[BUILD] fripholm's TG1 Zener Limiter boards - support thread

[scott2000]

I think I remember someone somewhere having some issue that was weird like this and they had the transformer primary wired up goofy or something.........just throwing stuff out there.....

 

[brewbacca]

So I just now tried this with a bench power supply, wired it backwards and quickly blew up c25 on one board. I though "ah ha!" and then realized I wired it backwards...

Tried with the other board and the bench supply set to +/- 28 v like I''d want with a draw of  0.03 amps on the negative rail and .06 amps on the positive, so I guess there must be something off with the power supply circuit somewhere. I'll email fripholm and get a copy of his psu pcb just so there's not weird variables in this.

 

[brewbacca]

Quick update: I received the PSU board from frip and got that working with a brand new power transformer, but I'm still running into the same issue with my TTx boards. One actually got worse (negative rail gets higher while the positive rail gets lower), but I think it's because I flipped the polarity during the bench test as noted in the post above.

 

[fripholm]

Sorry to hear that!

Maybe something else blew up on that board during the bench test with the polarity flipped. Most likely candidates are C21 and C26 and/or the transistors in the output stage. But you should first fix your PSU problem with the good board before troubleshooting this.

As I suggested earlier by email, you could try a TL783 on the PSU board as the positive regulator.

Could you post a bird's eye photo of your limiter boards? A few more eyes might see something you missed...

 

[brewbacca]

I replaced C12 and C26 on both boards and they're behaving the same again.

Picture attached.

I'll order a tl783 and take a look at that.

 

[scott2000]

So the bench power supply still gives the right readings on both boards  but the new psu does not??


how do you have your transformer wired? Have a diagram??? I know you mentioned series earlier.... ??

 

[brewbacca]

So with the antek transformer I had red-red and black-black connected to the 120V source (IEC jack, on/off switch etc), and then I had the secondaries in series - middle blue and green connected and the outer blue and green going to the power supply board ac inputs.

To rule out any weirdness with wiring, I switched to a simpler transformer (https://www.digikey.com/product-detail/en/hammond-manufacturing/187D56/HM4759-ND/2358269) with a single set of primaries/secondaries, and yeah, same issue, even with Frip's psu board.

The bench is working with the both boards just fine. I'd assume there's some sort of hefty regulation happening there.

Fuse should be slow blow, correct?

It seems like there some component on each board that's having some sort of gradual short?

 

[scott2000]

what is the current draw from these boards anyway? Did you measure that one resistor voltage drop now that you have frip's board?? The one after the bridge I think it was???

ll///you didn't wire the center tap to the psu board???

 

[fripholm]

ll///you didn't wire the center tap to the psu board???

Yes, the transformer's secondary center tap must connect to GND on the PSU board - at connector "AC" next to the AC label.

 

[Hobbyist]

Hello,

This is what i did and works great, have perfect 28- and 28+ .
I made also the mistake with wiring the secondaries wrong.

Connect 0v to ac

 

[brewbacca]

The missing center tap connection was the issue. I have a steady +/- 28V. Thanks guys!

I'm betting I'll have some issues getting my meters to work (they're the 2.4ma sifam that some other people in the thread have), but that will have to wait for a bit.

Thanks again everybody!

 

[Potato Cakes]

I have the fancy 2.4mA meters and they worked great after I did the calibration as per the instruction and then a little tweaking to my liking. I found that the gain reduction amount printed on the meter is pretty close, but it still should only be viewed as a guide for what is happening and not as a true representation of math and science.

Thanks!

Paul

 

[JCN1218]

Hi again everyone!

Took a little break from this project after it gave me a lot of trouble. I've got three almost complete projects on the bench right now though, and ideally I'd like to get them all working before I start any other projects.

Fripholm was nice enough to send me the LTspice files for the output section to help me do some initial troubleshooting. I found out that Q13 on the main board was installed incorrectly. After I fixed that, I couldn't find any problems with the output section of the main board, nothing seemed to be fried and voltage reading were just fine, however, to this day, I'm still not getting any kind of output. Fripholm's last email to me said to check my output transformer with a TS cable. I only really have TRS available to use with the test tones from my DAW, but I tried putting the input leads on all the combinations I could think of and still nothing.

I'm using a sowter 1365e, which are pretty pricey, so I'm hoping that they're not dead and I just have something wired up wrong. Could that flopped Q13 have damaged them enough so they aren't functioning?

 

[scott2000]

Yeah, the first thing I did once I saw it wasn't working was to check my transistors.

I did just do a quick test on the top of C29 and I'm not getting a DC voltage reading at all really, so something must have burned up due to the flipped BD139. Now I just have to track down the malfunctioning part(s). I haven't seen any smoke or anything during any power ups so far.

So what has happened since this post?? I was trying to follow your progress but it sorta dropped off until now......

 

[fripholm]

Fripholm's last email to me said to check my output transformer with a TS cable. I only really have TRS available to use with the test tones from my DAW, but I tried putting the input leads on all the combinations I could think of and still nothing.

