[BUILD] New FET/RACK Official Help Thread - Please read first post!

[Codedoval]

Hi folks,

I have completed a revision D build, and am experiencing an issue where there is a signal at the output without any input. It sounds like some self oscillation happening somewhere. The output knob on the front panel changes the frequency of the tone heard at the output when you turn it, and its seems to have limited range, and stabilizes at similar frequencies at either extreme of the knob.

I imagine I am missing something simple somewhere. I get expected readings at the qbias step of the troubleshooting guide.

I will spend some more time triple checking everything, but was wondering if the self oscillation was an obvious clue.

Any help is much appreciated, Thanks!

-Jeff

 

[scott2000]

It sounds like some self oscillation happening somewhere. The output knob on the front panel changes the frequency of the tone heard at the output when you turn it, and its seems to have limited range, and stabilizes at similar frequencies at either extreme of the knob.

Maybe start with the output section and make sure everything is correct. Searching for 1176 oscillation brings up a few things but it's not clear what the exact sound is you're hearing. Are the inputs disconnected or just no signal?
Maybe a pic could be useful in spotting anything obvious.
Good luck!
You could also record the sounds and post them here...

Or use them in your next club banger..

 

[DonnieDee]

Hi everyone,

Just got my rev a kit and was excited to start building it, got the power supply built but no power seems to be getting to the board, the ground is good, and there’s power getting to the gr switch section


Here’s my measurements, any help would be greatly appreciated. Haven’t populated anything other than power supply section.

Still very new to diy but eager to get better!


0,715v Ac when on front touching L lead with gr selected.

0.899 when In off position,

0 ohms resistance yellow transformer lead,

-0.288 v on -10 pad when checking power supply


+30 measures 0.657v

Measures the same when unplugged as when plugged in and on


0 volts on left pins on voltage switch pcb.

 

[Codedoval]

Maybe start with the output section and make sure everything is correct. Searching for 1176 oscillation brings up a few things but it's not clear what the exact sound is you're hearing. Are the inputs disconnected or just no signal?
Maybe a pic could be useful in spotting anything obvious.
Good luck!
You could also record the sounds and post them here...

Or use them in your next club banger..

Thanks scott2000 - I should have mentioned I had already looked for other oscillation issues here, but nothing seemed to fit.

A few other details:
A video of how the output control interacts with the oscillation:

The DC voltage at the CR8 was 30V after assembling the power supply, and is now right around 25V.
There is sound at the output with and without signal connected to the input.
The output is very hot - the lowest it goes when turning the output knob is around 12V AC. If I select any of the output settings on the meter, the needle pegs, or bounces rapidly at the high end of the scale depending on what specifically is happening at the output. (seems like the meter is working)

I have reflowed some components, but nothing changed.
My plan is to look at all the components, especially double checking resistor values. If that doesn't yield results, build a signal sniffer and look for where the input signal disappears.

I don't have a ton of experience, especially with troubleshooting a circuit. I have put together a couple of hairball bronze preamps without any issues. I imagine there is some small bug somewhere from user error.

For my own education, could self oscillation be a symptom of a wrongly biased transistor? Or, are there a bunch of different things that could cause it? I have experienced similar types of sounds with no-input mixer feedback, so maybe there is a feedback loop somewhere? I haven't found any solder shorts yet.

I grabbed a higher res audio recording for my next serious electronic music composition...

Thanks!

 

[scott2000]

Oh wow... That's crazy and awesome. Circuit bent like.....

Obviously something is wrong... Let us know how the component checks go. Make sure to double check wiring too...

 

[DonnieDee]

Hi everyone,

Just got my rev a kit and was excited to start building it, got the power supply built but no power seems to be getting to the board, the ground is good, and there’s power getting to the gr switch section


Here’s my measurements, any help would be greatly appreciated. Haven’t populated anything other than power supply section.

Still very new to diy but eager to get better!


0,715v Ac when on front touching L lead with gr selected.

0.899 when In off position,

0 ohms resistance yellow transformer lead,

-0.288 v on -10 pad when checking power supply


+30 measures 0.657v

Measures the same when unplugged as when plugged in and on


0 volts on left pins on voltage switch pcb.

Any insight would be really helpful thanks, I’m completely stuck!

 

[scott2000]

Any insight would be really helpful thanks, I’m completely stuck!

Is your meter set to dc when it needs to be?

 

[DonnieDee]

Is your meter set to dc when it needs to be?

Yes it is.

 

[scott2000]

Yes it is.

