Sound Skulptor forums

Build help and troubleshooting => CP4500 Stereo Bus Compressor => Topic started by: MRMNW on January 06, 2026, 02:12:11 PM

Title: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 06, 2026, 02:12:11 PM

Hi,
I was getting bad readings in the first stage of the setup guide:


V+: .890
V-: 15.67

-12: .711
+12: .174

I checked everything over and realised the THAT1246 was in upside down.

I've since replaced the 1246 but it's the same - so I'm guessing something else has blown.

Not sure where to look,
Any help much appreciated.

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 06, 2026, 04:19:31 PM

Remove all IC's from their sockets and check again.
If V+ and V- are correct, put the integrated circuits back in place, one by one, checking the voltages each time.
This will show you which one has failed.

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 06, 2026, 04:41:02 PM

Thank you. Including the VCAs?

I'm now getting 

V+ 4.59
V- 16.40

+12: 3.55
-12: .682

with no IC's in their sockets.

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 07, 2026, 10:16:37 AM

Including the VCAs?
Yes.

I'm now getting
V+ 4.59
V- 16.40



Do you mean V- equals minus 16.40V

Check that all the diodes are in the correct direction.
Check that all the diodes bridges BR1, BR2 are in the correct direction.
Check that all the electrolytic caps are in the correct direction.
Check that the 1R resistors R41, R52 are still at 1 ohm.
Look for solder bridges, forgotten solder joints, cold solder joints.

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 07, 2026, 11:42:32 AM

Yes, sorry V- = -16.40

I've triple checked all diodes, BRs and polar caps are correct. Not seeing any solder bridges or forgotten joints - reflowed all the joints too.
I have R52 as 5K1 in the parts list, do you mean R41 and R42?
R41 and R42 both read 1.2 ohm.
I've given it a good clean too with Isopropyl alcohol.

I'm still getting the same results.

Trying to add pictures but the spam filter won't allow me to embed them or post a link.

Thanks

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 07, 2026, 11:49:28 AM

pictures:

(https://i.postimg.cc/TLPHXN06/IMG-7633.jpg) (https://postimg.cc/TLPHXN06)

(https://i.postimg.cc/8JCnVX48/IMG-7634.jpg) (https://postimg.cc/8JCnVX48)

(https://i.postimg.cc/34wbHf1Q/IMG-7636.jpg) (https://postimg.cc/34wbHf1Q)

(https://i.postimg.cc/jWckmYz1/IMG-7637.jpg) (https://postimg.cc/jWckmYz1)

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 08, 2026, 11:57:03 AM

do you mean R41 and R42?
Yes, sorry.

Do you still get V+ at +4.59 when you only power the left PCB (A), without the front PCB being connected?
Check the diode D4.
Is R41 getting hot?

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 08, 2026, 01:23:11 PM

Still getting the same readings when it's just PCB (A). 
R41 Doesn't seem to be getting hot. 
I've replaced all the 1N4004 diodes.
Still the same. 

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 08, 2026, 05:59:28 PM

Are the fingers on the back connector clean and making a good contact?
Check that you didn't break the vias between the top and bottom pads of the components you replaced.
Is U23 getting hot? This is the last active device connected to V+.
If true, remove it.

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 08, 2026, 06:15:01 PM

78L12 AND 79L12 were in the wrong place! 

Thanks for your patience and help. (I may be back once I get past the first stage of setup!)

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 08, 2026, 06:32:41 PM

OK I'm up to step 8 on the setup guide. I have a reading of 0.5VAC on the DMM but the trimmer T4 isn't affecting the meter at all. Turned it a considerable amount in both directions. No movement at all. 

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 09, 2026, 03:25:25 PM

This is because you have the wrong trimmer in T4. It should be a 5k (502), your photo shows a 200R (201).

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 09, 2026, 04:03:17 PM

Bingo. Thank you. Another mistake! 

Title: Re: Bad power voltages and reversed THAT1246
Post by: MRMNW on January 09, 2026, 06:51:03 PM

All calibrated and working now, thanks JP

Title: Re: Bad power voltages and reversed THAT1246
Post by: JPK on January 10, 2026, 09:49:53 AM

(https://fonts.gstatic.com/s/e/notoemoji/16.0/1f44d/32.png)