I'm using a sowter 1365e, which are pretty pricey, so I'm hoping that they're not dead and I just have something wired up wrong. Could that flopped Q13 have damaged them enough so they aren't functioning?

You can also use TRS cables for the test. Make sure to disconnect the transformer from the circuit before testing. You basically connect your interface's output via TRS to the primary of the transformer and the transformer's secondary to the input of the interface. If you're getting a clean signal, the transformer is fine. If your interface has balanced I/O, you connect TIP to the PLUS side and RING to MINUS, SLEEVE goes to the transformer's chassis or shield if that wire exists.

You wrote this in an email back in November 2017:

-I'm using Sowter 1365 for output tx, the wiring scheme is Primary: Brown+, Blue-. Secondary: Green+, Grey-. It also has a black wire for electrostatic shield.

According to this, output cable TIP is BROWN, RING is BLUE - input cable TIP is GREEN and RING is GREY. BLACK goes to any SLEEVE. But before, make sure you're getting an input signal when one of the TRS cables goes straight from output to an input on your interface.

 

[JCN1218]

So what has happened since this post?? I was trying to follow your progress but it sorta dropped off until now......

Unfortunately, due to a combination of me being busy and just kind of generally discouraged about the project, not much. I've been recording a lot recently though, and this piece of gear is one of my bucket list items, so I'm feeling inspired to give it another try.

Your post did remind me of the C29. I just checked it again on my one board and I was reading very low VDC, but about 17.8VAC, which if I recall, was about what I kept getting at my output for whatever reason. Maybe there's a clue in there somewhere.

Edit:
Just rechecked my output transformers. No signal whatsoever when touching the primaries to a TRS cable as fripholm instructed. The secondaries are soldered to my XLR connector, so I was connecting back to my interface via a microphone cable. I managed to place the TRS so the tip and ring touched on pin 2 and ground and my test signal came through. This is true on both output transformers.

So I guess they're shot? It seems strange that they'd be so delicate, I once accidentally sent phantom power into an output transformer via a patch-bay misunderstanding and it's still kicking. But, then again, maybe I'm just in denial. Either way, I'm sure that's not the only issue that I'm facing with this project at the moment.

 

[Ilya]

A wild guess. Maybe the actual pinout of the transformers differs from what is there in the datasheet? You may try to check the windings with a DMM and see if they are not open. You can also check a signal with a scope before the transformer (AC coupled). This will show if you're getting anything from your output stage at all.

 

[JCN1218]

So I had a thought just now. My test signal was very low for my ears sake. Around -20dB. I cranked it up to -9dB and hey, a signal! It's a very low signal though. Tried it with some actual audio and got similar results. Sounds fine through the monitors, albeit incredibly quiet. The 1365e transformers I'm using are advertised as being 1:1.77 ratio, so I was expecting a volume increase. Maybe I got a batch with the color inverted? That being said, the signal I'm getting out of them is quite weak, so I'm not sure that's a good sign or not.

Edit:

Alright, I'm at a loss. Wired it up in reverse, since I realized I could actually do that. Got a signal, though still more quiet than I'd expect. I recorded some measurements and figured that for science, I should really see what my dB drop is when wired normally. Hooked it back up as I had been before, and suddenly I'm getting a full strength signal! Outputting -21dB from my interface and getting about -17dB back from the transformer. I have no idea why it suddenly decided to start working.

 

[scott2000]

Your post did remind me of the C29. I just checked it again on my one board and I was reading very low VDC, but about 17.8VAC, which if I recall, was about what I kept getting at my output for whatever reason. Maybe there's a clue in there somewhere.

Well I'm not sure about this piece without looking but ac voltage  shouldn't usually be around unless it's an audio signal.......and never that high even so.........

Something might be up for sure.........I don't know yet

calling fripholm.. 

is there a voltage reference schematic for this floating around????

 

[JCN1218]

Just did some more tests, I've been given new life for this project now that I know the output transformers work. Checking the voltages again and honestly, everything seems like it should be working. However, for some reason I keep getting a reading of 17.6VAC everywhere! Like, literally everywhere, even at the outputs of my power supply, leading me to believe that it has to be stemming from there, which is a shame since that's the one part of this build that I thought I had working.

Here are my thoughts:
1. Could be a grounding issue, despite my readings telling me everything is referencing ground. In my emails with fripholm he mentioned moving my star-ground point for the PSU to the output. I never got around to it, but I definitely will do that ASAP.

2. I never isolated my regulators on my power supply, which are attached to those large heatsinks. I never knew it was a thing that needed to be done. Not sure if it would be causing a problem like this, but I do have a couple of the insulator kits for those TO-220 regulators, so I should probably go ahead and do that.

I'm not really sure what else it could be. After all, I had initially thought my PSU was operating flawlessly, but now I'm reading AC at the outputs for some reason.