[Build] CAPI BT50, 500 Series EQ (a la 550), Official Support Thread

[sadmonkeysound]

You can check level at TP9. That is just before the output transformer. It should be around 9.5dB less than the expected final output level since the output transformer is utilized 1:3. If this is not the case you can lift the 2503 lead that is soldered to the GRY pad. That will take the output transformer out of circuit and will not load the opamp if there is an internal problem with the 2503.

Do that and let us know what you have at TP9.

All tests using an approximately 0dbu (0.775v at the +input pin to the module) 400hz sine wave, with the EQ in bypass, and the impedance adjustment jumper in the unmarked/non-600ohm position...

Shunt jumper in unmarked position: 0.257v/approx. -9.5dbu
Shunt jumper in +6 position: 0.518v, approx. -3.5dbu

Even though those are (I believe) the correct values, with the gray pad (red lead on Litz) lifted, I see a very slight increase in voltage at TP9, which makes sense to me given your explanation of what that would do.

Shunt jumper in +6 position: 0.540v, approx. -3.49dbu

Just to verify, this is the mapping I am doing for the red-top Litz transformer leads to the rev. B.2 PCB pads.

PAD	LEAD
Gray	Red
Blue	Pink
Yellow	Yellow
Red	Orange
Violet	Black
Green	Gray
Orange	White
Brown	Violet

 

[jsteiger]

OK I think you should try putting in the other output transformer. We can go down the road of testing the first one for an open winding or an internal short.

 

[sadmonkeysound]

OK I think you should try putting in the other output transformer. We can go down the road of testing the first one for an open winding or an internal short.

Okay, will do, and will report back after. Thanks for the help Jeff!

 

[sadmonkeysound]

Yep, works perfectly with the second output transformer, so that's good. Happy to run whatever tests you would like on the first one.

 

[jsteiger]

Yep, works perfectly with the second output transformer, so that's good. Happy to run whatever tests you would like on the first one.

OK. With the questionable output removed, make sure no leads are touching each other. Set your DMM to read a resistance in the lowest setting (less than 200 ohms).

Check DCR between each set of windings. Then check from one of those leads to all other leads. You should only have DCR between a set of windings and nothing to any other leads.

For a 2503-Litz:
Black to Red should be around 9 ohms. No DCR to any other lead.
White to Yellow should be around 9 ohms. No DCR to any other lead.
Gray to Pink should be around 9 ohms. No DCR to any other lead.
Violet to Orange should be around 9 ohms. No DCR to any other lead.

 

[sadmonkeysound]

OK. With the questionable output removed, make sure no leads are touching each other. Set your DMM to read a resistance in the lowest setting (less than 200 ohms).

Check DCR between each set of windings. Then check from one of those leads to all other leads. You should only have DCR between a set of windings and nothing to any other leads.

For a 2503-Litz:
Black to Red should be around 9 ohms. No DCR to any other lead.
White to Yellow should be around 9 ohms. No DCR to any other lead.
Gray to Pink should be around 9 ohms. No DCR to any other lead.
Violet to Orange should be around 9 ohms. No DCR to any other lead.

All the individual pairs have the right resistance, but Gray/Pink are shorted to Violet/Orange, which I presume is the issue here.

As an aside, the (now) working EQ sounds incredible, and I can't wait to get the second one working!

 

[Aj Q]

Hello just built 2 bt-50 kits with the litz transformers, I have the same issue as @sadmonkeysound with one of them. Really low output. All the bands seem to work as they should. Both of them sound great just about a 12db loss on one of them. Any way to order another transformer?

 

[sadmonkeysound]

Hello just built 2 bt-50 kits with the litz transformers, I have the same issue as @sadmonkeysound with one of them. Really low output. All the bands seem to work as they should. Both of them sound great just about a 12db loss on one of them. Any way to order another transformer?

Did you test the transformers for shorts with it off the board? It’s worth doing the test first to verify.

 

[Aj Q]

Did you test the transformers for shorts with it off the board? It’s worth doing the test first to verify.

Haven’t yet, but I did on the 2nd transformer before I built the second unit. It checked out, and the second unit works perfectly. I’ll try and dig into the suspect transformer soon. It also seems like the suspect transformer heats up quite a bit more than the one on the unit that’s working. I think all signs are pointing towards a short on the winding

 

[sadmonkeysound]

On mine, I was able to successfully repair the faulty transformer. Cutting into the tape very carefully, there was a very visible stray strand of copper wire causing the short. Removing that and re-taping resolved the issue. I don’t necessarily recommend doing it, and that transformer is now a backup in my parts box, but it is possible if you’re up for it and the risk.

