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Old 6th June 2012, 08:53 AM   #511
mogliaa is offline mogliaa  United Kingdom
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Hi Pete,
I was measuring THD on valves in common cathode mode with a CCS load taking the output through a capacitor, so DC was blocked. For whatever strange reason I had the ground floating switch on. I was also feeding the test jig with the interface output.
I did most of the measures at 10Vrms AC, however did some +20Vrms AC tests as well. Is this not the limit for floating measures?

Measures were fine till a member of the forum highlighted that THD on 26 was a bit high. Then I found that he was right by retaking measures and looking that the harmonic composition was very high indeed - and it wasn't the valve. Input signal in the soundcard was very low and distorted.

Need to retake some more tests this evening to answer your questions regarding the panel meter going blank.

Thanks
Ale
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Old 6th June 2012, 02:24 PM   #512
mogliaa is offline mogliaa  United Kingdom
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Just done a quick test over my lunch break and got the following tested:

1. Soundcard is working fine, no damage.
2. When producing a sine wave with Audiotester calibrated with oscilloscope to produce 1 Vrms @ 1KHz, then the interface panel voltmeter reads 2.17V and the output can be measured with my bench true RMS meter to be 2.17V
3. Interesting to see that when I just connect the GND clip of my scope to the GND terminal of the interface, then the meter goes blank and the signal fades.
4. This doesn't happen with my bench true RMS meter as of course is isolated from earth.
5. When measuring loop signal from PC OUT (1Vrms) to PC IN, the input sine is distorted and reduced in amplitude significantly.

Evidently something is damaged here...
any thoughts?

thanks
Ale
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Old 6th June 2012, 02:40 PM   #513
SY is offline SY  United States
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Do you have the floating/grounded switches set properly?
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Old 6th June 2012, 02:50 PM   #514
mogliaa is offline mogliaa  United Kingdom
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I think all went pear-shaped when I lifted the ground switch to float yesterday and measuring output swing above 20Vrms.
If I remember correctly the last tests were done with the switch in GND. That was the way I used it before and worked fine....

Even I tried plugging in an external signal generator (Ferrograph RTS2) and when plugged the same thing happened with the panel meter: went blank.

What other test do you suggest?
Thanks
Ale
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Old 6th June 2012, 06:53 PM   #515
mogliaa is offline mogliaa  United Kingdom
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With ground lifted meter works but signal is distorted heavily.
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Old 6th June 2012, 08:10 PM   #516
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Quote:
Originally Posted by mogliaa View Post
3. Interesting to see that when I just connect the GND clip of my scope to the GND terminal of the interface, then the meter goes blank and the signal fades.
That is interesting... the interface should (always) be grounded to the PC via the sound card. So you are saying the meter goes blank if you touch the ground of your scope to the INPUT BNC ground? Or the output? Or the sound card ground?

For the meter to go blank, I would suspect that the V5A power supply is being killed. So probably also the +15V supply. Can you confirm that with the DMM? See if the supplies collapse when you hit the GND switch.

If so, that would imply that somehow there is a short between the (-) input BNC and a supply. Which seems unlikely, as there would have to be several failed components (cap, resistor, protection diode).

The input shouldn't have been damaged, as long as DC didn't exceed 400V and AC didn't exceed ~100V. If it did, the protection diodes (D5-D8) and the input amp (U4) could be damaged. But I don't understand the meter thing...

Pete
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Old 6th June 2012, 08:29 PM   #517
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by pmillett View Post
That is interesting... the interface should (always) be grounded to the PC via the sound card. So you are saying the meter goes blank if you touch the ground of your scope to the INPUT BNC ground? Or the output? Or the sound card ground?
I only tested the output. Can make similar tests with input and ensure that it's grounded via soundcard BNC?
Will test voltages as you suggest.

How can you explain the distorted signal and the output amplification? E.g. When I feed the output buffer with the soundcard output at 1Vrms I get more than 2vrms!
Thanks for the help
Ale
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Old 8th June 2012, 01:12 PM   #518
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Quote:
Originally Posted by mogliaa View Post
I only tested the output. Can make similar tests with input and ensure that it's grounded via soundcard BNC?
Will test voltages as you suggest.

How can you explain the distorted signal and the output amplification? E.g. When I feed the output buffer with the soundcard output at 1Vrms I get more than 2vrms!
Thanks for the help
Ale
The gain is normal. It is designed to have 6dB of gain from the soundcard output, because many soundcards can't put out more than a volt.

I'm afraid you will need to do some debugging - there is no way I can answer "what's wrong?" without testing and measuring.

If grounding the OUTPUT causes a problem there must be a power supply issue. Perhaps you fried one of the ICs and it's drawing too much current, killing a power supply rail. Measure the voltages, pull out ICs (if you socketed them), etc.

Pete
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Old 8th June 2012, 01:50 PM   #519
mogliaa is offline mogliaa  United Kingdom
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Hi Pete,
Managed to do some other quick tests during my lunch break. Here is the summary:

1) Output driver works fine. Fed 500mVrms signal with external oscillator and get a clean 1Vrms output.

2) When checked ground continuity between signal ground and sleeve, found about 6K resistance. My BNC cables from the soundcard are connected to TIP and RING. I was expecting to have 0 resistance when GROUND switch is on?

3) Checked +5V and +15/-15V and are ok
4) When BNC are not plugged into the soundcard and connect oscilloscope ground to input ground there is no loss of power supply voltage and LCD is ok
5) When connected BNCs to soundcard, and connect the oscilloscope ground, then the LCD fades out and clearly the power supply drops to about 1.2V
Touched the output driver U8 (DRV134) and was really hot.So when removed U8 from IC socket power got back again on +5, +15 and -15 rails and LCD back to norm.

Could have been that at some point by mistake I swap I/O BNCs from the soundcard and damaged the interface output driver?

What other test do you suggest before I order some additional ICs?
I should be able to test this in more detail over the weekend

Thanks for the help
Ale
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Old 9th June 2012, 03:40 AM   #520
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Originally Posted by mogliaa View Post
Could have been that at some point by mistake I swap I/O BNCs from the soundcard and damaged the interface output driver?
Thanks for the help
Ale
AHA! That is entirely possible. That would explain all of the symptoms. It souds like the (-) output of the driver has popped, probably an internal short to a power rail. The (+) side must still be OK.

I would replace it and see if that fixes the problem.

Pete
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