Maybe another read through the instructions to make sure everything is installed correctly is in order.

0,715v Ac when on front touching L lead with gr selected.

Not sure what this is describing? "on front", "L lead"?

 

[DonnieDee]

Maybe another read through the instructions to make sure everything is installed correctly is in order.


Not sure what this is describing? "on front", "L lead"?

This is the ax votage touching the live lead on the iec jack.

 

[DonnieDee]

This is the ax votage touching the live lead on the iec jack.

Sorry ac voltage*

 

[jfurgason]

Hi all,

I've been having some difficulty with a Rev A build. I've gone through the entire troubleshooting guide and tested all the points and everything looks within an acceptable range but there is one value that sticks out, Q3 B. The guide says it should be around 0.051 VAC but I get nearly double that with 0.126 VAC. I saw that another post (from page 92, user adam.schw) was having a similar value show up on his Q3 but he mentioned accidentally swapping the 33n and 33j caps and I know mine are in their correct spots. I'm also noticing some significant harmonic distortion when a signal passes through the unit with the GR off and everything set to how it is for troubleshooting (ratio 20:1, meter set to GR, release full CW, attack full CCR (off), both in and out at noon), maybe this is related to the hot signal hitting Q3 B?

I've spent a good amount of time pouring over the component placements and I'm fairly sure that everything is where it needs to be, I've also re-flowed all of the solder joints for good measure. I've been able to follow all but the last step of the calibration setting; I cannot get the reduction meter tracking adjustment calibration step to show -10dB. The closest I've gotten it is to show about -15dB but then I hit the end of the trim pot. Here are the measurements I took after setting the q-bias following the troubleshooting guide, I will also attach a picture of the mainboard and a frequency graph of the harmonics I'm seeing. I would really appreciate any help or insight with this issue, thanks

Power 1 = 30.5 VDC
Power 2 = -9.62 VDC

Signal at input = 0.775 VAC
TP18 = -1.308 VDC [Q-bias has been set]

TP1 = 0.100 VAC
Q2 G = 0.082 VAC
Q3 B = 0.126 VAC ?
TP15 = 3.58 VAC

TP17 = 0.200 VAC
Q4 G = 0.200 VAC
Q5 B = 0.173 VAC
Q6 B = 0.173 VAC

BROWN = 3.14 VAC

OUT = 3.12 VAC

TP22 = [4:1 = 0.056 VAC | 8:1 = 0.119 VAC | 12:1 = 0.184 VAC | 20:1 = 0.388 VAC]
Q7 B = 0.389 VAC
Q8 B = 3.61 VAC
Q9 B = 0.424 VAC
Q10 B = 3.53 VAC

CR2 Anode = [4:1 = 0.529 VAC | 8:1 = 1.103 VAC | 12:1 = 1.684 VAC | 20:1 = 3.52 VAC]

TP21 = [4:1 = -1.518 VDC | 8:1 = -2.33 VDC | 12:1 = -3.16 VDC | 20:1 = -5.78 VDC]

DC Voltages on Q3 after the unit has been powered up for 10 minutes (these seem to be within expectation?)

C = 7.33 VDC
B = 7.42 VDC
E = 14.75 VDC

 

[jfurgason]

Update to my above post: I was able to get the unit to show the -10dB drop, I was failing to center the front panel trim (whoops) before starting the calibration process and now that is working as it should be. However, testing the unit showed a drop-off in low frequency, so I ran a test I saw posted a few pages back by sending in a 1kHz and a 100Hz signal and testing at the + in, TP17 (output wiper), 5002 Brown wire, and the 5002 Blue wire to check for differences in the levels. All values were checked with the chassis as ground, only adjusted the frequency on my generator but left everything exactly the same.

1kHz
+ in = 0.395 VAC
TP17 = 0.146 VAC
Brown = 2.30 VAC
Blue = 0.448 VAC

100Hz
+ in = 0.398 VAC
TP 17 = 0.147 VAC
Brown = 2.29 VAC
Blue = 0.046 VAC

So I'm having a huge loss at the Blue wire of the transformer, where should I be checking to correct this?

 

[Deadly Mix]

Sorry if this is already explained but there are 95 pages and I have a hot soldering iron on.

What do you mean by "Main components". I built the voltage switch now onto the next step (meter pcb, meter switch, meter resistors...).
It says the resistors from the main components. Is that "Rev A Components" or in the PS1 box?

