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

Help Support GroupDIY:

MiKa84

New member
Joined
Oct 13, 2017
Messages
2
Hi fellow DIY'ers,

Just finished a Rev F build. This is my third Hairball FET/Rack build, the first two went all smooth but on this one I'm having trouble.
When I turn it on the 75R 3W (R84) resistor in the power supply turns smoking hot after a few seconds. Obviously something is drawing way to much current. What have I done so far?
Checked all the components if they are the correct values - should be good on that
Checked the polarity on all electrolytic caps - good
At this point I'm a bit lost and I would very much appreciate it if someone that maybe had a similar issue before or someone that has an idea on where to look could chime in and point me in the direction on where to look next.
If it wasn't all on a pcb board I would disconnect the gain reduction or the signal path but since that is not an option I'm hoping for someone with a good idea on how to proceed without desoldering all the components.
Help is very much appreciated.

Thanks
Found the problem, I did swap Q8 & Q9.
Stupid mistake that brings a lot of problems.
 

Diamondj421

Well-known member
Joined
Sep 24, 2016
Messages
144
I’m sure this question has been asked already but my searches didn’t come up with anything. I’ve had my Rev D for about 5 years now and a week ago, the bulb in the meter died on me. The meter itself still functions properly though. I’m sure it could be something other than the bulb but I’m unaware of any other potential culprits.

Has anyone done this repair before? I haven’t opened it up yet. Figured I’d get my ducks in a row before tackling it so the units not down for longer than it needs to be.

Also, I see on the Hairball site that the meter currently in the unit uses a “12v fuse style lamp” but that there is also an LED version. Is it possible to convert the meter to LED? If not, does anyone have anymore details on a replacement bulb?
 

band19

New member
Joined
Dec 26, 2009
Messages
1
Hello all, I have started a rev Fet/rack rev F build and on my component sheet I can't see a r value for R82.?
I was also trying to find the PCB population map that is mentioned in the build thread but have not had any luck.

Any help appreciated

Ed
 

Diamondj421

Well-known member
Joined
Sep 24, 2016
Messages
144
I've spent a good amount of time scouring the web for a new meter bulb but can't seem to track down a replacement. Has anyone found these anywhere? Preferably in the US?
 

Mr.Mezmerize

Member
Joined
Dec 1, 2015
Messages
6
Location
Germany
Hi everybody!

I bought two Rev.F partial kits in April and I'm having trouble sourcing the MPSW06 and MPSW56 output transistors. I bought some on Ebay from China alongside the kit, but it seems like they were lost in shipping (5 weeks since the last update, even though they appear to be in the destination country).

Does anyone know another source for those parts, preferably in europe? I've had no luck with my ususal parts suppliers. Or are there any easy to get parts I could substitute?

I also contacted Hairball Audio about this a few days ago, but did not get any answer yet.

Any help would be greatly appreciated! Both units have been sitting here for a month, waiting to be powered up. :confused:

Thanks and cheers,
Lucas
 

jakekeisler7

New member
Joined
Apr 11, 2021
Messages
1
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?
 

SBAS

New member
Joined
Jul 6, 2021
Messages
1
Location
Illinois
Hello!

I recently put together a Rev A kit that I purchased a few years ago (PCB V1.12 - newer plug/play PSU) and I'm having low audio output issues.

Basically, build went fine - PSU checked out with +30VDC and -10VDC at respective points, device does input and pass signal, however, when attempting to do the very first calibration process (Qbias), I cannot output more than 1.2~VAC, not even close to the requested +11dB/2.75VAC (input set to .775VAC, Input knob set to 24). That's even with the output knob completely cranked/zeroed.

I went to the troubleshoot section of the Hairball site and started testing.

First test was for QBias Voltage - Requesting -1 VDC to -1.78 VDC at TP18 when setting QBias trim from mix to max. I did pass this with accurate voltages.

The next test requested setting the input knob to read 0.100VAC at TP1, and then check Q2 Gate, Q3 Base, and TP15. Q2-3 and TP15 were all very low voltages.

So this points me to the output circuit then? I guess my question is, where do I go and what do I look for from here? I'm currently double checking transistors and for cold joints, but nothing seems too obvious.

Lastly, unrelated, my meter does not seem to move at all. I'm assuming it's damaged? It lights up but that's it.

Thanks!
 

