[BUILD] CAPI 553F~500 Series~LC Equalizer~Official Support Thread

[Mishegas]

Can you desolder all of the leads for the 2503 and measure the DCR of each winding? Set your DMM to a range of less then 100 ohms.

red to brown= 8.5ohms
orange to yellow= 8.5ohms
green to blue= 8.5-  8.6ohms
violet to gray= 8.5 - 8.6ohms

I do have several 2503's around- just checked a couple for comparison- it would appear that the tranny in question is fine. Same DCR as the other 2503's...

 

[jsteiger]

Yes this is correct.

Can you measure the DC offset around the opamp in question? The red probe to the positive end of C11 and black probe to a solid ground reference. DMM set to measure DC voltage.

 

[Mishegas]

Thanks Jeff,
Should I re-solder the 2503 before doing this?

 

[jsteiger]

Yes. What load is coming after the EQ? At the output?

 

[Mishegas]

Yes this is correct.

Can you measure the DC offset around the opamp in question? The red probe to the positive end of C11 and black probe to a solid ground reference. DMM set to measure DC voltage.

Am I doing this with the op amps installed? With the jig attached/powered-up?

You'll have to forgive all my dullard questions...(however, what I lack in troubleshooting know-how- I make up for in test equipment)

 

[jsteiger]

Yes, opamps installed, unit under power. Set the jig for normal conditions so CMRR switch out.

 

[Mishegas]

...assuming that I'm doing this correctly

The + side of C11 is -5.4vdc ( it's fluctuating up to -5.5vdc and back)

 

[jsteiger]

OK that is a pretty big problem. It should not be more than a few mV typically less than 100mV. I would check all of the components around this opamp. Something might have gotten misplaced in the audio path.

 

[Mishegas]

I just went over the whole thing 3 times, re-tinned all solder joints,  (*all* of the passive components on the main PCB are identical to the BOM/overlays... I can't find anything wrong?)-

I then  pulled all the resistors and caps surrounding the DOA, checked them (all well within range) and put them back in the circuit. (sheesh!...after pulling the 2503 & the passive components this PCB is beginning look a little sorry...)

Anyhow, I put it  back in the jig...same thing.... **way hot op amp**!  :'(

I did (very quickly) shoot a test tone thru it- it's not passing audio at all (a little "tic" when engaging the EQ-in button, but that's it)

Anybody have any other suggestions? I do have a scope, etc. here. ANYTHING?

I'm just not sure how safe it is to leave the thing powered up while testing it? I'd rather not cook the DOA's (or anything else!)

Thanks Jeff for all the help!

 

 

[Mishegas]

Hi,

I quickly just put a 400hz tone thru the jig>553F and speed-probed it with a scope, the last points I can pick up a 400hz signal is R9/C8 & the +/- & o pins on the A1-DOA.

Dunno if this helps?

 

[jsteiger]

I will create a test points PDF but it will take me a day or 2. I have more open orders than I can get packed on today's agenda. I will post it ASAP though.

 

[jsteiger]

What opamp is this? Was it built from a kit? These symptoms point towards the opamp. Is it installed correctly? http://classicapi.com/catalog/DOA_Install.php

 

[Mishegas]

Hi Jeff,

2 SL Red Dots. 

I've also swapped out a bunch of other known-to-work GAR 2520's and 1731's from 4 different VP28's that have been humming along perfectly for a year or so....

That's what so moderately haunting about this...it really should be something somewhat obvious ( and seriously- I've gone over the PCB more times than I can count at this point...retinned 2X, bla,bla,bla)

I'm starting to feel like Gene Hackman at the end of "The Conversation" (forgive the ref )

 

[jsteiger]

Hmm yeah it is odd.

 

[jsteiger]

Mishegas, can you please post a few pics of the component side of the 553F PCB in question?

 

[Mishegas]

Here are some pics, sorry if it looks a bit messy (I pulled all the A2 surrounding components and tranny to test o.o.c.) I also tried to show it who's boss with some rosin/flux remover last night...

!!also!! I was checking the resistance on the sockets per your "pre-flight" instructions (for the 10th time) - I just happened to have the continuity "beep" on.
Are C and O sockets on A-2 supposed to be connected?...(55ohms) cuz they beeped!

Hope these are ok

 

[Mishegas]

and another

 

[Mishegas]

finally

 

[jsteiger]

Well, nothing is jumping out at me. Yes 55 ohms between C and O is correct. I am still thinking it may be 2503 related. Since it is all soldered in, please measure DCR between gold fingers #2 and #4 with the EQ not in a rack. It should be appx 25 ohms.

 

[Mishegas]

24.8 ohms on #2 and #4

I'm not sure if the pads are gonna take another tranny pull, but do you think I should just clip the leads and wire in another tranny? (w/heatshrink...PITA but, oh well...)