Also, this is not the first time this has happened. When i initially finished the mods, this channel sounded similar to the way it does now. At that time, i re-tinned a few solder joints, and it went away (not really sure what the exact problem was). I've tried doing the same this time to no avail. Does anyone have any idea what i should test (i only have a DMM) or look for that might be causing this problem?

Thanks

[ASi_TEK]

I posted this over at the asylum but have gotten no responses. Hopefully someone over here can help.

I have a PS Audio HCA-2 that i modded a few months ago (replaced a few caps in the signal path and power supply and replaced the internal speaker cables). This afternoon while dusting my power condition (which the amp is not plugged into, so i think this may just be a coincidence), the right channel swelled immensely in volume and then fell to a level lower than normal and sounds distorted/farty. The chan ...

Hi,

Is the unit going into protection at all???  Since you have two channels, they have indentical circuits (opamps, driver chips and mosfets) its time to start carefully taking voltage measurements, check the +12vdc reg, and the LM317/LM337, they should be -9, +30v, and then theres a +20v reg and check that rail. There are test points to easily do this on the board. Also, check the voltages at the coupling caps, it should be around +6v I beleive from the last time I checked. If one side is much different than the other, then the problem might enlie before the coupling caps), which is good for isolating where the problem might enlie.

The basic block diagram of the amp goes like this (I may not be 100% correct as I do not have the schematics, PS Audio will not release them).
Input stage opamp, coupling caps, DC servo (I think), Another larger package op-amp, Class D driver chip, output Mosfets IRZ34E's, output filtering, and then off to binding posts. Anyone correct me if I might have made a mistake.   I know it is kinda hard to test for voltages near the Mosfets the filter caps for the fets can get worn out or overheated fast I noticed, I have seen some very brown before. Also check the bottom of the board for burn spots, normal wear and tear of a HCA-2 in a entertainment center will show this, some people just dont get it, these amps still need ventilation!!!

This is a rather simple amp design , I really wish I could get the schematics for it to take it even further sonic wise..

Let me know what you find out  (also check the outputs binding posts for DC, and make sure the connections look good).

Just trying to help as I have a very decent exposure to this amp as I have professional modded about 7 of them in the past. Take care!

[gitarretyp]

Thank you so much for your help. I've tracked the problem down to a couple of suspect devices. There's a SMD transistor, Q18, that reads 57V on two pins on the same side of the device (cathode and anode?) and 0.5mV on the other on the good channel, while on the bad channel it's 57V, 57V, 2.68V. I have no idea what type of transistor this is. It's only marking is BK. Q19, the next transistor (a 2n5060) that comes off of the 2.68V side of Q18, reads 0.5mV at the cathode, 12.03 at the anode, and i couldn't get a good reading on the gate. The value wouldn't settle down. The same transistor in the other channel reads 0.5mV at gate and cathode and 12.03 at the anode.

I think it's likely Q18. Do you (or anyone else for that matter) have any thoughts?

Thanks again

[gitarretyp]

By the way, it doesn't go into protection. It runs just fine. Just the right channel is low in volume and distorts really badly on transients