MoltenReplica

New member
Joined
Feb 5, 2021
Messages
1
Hey y'all,
I'm working on a Rev A 2.0 kit. I'm on the calibration step, but I have no output signal. Working through the troubleshooting guide, I can see that I've got some issue in the preamp section. Q2-G is reading 0.03 VAC, with Q3-B at 0V. I'm not sure how to identify the problem at this point. I checked all the resistor values and everything appears to be soldered fine.
Thanks in advance!
 

AlexisPS

New member
Joined
Aug 13, 2021
Messages
1
Location
France
Hello!

I recently put together a Rev A kit that I purchased a few years ago (PCB V1.12 - newer plug/play PSU) and I'm having low audio output issues.

Basically, build went fine - PSU checked out with +30VDC and -10VDC at respective points, device does input and pass signal, however, when attempting to do the very first calibration process (Qbias), I cannot output more than 1.2~VAC, not even close to the requested +11dB/2.75VAC (input set to .775VAC, Input knob set to 24). That's even with the output knob completely cranked/zeroed.

I went to the troubleshoot section of the Hairball site and started testing.

First test was for QBias Voltage - Requesting -1 VDC to -1.78 VDC at TP18 when setting QBias trim from mix to max. I did pass this with accurate voltages.

The next test requested setting the input knob to read 0.100VAC at TP1, and then check Q2 Gate, Q3 Base, and TP15. Q2-3 and TP15 were all very low voltages.

So this points me to the output circuit then? I guess my question is, where do I go and what do I look for from here? I'm currently double checking transistors and for cold joints, but nothing seems too obvious.

Lastly, unrelated, my meter does not seem to move at all. I'm assuming it's damaged? It lights up but that's it.

Thanks!
Hi,
Did you manage to make it work? I seem to have almost the same problem (output voltages way lower than expected - around 0,035VAC and not moving with the output knob) and I didn't pass the TP18 test either. So I suppose the problem lies within the Q-Bias section or before. But all my joints seem to be ok, I check all the resistors, the transistors, the capacitors and their polarity... Even my VU Meter is at -∞ on GR mode instead of 0dBVU (maybe it's normal at this stage though).
I'm stuck at the moment...
Any idea?
Thank you!!
 

villamonster

New member
Joined
Aug 30, 2021
Messages
1
Location
NY
Halp! Does CR11 attach to the pad closest to the edge or the spot closer in over the image?
 

Attachments

  • 35B982A1-3461-474F-9F09-FE0B744B7CCE.jpeg
    35B982A1-3461-474F-9F09-FE0B744B7CCE.jpeg
    103.3 KB · Views: 12

johnnybregar

Member
Joined
Nov 22, 2013
Messages
15
I’m building a Rev A and managed to mix up my transistors and now I don’t know which ones are my matched pair for Q12/13. Is it critical that these are matched?
 
Last edited:

camshash

Member
Joined
Sep 4, 2021
Messages
20
Location
Michigan
Hi everyone, just wanted to post a quick solution to a problem I faced build a REV A.

I noticed my release knob was not affecting anything, Full CW or CCW resulted in no change.

Easy fix, accidentally mixed up the Output pot with the Release Pot. Release Pot is blue, Output pot is gray.

At first I thought they were the same as they they were the same size and since the input pot was blue, I assumed the output pot would be blue aswell, and the attack pot was grey, the release should be gray. Not the case, make sure you dont mix them up like I did haha, release function works exactly as it should now.
 
Joined
Sep 6, 2021
Messages
5
Location
California, USA
Hi folks, I am at my wit's end over here, and I'm hopeful somebody can help. I "completed" my latest rev D build this morning, but upon going to calibrate, I've found that I'm getting (effectively) 0 VAC at the XLR IN +/- terminals. I can get measurable voltage there by CRANKING the output of my interface, but that's not correct - I've measured the voltage at the XLR cable end when disconnected and it's 0.775 VAC. i am using the orange drop mod capacitors, but I don't think they're the issue (see below). I have not installed R-OPT.

In order to try to isolate the issue, I've tried...

1) All components installed - approx. 0 VAC at the XLR IN +/- terminals
2) Desoldering the XLR input jack and testing the pins of the jack directly with the XLR cable connected - 0,775 VAC at the XLR IN +/- terminals
3) Desoldering and removing the TPAD, as well as the input transformer. Reading the schematic, that should just leave the XLR input jack isolated - approx. 0 VAC at the XLR IN +/- terminals

I've tried these things with the mainboard on its own, only the mainboard attached to the chassis, with the chassis fully assembled, with the chassis fully assembled and powered on. I have also tried sending the signal to the XLR output jack, and testing at those terminals - that yields the expected voltage. I initially tested with the active link board installed, but removed it to eliminate as many variables as possible, The unit DOES pass audio - the input signal level is just ludicrously low.