It occurred to me the purpose for this sheet of paper in here may be my answer. I see the R77 listed on this paper under the 1k-9.1k list.
So do I go in the Rev A box and pull any of them from the 1k-9.1k bag then use the same logic for the other resistor?
That seems logical but I'm trying to be sure not to make any assumptions until I've built at least 1 or 2 of these things.

The components in the resistor bags do NOT all have the same color stripes as the rest in each bag. Is that a mixup during packing? Will I see instructions later in the build that tell me stripe specific details to pick? Or do the stripes not tell me anything at all and I'm just worrying too much?

 

[magnusp71]

Hey! I am a beginner so be patient with me. I am in the calibration stages, and I am having trouble with both the preamp and the VU meter.

I was able to get .1V AC at TP1, but the other areas were pretty inconsistent. It looks like i get a voltage on the Q2 Gate that steadily decreases to 0, except for occasionally it will show up around 0.3. The Q3 Base turns up as 2.3 and then starts increasing voltage. TP15 is 4.476 pretty consistently, which is okay I think. The instructions say that if the voltage is below 20% in a certain area, your problem area is there, but I'm not sure the exact next steps I need to be taking. Where should I start?

Hi man. Where do you find the TP’s?

 

[Deadly Mix]

I'm at step 1 of calibration.
- Steinberg Test Generator on a channel
- "External FX" plugin in that sends it out to analog next on that channel
- Swapped cables with my Neve 8801 and the Hairball 1176 so it sends and receives the signal instead of the Neve strip
- Switched my cheap little DMM to "200 ACV" (it only has a 200 and a 750 setting in ACV).
- Set 1176 knobs to: 24, 24, off, and 7 (as Hairball instructs)
- Placed my probes against against the + and - posts on the sides of my XLR output.
- DMM reads "5.8"

Any idea what's wrong?

Also, when will I know its safe to plug it in and see if it even works?

 

[JMan]

I'm slightly confused by your post, but I'm also very tired and my reading comprehension might be diminished at the moment lol

So first, are you saying you are trying to test the audio circuit without having the unit powered up? (This is where I think maybe I'm misunderstanding you because I'm tired). I think you need to power it up first, since it's not a passive circuit.

Second, did you build and test the power supply section before moving on to the rest of the build? As long as you know that the PSU is delivering the correct DC voltages, then you'll just want to do any double checking of the rest of the circuit that you feel is necessary (resistor values, orientation of polarized components and transistors, whatever you want to check) and then power up. This will be the smoke test, and if you were careful as you were building it -- which, you asked a lot of questions in your other thread, which is good, and so I think you were pretty meticulous -- it should pass. From there you will be able to run audio through it, do the calibration, any troubleshooting that might come up, etc.

 

[Deadly Mix]

I bought a better meter and that made setting up for calibration much easier (switched from the cheapest one you can imagine to a Fluke 179).

I was quickly able to adjust my input signal to 0.775 VAC
It was -18.14dB on Steinberg Test-Generator in Nuendo 11 using Lynx Aurora16 DA converter with a normalled Bittree patchbay between Lynx DAC and the 1176 input.

Irrelevant I suppose. What matters is that I got the test signal set correctly.

Then I set Input and Output dials to 12 o'clock.
Attack all the way CCW and Release all the way CW
20:1, GR, Q Bias all the way CW
All other trimmers (the 2 blue square inside and the "hole" in the face) at 12 o'clock

When I move over to the output terminals...
0.4mVAC
If I turn the ouput all the way up or down, I still get the same reading.

That's particularly confusing because, even if I just hold the probes in the air, I get a reading higher than that.

Any idea what I may have messed up on? What should I try to resolder? What did I screw up?

 

[guytanium]

Hey, I just built up a Rev A and completed all calibration without major issues once I fixed a bridge on a JST connector. While I was calibrating I did notice a very slow AC drain when monitoring the output level but just thought it may be caused by my inexpensive DMM.

Once calibrated I was so excited to test it out, I managed to pick up the wrong cable into my audio interface and sent phantom power into the output jack for a few minutes. Stupid mistake.

I am finding now that the 20:1 ratio does not work. It worked initially, then would slowly do less and less GR, but does nothing now. It looks like all other ratios work fine. I will replace the resistors on the ratio board for the 20:1 section. Is there anything else I should replace while I am at it? All capacitors are rated for 50V but I'm not sure if any of them are specific for the 20:1 ratio?

 

[Deadly Mix]

Any idea where to start troubleshooting? Anyone?
I'm about to start it's pair tomorrow and I'd feel a lot better if I get the first one in order before I start the next.