[BUILD] CAPI LC53A~500 Series~Love Child EQ Kit~Official Support Thread

[Tschem]

thx for your replay jeff!
i exported a +4dBu sine into the working unit and got more or less the same values like already another use measured in post #59. in my case 2nd reading: output pin of A1, DF on CB PCB  about 0.591 VAC.

when connecting the not-working unit, i'm measuring about 1.59 VAC. again i swapped the A1 with the one of the first unit, but nothing changes.  i don't know that this means and why the unit is actually more quiet.
i've looked at the resistors and all the other parts on main pcb but can't find a difference between the units.

 

[jsteiger]

OK so you are saying the unit with 1.59VAC at A1's output has a lower overall output?

 

[Tschem]

yes, thats right. when the  unit is switched on, i get an decrease in volume. what i've noticed is that if it is turned off, i get an increase!
right now the difference is less tragic than before, its around +- 2db, depending on whether i've turned it on or off. all i've done is to play around with RV1 a little bit...is it possible, that i've messed up my CMRR measurement?

 

[jsteiger]

OK. Lets concentrate on the unit that is fully built. Leave the other one alone for now.

The CMRR trimmer should never be touched except for the calibration procedure. Once that has been properly executed, I put a splotch of fingernail polish on the adjustment screw so it doesn't get moved.

With the load jumpers set to HiZ, the ON switched disengaged/out, the module should pass signal at or very near unity gain. The change should be within maybe +/- 0.25dB. Its been forever since I built one but anything much more than that and there is a problem somewhere.

With filters out, all cut/boost switches set to 0, engaging the ON switch should not really change the audio thru level at all. If there is any change in thru level, something is wrong.

 

[Tschem]

on the one which is fully build i experience exactly what you're saying, no change in volume whether it's off or on with 0 cut/boost, no matter which DOA i use.
i consider that one as finished.

but yes, on the second one (the one with the missing L1), there is a problem. ~3dB higher output with unit off, ~2 db lower output with unit on. so, not 7 dB less anymore like i said before. there's not much i changed except that i've swapped around the DOAs, but that hasn't changed anything. i also played around with RB1 trying to make a CMRR re-calibration, so i thought this could have contributed to the issue.

 

[jsteiger]

OK good 

So, on the trouble child, it sounds like the gain is coming from the A1 area. Let's concentrate on signal throughput with EQ disengaged, ON switch out. The gain here is a little odd to me since there is not much more than the ON switch between the receiver opamp's output and the A1 followers input. Besides that switch, the signal goes thru R3 on the CB PCB which is a 1k R. Even with R3 jumpered, the gain would not increase that much. What is your ACV at the input of A1?

 

[Tschem]

with switch out and ~ 1V input, i get 0,32V at R3 and  0,27V at each (+/-) input of A1.

 

[jsteiger]

I meant A1 of the CB PCB which is the discrete follower. I thought that was the amp that had gain at the output of it?

From a previous post, I gathered that A1 on the Main PCB was good since you had what was expected at the output of that amp.

 

[Tschem]

in and out of A1 of CB pcb is 0.313, also in and out of A2 is 0.313.
some more values: in of A3 + and - is 0,26V, A3 out is 0,53. same with A4.

 

[jsteiger]

Hmm, so I am confused. Where is the 1.59VAC reading that you had before? I thought that was at the output of A1 on the CB PCB?

 

[Tschem]

i'm getting confused too, i'm sorry for wasting your time, i will try to make things clear:

- at first, i had this -7dB issue, at that time i was measuring this 1,5 V at - C2.
- then i did some swapping around of DOAs and DFs, which had no effect on the good unit.
- then i also played around with RV1. this was the time i didn't encounter the -7dB issue, but the -2 dB when on / + 3 db when off.
- the voltages in the last post are correct for ~ 1V input (so for +4dBU i'm measuring about 0.5 V as before)

and because i'm so desperate, i've again swapped the DOAs and installed some factory-new ones i bought recently, so no self-build ones. these new ones are working properly on my pair of api 550a.
same behaviour: unit on-2dB, off -3 dB. so: no chance that there's an issue with A1 and A2 on the main pcb.

that's why i was asking: is it possible that this problem lies in CMRR adjustment / RV1 setting?

 

[Tschem]

one more update:
again, i swapped A1, A2, A3, A4 of cb pcb on both units, same behaviour. no difference in output on the good one, -2/+3 on the other one. all resistors on main and cb pcbs are identical.

 

[jsteiger]

With the known good working opamps in place, go back to the post around #59 and follow those steps. Inject a 1k sine wave at +4dBu and measure ACV at all the points specified.

 

[Tschem]

Input on both: on - : 1,67 ,  on + 1,79
c2 - : ~0,83 both
R2: 0,83 both
A1 in/out: 0,83 both
A2 in/out: 0,83both
now it gets interesting:
A3/4: in/out on BAD one 1,39
A3/4: in/out on GOOD ond 0,56
c12- : 0,693 on bad one
c12-: 0,56 on good one.

both measurements for units on and off.

units back in lunchbox: good one has same level on and off, bad one ~2-3 dB less when on, more when off.

 

[jsteiger]

So it looks like they are consistent at the in/out of A1 and A2 on the CB PCB. The difference and increase in level begins at the output of A4?

BTW, the signal hits A4 before A3 on the CB PCB.

 

[Tschem]

exactly, thats what i'm measuring!

 

[jsteiger]

Well, you did mention at "the in/out of A4" so I am trying to make sure. I did not see "In-" and/or "In+" when referring to A4.

If the increase is present at In+ of A4, I would check R10. It should be 14k3. With the exception of C6, R10 is the only series component between the output of A2 and the In+ of A4. R25 should be 10k, its the feedback R of A4. Also check R22, R23 and R24.

 

[Tschem]

i think you mean 14k7 for R10?
R10 on both units 14k7.
R25 on both units 10k.
R22, 23, 24 have the same correct colourcode on both units, but i can't measure the same values.

R22 on the good one: 7K
R22 on the bad one: 14,7

R23 on good and bad one: 6k4

R24 on good one: 3k
R24 on bad one: 3k3

that's just what i'm measuring, the colourcodes are correct with the values of the BOM!

 

[jsteiger]

Yes sorry, 14k7. That was a change I made but was still showing as 14k3 in my PCB software.

You cannot measure a resistor's value with both ends soldered in circuit. Yes color band verification is prolly the best way to go, unless something is really incorrect.

BTW, I did not see a confirmation of the ACV at the input(s) of A4?

 

[Tschem]

last time, i just measured the +in on A4/A3 so, that was the value i was talking about.

but, jeff, you will think that i'm crazy now or something but let me tell you this:
without doing anything, no swapping around DOAs or DVFs or whatever, the -7 dB problem is back.
so, i'm not measuring +/- 3 dB when turned off and on, right now i have equal gain when turned off and ~ -7bB when turned on again, like i experienced in the initial posts. i'm completely confused now, cause i didn't even touch my measurement toolchain, just turned signalgenerator and lunchbox off for a day.

any idea what could cause such issues? a defect condensor, or just a bad contact??  ???