 

[jsteiger]

Haven’t yet, but I did on the 2nd transformer before I built the second unit. It checked out, and the second unit works perfectly. I’ll try and dig into the suspect transformer soon. It also seems like the suspect transformer heats up quite a bit more than the one on the unit that’s working. I think all signs are pointing towards a short on the winding

You'll have to desolder the leads and measure DCR/check for shorts. Email me the full report.

 

[Aj Q]

You'll have to desolder the leads and measure DCR/check for shorts. Email me the full report.

Removed the transformer and measurements all seem to be in the correct range, also didn’t find any shorts between the windings. My issue is elsewhere.

 

[jsteiger]

Removed the transformer and measurements all seem to be in the correct range, also didn’t find any shorts between the windings. My issue is elsewhere.

Well, the good news is you have one good working unit to compare to.

100% rule out any outside influence. Rack slot, associated cabling, converter settings etc.

Make sure the shunt jumpers are set the same.

Swap opamps between the units (if they were built from kits).

Run signal thru the good unit and take ACV measurements at all of the Test Points making notes. Then do the same with the iffy unit and see where things go south. That will help narrow down where the issue is.

 

[Aj Q]

I’ve swapped rack slots and op amps, triple checked converter settings. I’m running over a cranborne 500adat so I don’t believe cabling is the issue. Jumpers are set the same, I have it in the louder jumper setting on both, that gets me to not loose much if any signal with the working one when put in the signal chain.
Do you sell the extender so I can check test points while sending audio through the unit?

 

[jsteiger]

Do you sell the extender so I can check test points while sending audio through the unit?

Indeed.

https://capi-gear.com/catalog/product_info.php?cPath=84_86&products_id=406

 

[Forcemusicgroup]

Bought one of the recent bt50 releases with the litz transformer.... Plug it in. No power... and no signal... all joints are double checked. Any idea where to start?

 

[jsteiger]

Bought one of the recent bt50 releases with the litz transformer.... Plug it in. No power... and no signal... all joints are double checked. Any idea where to start?

I have said many times, first thing is what opamps are you using and who built them?

 

[Forcemusicgroup]

False alarm. The filter pcb wasn't connected. However. There is a small reduction in sound when activated. Not much but slightly noticeable.
Using op amps that came with with the unit. All built by me.

 

[Aj Q]

@jsteiger, thanks for linking me to the extender, after comparing my working unit to the quiet unit, i found TP7 was where they started to differ. As i was looking at the board around tp7, I realized i made a stupid mistake while populating the transistors. Q4 was turned around backwards. flipped it the right way around and everything is working great! thanks!

 

[rtg603]

Hi all!
I just finished this build. It's my first from CAPI. I am however having a few issues. I sent an email to Jeff after I finished the build regarding the issues, but I have come to some better conclusions after tests the next day.

I initially thought the unit was not receiving power because the red LED was not illuminated when the switch was engaged. I thought I had the polarity wrong so I flipped it around just for testing and it still didn't illuminate. My next thought process was the switch had a short or cold joint. I couldn't see any noticeable shorts or cold joints. I didn't feel like desoldering the 90-degree header on the switch PCB to check further, so I just looked through the small gap to examine the joints. I checked the LED outside of the circuit and it illuminated so that rules out a bad LED. I'm not really sure where to check next.

Next issue, probably the biggest is a constant static noise when the unit is plugged in. I noticed that when I touch some of the knobs the static noise is amplified. I feel as if there is a short to ground somewhere. I've checked for shorts, but can't seem to find any. I've been constantly cleaning up joints and removing excess solder, but haven't seemed to hit the right joint yet.

With some small bits of testing I think the unit works, I tried a few boosts/cuts with the mid band and it seemed to work fine, just that annoying static noise. I'm not sure if this will help at all, but I was slightly sloppy on a single joint and got a thin layer of solder on the J3-14 contact and have not been able to remove it. I really hope this is not the cause. I'm going to be doing some testing in the meantime as I have some other ideas of things to test, but I'm hoping someone with a better background in electronics can maybe point my troubleshooting in the right direction.

Thank you!