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

[fripholm]

The trimmers on v2.5 definitely make calibration a lot easier but it isn't impossible with older revisions - unless something else is wrong or there are damaged traces from re-work etc.

What exactly isn't working?

 

[brewbacca]

I wasn't getting a healthy output but I think the transformer wiring change suggested above fixed that. The problem was that I started off with the calibration before I actually tried running audio in/out of the unit to see how it worked. I probably should have done that first.

I went ahead and tried the compressor out with the listed values for R7/R8 (47.7K, 9.1K) and R22 (12K) and sure enough there's audio and there's noticeable compression, so I may skip the calibration for now. It sounds about how I expected.

The last issue is these dang meters. I have the Sifam AL20-6-7 but there's no movement at all even after I adjust R88 all the way both directions. If I connect my DMM to the meter header, should I be testing for volts AC, DC, or ma? I'm wondering if I have a burnt out transistor or something, but it's the same on both boards.

 

[fripholm]

You're testing for DC. Without compression it should be zero volts. The more you compress, the more negative the voltage gets. The range is pretty small, though - about -0.1 to -0.7 Volts.

Already tried to connect it the other way around?

 

[JMan]

Are those 1mA meters?  For some reason I thought I remembered them being 2.4mA, but I could be wrong.

 

[brewbacca]

I did try connecting both ways, They are 2.4mA. Other people in the thread say they work, but I'm not sure if I need to use different resistor values or anything like that. (The meter lights are working great!)

In any case, I'm getting almost no DC reading on my DMM with leads in either direction.

 

[brewbacca]

I did a little probing along the Meter circuit and found that the voltage at the end of R91 where it connects to T16 is steady around -14.6 V and where R92 connects to T16 I'm getting around -10V which changes when I adjust the trimmer. I'm not getting any voltage at the collector for T16. I tried replacing T16, but still no voltage. This is with my DMM neg lead connected to the ground connection for the meter.

 

[fripholm]

The voltages you're getting around T16 seem about right. But the voltage at its collector should change with compression.

The original TG had the meter connected across diode D5. Does it move if you temporarily connect it there?

 

[brewbacca]

So, I did a bunch more testing (at first D5 didn't seem to have any variance over it, but then it did *shrug*), and then I decided to try switching the meter leads again and adjust and low and behold they work. It's just a little confusing because I initially connected the negative meter lead to the ground side of the meter connection, but ground is your voltage ceiling in that part of the circuit (probably not technically, but you know what I mean?) so positive should be connected on that side. I must have done something wrong when I was trying to flip it before.

It's always so nice when you get a new piece of equipment up after you've been having a lot of trouble with it 

Thanks everybody!

 

[brewbacca]

So it looks like I have one more issue. When I engage the compression, I get an awful lot of noise when there is no audio going through the compressor. Is this something that would be fixed by going through the calibration?

 

[fripholm]

Depending on how much noise you're talking about you may have run into a batch of noisy Zeners - as many others have. This thread is full of noise problems, mostly because of bad diodes.

Try a different brand of 1N751 or 1N5231. It's a bit of a gambling game, unfortunately...

 

[brewbacca]

Ok, I'll do some research on the thread, but it's a lot worse on one board than the other and they were built with the same parts at the same time, so not sure if that indicates another problem.

 

[warpie]

After reading this topic I'm wondering if a termination resistor is needed on the secondaries of the output transformer?

 

[fripholm]

This depends entirely on the output transformer you're using.

The currently last post in this thread by abbey says it all. Look at your frequency response. If it's flat within the audio range you're good without termination.

 

[warpie]

This depends entirely on the output transformer you're using.

The currently last post in this thread by abbey says it all. Look at your frequency response. If it's flat within the audio range you're good without termination.

Carnhill vtb2281 without a zobel or/and termination resistor. I haven't noticed any ringing up to 2MHz (that's the limit of my function gen).  The freq. response is about -0.5dB@20kHz

 

[brewbacca]

Got my new zeners and am going through the process of matching them. How close should I be shooting for? 3rd decimal place (eg 4.678v = 4.678v)?

 

[fripholm]

Got my new zeners and am going through the process of matching them. How close should I be shooting for? 3rd decimal place (eg 4.678v = 4.678v)?

Absolutely - if you can get them this close

4.677v would be a deviation of 0.02 percent and even 1 percent should be fine (about +/- 45mV in this case)...

 

[brewbacca]

Ok, I had to get a few more batches of zeners and it seems like the safest bet is the ones from Newark (Farnell). I used those and seemed to have some luck. With no input hooked up and the compression engaged, I have a pretty quiet unit, but when I hook up the input and when there's no signal, I am getting a noticeable amount of 60 hz esque hum. I'm assuming this is a ground loop issue (JOY!), but thought I'd ask in case anybody else has had this and fixed it?

I don't remember this from before, but it may have been there and I didn't notice due to the noise from the previous zeners.

 

[fripholm]

I'm assuming this is a ground loop issue...

Most likely.

See here or here for possible grounding schemes.

Also try rotating your power transformer and see if the hum changes in level. This doesn't fix a ground loop but can help reduce interference which is just another possible source of 50/60Hz hum.

 

[brewbacca]

Ok, making little bits of progress. I got the hum down by redoing the layout and being extra careful about the grounding scheme, but I still have a bit of hum. I have started to notice some static like noise in one channel but that only seems to happen with the output knob between 9:00 and 2:00. When the knob is half way, it's the loudest, and gets quieter the more further you get from halfway in either direction. Does that sound like it could be the self oscillating output?

 

[fripholm]

Does that sound like it could be the self oscillating output?

It is possible.

Have you tried a series resistor from the output to your output transformer? Does it change when you touch/move the wires of the output knob?