[rob@roblanter.com]

Hi JP,

I am getting distortion in channel A. I followed the tests in the "In case your CP4500 doesn't work as expected" I got some strange readings:

Step2:
TP2: -.7 VDC
TP4: -1.1 VDC
TP11: -6.5 VDC
TP12: - 6.0 VDC

Step 6:
U6 pin 1: -.7

Any suggestions?

Thanks,
Rob

[JPK]

Start by the usual suspects and IC's in the wrong place.
Are the +/-16V and +/-12V correct?
Remove all IC's except U21 and check again TP11 = 0VDC

[rob@roblanter.com]

Hi JP,

Usual suspects are fine, IC's are in the right place, I am getting +16.15V, all others are between 15V and 16V

I removed all IC's except U21, TP11 = 4.3DC

I am getting sound in channel A and all the controls are working, there is  just some distortion, where as Channel B is perfectly clean.

Thanks,

Rob

[rob@roblanter.com]

Hi JP,

Can you please help me debug this issue? 

Thanks,

Rob Lanter

[JPK]

I am getting sound in channel A and all the controls are working, there is  just some distortion, where as Channel B is perfectly clean.

Do you mean you get compression? and on channel B too?

TP11: -6.5 VDC

This is not compatible with the previous statement. With this value, the meter needle should be stuck to 20

[rob@roblanter.com]

Hi,

I get compression on Channel B.  I am not sure about channel A because of the distortion.

I had a typo in the previous reply...TP11 was 6.5VDC, NOT -6.5VDC, Then the second time I measured TP11 was 4.3DC.

Rob

[JPK]

Please redo the measurements. Check the negative voltmeter probe is well connected to the 0V.
It is just impossible to get any compression with +6.5V on TP11.
Do you see the meter needle moving when compressing?
Regarding the distortion try putting U11 (THAT1246) in U1, in case U1 is damaged.

[rob@roblanter.com]

Hi,

I am getting +4.6VDC on TP11. 
I switched U1 with U11...same problem.

I am definitely getting compression on both channels. Though Channel A has some slight distortion. 

I am attaching a short audio file so that you can hear what I am talking about.

https://www.dropbox.com/s/7yil2tongebu5gp/DistortionExample.mp3?dl=0

Any other suggestions as too what might be causing distortion?

Thanks,
Rob

[rob@roblanter.com]

Also, the meter needle does move when compressing. It moves as I would expect it to move when changing the Threshold.

Rob

[JPK]

Try swapping IC's between channels until the distortion changes side.
This in case the distortion is caused by a defective IC.

[rob@roblanter.com]

Hi JP,

Good news...I was able to move the distortion to the other side...the 1646 chip is bad...I will replace it and hopefully the problem will be solved. I am going to wait to replace it after I finish building the two 501 eq's I purchased in case I need to order more than just the IC 1646 chip. 

Thanks for your help.

Rob Lanter