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Old 29th January 2013, 02:58 AM   #11
rknize is offline rknize  United States
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From those voltages, the input/driver stage is operating correctly. From your picture, it seems that you measured the signal on the output tube side. Check the R15/25 to make sure they are the correct value. Also check R16/26 while you are there.
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Old 30th January 2013, 12:31 AM   #12
mpri is offline mpri  United States
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Thanks Russ for the help. I measured the resistors and R15/R25 were both 221K and R16/R26 were 100 and 99.8 ohms. I was hoping you were on to something. I also rechecked and reflowed some of the solder in the area with no change in the symptoms.

Could there be a problem with one of the coupling caps? I don't remember this as a problem right away. I think it came on slowly over a couple of months. But then I was so happy to hear it working that maybe I just didn't notice right away.

Thanks again all.

Mike
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Old 30th January 2013, 02:39 AM   #13
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It would be worth swapping them out as the next thing to try. After that, getting the scope out again and probing each stage would be next.
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Old 31st January 2013, 01:29 AM   #14
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Thanks again for the help.

Quick question: I have a pair of .22uf Auricaps on hand. Would attaching them in parallel with the other caps on board be a valid test to see if the originals are bad?

The Auricaps attached with alligator clip wire in parallel with the current .22uf MCaps would change the capacitance to .44uf, correct? I have read in another thread that George has said a coupling cap between .22 and .47 works the best.

My thought is maybe the parallel cap would either lessen or eliminate any difference between the caps to tell if that was the problem. In which case I would then replace them both. See I just don't want to unsolder components at will on the board and risk damage to it if I don't have to.

If not a valid test I just may do some more scoping first.

Although with the new concentric pot I am very happy with it now.

Thanks,
Mike
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Old 31st January 2013, 02:14 AM   #15
rknize is offline rknize  United States
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You can hook them in parallel with no ill effects. Just keep the leads relatively short or it may want to oscillate on you. If the cap is defective or was damaged by heat/etc, I would expect a LF roll off, rather than an overall attenuation. It's worth a shot, though.
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Old 1st February 2013, 02:03 AM   #16
mpri is offline mpri  United States
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Ok, thanks for the tip on the short leads. And yes, I guess I kinda would expect as you said a low frequency roll off if for instance a cap went lower in value. But I have read other threads on various sites where others have had low volume involving a coupling cap, or from a cold solder joint at a coupling cap, (which I have checked and re-soldered). So I hope it could be a good quick test to see if I should replace them before inflicting possible damage to the circuit board. If not then I'll have to break out the scope again and keep checking. I'll probably have time this weekend to check it. I'll post results either way just in case there are others that may have a similar issue, (I couldn't be the only one? Or am I?).

Thanks,
Mike
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Old 1st February 2013, 09:45 AM   #17
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Not trying to be funny, but you're sure it's not your speakers / speaker-cables / connectors?
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FLAC, Ethernet over Mains, SB3, GD Audio DAC 19. Tubelab SE with TJ Meshplate 45s. Fostex FE208ez in Kirishima Horns with Fountek NeoCD 2.0 Ribbon supertweeters (0.68uF & LPads).
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Old 1st February 2013, 02:12 PM   #18
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Re:
Quote:
you're sure it's not your speakers / speaker-cables / connectors?
Yes. When I test (scope, voltage, ohm readings) I move it to a bench and use other speakers and cables where it has the same issue.

Thanks for the suggestion, and all suggestions are wlecome.

Mike
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