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

Help Support GroupDIY:

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Sep 6, 2021
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California, USA
I don’t have a second rack unit - I’ve successfully built a couple of FET/500s previously. I got the active link so if I ever do build a second one it’s already be in place without cracking the unit open. This issue is super mysterious to me... Hopefully Mike or somebody sees this, otherwise I’ll send it into Hairball and see what they say. 🤷‍♀️

Okay, I found a hole in my testing regime. The XLR Input +/- terminals ARE shorting to chassis ground when and only when where is a cable inserted and that cable is connected to any other device. I'm going to try removing the jack and making an XLR F pigtail cable to attach to the board, and see what I get.

EDIT: I tried that, and it made no difference, so the issue is definitely somewhere on the board that's beyond. Shipped it to Mike at Hairball today, and I absolutely cannot wait to find out what happened here.
 
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Codedoval

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Feb 11, 2018
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2
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

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Feb 21, 2015
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Sunny...Sometimes Florida- USA
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

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Sep 12, 2021
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5
Location
Canada
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.
 
Last edited:

Codedoval

New member
Joined
Feb 11, 2018
Messages
2
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!
 

DonnieDee

Member
Joined
Sep 12, 2021
Messages
5
Location
Canada
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!
 

jfurgason

New member
Joined
Sep 20, 2021
Messages
2
Location
Los Angeles
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


IMG_3872.jpg
1k signal before L after R.jpeg
 

jfurgason

New member
Joined
Sep 20, 2021
Messages
2
Location
Los Angeles
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?
 

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