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Old 6th August 2014, 09:15 PM   #91
YashN is offline YashN  Canada
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Received the new 12AT7 from tubedepot, took ages to reach here.

Swapped it in, but still no luck with the right channel so the plan is to troubleshoot that whole right channel step-by-step.

I'll start with checking for shorts at the input then follow along the schematics and components.

If anybody has good information on how to do this, I'm all ears.
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Old 20th May 2015, 04:48 AM   #92
YashN is offline YashN  Canada
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Fingers crossed I can troubleshoot that problematic channel soon. Since outputs work, the issue should be localised to the right input. The amp is very heavy but the diagnosis shouldn't take an inordinate amount of time.
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Old 26th March 2016, 05:52 PM   #93
YashN is offline YashN  Canada
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Default My work area

Since I am always managing to make a mess in the dining room, in full view of any vistior/friend even after supposedly cleaning up, I was forced to set up a small new laboratory-slash-workshop area.

It's next to my synths and in the same room as the audio/video equipment, so DIY with music is possible, and it's familiar territory.

Click the image to open in full size.

Note the cunningly placed brown curtain, which handily hides all the salvage box underneath the table, although we're not immune to a cable peeking out.

On the table today is my DIY SET Tube Amp. I shudder at the thought of working on it every time: it's heavy. However, the Edcor transformers also make convenient 'feet' while I am diagnosing and working on the circuit.

The old Windows laptop, whose USB board doesn't work anymore is used solely as a way to access documentation on the net while troubleshooting, designing and researching or posting on audio forums, as well as for doing some LTSpice simulations.
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Old 27th March 2016, 11:18 AM   #94
wdecho is offline wdecho  United States
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One suggestion I have is to do resistance readings at all points between the 2 channels and compare. I had one dead channel on startup and found my problem this way. If at the same point of the audio chain you have a different reading between the good side and the bad side you have a section to start looking in. Diagnosing is first determining which section the problem is in then you can narrow it down to a few components. It is best to have the right test equipment but when you don't finding the problem can still be done but it is much easier when you narrow down the problem to fewer components.
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Old 27th March 2016, 12:12 PM   #95
cjkpkg is offline cjkpkg  United States
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The schematic luckily is elegantly simple.

Swap everything between L and R that you can even if you think you ruled something out.
What is your input source? Functioning properly?
Swap cables
Swap opts
Swap coupling caps
Double check all grid stoppers in suspect channel
Make sure input rcas are not shorted to ground

12at7 and the ccs are pretty robust which leads me to something simpler.
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Old 27th March 2016, 02:43 PM   #96
YashN is offline YashN  Canada
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Quote:
Originally Posted by wdecho View Post
One suggestion I have is to do resistance readings at all points between the 2 channels and compare. ... it is much easier when you narrow down the problem to fewer components.
Hey wdecho, thanks for the suggestion, great idea indeed.

Yesterday, I re-wired in simple Triode mode, this took me a long time, but at least I can probably diagnose easier with it.

Today, I measured the resistors on the right channel, they seem to all check out good.
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Old 27th March 2016, 02:48 PM   #97
YashN is offline YashN  Canada
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Quote:
Originally Posted by cjkpkg View Post
The schematic luckily is elegantly simple.
Indeed, and that's a good thing from tube amp designs. However, it's hard to know where to start with this kind of issue.

Thanks for the suggestion, it could be cool to swap the coupling caps I think, and perhaps other caps. When I diagnosed using George's method, both outputs work, so I think the OPTs are still OK, and the issue is narrowed down to the input section.

Quote:
12at7 and the ccs are pretty robust which leads me to something simpler.
Good to know, and a bit surprising for the CCS - I thought that since it was an IC, it could easily be damaged.

Swapping 12AT7 gave nothing different, so I think my tube is OK.

I also did a quick check for shorts with the inputs, and they're OK.
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Old 27th March 2016, 03:53 PM   #98
wdecho is offline wdecho  United States
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Quote:
Originally Posted by YashN View Post
Hey wdecho, thanks for the suggestion, great idea indeed.

Yesterday, I re-wired in simple Triode mode, this took me a long time, but at least I can probably diagnose easier with it.

Today, I measured the resistors on the right channel, they seem to all check out good.
When you take a reading say at the grid of the 12AT7 good channel compare it with the reading on the non working channel 12AT7 grid and see if they match and so on and so forth and see if every point on both channels are the same. When you run across the point that has a different reading than the good channel you have made progress.

Then you can do the same thing with voltage readings both DC and AC and compare. You have one channel you know is good so compare impedance and voltage readings between each channel. You are going to find something, voltage or resistance, that is not the same as the good channel. Then you will have localized the trouble then start looking at those components in that part of the circuit.

Last edited by wdecho; 27th March 2016 at 04:01 PM.
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Old 27th March 2016, 04:27 PM   #99
wdecho is offline wdecho  United States
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I am assuming you know the golden rule when working with a HOT amp. One hand in pocket.
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Old 27th March 2016, 04:43 PM   #100
cjkpkg is offline cjkpkg  United States
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Double check values for r10 13 14 and 19

What is each section running in mA? Measure above and below the triode section across the resistor once you confirm the values are correct. This will at least tell you if the ccs is functioning and that it's a signal issue instead. Separate between pure tube function vs how the signal travels.

Again double check grid stoppers that they aren't mis valued...

Lastly don't count on the label the resistors came in were correct...measure measure measure...
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