[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
141
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
141
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!
 

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