At this point, I'm at a loss for what else to try. Anybody have any ideas for what I could/should look at here before I consider sending it to Hairball for diagnosis? I've checked over my solder joints, and I don't see any obviously bad joints or anything shorting, and I've reflowed a lot of joints to be sure.
 
Last edited:

camshash

Member
Joined
Sep 4, 2021
Messages
20
Location
Michigan
Hi folks, I am at my wit's end over here, and I'm hopeful somebody can help. I "completed" my latest rev D build this morning, but upon going to calibrate, I've found that I'm getting (effectively) 0 VAC at the XLR IN +/- terminals. I can get measurable voltage there by CRANKING the output of my interface, but that's not correct - I've measured the voltage at the XLR cable end when disconnected and it's 0.775 VAC. i am using the orange drop mod capacitors, but I don't think they're the issue (see below). I have not installed R-OPT.

In order to try to isolate the issue, I've tried...

1) All components installed - approx. 0 VAC at the XLR IN +/- terminals
2) Desoldering the XLR input jack and testing the pins of the jack directly with the XLR cable connected - 0,775 VAC at the XLR IN +/- terminals
3) Desoldering and removing the TPAD, as well as the input transformer. Reading the schematic, that should just leave the XLR input jack isolated - approx. 0 VAC at the XLR IN +/- terminals

I've tried these things with the mainboard on its own, only the mainboard attached to the chassis, with the chassis fully assembled, with the chassis fully assembled and powered on. I have also tried sending the signal to the XLR output jack, and testing at those terminals - that yields the expected voltage. I initially tested with the active link board installed, but removed it to eliminate as many variables as possible, The unit DOES pass audio - the input signal level is just ludicrously low.

At this point, I'm at a loss for what else to try. Anybody have any ideas for what I could/should look at here before I consider sending it to Hairball for diagnosis? I've checked over my solder joints, and I don't see any obviously bad joints or anything shorting, and I've reflowed a lot of joints to be sure.
Have you confirmed that your input transformer is working as it should? Make sure there are no shorts in the windings
 
Joined
Sep 6, 2021
Messages
5
Location
California, USA
Have you confirmed that your input transformer is working as it should? Make sure there are no shorts in the windings
What would be the best way to test that? I assume connectivity checks between both sides of the transformer?

Still, with the transformer and TPAD removed, I would expect to be able to measure the VAC coming from the XLR cable at XLR IN +/-, as the connector should effectively be isolated on the board at that point... No?
 
Joined
Sep 6, 2021
Messages
5
Location
California, USA
Have you confirmed that your input transformer is working as it should? Make sure there are no shorts in the windings

Okay, so if the outside pin marked with the dot is pin 1, it's like this with the transformer soldered on the board (view from bottom of board)...

1 2 3 4
5 6.7 8

1 and 4 are connected. 5, 7 and 8 are connected.

Off the board, 1 and 4 are connected, and 5 and 8 are connected.
 
Last edited:

camshash

Member
Joined
Sep 4, 2021
Messages
20
Location
Michigan
Okay, so if the outside pin marked with the dot is pin 1, it's like this with the transformer soldered on the board (view from bottom of board)...

1 2 3 4
5 6.7 8

1 and 4 are connected. 5, 7 and 8 are connected.

Off the board, 1 and 4 are connected, and 5 and 8 are connected.
I'm not sure exactly the pin out for the transformer, but on a copper build I had there was a problem with the input transformer which made it so i had to crank my signal generator very high in order to see the test voltage at the inputs. Since you have the active link, do you have another FET rack you can troubleshoot with? Mike had me test the impedance between all transformer windings pads with either green ground pads.
 
Joined
Sep 6, 2021
Messages
5
Location
California, USA
I'm not sure exactly the pin out for the transformer, but on a copper build I had there was a problem with the input transformer which made it so i had to crank my signal generator very high in order to see the test voltage at the inputs. Since you have the active link, do you have another FET rack you can troubleshoot with? Mike had me test the impedance between all transformer windings pads with either green ground pads.
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. 🤷‍♀️
 

Latest posts

Top