Need help with an unusual problem

[Paul Birkeland]

You will want to turn the volume all the way down on the laptop, then up the minimum amount off zero volume that you can.  Plug that into your Eros (powered off is fine) and measure the AC voltage with your scope across the RCA jacks and then across the 47K resistors on the secondary side of the input transformer.  This will allow you to verify the ratio and those transformers ought to take a fair bit more signal than the typical MC cartridge without a ton of issues. 

If you see a 1:10 step-up, then let me know what the minimum voltage you have that comes out of the laptop and perhaps we can use it and your scope to check the EQ. 

You may also find that this all works a lot better if the laptop is running off its battery.

[Larpy]

I'm back to troubleshooting after a few days of kitchen renovating.  Lots going on right now.

The lowest waveform I could get on the oscilloscope from my laptop to the input of the Eros wasn't as low as I thought it should be:  99-114mV pk-pk (-3mV mean).  At the other end of the Sowter SUTs I measure 1v pk-pk (mean bounced around from -4v to -15v).

This would seem to indicate a proper 1:10 step up of the voltage.

By the way, at the output of the Eros, I measured 6v pk-pk.

The next step?

[Paul Birkeland]

OK, 6V P-2-P is about 2V RMS, which is plenty low for the Eros. 

I think it's worthwhile to to put low level tones into the input of the Eros again and measure the output.  Try a 3.8kHz, a 1kHz, and a 300Hz tone.  All the output voltages should be different.

[Larpy]

Got the sine waves from YouTube.

300hz at input 213-293mv pk-pk    at output 13-14v pk-pk
1Khz at input 217-245mv pk-pk         at output 7.12-7.32v pk-pk
4khz at input 126-174mv pk-pk    at output 10.8-11.4v pk-pk

Way above the other day's 6v at output.

[Paul Birkeland]

300hz at input 213-293mv pk-pk    at output 13-14v pk-pk
1Khz at input 217-245mv pk-pk         at output 7.12-7.32v pk-pk

This is what I would expect to see.  I chose the two frequencies that will be about half/double 1kHz.

4khz at input 126-174mv pk-pk    at output 10.8-11.4v pk-pk

Why did the input level go down?  That may be something that has to be looked at on its own.  Still, you have a bit less than half of the input voltage that you had a 1kHz and you have more output than expected, which indicates that your EQ is not properly working.

[Larpy]

Did some more snooping of my Eros with my new 'sillyscope.  I've concluded that the model I bought cannot detect AC signals below 100mV.  That means there's no way for me to adjust the level of an audio signal from my laptop into the Eros that matches the .4mv output of my cartridge. 

But my tests the other day seemed to confirm that my Sowter SUTs are indeed amplifying the signal by a factor of 10. 

So I decided to assume that the signal entering my Eros was .4mv and the signal after the Sowters was 4mv.  Can't measure either, of course, because they're too low for my 'sillyscope to detect.

I plugged my turntable into the Eros and played the first few tracks of the Cardas test LP:  a 1KHz tone and a 10Hz-30KHz sweep, both recorded (mastered?) at –14 dBs.  As expected, my little scope detected nothing at the input of the Eros and nothing after the Sowter 1:10 SUTs, but I got a clear signal at the outputs:

1KHz tone: 1.8v p-p
frequency sweep: 1v p-p from 10Hz to about 150Hz, where it rose to 1.5v p-p.  Stayed there until 7KHz, where it jumped to 2v p-p.  2.9v p-p at 11KHz, 3.5v p-p at 14KHz.  Then back to 1v p-p at 15K and then rising to 1.5v p-p at 20K.  I have no idea if this is expected behavior or not.

Then I decided to play a regular LP and measure the output of the Eros.  Highest measurement after a couple of minutes of watching the 'sillyscope was 6v p-p.  Is that normal and/or expected, given my cartridge's .4mv output?.

[2wo]

I would have a hard look at the RIAA, a misswired Or wrong component would mess up the eq.
 Also if you need to continue it might help to jump out the transformers or injected the signal after. This would rule them out and you can raise your signal level up a bit, get it out of the weeds so to speak... John

[Larpy]

Why did the input level go down?  That may be something that has to be looked at on its own. 

I assume that's simply because I used test tones from different "channels" on YT.  The first two I used came from the same YT source, but the third came from a different source that presumably put out a lower sine wave amplitude.

Today I'll follow PB's and John's advice and desolder, remove, and reinstall the components in my RIAA EQ stage.  See if that makes a difference.

Thank you for your patience.

[Doc B.]

Don't remove and replace EQ components unless they are determined to be in the wrong place. Just confirm.every position and reflow the connections first.

[Larpy]

OK, I'll exercise caution and restraint.

To clarify, I'm looking to measure the output of my Eros at 300hz, 1khz, and 4Khz, with the voltage at 300hz double that of the voltage at 1K and the voltage at 4Khz half of what it is at 1khz?  Getting such results will indicate that my RIAA eq is functioning properly?

[sl-15]

I read that you are using youtube videos for the tones. Here is a website that will work a bit better. It is a tone generator and you can choose the output and frequency. That way all the volumes for the different frequencies are the same. Hope it helps. https://www.szynalski.com/tone-generator/

[Larpy]

OK, my latest update: I reflowed the solder connections in my RIAA network and got the following measurements at the Eros' output jacks using sine waves from the site Stefan recommended (thank you for the recommendation Stefan; it's a great site and much easier to use than YT).

300Hz  8.7v p-p Right channel; 7.7v p-p Left channel
1KHz    4.7v p-p Right Channel;  4.1v p-p Left channel
4kHz    2.37v p-p Right channel;  2.0v p-p Left channel

These measurements were taken with a feed from my laptop with its volume set as low as it would go and the volume output at the tone generator site set at 9%.  If I turned the volume of the tone generator any lower I couldn't get a reading at 4KHz on the oscilloscope.

I played a sine wave and checked the difference in AC voltage between the input to the EF86 and the output jacks and in order to get a measurement on the oscilloscope at the EF86 I had to turn the volume of the input signal up quite a bit, but what I measured was 574mV p-p at the EF86 grids and 102 p-p at the output jacks.  Is that expected behavior?

And that reminds me: I've been regularly injecting far larger AC signals into my Eros than it would see from playing an LP.  Could that harm the circuit?  The Sowter SUTs?  I checked the resistance across their primaries and secondaries and they still measure in spec.

[Doc B.]

The issue with signals that are 100x what is intended is that you are looking at severely distorted (clipped) waveforms to get your voltage readings since the 6922 will be way overdriven. The results will not be very accurate. Sorry for giving a recommendation for a cheap scope that apparently doesn't have the needed sensitivity. I have only used mine for automotive and motocycle stuff, not low level audio signals.

That said, the result does sound like the preamp is amplifying properly. It could be that the low signal level you get once the Eros is put back in the system is a problem somewhere before or after the Eros. Maybe a cabling issue from the tonearm/cartridge or the wrong input setting on the Moreplay.

[Larpy]

Can a 6922 tube be damaged by being overdriven the way I have been doing?

[Larpy]

Hooked my Eros back up to the audio system and, unfortunately, I'm hearing the same distortion on loud passages I've been hearing for months now.  I recorded about a minute of an LP and once again got an unsymmetrical waveform.  See attached.

I watched my little 'sillyscope while I listened to the LP and the massed strings crescendo that sounded distorted measured around 9v p-p.

So I'm afraid I'm back at square one.  Assuming a 9v p-p signal doesn't indicate the Eros is being pushed into overload, that is.