same amp, new prob MTX 8302 LP filter

[damage]

Hi!

High-pass and Full-range filters work. Low-pass filter makes a loud thump when engaged and then there is no sound.

It pops when I change the RCA output xover.

 

[Perry Babin]

Measure the DC voltage on the output pins of the op-amps near the RCA end of the board. Do any have more than a few millivolts of DC? If so, does the offset vary significantly when adjusting the gain?

What is the DC voltage on pins 1 and 8 of U6?

Use the RCA shield as the reference (black meter probe on RCA shield).

 

[damage]

I cannot tell which one is U6 because the board is marked underneath the opamps.

 

[Perry Babin]

U6 is the 8 pin SIP IC.

 

[damage]

The NE5532 right behind the gains has significant voltage change on the outputs when gains turned. The rest appear to have 3mV of offset.

U6
1: -.75V
2: 3.2V
3: .4mV
4: -2.6V
5: -57mV
6: -22mv
7: .4mV
8: -.75V

 

[Perry Babin]

Pull U6 and install a jumper from pin 1 to pin 8. Does the amp work normally (low frequency crossover working, no pop, no DC offset)?

 

[damage]

Hello!

Yes, after removal it appears to work normally. Slight pop when engaging the switch, but nothing like it was before. Offset is now normal on the NE5532.

 

[Perry Babin]

Replacing the THAT 2155 will probably solve the problem The 2155 is obsolete. You'll have to replace it with a 2180 (mouser - search for THAT 2180).

The amp will work without it but the remote level controller and the compressor (if it has one) won't work.

 

[damage]

Hi!

I'll get a replacement. At least the compression circuit is useful.

Thanks Again Perry!