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SOHA II Builders Thread - Page 86

post #1276 of 1687

back of the envelope so to speak

I did a little sketch of what I have envisioned for a case while sitting on a bumpy flight into the windy city.

Attachment 28512

I am thinking that the case will be a lighter color wood such as a flamed maple and the corners / feet will be something dark like ebony. We will see if I can actually pull it off and still keep the costs relatively in check.

I'm thinking I will make the top and bottom out of aluminum plate. Somehow I have to work some ventilation in and I have a couple of interesting ideas for that but I'm quite sold yet.

Hopefully I can get this baby up and running soon, I'll start working the case in the mean time.
LL
post #1277 of 1687
I will be using perforated sheet metal as the top plate: Amazon.com: Perforated Cold Rolled Steel 12" x 12" Sheet, .060" Thick, 0.078" Round Perfs on 0.125" Centers: Industrial & Scientific

$7.68 free shipping. Its not stainless steel though, so I'll have to paint it.
post #1278 of 1687
Quote:
Originally Posted by Eokboy View Post
I will be using perforated sheet metal as the top plate: Amazon.com: Perforated Cold Rolled Steel 12" x 12" Sheet, .060" Thick, 0.078" Round Perfs on 0.125" Centers: Industrial & Scientific

$7.68 free shipping. Its not stainless steel though, so I'll have to paint it.
Nice find - It looks like they also carry aluminum if you didn't want to paint it
post #1279 of 1687

New opamp voltages

Hey Runeight,

I replaced the opamps and it looks like the left side is in better shape now. Here are the voltages of the top four pins as looking at the part from above
from left to right:

14.7V, 15.1V, 3.2V, 0V

and the bottom 4 pins (L-R from above)

14.7V, 0V, 0V, -15V

I checked the voltage drop across R10L and was able to dial in the current so I think we have this side back on track.

I also swapped out the opamp on the right side but i didn't get much change in the voltages and think I still have a problem.

-13.9V, -14V, -14.5V, 0V

and the bottom 4 pins

-13.9V, -13.5V, -13.5V, -15V

Any ideas of where to look next?
post #1280 of 1687
Quote:
Originally Posted by regal View Post
Are you sure your heatsinks are isolated?
Sorry for the delay, checked the isolation of the heatsinks. The IC's seemed to be installed correctly with mica sheet and the nylon washers. However i am getting a reading of 0.5v on the heatsinks themselves. Still getting the crazy readings on the test points. the LED's still light fine. and am still getting an 18.6V reading on the screws on the yellow and black wires coming into the board from the T1.
post #1281 of 1687
Quote:
Originally Posted by tuna320 View Post
Sorry for the delay, checked the isolation of the heatsinks. The IC's seemed to be installed correctly with mica sheet and the nylon washers. However i am getting a reading of 0.5v on the heatsinks themselves. Still getting the crazy readings on the test points. the LED's still light fine. and am still getting an 18.6V reading on the screws on the yellow and black wires coming into the board from the T1.
If you are reading 0.5V on the heatsinks they aren't isolated.
post #1282 of 1687
Quote:
Originally Posted by regal View Post
If you are reading 0.5V on the heatsinks they aren't isolated.
thanks for the quick response. Is the problem the heatsinks isolation from the IC or from each other. Any suggestions on how to better isolate the heatsinks.
post #1283 of 1687
Quote:
Originally Posted by tuna320 View Post
thanks for the quick response. Is the problem the heatsinks isolation from the IC or from each other. Any suggestions on how to better isolate the heatsinks.
Best practice: before soldering the IC attach the heatsink, then use your DMM to make sure it is isolated, if its not try different nylon washers, many aren't long enough. You have to check at a bare metal spot on the heatsink, many are coated and you can't get a reading.
post #1284 of 1687
Quote:
Originally Posted by runeight View Post
Yes, you're right. Did you use the right transistors??? If so, then one or both Q1/Q2 are still bad.

You are getting the correct voltage on Pin 1.
Sorry about the delay -
The replacement parts got lost in the mail...
I replaced Q1 and Q2 on both side a second time around, taking all precautions to make sure they didn't get damaged in the process.

I am getting the same measurements, the difference now being that I get a 3.3v drop on one side. (0.2v on the other)

I also received a PM this morning - it seems that another head-fi'er is experiencing the same issue.
post #1285 of 1687
My SOHA II lives!

I accidentally shorted R6 and Q4, smoked the trimpot. Good thing I have a 500R one.

+/-18V LV supply, 90V B+, 7mA tail current, using BD137 output transistor.
post #1286 of 1687
Thread Starter 
Quote:
Originally Posted by JamesL View Post
Sorry about the delay -
The replacement parts got lost in the mail...
I replaced Q1 and Q2 on both side a second time around, taking all precautions to make sure they didn't get damaged in the process.

I am getting the same measurements, the difference now being that I get a 3.3v drop on one side. (0.2v on the other)

I also received a PM this morning - it seems that another head-fi'er is experiencing the same issue.
Well then, we'll have to be a bit more exhaustive in our debugging. I hate to ask, but if you would kindly measure with respect to ground:

1. All pins on both tubes.
2. All test points
3. All opamp pins

I will transfer these to a paper schematic and see if I can figure out what gives.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1287 of 1687
Quote:
Originally Posted by JamesL View Post

...

I also received a PM this morning - it seems that another head-fi'er is experiencing the same issue.
The other head-fi'er it's me .

I'm having the same problem as JamesL, and also replaced Q1 and Q2 and obtained the same results.

I was planning to spend a few hours debugging the amp over the weekend and see if I can solve this by myself.

Since I'm getting a voltage drop between TB+L and TP of only a few milivolts, I'm assuming (and please correct me if I'm wrong) that almost no current is getting through the tubes). So I believe that there must be something wrong with the CCS, but can't figure out what.

If I can find some time before the weekend I will try to document all my results exhaustively and post them here, hopefully we can work this out together .

Thanks!!!!
post #1288 of 1687
It is really easy to have your probes slip and short R6 and Q4 on the left channel. Found out I fried the trimpot when there is no voltage across R6, and the fried trimpot measured in the megaohms.
post #1289 of 1687
Quote:
Originally Posted by Eokboy View Post
It is really easy to have your probes slip and short R6 and Q4 on the left channel. Found out I fried the trimpot when there is no voltage across R6, and the fried trimpot measured in the megaohms.
That's the first thing I'm going to check.

Many thanks for the advice!!!
post #1290 of 1687

Parts Sourcing

Hi all. I'm trying to source parts for a SOHAII and everyone seems to be out of the MC7815 and MC7915 regulators. Mouser, Newark, and Digikey are all out. Are you guys hording them? Any suggestions where to find some? Are there any alternative parts?

For example, could I use this instead of the 7815 and this for the 7915?

Thanks in advance.
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